34,35
                                      ok
                                      ok
                                      yP
                                       6
o-Xylene
                                      18
                                       2
                                      32
                                      35
                                      ok
                                      ok
                                      yP
                                       7
p-Xylene
                                      14
                                       1
                                      33
                                      35
                                      ok
                                      ok
                                      yP
                                       6
1,2,3-Trimethyl benzene
                                       9
                                       2
                                      36
                                      39
                                      ok
                                      ok
                                      yP

1,2,4-Trimethyl benzene
                                      11
                                       2
                                      37
                                      39
                                      ok
                                      ok
                                      yP

1,3,5-Trimethyl benzene
                                      21
                                       1
                                      38
                                      39
                                     note
                                     note
                                      yP
                                       8
Naphthalene
                                       5

                                      40

                                      +1
                                      ok
                                      Prm

Tetralin
                                       5

                                      40

                                      +1
                                      ok
                                      Prm

2,3-Dimethyl naphthalene
                                       4

                                      40

                                      +1
                                      +1
                                      Prm

Acetylene
                                       4
                                       7
                                      41
                                      42
                                      -1
                                      -1

                                       9
Methanol

                                       3

                                      43
                                      ok

Ethanol

                                       3

                                      43
                                      ok

Isopropyl alcohol

                                      10

                                      44
                                      ok
                                      ok

t-Butyl alcohol

                                       7

                                      45
                                      ok
                                      +1
                                      yN

1-Octanol

                                       4

                                      46
                                      ok
                                      +1

2-Octanol

                                       3

                                      46
                                      ok
                                      +1

3-Octanol

                                       3

                                      46
                                      ok
                                      +1

Ethylene glycol

                                       5

                                      47
                                      ok
                                      ok

                                      10
Propylene glycol

                                      12

                                      48
                                      ok
                                      ok

                                      10
Dimethyl ether

                                       4

                                      49
                                      ok

Diethyl ether

                                       6

                                      49
                                      ok
                                      ok
                                      yN

Methyl t-butyl ether

                                       4

                                      50
                                      ok

1-Methoxy-2-propanol

                                       6

                                      51
                                      ok
                                      +1

2-Ethoxyethanol

                                       3

                                      52
                                      ok

2-(2-Ethoxyethoxy) ethanol

                                       3

                                      52
                                      ok

2-Butoxyethanol

                                       7

                                      53
                                      ok
                                      ok

2-(2-Butoxyethoxy)-ethanol

                                       3

                                      54
                                      ok
                                      ok
                                      yN

Methyl acetate

                                       7

                                      55
                                      ok
                                      ok

Ethyl acetate

                                       9

                                      56
                                      ok
                                      ok

                                      11
Isopropyl acetate

                                       3

                                      57
                                      ok
                                      ok

t-Butyl acetate

                                       6

                                      57
                                      ok
                                      +1
                                      yN

Methyl isobutyrate

                                       7

                                      58
                                      ok
                                      ok

Methyl pivalate

                                       6

                                      59
                                      ok
                                      ok

n-Butyl acetate

                                       8

                                      60
                                      ok
                                      +1

Dimethyl carbonate

                                       6

                                      61
                                      ok
                                      ok
                                      yN

Methyl isopropyl carbonate

                                       5

                                      61
                                      ok
                                      ok
                                      yN

Propylene carbonate

                                       7

                                      62
                                      -1
                                      ok
                                      yN

1-Methoxy-2-propyl acetate

                                       6

                                      63
                                      ok
                                      +1
                                      yN

Dimethyl succinate

                                       6

                                      64
                                      ok
                                      ok
                                      yN

Dimethyl glutarate

                                       6

                                      64
                                      ok
                                      ok
                                      yN

Texanol(R) isomers

                                       4

                                      65
                                      ok
                                      ok

Furan
                                       8

                                      66
                                     note
                                     note
                                      yP
                                      12
2-Methyl furan
                                       2

                                      67

                                      ok
                                      ok
                                      yP

3-Methyl furan
                                       6

                                      67

                                      ok
                                      ok
                                      yP

