RESPONSE TO SIGNIFICANT PUBLIC COMMENTS ON THE PROPOSED CLEAN AIR MERCURY RULE Received in response to:

Proposed National Emission Standards for Hazardous Air Pollutants; and, in the Alternative, Proposed Standards of Performance for New and Existing Stationary Sources: Electric Utility Steam Generating Units ( 69 FR 4652; January 30, 2004)

Supplemental Notice for the Proposed National Emission Standards for Hazardous Air Pollutants; and, in the Alternative, Proposed Standards of Performance for New and Existing Stationary Sources: Electric Utility Steam Generating Units ( 69 FR 12398; March 16, 2004)

Proposed National Emission Standards for Hazardous Air Pollutants; and, in the Alternative, Proposed Standards of Performance for New and Existing Stationary Sources, Electric Utility Steam Generating Units: Notice of Data Availability ( 69 FR 69864; December 1, 2004)

Docket Number OAR 2002 0056 6.0 MERCURY EMISSIONS MONITORING US Environmental Protection Agency Emissions Standards Division Office of Air Quality Planning and Standards Research Triangle Park, North Carolina 27711 15 March 2005 i General Outline 1.0 INTRODUCTION AND BACKGROUND 2.0 APPLICABILITY AND SUBCATEGORIZATION 3.0 PERFORMANCE STANDARDS FOR COAL FIRED ELECTRIC UTILITY STEAM GENERATING UNITS 4.0 PERFORMANCE STANDARDS FOR OIL FIRED ELECTRIC UTILITY STEAM GENERATING UNITS 5.0 MERCURY CAP AND TRADE PROGRAM 6.0 MERCURY EMISSIONS MONITORING 7.0 IMPACT ESTIMATES 8.0 COMPLIANCE WITH EXECUTIVE ORDERS AND STATUTES 9.0 NODA 10.0 OTHER Appendix A LIST OF COMMENTERS 6 1 6.0 MERCURY EMISSIONS MONITORING 6.1 SELECTION OF MONITORING METHOD Comment:

Several commenters ( OAR 2002 0056 2429,  2634,  2718,  2922,  3565) noted that in the proposed MACT in subpart UUUUU, EPA's proposed 40 CFR part 75 revisions include provisions for use of both Hg CEMS and a sorbent trap monitoring system meeting Method 324. However, EPA also proposes to prohibit new units that commence commercial operation more than 6 months after publication of the final rule from using Method 324. The commenters do not believe that either of EPA's proposed alternatives to restrict use of Method 324 is appropriate or justified.

Response:

EPA agrees with the commenters. In light of comparative field data, EPA believes that monitoring using sorbent media should be as similar as possible to monitoring using Hg CEMS. Therefore, today's final rule allows the sorbent trap methodology to be used for new units.

Comment:

One commenter ( OAR 2002 0056 2375) stated that if the EPA promulgates a requirement that all sources above a certain emissions threshold monitor with CEMS, the commenter recommended that the threshold be 76 lb/ year.

Response:

EPA's rule will require continuous monitoring ( Hg CEMS or sorbent trap monitoring systems) for all units with annual Hg mass emissions greater than 29 lb/ yr. For units with Hg emissions of less than or equal to 29 lb/ yr, a low mass emitter option is provided that is less rigorous but still environmentally conservative.

Comment:

Numerous commenters ( OAR 2002 0056 1596,  2101,  2429,  2485,  2634,  2718,  2827,  2850,  2861,  2918,  2922,  2932,  2948,  3406,  3432,  3455,  3536,  3539,  3565) expressed concern that EPA's proposal is unfairly and unjustifiably biased against the sorbent trap method. The commenters did not support Alternative # 1, because it restricts the use of sorbent traps to low emitting units. Commenters were generally more receptive to Alternative # 2, except for the proposed quality assurance/ quality control ( QA/ QC) procedures for sorbent trap systems ( most notably the quarterly relative accuracy testing), which they found to be inappropriate, 6 2 overly burdensome, costly, and time consuming. Several commenters stated that EPA has no justification for restricting the use of the sorbent trap method because it has been shown during EPA sponsored mercury ( Hg) monitoring demonstrations that the method can achieve accuracies comparable, and in some cases better than those achieved by Hg CEMS. Other commenters recommended that the type of QA/ QC procedures prescribed for sorbent trap systems should be more specific to the sorbent trap technology and should be more clearly defined. Finally, a number of commenters objected to the proposal to report the higher of the two Hg concentrations from the paired sorbent traps, and recommended that the results be averaged instead.

Response:

Section 75.81( a) of the final rule adopts a modified version of Alternative # 2, which allows the use of sorbent trap systems for any affected unit, provided that rigorous, technologyspecific QA procedures are implemented. The operational and QA/ QC procedures for sorbent trap systems are found in section 75.15 and in appendices B and K of the final rule. EPA also has incorporated the recommendation of the commenters to use the average of the Hg concentrations measured by the paired sorbent traps meeting specified criteria. And in cases where one of the traps is accidentally lost, damaged or broken, the owner or operator would be permitted to report the results of the analysis of the other trap, if valid.

Recent field test data from several different test sites indicate that sorbent trap systems can be as accurate as Hg CEMS. Recent field tests have answered questions regarding which substances in the flue gas can interfere with accurate vapor phase Hg monitoring by sorbent traps. Sorbent trap technology also has evolved, with the addition of a third section that enables the traps to be subject to enhanced QA procedures. And the Agency has been working with industry and equipment manufacturer representatives to develop new QA procedures that are more relevant to the operation of a sorbent trap system. These improved QA procedures are included in the final rule. In view of this, EPA believes that it is appropriate to extend the use of sorbent trap systems to all affected units.

EPA notes that although the restrictions on the use of sorbent traps have been removed, there are some inherent risks associated with the use of this technology. For instance, because sorbent traps may contain several days of accumulated Hg mass, the potential exists for long missing data periods, if the traps should be broken, compromised, or lost during transit or relative accuracy test audit ( RATA) of a sorbent trap system is performed, the results of the test cannot be known until the contents of the traps have been analyzed. If the results of the analysis are unsatisfactory, the RATA may have to be repeated. This also may resulting in a long missing data period. However, EPA believes that these undesirable outcomes can be minimized by following the proper handling, chain of custody, and laboratory certification procedures in the final rule. The use of redundant backup monitoring systems can also help to reduce the amount of missing data substitution.

Comment: 6 3 One commenter ( OAR 2002 0056 4891) stated that any monitoring method that meets the EPA criteria should be allowed. As noted above, accurate Hg monitoring technologies, including accurate continuous emissions monitoring systems for Hg, are not yet commercially available. When such equipment will be successfully tested and commercially available is not known. To facilitate the use of new Hg monitoring technologies as they are developed, the commenter urges EPA to allow power plant owner/ operators subject to the Utility Mercury Reduction Rule ( UMRR; now known as the Clean Air Mercury Rule, CAMR) to use any monitoring method that meets EPA's standard criteria for reliability and accuracy. Unduly limiting the options available for monitoring emissions would only serve to drive up the cost of what is already an extremely expensive regulatory scheme. Given the current lack of accurate Hg emissions monitoring methods, it will be important for facilities to have the flexibility to use monitoring methods that are developed in the future as a result of the adoption of UMRR.

Response:

EPA agrees with the commenter and has specified a performance based approach for monitoring criteria. The performance based approach allows for use of various suitable sampling and analytical technologies while maintaining a specified and documented level of data quality.

6.2 MISSING DATA PROCEDURES Comment:

Several commenters ( OAR 2002 0056 2634,  2718,  2861,  2922,  3565) were concerned that EPA had overlooked a potential problem with the requirement to subject Hg CEMS and Method 324 to periodic RATAs and/ or other audits that rely on comparison to EPA test methods for Hg ( including Method 29 and the Ontario Hydro method). Unlike the instrumental reference methods routinely used to QA SO2 and NOx CEMS, the available Hg test methods can take days to complete and weeks for the return of test results from the laboratory. Under the current general provisions, a monitor that fails a RATA would be deemed out of control beginning with the hour that the required test was conducted until the hour that the test is successfully passed. This construct could lead to significant implementation problems with respect to missing data and requirements to calculate and report data. If a source does not know until weeks after a RATA is completed whether the test was passed, the source has no means of minimizing missing data associated with a failed test. Similarly, once it is clear that a test has been failed, the source must schedule and perform a new test and wait for results before determining whether the monitor is back in control and the data are valid. Under these procedures, monitoring systems that fail a RATA will have significant amounts of missing data due simply to the delay in obtaining testing results. Until a method is developed that will allow for onsite results of Hg RATA testing, EPA must provide special rules to avoid these unavoidable implementation problems. 6 4 Response:

EPA Agrees with the commenters. Based on field testing., EPA intends to develop adequate criteria for a performance based instrumental reference method for Hg. Initial evaluations of such a method have already begun.

Comment:

Several commenters ( OAR 2002 0056 2922,  2634,  2718) stated that section 63.10020( c) states that any period for which a monitoring system is out of control and data are not available constitutes a " deviation." The commenters object to labeling each period when a monitor fails a QA/ QC test and is therefore " out of control" as a " deviation" of the requirement to monitor. Monitoring systems no matter how well maintained will occasionally fail a QA/ QC test. As long as the source takes appropriate action, no deviation from the requirement to monitor has occurred. The commenters are especially concerned about this provision given the uncertainty surrounding the ability of the Hg CEMS to satisfy the proposed standards in PS 12A on an ongoing basis.

Response:

EPA has decided to control Hg emissions using a cap and trade approach rather than by using maximum achievable control technology ( MACT, 40 CFR part 63).

Comment:

Several commenters ( OAR 2002 0056 1854,  2634,  2718,  2721,  2891,  2922,  3403,  3455,  3565,  2855) stated that the proposed missing data procedures seem to be unduly harsh and appear to be unfairly biased against the use of the sorbent trap method. The commenters indicated that the missing data routines should properly consider the uncertainties associated with Hg monitoring, i. e., there is a lack of evidence that high percent monitor data availability ( PMA) is achievable with these monitoring systems. Other commenters suggested that EPA should remove the maximum potential concentration ( MPC) provision altogether for Hg monitors and fill in all missing data periods using average concentrations until more confidence is gained in the reliability of Hg monitors.

Response:

The final rule retains the basic missing data substitution approach for Hg that was proposed. This approach has worked well in the Acid Rain and NOx Budget Programs. The conservative nature of the missing data routines has provided a strong incentive to sources to keep their monitoring systems operating and well maintained. However, the PMA cut points in the final rule have been loosened slightly to account for the present lack of long term Hg monitoring experience in the U. S. EPA will continue to collect and analyze CEMS and sorbent 6 5 trap data from various field demonstration projects and will evaluate the performance of certified Hg CEMS operating on similar source categories ( e. g., waste combustors). If the data indicates that the PMA cut points should be changed for Hg CEMS or sorbent traps, the Agency will initiate a rulemaking for that purpose.

The suggestion to remove the MPC provisions and to fill in all missing data periods using average concentrations until EPA develops better procedures was not incorporated in the final rule for two reasons. First, when add on emission controls that reduce Hg emissions either malfunction and are taken off line, uncontrolled Hg emissions will result. If the Hg CEMS or sorbent trap system is out of control during the control device outage, an appropriate substitute data value must be used to represent uncontrolled Hg emissions and provide an incentive to fix the Hg monitoring system. The MPC concept has successfully been used in the Acid Rain and NOx Budget Programs.

Second, EPA does not agree with the commenters that using the MPC for certain missing data periods is always unduly harsh or punitive. For the initial Hg MPC determination, the March 16, 2004 SNPR provided three options: ( 1) use a coal specific default value; or ( 2) perform site specific emission testing upstream of any control device; or ( 3) base the MPC on 720 hours or more of historical CEMS data on uncontrolled Hg emissions. The Agency believes that these options provide adequate opportunity for affected units to develop appropriate MPC values.

Regarding the missing data routines for sorbent trap systems, available field test data have indicated that these systems are capable of performance that is equivalent to a CEMS. In view of this, EPA believes that sorbent traps should be treated on a a more equal footing with Hg CEMS in many areas, including the missing data provisions.

Finally, EPA notes that a new missing data policy has been posted on the Clean Air Markets Division web site. The policy allows the four tiered missing data algorithms to be applied hour by hour, in a stepwise manner, based on the PMA. Previously, the Agency's policy had been to determine the PMA at the end of the missing data period and to apply a single substitute data value ( sometimes the MPC, if the ending PMA was < 80 percent) to each hour in the missing data block. This new, more lenient interpretation of the 40 CFR part 75 missing data requirements will result in more representative missing data substitution and minimize the use of the MPC.