2,5-Dimethyl furan
                                       2

                                      67

                                      ok
                                      ok
                                      yP

Benzyl alcohol
                                       6
                                       3
                                      68
                                      69
                                      ok
                                      ok
                                      yP

Formaldehyde
                                      33
                                       8
                                      70
                                      71
                                      ok
                                      ok

Acetaldehyde
                                      18
                                       5
                                      72
                                      73
                                      ok
                                      ok

Acrolein
                                       3

                                      74

                                      ok
                                      ok

Methacrolein
                                      12

                                      74

                                      ok
                                      ok

Benzaldehyde

                                       3

                                      75
                                      ok
                                      ok

Acetone
                                       7
                                      10
                                      76
                                      77
                                     note
                                     note
                                      QY
                                     13,14
Methyl ethyl ketone
                                       6
                                       5
                                      78
                                      79
                                      ok
                                      ok
                                      QY
                                      13
2-Pentanone
                                       1
                                       4
                                      78
                                      80
                                      ok
                                      ok
                                      QY

2-Heptanone
                                       1
                                       3
                                      78
                                      81
                                     note
                                     note
                                      QY
                                      15
4-Methyl-2-pentanone

                                       8

                                      81
                                      ok
                                      ok

Cyclohexanone

                                       9

                                      82
                                      ok
                                      ok

Methyl vinyl ketone
                                       6

                                      83

                                      ok
                                      ok

o-Cresol
                                       1

                                      84

                                      ok
                                      ok
                                      Prm
                                      16
m-Cresol
                                       1
                                       1
                                      84
                                      85
                                      -2
                                      ok?

                                     16,17
p-Cresol
                                       1

                                      84

                                      ok
                                      ok?

                                     16,18
Para toluene isocyanate

                                       6
                                      86

                                      ok
                                      ok
                                      Prm

2,4-Toluene diisocyanate

                                       7
                                      87

                                      ok
                                      ok
                                      Prm

2,6-Toluene diisocyanate

                                       2
                                      87

                                      ok
                                      ok
                                      Prm

N-Methyl-2-pyrrolidone

                                       6
                                      88

                                      ok
                                      ok
                                      yN

N-Propyl bromide

                                       6
                                      89

                                      -1
                                      +2

                                      19
N-Butyl bromide

                                       6
                                      89

                                      -1
                                      +2

                                      19
Trichloroethylene

                                       8
                                      90

                                      +2
                                      +2

                                      20
trans 1,3,3,3-Tetrafluoropropene
                                  (See notes)
                                      ok
                                      ok
                                      yN
                                     29,30
2,3,3,3-Tetrafluoropropene
                                  (See notes)
                                      ok
                                      ok
                                      yN
                                     29,31
1,3-Dichloropropene mixture
                                       6
                                       3
                                      91
                                      92
                                      ok
                                      ok

                                      21
2-(Chloromethyl)-3-chloropropene

                                       1

                                      93
                                      -1
                                       ?

                                      20
Chloropicrin
                                       6
                                       5
                                      94
                                      95
                                      ok
                                      ok

Chlorine
                                       2
                                       1

                                      96
                                      ok
                                      ok

                                      22
Hexamethyldisiloxane

                                       3

                                      97
                                      ok

                                      Prm

Hydroxymethyldisiloxane

                                       4

                                      97
                                      ok

                                      Prm

D4 cyclosiloxane

                                       3

                                      97
                                      ok

                                      Prm

Carbon disulfide

                                       4

                                      98
                                      ok
                                      ok
                                      Oth
                                      21
Methyl isothiocyanate

                                       4

                                      98
                                      ok
                                      ok
                                      Oth
                                      21
Dimethyl sulfoxide
                                       2
                                       6
                                      99
                                      100
                                      -2
                                      ok
                                      Oth
                                      23
S-ethyl dipropylthiocarbamate (EPTC)

                                       5

                                      101
                                      ok
                                      ok
                                      Oth
                                      21
Ethanolamine

                                     5 [f]

                                      108
                                    ok [g]

                                     Init
                                      27
2-Amino-2-methyl-1-propanol

                                     7 [f]