Comment:

One commenter ( OAR 2002 0056 3455) stated that, in reference to Appendix A to the Preamble Proposed Changes to Parts 72 and 75, ( Proposed Rules March 16, 2004); page 12418, the section dealing with missing data seems to be either incorrect or unduly harsh; in either case, the section requires clarification. The procedure states that missing data starts when the traps that first caused the problem were put into service, but it further states that this missing data period 6 6 ends only when the next valid Hg concentration data are first obtained. Because the data is not " obtained" until the traps have been analyzed and the reports sent back to the owner, this would mean that the missing period data would not only include time that the failed traps were in service, it would also include the time period that the next valid set of traps were in service.

Response:

The wording in the final rule has been clarified to say that the missing data period would end on the commencement of operation of another pair of sorbent traps that contain valid Hg concentration data.

Comment:

Several commenters ( OAR 2002 0056 2634,  2718,  2922) stated that the proposed Method 324 needs to include provisions for data availability and missing data. Because it is not reasonable for EPA to expect 100 percent data capture from any method, EPA must specify at what point sorbent trap data would need to be filled in and what method would be used. EPA might also consider providing alternative minimum data collection and missing data requirements that would to apply simply to Hg data, regardless of the method of collection.

Response:

See the Missing Data discussion in the preamble.

Comment:

Several commenters ( OAR 2002 0056 2634,  2718,  2922) stated that EPA needs to clarify how periods of startup, shutdown, and malfunction are to be treated in data collection and reporting. The proposed rule appropriately states that deviations that occur during periods of " startup, shutdown, or malfunction" (" SSM") are " not violations" if the source was operating in accordance with its SSM plan. However, the proposal does not explain whether or how those data would be excluded from the compliance calculation, or how they would be treated with respect to the data collection requirements. Presumably EPA does not mean that any 12 month rolling average in excess of the standard that includes a period covered by an SSM plan is not a violation. On the other hand, data collected during periods of SSM are not representative of normal operations and should not be included in data averages used to determine compliance and missing data substitution procedures. EPA needs to give additional thought to these issues and provide clear instructions in the rule for how such periods would be treated.

Response:

These comments pertain to the January 30, 2004 NPR, in which both a MACT rule and a NSPS rule were proposed for Hg. The proposed MACT approach has not been selected for 6 7 promulgation. The NSPS has been finalized as a series of amendments to 40 CFR part 60, subpart Da. The NSPS clearly states that data recorded during periods of unit startup, shutdown, and malfunction are not included in the calculation of the 12 month rolling average Hg emission rate. However, the owner or operator is required to report the number of hours excluded from the calculations for those reasons.

Comment:

Several commenters ( OAR 2002 0056 2634,  2718,  2922) stated that EPA proposes to require sources that utilize a FGD system to maintain records of scrubber operating parameters for each Hg missing data period in order to show proper operation of the scrubber. Because recording of FGD parameters generally is not automated, this requirement could become very burdensome if there is a significant amount of missing data. As a result, the commenters request that EPA consider allowing sources the option of utilizing parameters other than control device operating parameters, such as documented compliance with an SO2 permit limit using the SO2 CEMS, to establish proper operation of the FGD during Hg missing data periods.

Response:

The final rule allows quality assured SO2 data to be used to demonstrate proper operation of an FGD.

Comment:

Several commenters ( OAR 2002 0056 2634,  2718,  2922) stated that proposed data collection and missing data scheme under section 63.10008( d)( 4) has several significant flaws. The first flaw is the assumption that these minimum criteria are reasonable requirements for Hg CEMS. There are no data to support the assumption that Hg CEMS will be capable of operating within the specified performance criteria for 18 hours a day for 21 days each month. Sources should not be penalized for failing to meet minimum criteria that may not in fact be achievable. As a result, EPA will need to continue to review this requirement in light of additional data collected between now and the first compliance deadline. EPA also should revise the rules to allow use of Method 324 as a backup for any source that chooses that option, and should allow data from that method to be used to meet the minimum data requirements in lieu of a Hg CEMS. The second flaw in the rule, even if you assume that the Hg CEMS can meet the minimum criteria, is the failure of the rule to distinguish between unit operating hours and non operating hours in determining if a day is complete. If the minimum requirement applies regardless of unit operation, sources will be required to try to keep monitors running and quality assured, and to calculate mass Hg, even if the unit is not operating in order to collect enough data for a complete day. That is contrary to section 63.10020( a) (" you must monitor continuously at all times that the affected source is operating"). That would also result in months with little operation, and therefore very little actual Hg mass emissions data, being counted in the 12 month rolling average with the same weight as months with significant operation. If, on the other hand, the source does 6 8 not collect that data, any day the unit does not operate and most days involving a startup or shutdown are likely to be " incomplete" and not count towards the required 21 days for a complete month. Under the missing data provisions, that approach would mean that valid data would be thrown out simply because there was not enough unit operation in the month. Sources should not be required to operate monitors when they are not operating, and should not be penalized for non operation. As a result, EPA should give more thought to whether the data collection and missing data provisions associated with the calculation of " monthly" averages is the best approach.

Response:

The proposed MACT rule has not been selected for promulgation. However, the proposed NSPS rule for Hg, which contained the same data capture requirements as the MACT rule, has been finalized. In the final rule, the minimum data capture requirement for the Hg monitoring systems is 75 percent of the unit operating hours in each month. If this requirement is not met for a particular month, a substitute Hg emission rate must be reported. Compliance with the NSPS emission limit for Hg is determined on a 12 month rolling average basis. The rolling average is weighted according to the number of valid hours of Hg data collected in each month, except when the 75 percent data capture requirement is not met. When that occurs, the substitute Hg emission rate for that month is weighted according to the number of unit operating hours in the month. Months with zero unit operating hours are not included in the rolling average calculations.

Comment:

Several commenters ( OAR 2002 0056 2634,  2718,  2922) stated that EPA proposes to revise 40 CFR 75.20( d) to include Hg CEMS in the list of non redundant backup monitoring systems that can be used for up to 720 operating hours without a RATA. The commenters request that EPA revise this section, and other sections regarding use of backup monitoring systems and missing data, including 40 CFR 75.80( f), to allow use of sorbent trap monitoring systems as a backup to Hg CEMS and vice versa. The EPA should also allow use of additional paired traps as a backup to a sorbent trap monitoring system.

Response:

The final rule does not prohibit you from using sorbent traps as redundant backup monitors.

6.3 SORBENT TRAP OPERATION AND QA/ QC In view of the many comments received regarding a large number of testing and QA provisions in proposed Method 324, EPA has decided to revise and rename proposed Method 324 as Appendix K to Part 75 in the final rule. Based on comments received and experience 6 9 gained from field tests since proposal, Appendix K retains certain provisions and revises others in proposed Method 324 to include detailed, performance based QA standards and procedures for sorbent trap monitoring systems.

Comment:

One commenter ( OAR 2002 0056 3455) stated that, in reference to section 75.15( a)( e), Page 12417, conducting proportional sampling with a Method 324 sampling train may be conducted at a single point. If the velocity profile changes with load, as may sometimes be the case, proportional sampling by determining the velocity at a single point may misrepresent the total gas flow in the stack.

Response:

40 CFR part 75, Appendix K, requires that the output from a part 75 certified stack gas flow monitor be used to maintain a proportional sample flow rate through a sorbent trap or cartridge. All affected sources under the final rule must already comply with the 40 CFR part 75 Acid Rain Program monitoring requirements which include maintaining and quality assuring a stack gas flow monitor. A part 75 certified flow monitor must pass a two load RATA annually and a three load RATA every 5 years, and is, therefore, representative of the stack gas flow rate across different loads.

Comment:

One commenter ( OAR 2002 0056 3455) stated that, in reference to Method 324 ( 40 CFR part 63), does not meet EPA's definition of a CEMS ( page 12453, section 72.2 Definitions).

Response:

It is true that in section 72.2, a sorbent trap monitoring system is not included in the definition of a CEMS, but rather is defined separately as an " excepted" monitoring system. However, EPA believes that this distinction is more semantic than substantive, since a sorbent trap system samples the stack effluent continuously, and data from the system are combined with hourly readings from a certified 40 CFR part 75 flow monitor to give a continuous record of Hg mass emissions. Relative accuracy test data have shown that a sorbent trap system can measure Hg concentration as accurately as a CEMS.

Comment:

One commenter ( OAR 2002 0056 2867) notes that EPA also stated, for the purposes of applying Method 324, " an intermediate sampling rate of 0.3 to 0.5 L/ min through each sorbent trap would be used when the unit is operating at the normal load level, whether low, mid, or high. The sampling rate would then be increased or decreased, as appropriate, by 0.1 L/ min when the 6 10 unit operates at the other two load levels" ( 69 FR 12417). The commenter believed that this sample rate adjustment procedure is not appropriate. The proportional sampling is not necessary, and requiring a step change to the sample rate adds further complication without any appreciable benefit. According to the commenter, a mathematical analysis of the difference between constant flow sampling and proportional flow sampling shows a negligible difference between the two, with a percent difference of 1.45. The commenter submitted an attachment with the simulated data ( based on a realistic scenario). The commenter stated that since the method allows for a ± 25 percent change in proportional flow sampling as mentioned above in ( 69 FR 4736), the percent difference between constant flow sampling and proportional flow sampling is negligible at 1.45 percent, and well within the ± 25 percent range. The commenter recommended that proportional sampling not be required because of the added complications in the design of the sampling process, and without the appreciable increase in the accuracy of the measurements. In the undesirable alternative that the EPA requires proportional flow sampling, the commenter recommended the sampling should be done on a continuous proportional basis and not on a 3 step proportional basis. The commenter submited further explanation of the reasons proportional flow sampling is unnecessary as follows. Since Method 324 measures the concentration of the Hg in the stack, it is independent of the flow rate. The procedure included in the rule to calculate the total mass of the mercury emissions includes the stack flow rate as follows: "( 3) If you use Method 324 ( 40 CFR part 63, appendix A), determine the 12 month rolling average mercury emission rate according to the applicable procedures in paragraphs ( d)( 3)( i) through ( v) of this section. ( i) Sum the mercury concentrations for the emission rate period, (

g/ dscm). ( ii) Calculate the total volumetric flow for the emission rate period, ( dscm). ( iii) Multiply the total mercury concentration times the total volumetric flow to obtain the total mass of mercury for the emission rate period in micrograms. ( iv) Calculate the mercury emission rate for an input based limit ( lb/ Tbtu) using Equation 4 of this section. ( v) Calculate the mercury emissions rate for an output based limit ( lb/ MWh) using Equation 5 of this section," ( 69 FR 4724). The commenter pointed out that since any changes in stack flow rate will be taken into account in this calculation, proportional sampling is unnecessary. The commenter asks EPA to clarify their intent on requiring proportional flow for sampling periods greater than 12 hours. The commenter contends that this is not a necessary requirement for accurate measurement of the Hg emissions.

Response:

When stack gas flow changes, Hg concentration may also change. Therefore, the final rule requires flow proportional sampling to better ensure a representative sample. The 3 step adjustment procedure in the proposed rule has not been finalized. Rather, the ratio of the stack flow rate to sample flow rate must be kept constant (+ 25 percent) from hour to hour.

Comment:

One commenter ( OAR 2002 0056 2063) stated that, in reference to the Preamble Proposed Changes to Parts 72 and 75, ( Proposed Rules March 16, 2004), Method 324, since Method 324 is a Reference Method and the main vulnerability on an ongoing basis is 6 11 the sample volume measurement, quarterly volume measurement calibrations should be required, rather than additional Reference Method ( RAA) testing.

Response:

The final rule does not require quarterly RAAs, but has instituted quarterly calibration checks of dry gas meters, and sample specific volume QA/ QC.

Comment:

One commenter ( OAR 2002 0056 0544) suggested that should sorbent tubes be utilized for monitoring purposes, the sorbent media should not be restricted to iodated activated carbon only. There is a sorbent media used to test for Hg vapor in ambient air method listed as NOSH 6009 used in industrial hygiene. These sorbent tubes are readily available from SKC Inc, manufacturer, located in Pennsylvania, part number 226 17 1A. These tubes have been used by many Industrial Hygienists for many years and are time proven for their reliability.

One commenter ( OAR 2002 0056 2867) asserted that EPA should publish the QA/ QC for the digestion procedure in this rule and disclose the entity that has developed the analytical procedure and written Method 324. The commenter believed this will increase the understanding of the intent in requiring this sampling method and analytical procedure. The commenter stated the current digestion method appears to be biased towards the use of one vendor's procedure. The commenter feels this may give them an unfair market advantage in supplying the traps to the industry. The commenter believed alternate analytical procedures should be allowed, so that one company does not control the market.

Response:

The sorbent media monitoring requirements ( formerly Method 324) have been revised to make them performance based, thus providing significant additional flexibility in choice of sampling and analytical approaches, as well as performance criteria used to assess the quality and validity of the monitoring data generated. There are numerous potential approaches for sample preparation, depending upon the analytical technique selected. The performance based approach intentionally avoids specifying that level of detail.

Comment:

One commenter ( OAR 2002 0056 3546) stated that they had recently conducted additional Hg sampling at the Navajo and Coronado Generating Stations to enhance their understanding of Hg emissions from these two coal fired facilities and the challenges of testing for Hg in flue gas. The commenter notes that both stations were part of the 1999 ICR where the Ontario Hydro method was used for Hg sample collection. Attachment 1 to the commenter's letter contains the test results and a brief description of sampling protocols. 6 12 Response:

The Agency appreciates the data and is evaluating it.

Comment:

One commenter ( OAR 2002 0056 2101) stated that, in reference to Method 324, if a separate particulate filter is used in front of the trap, this filter may easily absorb gaseous Hg species during a long sample run. Is the inlet filter material analyzed along with the sorbent material? If so, the method measures particulate Hg as well as gaseous. Unless iso kinetic sampling is done, the particulate fraction may be over or under represented. If the particulate filter material is not analyzed, the danger exists that gaseous Hg has deposited on it and will be discarded along with the material. This would cause serious under representation of the gaseous phase Hg.

Response: 40 CFR part 75, Appendix K in the final rule specifies that sorbent media should be the first thing to contact stack gas.

Comment:

One commenter ( OAR 2002 0056 2101) stated that, in the proposals, CEMs require that stack flows be monitored exactly in order to allow calculation of an accurate emission rate. However, in the proposed Method 324, even when variable flow rates that track emission volumes are required ( only for samples of > 12 hours duration), the sampling flow rates need agree only within ± 25 percent of stack velocity and only over a 3: 1 dynamic range. This wide error allowance will produce inaccurate mass emission calculations.

Response:

Appendix K of 40 CFR part 75 requires that the output from a part 75 certified stack gas flow monitor be used to maintain a proportional sample flow rate through a sorbent trap across all load levels, not just the three specified in the SNPR. Most of the affected sources under the final rule are subject to the Acid Rain Program or to the NOx Budget Program ( or both) and already have the required stack gas flow monitor. The final rule retains the requirement to maintain a constant (+ 25 percent) ratio of stack gas flow rate to sample flow rate from hour tohour EPA will evaluate sorbent trap system data as time goes on, to see whether the + 25 percent criterion needs to be tightened.

Comment:

Two commenters ( OAR 2002 0056 2485,  3455) noted that, in reference to proposed Method 324 ( 40 CFR part 63), section 8.2.1 ( Sample Collection), page 4738, an isolation valve is 6 13 shown in Figure 324 1. The method should suggest closing the valve to prevent negative or positive flow due to stack gas pressure. This would also allow the tube to heat to the correct temperature before commencement of sampling.

Response:

The sorbent media monitoring requirements ( formerly Method 324) have been revised to make them performance based providing additional flexibility as well as performance criteria used to assess the quality and validity of the monitoring data generated. The drawing referenced by the commenters is now offered as one example of a suitable sampling train configuration. There are numerous potential approaches for gas control and the performance based approach intentionally avoids specifying that level of detail.

Comment:

One commenter ( OAR 2002 0056 2101) noted that, in reference to Method 324, section 8.2.3 ( Flow Rates), because the method returns a true dry concentration, the instantaneous stack flow rate must also be a true dry value. However, the velocity measurement is a normally wet value. This requires one of the following:

° Verification that stack moisture levels do not vary significantly with velocity or over time; ° A moisture analyzer to allow calculation of dry flow rate; ° Some other precise and accurate method of determining current moisture levels; or ° An error analysis showing that the worst case errors introduced by failing to perform this correction are not significant.

Response:

The final rule requires that Hg concentrations and stack gas flow rates used to calculate Hg mass emissions must be on the same moisture basis.

Comment:

Two commenters ( OAR 2002 0056 2101,  3455) stated that, in reference to proposed Method 324 ( 40 CFR part 63), section 8.2.6 ( Moisture Knockout), page 4738, the section should state that data for the entire run period must be invalidated in the event that the leak check fails. This is because the dry gas flow meter may have been sampling after the sorbent cartridge, not through it, resulting in low Hg readings.

Response:

EPA agrees. Table K 1 in Section 8.0 of Appendix K specifies that failure to meet the 6 14 post test leak check criterion will invalidate the sorbent trap monitoring data.

Comment:

Two commenters ( OAR 2002 0056 2101,  3455) stated that, in reference to proposed Method 324 ( 40 CFR part 63), section 8.3.4 ( Field Spikes), page 4738, this section gives the impression that the Hg is spiked onto the cartridges before exposure. ( This procedure should possibly be renamed to lab spike, rather than field spike.) This procedure yields very little information. Unless the sample gas matrix actually causes the removal of existing Hg on the trap, or interference with the analysis, this procedure will always generate 100 percent recoveries. Even if Hg is spiked onto the cartridges after sampling, this does not provide definitive proof that Hg was being properly captured under actual sampling conditions. Commenter OAR 2002 0056 2101 suggested that a more valid type of spike test would be to spike the cartridges in situ, under actual sampling conditions, towards the end of a lengthy sampling period. This could be done by adding a high concentration of a gas containing elemental ( or ionic) Hg. Spiking would be done at a low flow rate ( e. g., 100 ml/ m). This spike gas would be sent to the probe, directly in front of the cartridges and the spike duration would be chosen to produce a significant additional loading on the cartridge. Because the spike gas is produced at a rate significantly below the cartridge sampling rate, all of the spike would be drawn through the cartridge and none would be lost out of the tip of the probe. Only in this way can one be assured that Hg is being captured quantitatively throughout the entire measurement period. The spiking gas would have to be introduced at a known flow rate and with a known concentration. A conventional saturated Hg vapor generator would suffice.

Response:

In the final rule, Appendix K requires spiking of the third section of each sorbent trap with elemental Hg prior to sampling. The purpose of the spiking is to serve as a QC check of the laboratory performing the analyses. EPA does not agree with the commenter's proposed spiking technique. Unless one can be assured of 100 percent recovery of the Hg in each trap ( the Agency does not believe this is possible), spiking directly in front of the sorbent trap will cause the spiked Hg to become mixed with the sampled Hg, making it impossible to separate the two.

Comment:

Two commenters ( OAR 2002 0056 2485,  3455) stated that, in reference to proposed Method 324 ( 40 CFR part 63), section 10.1 ( Calibration and Standardization), page 4739, the standards should contain the same quantity of leaching agent as the samples being analyzed.

Response:

The sorbent media monitoring requirements ( formerly Method 324) have been revised to make them performance based, thus providing significant additional flexibility in choice of 6 15 sampling and analytical approaches, as well as performance criteria used to assess the quality and validity of the monitoring data generated. There are numerous potential approaches for sample preparation, depending upon the analytical technique selected. The performance based approach intentionally avoids specifying that level of detail.

Comment:

One commenter ( OAR 2002 0056 2867) notes that Method 324, section 11.1 states, " The sorbent traps are received and processed in a low mercury environment ( class 100 laminar flow hood and gaseous Hg air concentrations below 20 ng/ m3) following clean handling procedures" ( 69 FR 4739). The commenter recommended that the EPA not require the use of the class 100 laminar flow hood. The commenter's experience with EPA Method 1631 for low level Hg in water indicates that processing samples in a class 100 laminar flow hood is unnecessary to maintain Hg contamination below stated levels. However, the room in which samples are processed may require restricted activity to keep contamination below 20 ng/ m3. The commenter submitted the lab should be able to achieve and maintain a low Hg environment by the means it deems necessary. In the event that the EPA chooses to require the class 100 laminar flow hood, the commenter requested EPA to specifically define the underpinnings for the use of a class 100 laminar flow hood and define the requirements of a class 100 laminar flow hood.

Response:

As noted previously, the sorbent media monitoring requirements have been revised to utilize a performance based approach. With this approach, is up to individual laboratories to utilize whatever measures they find necessary to achieve acceptable background Hg levels in order to achieve the performance criteria for the measurement.

Comment:

One commenter ( OAR 2002 0056 2867) notes that section 11.14 of Method 324 states, " A field blank is performed by assembling a sample train, transporting it to the sampling location during the sampling period, and recovering it as a regular sample. These data are used to ensure that there is no contamination as a result of the sampling activities. A minimum of one field blank at each sampling location must be completed for each test site" ( 69 FR 4740). The commenter believed the requirement to assemble an entire sample train to make a field blank is excessive. The commenter asserted a field blank does not need to be attached to a separate sample train. The commenter stated simply handling the field blank trap along with the actual test sample trap at the test location and then sending it to the lab for analysis will fulfill the goals of the field blank. The commenter recognizes that the requirements for a field blank should be based on actual test data. The commenter requests that the EPA publish the test data on which the field blank requirements are based. The commenter stated that EPA should provide data that indicate that the process of sample train preparation and sample recovery can introduce significant contamination into the sampling procedure, if this is indeed a requirement. 6 16 Response:

As noted previously, the sorbent media monitoring requirements have been revised to utilize a performance based approach. Blanks, including reagent blanks, method blanks, and field blanks can be used at the tester's option to assess sources and levels of sample contamination. Blank correction, however, is not allowed.

Comment:

Two commenters ( OAR 2002 0056 2101,  3455) noted that, in reference to proposed Method 324 ( 40 CFR part 63), section 11.14 ( Field Blanks), page 4739, this section does not say what is to happen with results that exceed 30 percent of the measured value. The data should be considered invalid.

One commenter ( OAR 2002 0056 3455) stated that, in reference to proposed Method 324 ( 40 CFR part 63), section 11.14 ( Field Blanks), page 4739, Table 324 2 the table should be completed so the corrective action is available for all QA/ QC failures.

Two commenters ( OAR 2002 0056 2485,  3455) stated that, in reference to proposed Method 324 ( 40 CFR part 63), section 11.4 ( Mercury Reduction and Purging), page 4739, the suggested field blank of 30 percent of the measured value is much too high. Surely it would be more sensible to define the field blank in absolute mass as per table 324 2. If a trap has a max capacity of 1,800

g then this would equate to 540

g, which would be a serious contamination issue.

Response:

As noted above, the sorbent media monitoring requirements have been revised to utilize a performance based approach. Blank samples are not required, but are rather used at the tester's option to assess sources and levels of sample contamination. Blank correction is not allowed and there are no criteria for acceptable blank levels.

Comment:

Two commenters ( OAR 2002 0056 2485,  3455) stated that, in reference to proposed Method 324 ( 40 CFR part 63), section 11.3 ( Dilution Step), page 4739, the dilution volume is not specified but potentially very large dilutions are required to fit into the calibration range of Method 1631.

Response:

As previously noted, the sorbent media monitoring requirements have been revised to utilize a performance based approach. With this approach, it is up to individual users to select 6 17 suitable analytical techniques and associated sample preparation steps in order to achieve the performance criteria specified for the measurement and thus dilution steps are no longer specified.

Comment:

One commenter ( OAR 2002 0056 2867) stated that sections 11.0 through 11.4 define the analytical procedure of Method 324 and use a water digestion method. The commenter submitted that the rule should include an avenue for approval of an alternate ASTM analytical procedure, if the alternate method is shown to provide comparable results ( similar to use of ASTM methods for the analysis of coal and ash).

One commenter ( OAR 2002 0056 2485) noted that, in reference to Method 324, section 11.4, the selection of the EPA 1631 as the preferred method to analyzed the sorbent tubes has no scientific justification. This method is extremely complex and only the most specialized laboratories have the necessary resources to run this procedure. The commenter also noted that, in reference to Method 324, section 11.4, the Method 1631 method has many requirements that are not appropriate or needed to analyzed sorbent traps. The 1631 method specifies the use of amalgamation on gold to decrease detection limits. This is not necessary because the concentrations in the final solution are sufficiently high. The amalgamation step complicates the measurement and introduces more error.

Two commenters ( OAR 2002 0056 2485,  3455) noted that, in reference to Method 324, section 11.4, the Method 1631 method has many requirements that are not appropriate and needed to analyzed sorbent traps. The 1631 method specifies an oxidation using BCL. Is this step necessary as the Hg species will be already digested during the leaching process? What is the purpose of the BCL?

The commenters also noted that, in reference to Method 324, section 11.4, the Method 1631 method has many requirements that are not appropriate and needed to analyzed sorbent traps. The addition of NH2OH after the BrCl oxidation to remove free halogens is clearly specified in the 1631 method to overcome damage to the gold trap and prevent low collection efficiency ( M163l, section 11.2). Method 324 states that this stage is omitted allow the free halogens will still be present. What is the justification in omitting such a critical stage of the 1631 method? Furthermore the sorbent leaching step uses a concentrated HN03/ H2S04 mixture at elevated temperature in a closed vessel. As mentioned in Method 324 in section 11.2 significant quantities of noxious and corrosive gases are released. These gases are likely to be NOx fumes which may combine with BrCl to produce NOCl fumes which may also attack the gold trap and lower its collection efficiency.