                                      109
                                    ok [g]

                                     Init
                                      28
Isopropyl amine

                                     1 [f]

                                      108
                                    ok [g]
                                      +2
                                     Init
                                      27
t-Butyl amine

                                     1 [f]

                                      109
                                    ok [g]
                                      +2
                                     Init
                                      28
Methyl Iodide
                                       7
                                     3 [f]
                                      110
                                      111
                                      ok
                                      +1
                                      Oth
                                      26
                               Complex Mixtures
Safety-Kleen mineral spirits "A" (type I-b, 91% alkanes)

                                       3

                                      102
                                      ok
                                      +1

                                      24
Safety-Kleen mineral spirits "B" (type II-c)

                                       3

                                      102
                                      ok
                                      +1

                                      24
Safety-Kleen mineral spirits "C" (type II-c)

                                       3

                                      102
                                      ok
                                      +1

                                      24
Safety-Kleen mineral spirits "D" (type II-c)

                                       3

                                      102
                                      ok
                                      +1

                                      24
Exxon Exxol(R) D95 Fluid

                                       6

                                      103
                                      ok
                                      ok

                                      24
Exxon Isopar(R) M Fluid

                                       5

                                      103
                                      ok
                                      ok

                                      24
Oxo-decyl acetate

                                       5

                                      104
                                      ok
                                      ok

                                      24
VMP naphtha

                                       4

                                      105
                                      ok
                                      ok

                                      24
Kerosene

                                       4

                                      105
                                      ok
                                      ok

                                      24
Dearomatized Alkanes, mixed, predominately C10-C12 (ASTM1C)

                                       2

                                      106
                                      ok
                                      ok

                                      24
Synthetic isoparaffinic alkane mixture, predominately C10-C12 (ASTM3C1)

                                       4

                                      106
                                      ok
                                      ok

                                     24,25
Reduced aromatics mineral spirits (ASTM1B)

                                       3

                                      106
                                      ok
                                      ok

                                      24
Regular mineral spirits (ASTM1A)

                                       3

                                      106
                                      ok
                                      ok

                                      24
Aromatic 100

                                       4

                                      107
                                      ok
                                      ok