Response:

As previously noted, the sorbent media monitoring requirements have been revised to 6 18 utilize a performance based approach. With this approach, it is up to individual users to select any suitable analytical technique as long as it can achieve the performance criteria specified for the measurement.

Comment:

One commenter ( OAR 2002 0056 2485) noted that, in reference to Method 324, section 11.4, Method 1631 has many requirements that are not appropriate and needed to analyzed sorbent traps. The method was designed for trace levels of Hg in water not sorbent tubes. The concentration range of the method is too low ( 0 100 ng/ L) for the expected mass of Hg on sorbent traps. For example if the flue gas Hg concentration was 1

g/ m3 sampling for 4 hours at 0.4 L/ min would yield 96 ng Hg. Insufficient information is provided in the method to calculate the final concentration of Hg in solution after sample preparation. A simple calculation however would suggest that the mass of Hg collected would have to be diluted using 1000 ml to produce 96 ng/ L to be in the concentration range of the method. If one considers the maximum allowable mass collected on the small trap ( 150

g) a dilution volume of l, 500,000 ml. The large sorbent trap specifies 1800

g capacity so a dilution volume of l8,000,000 ml would be required. Although these calculations represent the upper capacity limits they surely demonstrate that 1631 is not an appropriate method for the analysis of sorbent tubes and the potential error of such high dilutions Response:

As previously noted, the sorbent media monitoring requirements have been revised to utilize a performance based approach. With this approach Method 1631 is no longer specified; it is up to individual users to select an suitable analytical techniques and associated steps in order to achieve the performance criteria for the measurement.

Comment:

Two commenters ( OAR 2002 0056 2101,  3455) noted that, in reference to proposed Method 324 ( 40 CFR part 63), section 11.6 ( Instrument Calibration), page 4739, Method 1631 uses a calibration factor approach to calibration. It does not rely on r2 values. The method should standardize on either a weighted or unweighted regression approach to calibration because the two approaches may yield significantly different results. Note that the Method 1631 analytical approach is often used with weighted, ( 1/ r2) least squares curve fitting.

Response:

Numerous commenters stated that Method 1631 is inappropriate for the analysis of sorbent trap samples. EPA concurs, and all references to Method 1631 have been removed in the final rule. The final rule allows any suitable analytical technique to be used. For the calibration curve of the analyzer, the rule simply requires an r2 value greater than or equal to 6 19 0.99 and requires each calibration point to be within 10 percent of the true value.

Comment:

One commenter ( OAR 2002 0056 2889) stated that, in reference to Method 324, section 11.7, refers to section 15, but likely should refer to section 12.

Response:

Although the commenter is correct, the revisions to the sorbent monitoring procedures have eliminated the need for this reference.

Comment:

Two commenters ( OAR 2002 0056 2485,  3455) stated that, in reference to proposed Method 324 ( 40 CFR part 63), section 11.8 ( Continued Calibration Performance), page 4739, 10 percent drift on a calibration standard is too high. If the CV AAS or CV AFS has drifted 10 percent then the probability of passing a RA test is lessened dramatically.

One commenter ( OAR 2002 0056 2867) noted that, in reference to section 11.9 of Method 324, EPA states, " The QA/ QC for the analytical portion of this method is that every sample, after it has been prepared, is to be analyzed in duplicate with every tenth sample analyzed in triplicate" ( 69 FR 4739). The commenter claims these are highly excessive requirements for the QA/ QC of the analytical portion of the method. The commenter recommended that the QA/ QC be reduced to one duplicate every tenth sample analyzed, as allowed under other EPA's QA/ QC programs. In other EPA monitoring programs such as the methods for water analysis, QA/ QC requirements are limited to a duplicate and a spike for duplicate for every tenth sample. The commenter submits that if required, EPA could adopt the practices used in water monitoring programs.

One commenter ( OAR 2002 0056 2867) stated that, in reference to section 11.9 of Method 324, the sorbent trap laboratory blank requirement of 3 percent analysis set of 20 sorbent traps also seems excessive. The commenter recommended one blank per every tenth sorbent trap.

One commenter ( OAR 2002 0056 2889) stated that, in reference to Method 324, section 11.10, should also require the independently prepared samples to be within 10 percent of the expected value.

Response:

As previously noted, the sorbent media monitoring requirements have been revised to utilize a performance based approach and this level of detail is no longer specified. Laboratories conducting analyses of the sorbent media cartridges for Hg must either be ISO 6 20 certified or accredited or must perform the spike recovery study described in Appendix K of 40 CFR part 75 annually. Thus, adequate QC procedures will be in place to ensure the quality of the data.

Comment:

One commenter ( OAR 2002 0056 2889) stated that, in reference to Method 324, section 11.13, should in the last sentence refer to 10 percent of the measured sample results.

Response:

The commenter is referring to a requirement for solution blanks. As noted above, the sorbent media monitoring requirements have been revised to utilize a performance based approach. Blank samples are not required, but are rather used at the tester's option to assess sources and levels of sample contamination. Blank correction is not allowed and there are no criteria for acceptable blank levels.

Comment:

One commenter ( OAR 2002 0056 2889) stated that, in reference to Method 324, section 13.0, should specify the consequences of failing to achieve the required sample rate per stack flow. Another commenter ( OAR 2002 0056 2485) stated that, in reference to Method 324, Table 324 2, the table should be completed so the corrective action is available for all QA/ QC failures.

Response:

EPA agrees. Table K 1 in Appendix K specifies that failure to achieve certain performance or acceptance criteria will invalidate the sorbent trap monitoring data.

Comment:

One commenter ( OAR 2002 0056 2485) stated that section 13.0 of Method 324 is confusing.

Response:

EPA agrees and has clarified the flow proportional sampling requirements in Appendix K of 40 CFR part 75.

Comment:

One commenter ( OAR 2002 0056 3455) stated that, in reference to proposed Method 324 ( 40 CFR part 63), section 13.0 ( Constant Proportion Sampling), page 4740 and section 6 21 8.2.4 ( Constant Proportion Sampling) Option 1 & Option 2, these sections are confusing. If these criteria were to be followed, a source could set their flow rate based on 75 percent of maximum possible stack flow and be in compliance continuously from 50 percent of flow to 100 percent of flow. However, the commenter believes that this would result in significant under reporting of full load data and over reporting of reduced load data. The results of this could greatly reduced Hg emission rates. Load following is the only way to make this work and it should have the same standards as Method 5 sampling (+/ 10 percent). Simply picking a sample flow rate as suggested in the amended sections is not appropriate either.

Response:

EPA understands the commenter's theoretical concern and will evaluate additional data from actual units to see whether the permissible deviation criteria needs to be tightened in the future.

Comment:

Several commenters ( OAR 2002 0056 2634,  2718,  2861,  2922,  3565) stated that several provisions, including sections 13 and 14, purport to contain requirements for calculations and data analysis. These sections are inadequate. These sections should contain all of the equations and calculations needed to conduct the method and arrive at Hg emissions values that are either in the units of applicable standards or that can be converted to those units. Procedures for incorporating blank determinations should also be included.

Response:

EPA has addressed the commenters' concerns in finalizing the sorbent monitoring procedures; however, the final procedures do not include provisions for blank correction.

Comment:

Several commenters ( OAR 2002 0056 2634,  2718,  2861,  2922,  3565) stated that the proposed Method 324 appropriately requires leak checks of the sampling line with and without the sorbent trap in place. The checks are performed using a rotameter. The commenters are concerned that with a nominal flow rate of 0.4 L/ min ( see Table 324 1), 2 percent of that flow rate ( which is the maximum leakage allowed under the proposal) may be too low to be accurately read on a standard rotameter. Accordingly, the commenters suggest that EPA consider revising the Method 324 to quantify the leak rate based on readings from the dry gas meter over a period of at least 1 minute.

Response: 6 22 EPA understands the commenters' concerns and has revised the leak check procedures to clarify that the dry gas meter should be used to quantify the leak rate. The leak check specifications have also been revised to allow a leak rate up to 4 percent of the planned ( pre test leak check) or 4 percent of the average sampling rate ( post test leak check) which is consistent with Method 5 ( 40 CFR 60, Appendix A).

Comment:

One commenter ( OAR 2002 0056 2867) pointed out that Table 324 1 ( 69 FR 4738) of Method 324 states that the maximum sample duration for the large sorbent trap is 10 days. The commenter recommended that the maximum sample duration not be limited at this time. The commenter stated that requiring short sampling periods adds to the cost of the monitoring process, as the traps will have to be changed more frequently. The commenter's experience indicated that changing the sample trap results in approximately 30 minutes of unmonitored time. The 10 day sorbent trap stipulation as proposed would lead to increases in the amount of unmonitored time, as compared to a longer sample period, which requires changing less frequently. The commenter submitted that the use of shorter length traps also increases handling, analysis, and operations support compared to a longer sampling trap. This extra handling and analysis will also introduce more opportunity for error. The commenter recommended that the sample period comport with capabilities of the carbon traps. The commenter's experience shows that the traps can be designed and used for longer sampling periods. In recent tests, the commenter compared the results of two smaller sorbent traps ( one 10 day and one 11 day sorbent trap) to one large trap ( a 21 day sorbent trap). The commenter claimed the test demonstrates that the results are comparable and not dependent on the length of the sample period or size of the trap ( the commenter submitted an attachment with the test results). The commenter pointed out that the percent difference between the weighted average concentration measured in the two smaller traps versus the larger trap is 13.7 percent. This is well within the RA range proposed by EPA of 20 percent when comparing methods to Ontario Hydro ( 69 FR 12419). The commenter has plans for further development of Method 324 to lengthen the sample duration. The commenter envisioned that the capabilities of the sorbent technology will improve with time, leading to longer duration sample collection. In this context, the commenter recommended that EPA should instead focus on the accuracy of information and not arbitrarily limit the sampling period to 10 days or a month.

Response:

EPA has revised the sorbent monitoring procedure to be performance based and has eliminated the 10 day limit on sampling duration leaving it up to the tester to identify a sampling duration that provides the correct balance between convenience and performance.

Comment:

One commenter ( OAR 2002 0056 2485) stated that, in reference to Method 324, Table 6 23 324 2, the table should be completed so the corrective action is available for all QA/ QC failures.

Response:

EPA agrees. Table K 1 in Section 8.0 of Appendix K specifies that failure to achieve the performance or acceptance criteria will require corrective action in some cases, and in other cases will invalidate the sorbent trap monitoring data.

Comment:

Several commenters ( OAR 2002 0056 2634,  2718,  2861,  2922,  3565) stated that Table 324 2 sets out the QC requirements for samples. The commenters are concerned that some of these requirements are excessive or not sufficiently explained or studied. For example, the requirement for laboratory blanks could be excessive for large trap lots. And, without more data, it is not possible to determine whether the paired train criterion ( which is not set out anywhere else in the rule) is subject to the same problems as PS 12A, section 8.6.6. Finally, the field spiking requirement is not sufficiently explained. These problems are explained more fully in RMB's comments.

Response:

As noted previously, the sorbent media monitoring requirements have been revised to utilize a performance based approach. Blank samples are not required, but instead are used at the tester's option and, therefore, there are no criteria for acceptable blank levels. The paired train performance criterion has been based on actual levels achieved during several EPA demonstrations as well as stakeholder supplied data. The field spiking requirement has been replaced with a cartridge spiking requirement for QA and normalization of the data; this new procedure has been adequately detailed.

Comment:

Several commenters ( OAR 2002 0056 2634,  2718,  2861,  2922,  3565) stated that because Method 324 is a test method itself, they believe that EPA could justify a rule that did not require a RATA for validation of the sorbent trap system. The commenters are not aware of any other instance where EPA had required that an EPA test method be compared to another test method using RATA procedures prior to use to determine compliance. Use of the RATA in this case is unusual given the mixed performance of the Ontario Hydro method that would be used for the RA test. Because Method 324 is a new method that has not been employed in practice, the commenters are not objecting to an annual RATA requirement. Such testing in the early years of the program could provide valuable information for improvement of both methods. However, the commenters would object to any more frequent testing. Method 324 already provides significant QA/ QC in its sampling and analysis procedures and additional RATA testing would be unwarranted. 6 24 Response:

EPA does not believe that the sorbent media measurement procedure has the quality to serve as a reference method. However, we are confident in its performance as a monitoring technique. We have reconsidered the RATA requirements for sorbent monitoring systems and concluded that a yearly RATA plus enhanced performance based QA measures in Appendix K should suffice to provide accurate measurements.

Comment:

Several commenters ( OAR 2002 0056 2634,  2718,  2861,  2922,  3565) stated that similar to proposed PS 12A, section 8.1.1 of Method 324 proposes to require testing for SO2 and NOx stratification at the proposed installation location. This requirement is not consistent with other EPA regulations and should be revised to deem the location suitable as long as the RATA is passed. As in EPA's other rules, stratification testing should only be required if the source uses a wet control device ( or is otherwise expected to have stratification) and exercises the option to use the short measurement line or a single measurement point during RATA testing.

Response:

The provisions of Method 324 have been revised and placed in 40 CFR part 75, appendix K. Appendix K suggests ( but does not require) that stratification testing be used to site Hg monitors.

Comment:

Several commenters ( OAR 2002 0056 3455, 2485) noted that, in reference to proposed Method 324 ( 40 CFR part 63), section 8.1.6 ( Pre Test Leak Check), page 4738, a leakage rate of less than 2 percent of the recommended flow rate of 0.4 L/ min equates to less than 0.008 L/ min. This flow rate is not measurable using a flow rate in the range of the method ( 0  0.8 L/ min). A leak check under vacuum would be more effective and accurate.

Response:

EPA agrees that the leak checks should be conducted under vacuum. The pre test leak check specifies ~ 15" Hg vacuum, while the procedures have been revised to specify that the post test leak check be done at the maximum vacuum reached during sampling. In addition, the leak check specifications have been revised to permit a leak rate up to 4 percent of the planned ( pretest leak check) or 4 percent of the average sampling rate ( post test leak check) to be consistent with Method 5 ( 40 CFR 60, Appendix A).

Comment: 6 25 One commenter ( OAR 2002 0056 2485) noted that, in reference to Method 324, section 1.0, the title is misleading as the sorbent trap is not dry during sampling and it is quite likely that a fraction of the particulate Hg is sampled because no filtration is used.

Response:

The provisions of Method 324 have been revised and placed in 40 CFR part 75, appendix K.

Comment:

One commenter ( OAR 2002 0056 2485) stated that, in reference to Method 324, section 6.1.2, presumably, the large and small sorbent traps have the same type of sorbent material. Suggesting the use of larger sorbent tubes at higher duct temperatures would imply that breakthrough of Hg is potentially a problem at temperatures of 375 ° F. The length of the probe holding the sorbent tubes is not specified. It is quite common for the flue gas to be saturated with water so unheated sorbent traps may be subjected to condensation. Surely it is advisable to heat the sorbent traps above the stack gas temperature for all applications.

One commenter ( OAR 2002 0056 3455) stated, in reference to proposed Method 324 ( 40 CFR part 63): It is quite common for the flue gas to be saturated with water so unheated sorbent traps may be subjected to condensation. Surely it is advisable to heat the sorbent traps above the stack gas temperature for all applications. Our opinion is the specified operating temperature of the sampling probe for Method 324 is too low. Experience and data taken in the field have pointed out that a minimum operating temperature of 400 ° F is required to transport oxidized Hg, unless the sample probe is recovered along with the sorbent tubes.

One commenter ( OAR 2002 0056 2867) stated that their experience indicates that further development of the Method 324 for use on a wet stack is necessary and can be accomplished in the days ahead. The commenter noted that the original sampling probe does not work in a wet stack. Condensation builds up in the trap, and renders the sample suspect. The commenter stated that in order to sample in a wet stack, the trap must be inside a heated probe. The commenter pointed out that Method 324 addresses this issue in section 6.1.2 and states that the sampling probe must be heated in duct temperatures less than 200 ° F, which equate to wet stacks.

Several commenters ( OAR 2002 0056 2634,  2718,  2861,  2922,  3565) stated that one important aspect of the Method 324 measurement is avoiding condensation in the sorbent trap by heating the sampling probe in those conditions where the gas stream may fall below the condensation point. Section 6.1.2 requires use of a heated sampling probe for effluents below 200 ° F as measured with a thermocouple. The commenter suggest the lower boundary of the range be increased to 250 ° F to ensure that no water droplets form in the sorbent trap.

One commenter ( OAR 2002 0056 2867) requests that, in reference to Method 324, 6 26 section 6.1.2, EPA confirm that using a heated probe does not introduce differences into the procedure for obtaining a dry stack sample and wet stack sample. The commenter contends that any introduced differences are not quantified at this time, and should be investigated further. Questions the commenter requests be answered include:

° What are the differences between the heated probe and normal probe? ° Will the differences introduce any variations into the dry/ wet stack sampling process? ° Does the heated probe effectively capture the Hg in the stack? ° If the heated probe is used on the wet stack, can it be used on all stacks to avoid introducing differences between the wet stack/ dry stack methods? ° Is this a sufficient work around for the wet stack problem?

Response:

EPA agrees that the commenters' concerns are all valid. However, the revised sorbent monitoring requirements are performance based and so achievement of the performance criteria will assure that these issues have been effectively managed for each sample. The final sorbent monitoring provisions advise the user that these are important issues to consider in developing their sampling strategy.

Comment:

One commenter ( OAR 2002 0056 2101) stated that the preamble should clearly say that Method 324 is intended as a measurement method, and not a reference method. This commenter asked if, in reference to Method 324, section 1.0 ( AF vs AA), tests have been done to confirm that atomic absorption ( AA) gives answers equivalent to atomic fluorescence ( AF) under all circumstances? Method 1631 is a performance based AF method that allows substitution of AA provided that QA/ QC criteria are met, however this method was originally intended for simple matrices like waste waters, not extracts from flue gas. ( It should be noted that this reviewer is not aware of any laboratories that have achieved true Method 1631 class performance with AA based systems, even for the simpler matrices.) EPA should exercise extreme care to ensure that AA techniques will always yield identical results to AF techniques.

One commenter ( OAR 2002 0056 2485) stated that Method 324 should also be subjected to the same RA test as the HgCEM.

One commenter ( OAR 2002 0056 2867) stated that Method 324 should be considered as a viable reference method. The Ontario Hydro method is the currently accepted reference method for measuring Hg concentration in a flue gas. EPRI's experience shows that Method 324 has less variability than the Ontario Hydro method in side by side comparisons ( as previously seen in Attachment 1). Method 324 is also a continuous monitoring system, whereas the Ontario Hydro method measures the Hg concentration for a snapshot of time. Thus, Method 324 has 6 27 considerable advantages over the Ontario Hydro method as a reference method. Once EPA incorporates Method 324 as a reference method, a natural extension will be to use it to monitor Hg emissions all the time or as a test audit for Hg CEMS. This argument is enhanced by the underpinnings in other CEMS activities, where the reference method is used to periodically check the daily monitoring method or can be used for daily monitoring in lieu of CEMS.

One commenter ( OAR 2002 0056 2867) stated that for applications where Hg CEMS are used and for Hg monitoring and audits to be advanced, a real time instrumental reference method for RA audits is needed. EPA must develop such an instrumental reference method for Hg.

Several commenters ( OAR 2002 0056 2485,  3455) suggested that Method 1631 was not an appropriate method to analyze sorbent traps for many reasons.

One commenter ( OAR 2002 0056 2485) noted that, in reference to Method 324, section 2.0, this section states that other recognized procedures can be used for the analysis of sorbent tubes such as ASTM D6784 02 and method 29. Neither of these methods is suitable for this application. Both methods use various impinger solutions none of which relate to the chemistry used to leach the sorbent tubes. Suggesting the use of these methods will ultimately create confusion and the method open to interpretation. For example the ASTM method has digestion and preparations for KCl, H2SO4/ KMnO4 and H2O2/ HNO3 impingers. Which digestion or procedure should be used for the sorbent tubes? Furthermore CV AAS and CV AFS should ideally have matrix matched standards for calibration procedures so what type of standardization should be used for sorbent tube leachate? The CEN 13506 methods, EPA 245.7 and 245.1 are more appropriate options because they do not use amalgamation and have wider dynamic ranges.

One commenter ( OAR 2002 0056 2040) proposes an alternative analytical technique to analyze Hg sorbent tubes for proposed Method 324. This technique involves direct ( no sample preparation) Hg analyzer Lumex RA 915+ with Attachment Pyro915/ RP91C for testing of iodated carbon sorbent from the tubes as an alternative to chemical digestion with following atomic fluorescent analysis of tubes. The advantages of this technology compare to proposed in Method 324 as follows:

° Technology is based on field portable Atomic Absorption spectrometry with Zeeman correction coupled to a furnace heated to 800 degrees C wherein Hg is converted from a bound state to the atomic state by thermal decomposition in a two section furnace. In the first section of the furnace the " light" Hg compounds are preheated and burned. In the second section a catalytic afterburner decomposes " heavy" compounds. ° Direct, onsite testing of tubes. Save time for shipping, generating of chain of custody, and wait for laboratory results. ° Testing results will be available within 1 hour after method 324 testing tube removed from the sampler. Analytical throughput 20 tests per hour. ° No chemical waste generated. ° NIST traceable standards used for multipoint calibration. 6 28 ° Detection Level to 0.5

g/ kg is 10 times lower than concentration of Hg expected for Method 324 tubes. ° No compressed gases required.

Response:

In view of the many comments received and the Agency's own field testing, EPA has decided to rename proposed Method 324, and to revise it with detailed, performance based QA standards and procedures for sorbent trap monitoring systems. These new standards and procedures are now included in 40 CFR part 75, Appendix K, of the final rule. EPA believes that by taking this action, there will be less confusion and more convenience for users affected by the Hg cap and trade program. Because the sorbent trap monitoring system requirements in Appendix K are performance based, the results of any analytical technique meeting the performance criteria should be comparable to any other analytical technique meeting the same performance criteria.

EPA did not propose Method 324 as a reference method, but rather as a set of analytical and QA procedures for sorbent traps to be used in a Hg monitoring program. The Ontario Hydro Method is the reference method for Hg in part 75 of the final rule. However, EPA is developing a Hg instrumental reference method as part of the Agency's field tests of Hg monitoring systems.

Comment:

One commenter ( OAR 2002 0056 3455) noted that, in reference to Appendix A to the Preamble Proposed Changes to Parts 72 and 75, ( Proposed Rules March 16, 2004); page 12417, EPA stated that sorbent trap systems can't be calibrated with cylinder gas ( in description of Alternative 2, page 12417); although this may be true, it does not mean that their performance can't be checked. The lack of QA/ QC and method validation is a very disturbing approach to proposing a new sampling method. It is common knowledge in the measurement industry that carbon traps can become passivated in the presence of flue gas constituents. In addition to becoming passivated, traps are also prone to re releasing Hg after long term exposure to flue gas constituents, thus creating a situation that would seriously under report Hg concentrations, and unfortunately, the use of the second section in the trap does nothing to address either of these problems.

Two possible solutions to this problem exist: ( 1) one trap in a pair could be spiked and placed in a dual sample train, when the train is analyzed the spike could be subtracted and the result should then match with the second train that was not spiked; or ( 2) one of the traps in the paired sample could be exposed to a Hg calibration gas near the end of its sample period, this spike could then be subtracted from the value of the spiked trap and that value compared to the trap that was not spiked. If the value of the reference trap and the spiked trap minus the spike do not match ( i. e., within +/ 20 percent) then the data should be invalidated. 6 29 One commenter ( OAR 2002 0056 2101) stated that, in reference to Method 324, the periodic RATA test against a reference method will not be effective in ensuring that the method is producing accurate results. This type of test is not capable of detecting cartridge passivation during lengthy Method 324 runs. Passivation is expected to occur relatively late in a sample run. Total sample volumes passed through a collection cartridge during a RATA test may be only a few percent of the total volumes that will be passed through the cartridges during normal sampling.

Response:

Extremely high stack gas temperatures ( greater than about 350 deg F) required for passivating sorbent material are not expected. On wet stacks, 40 CFR part 75, appendix K, states that sorbent traps may need to be heated above the dew point to prevent condensation. Therefore, matrix effects that cause passivation or loss of Hg trapping efficiency should not be a problem. However, EPA will be examining this issue further during our field testing of sorbent media.

Comment:

One commenter ( OAR 2002 0056 2867) stated that in the SNPR Section B. 3., " Use of Mercury CEMS and Sorbent Trap Systems," the EPA proposes in Alternative 2 that the sorbent trap method can be used by any source, with an annual 9 run RATA and quarterly 3 run RAA required for QA. The commenter maintains that these QA requirements are excessive and the results ( pass/ fail/ bias) will remain unknown for days and weeks. The commenter also stated that elsewhere, the EPA stated, " For sources with annual Hg emissions below the specified threshold value, the QA requirements for sorbent trap monitoring systems would be less, with only an annual RATA being required" ( 69 FR 12417). The commenter recommends that the QA requirements for sorbent traps  all units, irrespective of emission rates  be consistent and limited to an annual RATA, although the RATA may be unnecessary if Method 324 is ultimately is adopted as a reference method. The commenter stated that the accuracy of the sorbent trap method does not decrease with increasing unit size or larger Hg emission rate. The commenter also stated that the EPA also stated " the Agency is willing to consider replacing the RAA requirement with another type of substantive quarterly QA test, if commenters who favor the use of sorbent trap systems are aware of, and can provide details of, any such test or procedures" ( 69 FR 12417). The commenter recommends that if quarterly checks are deemed necessary, the EPA might include semiannual analysis of spiked and blank traps, and quarterly sample flow checks and pump calibrations.