                                      24
[a]	Number of chamber experiments. "Single" refers to single VOC - NOx or VOC + CO or alkane - NOx irradiation; "Rct'y" refers to incremental reactivity experiment.
[b]	Figure number in Appendix C where mechanism performance for single VOC or incremental reactivity experiments is shown.
[c]	Qualitative indication of ability of mechanism to fit initial rates of O3 formation and NO oxidation ("rate") and final ozone yields ("Max"). The former is more indicative of model performance under relatively high NOx, MIR-like conditions, and the latter is indicative of model performance in simulating O3 impacts under lower NOx conditions. If blank, then data are not available to evaluate this aspect of the mechanism. Codes used are as follows:
   ok	Model simulates most of the data within the experimental variability, with low or no apparent biases.
   n	Model tends to overpredict (+) or underpredict (-) O3 yield or O3 formation and NO oxidation rates. 1 = moderate but apparently consistent biases; 2 = potentially significant biases
   ?	The data are insufficient to evaluate this aspect of the mechanism.
   note	See note for a discussion of the model performance.
[d]	Indicates if mechanism was adjusted to fit the data in this evaluation. Codes indicate the type of adjustment, as follows:
   yN	Nitrate yield in peroxy + NO reaction(s) adjusted
   yP	Yield of photoreactive products(s)adjusted. For aromatics, this is the AFG1/AFG2 ratio.
   Or	Radical yield in O3 and O[3]P reaction adjusted.
   QY	Quantum yield for photodecomposition adjusted
   Prm	Parameterized mechanism used with various parameters adjusted.
   Oth	Other adjustments were made. See references in notes.
   Init	Initial reactant concentration injected. See Note [g].
[e]	Notes and comments concerning the evaluation results of mechanism adjustment, as follows:
   1 The model significantly overpredicts the direct reactivity measured by Carter and Malkina (2002) (see "Iso-Octane" on Figure 15)
   2 The model tends to overpredict O3 in the -pinene - NOx experiments but gives reasonably good simulations of the reactivity experiments.
   3 Model overpredicts O3 formation rate in the arc light benzene - NOx experiments but tends to underpredict it in the blacklight runs. Model gives reasonably good simulation of most of the incremental reactivity experiments.
   4 The model underpredicts the direct reactivities measured by Carter and Malkina (2002), but not as much as SAPRC-99 (see Figure 15).
   5 Model overpredicts initial NO oxidation and O3 formation rates in the toluene - NOx blacklight experiments but gives good simulations to these experiments in the TVA chamber and in chambers using arc lights. Model gives good simulations of the incremental reactivity results. The model significantly overpredicts the reactivity seen in the toluene - NOx experiments in the UNC outdoor chamber, but this could be due to chamber characterization problems. The model underpredicts the direct reactivities measured by Carter and Malkina (2002), but not as much as SAPRC-99 (see Figure 15).
   6 The model underpredicts the effects of CO addition on O3 formation, though to a lesser extent than SAPRC-99 (see Figure 8).
   7 The model somewhat overpredicts the reactivity seen in the o-xylene - NOx experiments in the UNC outdoor chamber, but this could be due to chamber characterization problems.
   8 The model tends to overpredict O3 formation in experiments in the EC but gives good simulations to results of more recent experiments using both blacklights and arc lights. The model somewhat overpredicts the direct reactivities measured by Carter and Malkina (2002), but the discrepancy may be within the uncertainty of the measurement.
   9 The model consistently underpredicts the reactivities in the acetylene - NOx experiments, but the underprediction of the incremental reactivity results is relatively small. The SAPRC-99 mechanism, which uses a higher rate constant for OH + acetylene based on an earlier recommendation by Atkinson (1994), simulates the data with much less bias. This rate constant was changed to reflect the recommendation in the NASA (2006) evaluation.
   10 Even though the mechanism was not changed, the model performs somewhat better than SAPRC-99 in simulating some experiments.
   11 The model somewhat underpredicts the direct reactivity measured by Carter and Malkina (2002) (see Figure 15).
   12 The model gives reasonably good simulations of earlier ITC experiments but tends to underpredict maximum O3 in the new arc light and blacklight experiments carried out for this project.
   13 See base mechanism documentation (footnotes to Table A-2) for a discussion of the adjustments made for this compound.
   14 The reactivity is somewhat underpredicted in the acetone - NOx experiments with the arc lights and in the outdoor chamber but is well simulated in the blacklight chamber runs. The incremental reactivity results are reasonably well simulated.
   15 The model evaluation results for this compound are highly variable. O3 formation in the single heptanone - NOx experiment is significantly underpredicted, but the reactivity in one of the two incremental reactivity experiments is overpredicted, while the other two incremental reactivity experiments are reasonably well simulated. The latter were used as the basis for adjusting the photolysis rate, which was an important uncertain parameter in affecting the results. Note that the photolysis rate adjusted to fit this compound affects the base mechanism because the photolysis rate used for the lumped higher ketone product, PROD2, is based primarily on the photolysis rate derived for this compound.
   16 The parameterized mechanism used for all the cresol isomers was adjusted to fit the data for o-cresol based on the model simulation of a single experiment. The ([O3]-[NO]), cresol and PAN data in that one experiment are simulated reasonably well.
   17 The model underpredicts O3 formation in the m-cresol - NOx experiment and gives an incorrect prediction of the effect of o-cresol addition on the initial NO oxidation and O3 formation rate in the reactivity experiment, but simulates the effect on final O3 reasonably well.
   18 The one p-cresol - NOx experiment is simulated reasonably well.
   19 No attempt was made to adjust the approximate mechanisms to fit the chamber data. The performance indicates the performance of the approximation of using chlorine mechanisms to estimate mechanisms for Br-containing species.
   20 No attempt was made to adjust this mechanism to improve simulations of the data.
   21 See Carter and Malkina (2007) for a discussion of adjustments made to these mechanisms to fit the chamber data.
   22 The effects of Cl2 on n-butane - NOx irradiations and the effects of multiple Cl2 additions to a surrogate - NOx experiment were reasonably well simulated.
   23 The initial NO oxidation and O3 formation rates are significantly underpredicted in the DMSO - NOx experiments but are only slightly underpredicted in the MIR-type incremental reactivity experiments. Except for one experiment where the model overpredicts DMSO reactivity, the effects of DMSO on final O3 in the lower NOx reactivity experiments are reasonably well simulated.
   24 The mechanism evaluation with these complex mixtures primarily tests how well the mixtures are represented by the distribution of model species used to represent them. No mechanism adjustments can be made to improve simulations in these cases.
   25 The model performance with this version of the mechanism is considerably better than SAPRC-99, which had a consistent bias in underpredicting the O3 impact (overpredicting the O3 inhibition) of this mechanism.
   26 See Carter (2007) for a discussion to the adjustments made to this mechanism to fit the chamber data. The mechanism tends to underpredict the amount of ozone inhibition caused by the addition of methyl iodide at the end of some of the experiments.
   27 See Note [g] and Carter (2008a) for a discussion of the amine experiments. Although the amount of initial amine in the experiments are adjusted to fit the effects of the amine on the initial rates of NO oxidation and O3 formation in the experiments (see Note [g]), the model does not predict the observed inhibition of ozone at the end of the lower NOx experiments.
   28 See Note [g] and Carter (2008a) for a discussion of the amine experiments. Because these compounds significantly inhibited the rates of O3 formation, the ability of the mechanism to simulate the effects of the amine on final O3 yields could not be determined.
   29 See the report cited in the other note for the results of the evaluation experiments for these compounds, which were carried out after the list of experiments to be included in this report was finalized.
   30 See Carter (2009a) for a discussion of the experiments used to evaluate the mechanism for this compound. Good simulations of MIR and MOIR/2 reactivity experiments were obtained after adjusting the overall nitrate yield.
   31 See Carter (2009b) for a discussion of the experiments used to evaluate the mechanism for this compound. Good simulations of MIR and higher ROG/NOx reactivity experiments were obtained after adjusting the overall nitrate yield.
[f]	These experiments that were not in the original (August, 2007) version of the report.
[g]	Amount of injected amine available for reaction is uncertain because initial concentration of the amine in the gas phase could not be measured, and because of expected absorption of amines on the walls and other heterogeneous effects. The fits indicated on the table reflect the fits obtained after adjusting initial amount of amine to fit the initial rates of NO oxidation and O3 formation in the experiments, and the adjusted initial amine concentrations were always lower than the calculated amount injected. See Carter (2008a). The figures show the fits with the amount of initial amine adjusted to best fit the data.