Response:

See Sorbent Trap Operation and QA/ QC discussion in the preamble.

Comment: 6 30 Several commenters ( OAR 2002 0056 2079,  2485,  2634,  2718,  2861,  2922,  3455,  3565) were generally opposed to the proposed quarterly relative accuracy audits ( RAAs) for sorbent trap systems as being too costly and of little value. A number of commenters suggested that EPA should revise proposed Alternative # 2 and specify QA procedures that are meaningful to the type of measurement system that the sorbent trap actually is. For example, the volume of stack gas sampled by the system is an important parameter in determining the Hg concentration. Therefore, procedures for quality assuring the measurement of the sample volume could be implemented.

Some commenters favored allowing the use of proposed Method 324 for all affected units, and stated that because Method 324 is itself a test method, it does not need additional QA procedures. Two commenters suggested that EPA should even take steps to make Method 324 a reference method. However, numerous other commenters objected to various provisions of proposed Method 324 and offered suggestions for improving it. Some of the chief objections raised were as follows:

 The allowable analytical techniques and procedures in the method are too exclusive, and in the case of Method 1631, inappropriate. Other analytical methodologies should be allowed;

 The impinger and dessicant method of moisture removal is inadequate;

 The leakage rate prescribed for the leak checks checks may be too low to measure;

 The method allows constant rate sampling for collection periods less than 12 hours, which may introduce bias if unit load changes during the collection period;

 The specification for flow proportional sampling ( adjust sample flow rate to maintain proportional sampling within + 25 percent of stack gas flow rate) is not stringent enough and can lead to inaccurate concentration measurement;

 The frequency for dry gas meter calibration is unspecified; and  The method does not include chain of custody procedures.

A number of commenters suggested that EPA should not require the use of paired sorbent traps and should allow the use of single sorbent traps.

Several commenters objected to the proposal in section 1.5.4 of Appendix B that laboratories performing Method 324 be certified by the International Organization for Standardization ( ISO) to have proficiency that meets the requirements of ISO 9000. One commenter stated that having a good blank and matrix spike program in place is much more indicative of a good QA/ QC program for Hg measurement than ISO 9000 certification. Another commenter favored ISO certification, but not according to ISO 9000. The commenter recommended that ISO 17025 be required instead, because it requires the laboratory to demonstrate proficiency, rather than simply having an acceptable protocol for the analyses.

One commenter stated that EPA has not explained the appropriateness of applying a bias 6 31 test and adjustment factor to Method 324, when it has already satisfied the same standards for bias and precision as the Ontario Hydro Method under EPA Method 301. Another commenter suggested that it does not make sense to subject Hg monitors to a bias adjustment factor under Appendix A, section 7.6 when paired reference method trains are allowed to differ by 10 percent relative deviation ( RD), based on a flawed definition of RD. The commenter asserted that it is not reasonable to suggest that a Hg monitor is biased by comparing its readings to a pair of reference method tests that can differ by 20 percent.

One commenter ( OAR 2002 0056 2867) stated that their current experience with Method 324 ( QSEMS) indicates that component reliability has to be improved. The commenter's experience indicates issues with the following:

° Flow meter ( Totalizer)  Typical problems encountered are a delayed start to recording the data and discontinuous recording of data after several days. ° Sampler Pump  Readings vary by ± 25 percent when set at constant flow. The flow rate also decreases overtime as particulates build up. ° Currently O 2 cannot be measured and recorded with this device. Correcting to 3 percent O 2 is the typical way to compare individual test runs and to allow the detection of leaks on line during the sampling period. ° The carbon trap plugs in high ash situations. The method will not work upstream of an ESP without an inertial separator or other method to remove the ash. ° For wet stack application the probe will need modification.

The commenter asserted that these changes are required before QSEMS can be relied on to demonstrate compliance. The commenter notes the industry is investing time and money into the development of these systems, to enhance reliability by 2010. The commenter contends it is important that the compliance deadline not be moved earlier than 2010.

Response:

In view of the many comments received and the Agency's own field testing, EPA has decided to rename proposed Method 324, and to revise it with detailed, performance based QA standards and procedures for sorbent trap monitoring systems. These new standards and procedures are now included in 40 CFR part 75, appendix K, of the final rule. EPA believes that by taking this action, there will be less confusion and more convenience for users affected by the Hg cap and trade program. Today's rule also revises both the definition of a sorbent trap monitoring system in section 72.2 and the general guidelines for sorbent trap monitoring system operation in section 75.15, to be consistent with the QA requirements of Appendix K.

The final rule retains the annual RATA and bias test requirements for sorbent trap monitoring systems, but the proposed quarterly RAA requirement has been withdrawn. The requirements to use paired traps and flow proportional sampling have also been retained. Finally, the ISO 9000 certification requirement for the laboratory performing the Hg analyses 6 32 has been replaced with a requirement for the laboratory to either comply with ISO 17025 or to comply initially, and annually thereafter, with the spike recovery study provision in section 10 of 40 CFR part 75, Appendix K.

Several commenters recommended that EPA should require QA procedures for sorbent traps that are more meaningful and reasonable than the procedures in the SNPR. EPA agrees with these comments, and based on the recommendations received, today's rule specifies such procedures in Appendix K. Many provisions of Method 324 have been included in Appendix K without modification, but other provisions of the method have been modified and some new QA procedures have been added to address concerns expressed by the commenters. Some of the more significant differences between Method 324 and Appendix K are as follows:

Appendix K allows the use of any sample recovery and analytical methods that are capable of quantifying the total vapor phase Hg collected on the sorbent media. Candidate recovery techniques include leaching, digestion, and thermal desorption. Candidate analytical techniques include ultraviolet atomic fluorescence, ultraviolet atomic absorption, and in situ X ray fluorescence;

Appendix K requires that each sorbent trap be comprised of three equal sections, the first one for sample collection, the second to assess " breakthrough", and the third to allow spiking with elemental Hg, for QA purposes;

Appendix K specifies the frequency of dry gas meter calibration, and the appropriate calibration procedures;

Appendix K requires ASTM sample handling and chain of custody procedures to be followed.

Spiking of the third section of each trap with elemental Hg is required before each data collection period begins.

The laboratory performing the analyses must demonstrate the ability to recover and quantify Hg from the sorbent media

The measured Hg mass in the first and second sections of each trap is adjusted ( normalized), based on the percent recovery of Hg from the third (" spiked") section.

EPA believes that if these procedures are implemented, this will ensure the quality of the data from sorbent trap systems.

The final rule retains the requirement to use paired sorbent traps. The SNPR proposed the use of paired sorbent traps for the same basic reason that paired Ontario Hydro trains are required for RATA testing, i. e., it provides an important check on the quality of the data. The proposed rule would have required the higher of the two Hg concentrations obtained from the paired traps to be used for reporting. However, the final rule requires the results from the two traps to be averaged if they meet specified criteria, and allows the results from one trap ( if those results are valid) to be reported in cases where the other trap is accidentally damaged, broken or lost during transport and analysis. Thus, using paired sorbent traps provides a relatively 6 33 inexpensive means of ensure against data loss should one of the traps become lost or damaged.

The commenters generally objected to the proposed quarterly relative accuracy ( RA) testing of sorbent traps, believing it to be unnecessary and costly. After consideration of recent field data comparing the sorbent traps to Hg CEMS, EPA agrees that sorbent trap systems should be treated more similarly to Hg CEMS. Therefore, the final rule removes the quarterly RAA requirement, and requires only that an annual RATA be performed on a sorbent trap monitoring system.

One commenter objected to the proposed bias test requirement for sorbent trap systems, citing the fact that Method 324 had satisfied the same standards for bias and precision as the Ontario Hydro Method under EPA Method 301. EPA does not agree with this comment. The fact that Method 324 met the bias and precision requirements of Method 301 does not imply that Hg sorbent traps will not exhibit low bias with respect to a Hg reference method during a RATA. The bias test in section 7.6 of 40 CFR part 75, appendix A, is a one tailed t test, which, if failed, requires a bias adjustment factor ( BAF) to be applied to the subsequent emissions data.

EPA also does not agree with the commenter who stated that bias adjustment is not appropriate for sorbent trap systems because of the allowable 10 percent RD between the paired reference method trains. The part 75 bias test determines systematic error, not random error, whereas RD and relative accuracy are metrics used to quantify random error in the measurement.

Comment:

One commenter ( OAR 2002 0056 3455) suggested that, in reference to Method 324 ( 40 CFR part 63), section 1.1.2 ( Applicability), page 4736, a Hg RATA should be performed on the same long term time basis as the method's use of a CEMS, i. e., an applicable RATA time period equal to or greater than the longest averaging period. This may mean multiple Ontario Hydro paired train runs during the Method 324 extended sampling time period. Moisture corrections from the Reference Method should be applied to Method 324 data for the entire averaging period. A short term averaging period for correlation to the Reference Method does not provide long term assurance of carbon trap performance.

Response:

The final rule requires that a minimum of 9 runs be used to calculate a RA for either Hg CEMS or sorbent traps. For the RATA of a Hg CEMS using the Ontario Hydro Method, or for the RATA of a sorbent trap system ( irrespective of the reference method used), the minimum time per run must be long enough to collect a sufficient mass of Hg to analyze. EPA has decided to implement this approach as a compromise between the desire to test the sorbent traps' long term performance and the practicality of performing a potentially week long reference method test, with the Ontario Hydro Method. 6 34 Comment:

One commenter ( OAR 2002 0056 3455) stated that, in reference to proposed Method 324 ( 40 CFR part 63), section 1.1.2 ( Applicability), page 4736, performance verification criteria must be established to assure that 100 percent cartridge trapping efficiency is maintained:

° Over the full permitted temperature range of the sampling train and over the full stack gas temperature range; ° Over the full flow rate range of the sampling train ( especially the combination of highest temperature range with highest flow rate  which would be expected to create the highest probability of breakthrough.); and ° Over the full range of stack gas compositions. This becomes particularly important in cases where control technology is employed.

Response:

The extremely high stack gas temperatures required for passivating sorbent material are not expected to be encountered in utility stacks. On wet stacks, Appendix K of 40 CFR part 75 states that sorbent traps may need to be heated above the dew point to prevent condensation. Therefore, matrix effects that cause passivation or loss of Hg trapping efficiency should not be a problem. However, the Agency will be investigating this issue further during our field testing of sorbent media.

Comment:

Commenter OAR 2002 0056 2922 stated that section 75.15 contains the Special Provisions for using the sorbent trap monitoring method ( Method 324). Section 75.15( e) specifies proportional sampling and then further explains the proportional sampling procedure as a change in the sampling rate in relation to load. This procedure is flawed and is in conflict with Method 324. Any proportional sampling method will require stack flow rate or load input into the sampling device. The sorbent traps will be located at a stack or duct location and manually making the required flow changes, at odd times of the day, will just not be feasible. EPA should make this section consistent with Method 324 and allow for automated input of stack flow or load data into the sorbent sampling system and allow for automated flow rate adjustment.

Response:

EPA agrees with the commenter and the final rule requires the flow control valve and air tight sample pump to be controlled by the 40 CFR part 75 certified flow monitoring system. The final rule also requires that the data acquisition and handling system ensure that the sampling rate is proportional to the stack gas volumetric flow rate.

Comment: 6 35 Commenter OAR 2002 0056 2922 stated that section 75.15( i) should have the words " or partial hours" added to the first sentence.

Response:

EPA believes that the suggested change is unnecessary in light of the definition of " unit operating hour" in section 72.2 which includes " any hour ( or fraction of an hour)."

6.4 QA/ QC PROCEDURES FOR HG CEMS Comment:

One commenter ( OAR 2002 0056 3406) stated that given the early stages of the development of Hg CEMS technology, it is inappropriate to specify a mandatory performance specification. The commenter believed facilities subject to this requirements should have the option of proposing and complying with an alternative performance standard that provides substantially similar assurances.

Response:

The final rule contains an alternative relative accuracy performance specification for low emitting sources.

Comment:

One commenter ( OAR 2002 0056 3455) suggested that, in reference to PS 12A, the final rule should change the cycle time ( from the proposed fixed 15 minutes) to a field determined cycle time. Elemental Hg can achieve a 15 minute cycle, however, currently that cycle time would be difficult for oxidized Hg to achieve. EPA should continue to work with vendors with a goal of achieving 15 minute cycle times for all forms of Hg. A TGM blend will also take longer.

Response:

EPA's field testing indicates that 15 minute response times and cycle times are achievable for both elemental and oxidized Hg.