in footnotes to the table. The figures in Appendix C show the model performance for both SAPRC-99 and SAPRC-07, but Table 23 only summarizes the results for SAPRC-07, though cases where the mechanisms differ significantly are noted.
For the vast majority of the VOCs and mixtures, the performance for SAPRC-07 was similar to that of SAPRC-99, though as indicated on Table 23, mechanism parameters had to be adjusted to obtain the comparable fits. This is expected, since for most VOCs the mechanisms were not significantly changed. Exceptions and other cases worth noting are as follows:
Alkylbenzenes. As discussed above, significant changes were made to the ring-opening mechanisms for the alkylbenzenes and the model species used to represent the non--dicarbonyl ring opening products. However, these changes did not result in significant changes in the ability of the model to simulate aromatic - NOx experiments or incremental reactivity experiments with aromatics, presumably because portions of both mechanisms were optimized to simulate essentially the same database. Like SAPRC-99, the updated mechanism also tended to underpredict the effects of adding CO to aromatics - NOx irradiations, but to a lesser extent than SAPRC-99. This is shown on Figure 8, which shows results of selected toluene and m-xylene - NOx experiments where CO was added. Likewise, as discussed below, the updated mechanisms also tended to overpredict direct reactivities of benzene and toluene, but again to a lesser extent than SAPRC-99. Therefore, the overall performance of the updated mechanism is an improvement over SAPRC-99 in some at least some respects, but problems in simulating all of the data still remain.
Alkenes. As also indicated above, it was necessary to assume lower radical yields in the reactions of O3 and O[3]P with alkenes than indicated by current laboratory data in order to obtain unbiased simulations of the data. This was the case for SAPRC-99 and is also the case with this updated mechanism. However, the extent to which the yields had to be adjusted to lower values was somewhat less with this version of the mechanism, so at least in this respect the problem may be slightly reduced.
Acetylene. The current NASA (2006) evaluation recommends an OH + acetylene rate constant that is lower than that used in SAPRC-99 by ~17% at ambient temperature and pressure. This caused a negative bias in the model simulations of the acetylene experiments that was not the case for SAPRC-99. However, this bias was not so large that it was considered appropriate to adjust the rate constant, and there were not other uncertain aspects of the mechanism considered to be appropriate to adjust to remove this bias.
Higher Ketones. The photolysis rate used in the base mechanism for the lumped model species used to represent the more reactive ketone products, PROD2, is based on overall quantum yields derived based on model simulations of the experiments with methyl propyl ketone, methyl isobutyl