Comment:

One commenter ( OAR 2002 0056 3455) suggested that, in reference to PS 12A, the CEMS should be challenged with a calibration gas blend of Hg0 and Hg+ 2 at a frequency consistent with 40 CFR part 60, Appendix B, PS 2, introduced at the sample acquisition point, prior to any filtration; to access sample transport for data validation. 6 36 Response:

EPA agrees with the need to periodically check the performance of the entire monitoring system. EPA's field testing has shown that a single injection performed daily using the appropriate concentration of oxidized Hg is sufficient to check the upscale calibration of the Hg CEMS from the probe tip through the analyzer and the efficiency of the HgCl2 to Hg converter. The final rule allows either elemental or oxidized Hg to be used to perform daily calibration checks.

Comment:

Commenter OAR 2002 0056 2922 stated that in section 60.4171( c)( 2) it is not clear whether a source installing a new Hg monitoring system must submit a Certification Application. In many instances, the Hg monitoring system will be a completely separate Hg/ diluent system that uses full concentration, dry basis analysis. The data acquisition system and other inputs, like stack flow rate will be associated with a previously certified part 75 system. It is not clear whether such a system would require a certification application to be submitted. It is also not clear how much of the system would need to be included in the certification application.

Response:

Section 60.4171( c)( 2) states that a certification application is not required if the system has been previously certified under the Acid Rain Program or under an applicable State or Federal NOx mass emission reduction program that adopts the requirements of 40 CFR part 75, subpart H. Therefore, e. g., if a flow monitoring system was previously certified under, e. g., the Acid Rain Program ( part 75), a certification application for that flow monitoring system is not required.

The final rule removes all requirements for a Hg emission rate ( or Hg diluent) monitoring system. However, either a sorbent trap monitoring system or a Hg concentration monitoring system is required by the final rule. A Hg concentration monitoring system consists of a Hg pollutant concentration monitor and an automated data acquisition and handling system. Because neither a Hg concentration monitoring system nor a sorbent trap monitoring system was required by 40 CFR part 75 until this rulemaking, a certification application must be submitted for either of these systems.

Comment:

Several commenters ( OAR 2002 0056 1969,  1975,  2040,  2046,  2063,  2068,  2073,  2079,  2160,  2181,  2206,  2224,  2244,  2252,  2259,  2260,  2267,  2296,  2365,  2379,  2380,  2429,  2485,  2578,  2718,  2721,  2827,  2830,  2833,  2835,  2844,  2862,  2867,  2877,  2889,  2891,  2898,  2900,  2918,  2922,  2929,  2947,  3200,  3413,  3436,  3443,  3444,  3454,  3458,  3487,  3559,  3568) were in general agreement on the following points. 6 37 Although many vendors of Hg CEMS have recently upgraded their instrument systems and these changes should eventually improve the accuracy and reliability of Hg CEMS and reduce the labor needed for instrument maintenance, these new instrument systems have not been tested extensively in demonstration programs. Therefore, the ability of these instrument systems to achieve the proposed relative accuracy, calibration error, and calibration precision requirements has not been adequately demonstrated. Therefore, EPA does not yet have a basis or data to guide the setting of specifications for calibration error, linearity, or relative accuracy. It appears that the proposed performance specifications mirror those for SO2 and NOx monitoring. EPA should commit to collecting data and evaluating these specifications as soon as calibration gases are available, so that the specifications can be adjusted if necessary, prior to program implementation. EPA should require operators of Hg CEMS to conduct procedures that include but are not necessarily limited to daily zero and span audits, quarterly relative accuracy tests and three point elemental Hg linearity tests, and absolute calibration audits. Analytically, there is clearly a need to challenge the entire system often with a form of oxidized Hg. This Hg chloride ( HgCl2) reference gas would be highly desirable to check integrity of the sample interface. However, further research needs to be required to enable the development of an accurate oxidized Hg standard. One device, the Hovacal, may have the potential of delivering known concentrations of HgCl2. EPA should recognize and accept this type of calibration system in the proposed regulation. There are concerns with the proposed RATA process, particularly the length of time and amount of money that may be required to comply with the Hg monitoring requirements on an annual basis. The final monitoring requirements must be technically achievable and capable of measuring Hg emissions with precision, reliability, and accuracy in a cost effective manner. The decision to report Hg concentration on dry or wet basis needs more consideration, as well as, the evaluation of gaseous interferences. Lastly, many of the equations and calculations are incomplete or contain errors and many sections need further clarification.

Response:

In the final rule, the same tests are required for initial certification and on going QA of Hg CEMS as were proposed in the SNPR. However, note the following changes to some of the procedures and performance specifications:

For the 7 day calibration error test, either elemental Hg standards or a NISTtraceable source of oxidized Hg ( referred to as " HgCl2 standards" in the SNPR) may be used;

Quarterly 3 level " system integrity checks" ( which were called " converter checks" in the SNPR) using a NIST traceable source of oxidized Hg may be performed in lieu of the quarterly linearity checks with elemental Hg;

Daily calibration error checks may be performed using either elemental Hg standards or a NIST traceable source of oxidized Hg. The daily performance specification has been made the same as for the 7 day calibration error test;

The monthly converter check at three points has been replaced with a weekly 6 38 system integrity check at a single point, and the weekly test is not required if daily calibrations are performed with a NIST traceable source of oxidized Hg.

When the Ontario Hydro Method is used, paired trains are required, the results must agree within 10 percent RD, and the results should be averaged.

Note that EPA plans to analyze RATA data from Hg monitors and may initiate a future rulemaking to adjust the relative accuracy performance specifications and to propose a performance based RATA incentive system similar to the reduced frequency incentive system in 40 CFR part 75 for SO2, NOx, CO2, and flow monitors.

EPA disagrees with the commenters who stated that there is no data available to justify the proposed performance specifications for Hg monitors. Such data have been collected from several field test sites and for several different types of Hg concentration monitors, which show that Hg CEMS can meet the proposed calibration error and linearity standards, and can meet a 20 percent RA standard. Therefore, except for the daily calibration error specification, which has been tightened based on the available data, the final rule promulgates the proposed calibration error, linearity check, and RATA performance specifications, as proposed.

EPA has retained the requirement to check the converter periodically with HgCl2 standards, because it is essential to ensure that all of the vapor phase Hg is being measured. The frequency of the check ( which is referred to as a " system integrity check" in the final rule) has been increased from monthly to weekly, based on supportive comments to check the entire system more often, but the requirement to perform a 3 point check has been reduced to a singlepoint test. And the weekly test is not required if a NIST traceable oxidized Hg source is used for daily calibrations.

There are several different devices available that can provide oxidized Hg, including the HOVACAL and the MerCAL. The HOVACAL has been successfully applied in the laboratory and field to generate and deliver known concentrations of HgCl2 to Hg CEMS to achieve the requirements of the 40 CFR part 75 system integrity check. Moreover, oxidized Hg gas standards such as are produced by the HOVACAL and MerCAL are currently scheduled to be independently tested by NIST, to verify their suitability as reference gas standards.

Comment:

Commenter OAR 2002 0056 2922 stated that the requirement of proposed Appendix A, section 2.2.3 cannot be met. The commenter is not aware of any device, other than the Hovacal that might be able to deliver " known concentrations of HgCl2" as required. The performance of the Hovacal does not appear to be well documented.

Response:

Several different sources of gaseous, oxidized Hg are available and have been 6 39 demonstrated, incuding the HOVACAL. The HOVACAL has been successfully applied in the laboratory and field to generate and deliver known concentrations of HgCl2 to Hg CEMS to achieve the requirements of the converter check as called for in the 40 CFR part 75 Hg CEMS precertification requirement. Moreover, oxidized Hg gas standards such as the HOVACAL and MerCAL are currently scheduled to be independently tested by NIST to verify their suitability as reference gas standards.

Comment:

Commenter OAR 2002 0056 2922 stated that in Appendix A, section 3.1, the specification should read " 5% of the span value if the span value is 20 micrograms/ dscm or greater, or 1 microgram/ dscm if the span value is less than 20 micrograms/ dscm." The specification as now written penalizes any monitor with a span value between 10 and 20 micrograms/ dscm.

Response:

Appendix A, section <IP_ADDRESS> defines span for Hg CEMS in multiples of 10 micrograms/ dscm. Therefore, there will be no Hg span values between 10 and 20 micrograms/ dscm. Appendix A, Section 3.1 is retained, as proposed, in the final rule.

Comment:

Commenter OAR 2002 0056 2922 stated that Equation F 29 appears to make the calculation correctly. However, the description in section 9.1.2 is difficult to understand. A complete set of equations and nomenclature should be provided instead of the cross references and replacement values.

Response:

Proposed Equation F 29 applies only to Hg diluent monitoring systems. In the final rule, EPA has deleted Equation F 29 and all provisions related to this type of monitoring system. The final rule requires Hg mass emissions to be determined as the product of the Hg concentration and the stack gas flow rate.

Comment:

Commenter OAR 2002 0056 2922 stated that, both PS 12A and Method 324 are designed to measure vapor phase Hg. Consistent with that, EPA should make clear in the 40 CFR part 75 RATA requirements ( as EPA did in PS 12A, section 8.6.2) that the filterable portion of the reference method sample is not included when making the comparison to the CEMS. Consistent with that change, EPA should also remove the requirements in sectioin 75.59( a)( 7) to record and report RATA results related to particle bound Hg, or justify the collection and 6 40 submission of that additional data.

Response:

The final rule clearly states in 40 CFR 75.22 that only the vapor phase Hg, and not the filterable portion of the reference method sample is included when making the comparison to the CEMS. However, EPA has retained in section 75.59( a)( 7) the recording and reporting of both gaseous and particle bound Hg, when particle bound Hg is provided by the reference method. The particle bound Hg emissions data is required in today's final rule to provide information to assess the need for possible future regulation of particle bound Hg.

6.5 MERCURY MONITOR AVAILABILITY Comment:

One commenter ( OAR 2002 0056 3560) stated that no correlation has been established between the basis for the standards and the required test methods. Plants that submitted Hg data to the EPA used the Ontario Hydro Method to calculate their Hg emissions. EPA then used these data, based on the Ontario Hydro Method, to create the Hg standards. Whether the proposed monitoring systems are comparable to the Ontario Hydro Method, which was used to set the Hg emissions limits is unknown. If they are not, the Hg emissions limits could result in a standard that is not obtainable from the outset. This would leave facilities open to notices of violations/ penalties when their failure to comply is a result of an inconsistency between analytical methods   the Ontario Hydro Method used to set the Hg emissions limits versus Hg CEMS, sorbent trap monitoring systems, and long term sampling monitoring that are to be employed to determine if units are meeting the emissions limits for compliance purposes. Discerning whether the test methodology utilized to create the Hg emission standards ( Ontario Hydro Method) correlates to any of the required test methods, and especially when Hg CEMS are not yet commercially available, is impossible.

Response:

EPA disagrees and believes Hg CEMS and sorbent trap monitoring systems have been demonstrated to be reasonably comparable to the Ontario Hydro Method through various field demonstrations.

Comment:

One commenter ( OAR 2002 0056 1854) noted that the analyzer can be no more than 100 feet from the sample probe. The commenter state that this means, in their case, that QA and maintenance procedures must take place on the stack platforms where personnel will be exposed to extreme weather conditions from time to time. The commenter also stated that their experience would also indicate that additional staffing by specially trained personnel will be 6 41 required because these monitors cannot be left unattended for long periods of time.

Response:

Mercury CEMS demonstration testing conducted by EPA so far has provided evidence from a number of CEMS at a number of different utilities that the analyzer does not have to be located on the stack platform. Regarding on going system maintenance, the level required will ultimately depend on the monitoring system and the emission characteristics. We have noticed a marked improvement in amount of time needed for hands on monitor attendance over the time frame of demonstrations, including automation of daily check procedures and capability for remote system adjustment, and expect significantly more improvement in the coming months.

Comment:

One commenter ( OAR 2002 0056 3548) stated concerns about the technical feasibility of operating and maintaining CEMS for Hg. The proposed monitoring requirements are beyond the capabilities of current monitoring equipment. Although the technology is developing rapidly, the proposed technology is analogous to EPA proposing to require 40 CFR part 75 monitoring requirements for flow, SO2, and NOx in 1970 with a compliance deadline of 1974. This being the case, it is imperative that EPA provide as much flexibility as possible in allowable monitoring methods.

Response:

EPA believes that Hg CEMS and sorbent trap monitoring systems meet the proposed monitoring requirements. The final rule contains flexibility by allowing sources to account for their Hg emissions by using Hg CEMS, sorbent trap monitoring systems ( or a combination thereof), and, in some cases, using low mass provisions.

Comment:

One commenter ( OAR 2002 0056 2915) stated concerns about the technical feasibility of operating and maintaining CEMS for Hg. The commenter's concerns regard the precision, reliability, and accuracy of the monitoring alternatives identified by the EPA in the proposed Hg rule. One area of major concern involves the proposed detection limits. The commenter asserts that as the allowable Hg emissions levels grow smaller ( particularly in the case of new or well controlled existing units), it becomes technically more difficult to measure Hg levels in the emissions from an electric generating unit ( EGU) and to determine how the inherent measurement uncertainties will impact an EGU's compliance demonstration with the Hg limits. Further, should CEMS for Hg be required by the Hg rule, the commenter would have concerns with the proposed RATA process, particularly the length of time and amount of money that may be required to comply with the Hg monitoring requirements on an annual basis. The commenter asserts that the final monitoring requirements must be technically achievable and capable of measuring Hg 6 42 emissions with precision, reliability, and accuracy in a cost effective manner.

Response:

EPA believes that Hg CEMS will provide adequate precision, reliability, and accuracy for emissions trading, however, may not be necessary for all units. Consistent with the low mass emissions ( LME) provisions for SO2 and NOx, the final rule provides a less rigorous monitoring option for low Hg emitters.

Comment:

One commenter ( OAR 2002 0056 2830) stated it is not appropriate to require CEMS monitors on new units if operations begins more than six months after publication of the final rule. CEMS have not been commercially demonstrated, and as commented on earlier, the commenter believes that they cannot be commercially demonstrated within 4 years of the rule being finalized.

Response:

EPA disagrees with commenter and believes that Hg CEMS and sorbent trap monitoring systems can be commercially demonstrated before rule implementation. However, the requirement for new units to use Hg CEMS has been withdrawn. New units may use sorbent trap monitoring systems instead, under both 40 CFR part 75, subpart I, and under 40 CFR part 60, subpart Da.

Comment:

One commenter ( OAR 2002 0056 2429) requested the EPA provide additional technical information on both CEMS that meet PS12 requirements and sorbent traps that meet Method 324 requirements for various plant configurations and conditions such as wet stacks. Their units are equipped with wet scrubbers, fabric filters, and low NOx burners.

Response:

EPA will provide additional information when available.

Comment:

One commenter ( OAR 2002 0056 2899) stated that currently no Hg CEMS have been demonstrated to be accurate and reliable. The commenter stated that although continuous systems are available from different manufactures none have been used in continuous operation for an extended period of time. The commenter asserts that most have been used in pilot or short term full scale tests and generally compared to the Ontario Hydro impinger method, which is a short term test ranging from minutes to hours, and this gives only snapshots of the continuous 6 43 monitors performance. The commenter noted the current generation of continuous monitors are based on wet chemistry requiring almost constant maintenance and calibration. The commenter stated that although dry chemistry systems are under development and testing none are yet commercially viable. According to the commenter, at present continuous Hg monitors not are ready to be used for continuous compliance. The commenter stated that the EPA should allow the option of using a periodic measurement system such as EPRI's QuickSEM or other system instead of a continuous monitor for compliance with this rule.

Response:

EPA disagrees with the commenter and believes that field tests have demonstrated Hg CEMS to be accurate and reliable. The Hg CEMS have performed adequately for several months and meet the Ontario Hydro Reference Method specifications. Furthermore, several dry chemistry Hg CEMS are currently being tested at sites that represent the most challenging conditions and the Agency plans to share with industry the results of such experiences to facilitate the selection of appropriate monitoring methodology. EPA is also confident that substantial advancement of Hg CEMS will occur before the implementation of the rule and as other monitoring techniques may become available, is allowing the use of systems that can meet performance based specifications.

Comment:

One commenter ( OAR 2002 0056 5495) stated that CEMS for measuring low Hg emissions from waste coal is not a proven technology.

Response:

EPA disagrees with the commenter and believes that field tests have demonstrated Hg CEMS to be accurate and reliable at low Hg concentrations. The Hg CEMS have performed adequately for several months and meet the Ontario Hydro Reference Method RATA specifications at two low Hg concentration ( 0.5  2 µ g/ dscm) coal fired sources. EPA is also confident that substantial advancement of Hg CEMS will occur before the implementation of the rule and as other monitoring techniques may become available, is allowing the use of systems that can meet performance based specifications.

Comment:

One commenter ( OAR 2002 0056 2830) agreed with the EPA that compliance be monitored through the use of CEMS or other continuous measurement methods ( e. g., sorbent trap) for all affected sources. However, the commenter is concerned that monitoring and recording technology has not evolved to the level of reliability necessary to collect emissions data for compliance purposes. As entered in the EPA Docket, the EPA funded project termed " Long Term Evaluation of Mercury Continuous Emission Monitoring Systems"  December 11, 6 44 2003, it was determined that the current capabilities of Hg CEMS are not at a level that can be relied on in a regulatory compliance environment. In almost all cases, the analyzers failed to meet the 20 percent relative accuracy criteria for the first two phases. In Phase III, there were some instruments that passed the relative accuracy criteria. The Carbon Bed technology was only tested in Phase ill of the program and did exceptionally well as compared to the other CEM systems and against the Ontario Hydro Method. However, there are concerns that the facility was an " optimum test site." A substantial test for Hg analyzer systems should come under test conditions that are strenuous and challenging in order to identify shortcomings of the systems. The test should be completed at a facility where the sampling location has a representative particulate loading and sulfur concentrations. The systems should also be challenged under wet stack conditions.

Response:

EPA is currently field testing Hg CEMS and sorbent trap monitoring systems under strenuous and challenging conditions where the sampling location is under wet stack conditions and has a representative particulate loading and sulfur concentration. Preliminary data from these field tests seems to indicate that monitoring and recording technology are getting closer to the level of reliability necessary to collect emissions data for compliance purposes, supporting the Agency's confidence that by the end of the demonstration technology will provide the answers to the challenges presented.

Comment:

One commenter ( OAR 2002 0056 3469) stated that lack of control and monitoring technology impedes speedy compliance. The EPA's proposed Hg monitoring technologies ( e. g., CEMS 12A and Method 324) are not yet commercially available and do not yet provide accurate data. It is not known when they will be successfully tested and commercially available.

Response:

EPA disagrees with commenter; Hg CEMS and sorbent trap monitoring systems are commercially available and provide accurate data, as demonstrated by various completed and ongoing field tests. In addition, EPA believes these technologies will significantly advance before compliance is necessary.

Comment:

Is it currently feasible, or will it be feasible within the compliance timeframes of the proposed rule, to accurately monitor a source's Hg emissions by species?

Response: 6 45 The final rule requires the measurement of total vapor phase Hg, but does not require separate monitoring of speciated Hg emissions ( i. e., elemental and ionized Hg). Because of the potential impact of Hg speciation on local versus broader geographical deposition, the Agency considers separate monitoring of these emissions as a need to be addressed. However, at least two current monitoring technologies can accurately monitor speciated Hg emissions. The Agency will continue to test speciated Hg monitoring technologies. If these technologies are adequately demonstrated, the Agency may consider a proposed rulemaking within four to five years after program implementation, which should provide enough lead time for development and installation of these monitoring systems.

6.6 MERCURY DILUENT SYSTEMS Comment:

One commenter ( OAR 2002 0056 3455), in reference to section 72.2  Definitions, page 12453, asked why the proposed Hg emissions units of measurement are the same as NOx  diluent? The Hg concentration measurements are orders of magnitude below NOx emissions, thus applying a diluent correction with the additional uncertainties of measurement further complicates the direct emissions reporting uncertainties. Mercury is a resident pollutant in the fuel, it can be measured, and measurement should parallel the same regulation requirements as SO2.

Response:

The final rule removes all mention of Hg diluent monitoring systems and requires the hourly Hg mass emissions to be calculated in the same manner as is done for SO2 under the Acid Rain Program, i. e., as the product of the Hg concentration and the stack gas flow rate. The final rule also better accommodates Hg analyzers that measure on a wet basis.

EPA believes that the rule can be considerably simplified and shortened without losing any flexibility by deleting the provisions related to Hg diluent monitoring systems and allowing only Hg concentration monitoring systems and sorbent trap systems to be used. Therefore, the final rule removes all mention of Hg diluent monitoring systems and requires the hourly Hg mass emissions to be calculated in the same manner as is done for SO2, i. e., as the product of the Hg concentration and the stack gas flow rate.

6.7 LOW EMITTING UNITS Comment:

One commenter ( OAR 2002 0056 2900) questioned the need for continuous monitoring if the EPA requires coal fired EGUs to meet MACT standards. The commenter urges the Agency to allow periodic monitoring and parametric monitoring approaches rather than restricting sources to the use of Hg CEMS or sorbent trap monitoring. 6 46 Response:

The Agency has decided to use a cap and trade program to control Hg emissions. Complete and accurate accounting of Hg emissions is required for a credible cap and trade program. Therefore, Hg CEMS or sorbent traps are required in the final rule. However, qualifying, low emitting sources may comply with today's monitoring requirements by using conservative default Hg emission factors and annual or semi annual stack testing.

Comment:

Numerous commenters ( OAR 2002 0056 2101,  2162,  2267,  2634,  2718,  2861,  2867,  2900 2918,  2922,  3432,  3509,  3513,  3565,  2855) requested that EPA provide a less rigorous, cost effective monitoring option for low emitting units. Affected units could meet a low emitter criterion based on a combination of unit size, operating time, and/ or control device operation. Any marginal decrease in accuracy from less rigorous monitoring would have a minimal impact overall, because these units represent only a small percentage of the nationwide Hg mass emissions.

Response:

Consistent with the low mass emissions ( LME) provisions in section 75.19 for SO2 and NOx, sections 75.81( b) through ( g) of the final rule provide a less rigorous monitoring option for low Hg emitters. These provisions allow sources with estimated annual emissions of 29 lb/ yr ( 464 oz/ yr) or less, representing about 5 percent of the nationwide Hg mass emissions, to use periodic emission testing to quantify their Hg emissions, rather than continuously monitoring the Hg concentration. For units with Hg emissions of 9 lb/ yr ( 144 oz/ yr) or less, annual emission testing is required. For units with Hg emissions greater than 144 oz/ yr but less than or equal to 464 oz/ yr, semiannual testing is required. For reporting purposes, the owner or operator is required to use either the highest Hg concentration from the most recent emission testing or 0.50 µ g/ scm, whichever is greater. If, at the end of a particular calendar year, the reported annual Hg mass emissions for a unit exceed 464 ounces, the unit is disqualified as a low mass emitter and the owner or operator must install and certify a Hg CEMS or sorbent trap monitoring system within180 days of the end of that year. The final rule also contains special low mass emitter provisions for common stack and multiple stack exhaust configurations.

The Agency believes that a low mass emitter provision can be beneficial to both EPA and industry. It is cost effective for industry, in that it allows periodic stack testing to be used to estimate Hg emissions instead of requiring continuous emission monitoring. In the context of a cap and trade program, a low emitter provision can provide environmental benefit, because it requires conservatively high default emission factors to be used for reporting, thereby removing Hg allowances from circulation. Also, allowing a subset of the affected units to use less rigorous monitoring reduces the administrative burden of program implementation, allowing EPA to focus its attention on the higher emitting sources. 6 47 Selecting an appropriate low emitter cutoff point is of critical importance. On the one hand, if the cutoff point is too low ( i. e., too exclusive) this would not be cost effective for the regulated sources and would greatly increase the burden on the regulatory agencies to implement and maintain the program. On the other hand, if the cutoff point is too high ( i. e., too inclusive), this would create inequities in the trading market.

Over the years, EPA has used a de minimis concept to either exempt low emitting sources from monitoring or to allow these sources to use less rigorous, lower cost techniques to monitor emissions instead of installing CEMS:

In the preamble of the 1993 Acid Rain Program ( ARP) final rule ( see 58 FR 3593, January 11, 1993), EPA's Acid Rain Division ( now the Clean Air Markets Division) first used the de minimis concept to exempt certain new utility units from the Acid Rain Program ( i. e., units less than or equal to 25 MW that burn only fuels with a sulfur content less than or equal to 0.05 percent by weight);

EPA also allows gas fired and oil fired peaking units to use the less costly methodology in 40 CFR part 74, appendix E, to estimate NOx emissions instead of using CEMS, because the Agency's analyses indicated that projected NOx emissions from these units represent less than 1 percent of the total NOx emissions from Acid Rain Program units.

In 1998, EPA promulgated LME provisions in section 75.19 for SO2 and NOx ( see 63 FR 57484, October 27, 1998). These provisions require the use of conservatively high default emission rates to quantify SO2 and NOx emissions. EPA determined the appropriate SO2 and NOx mass emissions thresholds or " cutoff points" for unit to qualify as a low mass emissions methodology, considering inventory and regulatory changes that had taken place since the original1993 Acid Rain rulemaking. The selected threshold values were based on a de minimis concept, i. e., the SO2 and NOx emissions from the units that could potentially qualify to use the LME methodology represented less than or equal to 1 percent of the emissions from all affected units.