Figure 8.	Experimental and calculated effects of CO addition in representative toluene and m-xylene NOx experiments carried out in the UCR EPA chamber.

ketone and 2-heptanone. The data indicate that the quantum yields decrease with carbon number, but the extent of the decrease with the higher ketones is somewhat greater with the updated mechanism than SAPRC-99. This resulted in a lower overall photolysis rate for the PROD2 model species. However, the very limited and somewhat consistent data for 2-heptanone provides the only information about ketones with greater than 7 carbons, which are used as the basis for the even larger ketones that are important in alkane and other photooxidations. More data are needed to place the estimates for photolysis rates of higher ketones on a firmer basis.
Alkyl Bromides. Since developing mechanisms for bromine species was beyond the scope of this project, the mechanisms for the few bromine-containing compounds that are represented are estimated based on mechanisms derived for the corresponding chlorine-containing compounds. The chamber data for n-propyl and n-butyl bromides, shown on Figure C-89, indicate that this may somewhat underestimate the rate of initial NO oxidation and O3 formation but significantly overestimate the impact of the compound on final O3 yields. The implication of this on predicted MIR values for these compounds is uncertain, but it is clear that low NOx reactivities of these compounds are almost certainly overestimated.
Isoparaffinic Alkane Mixtures. Carter and Malkina (2005) noted that the SAPRC-99 mechanism predicted much more O3 inhibition in the incremental reactivity experiments with the synthetic isoparaffinic alkane mixture (designated ASTM3C1) than observed experimentally, and concluded that there is a problem either with the characterization of such mixtures or with branched alkane mechanisms in general. However, although neither the composition of the mixture nor the branched alkane mechanisms were changed, the updated mechanism now gives much better simulations of these data, as shown on Figure C-106. This indicates that reactivities of compounds such as these, and presumably others, can be sensitive to changes in the base mechanism that has nothing directly to do with the mechanisms for the compounds themselves.
Amines. The mechanisms for amines were newly derived for this mechanism; SAPRC-99 either has no mechanisms for these compounds or uses highly approximate "placeholder" mechanisms. The amine mechanisms derived for SAPRC-07 are and are discussed above in the "Mechanisms for Individual VOCs" section and in more detail by Carter (2008a). A series of incremental reactivity environmental chamber experiments were carried out with 2-amino-2-methyl-1-propanol (AMP) and ethanolamine and one experiment each was carried out for t-butyl amine and isopropyl amine (Carter, 2008a), and selected results are shown on Figure C-108 and Figure C-109. As discussed by Carter (2008a), because of analytical problems no data were available concerning the amount of amines present in the gas phase at the start of the experiments, and if the amount of initial amine derived from the calculated amounts injected is assumed, then the effects of the amines on the results of the experiments are significantly overpredicted. This is even if it is assumed that the amines react rapidly with HNO3 formed in the experiments to form the corresponding salt, which is also assumed to be removed from the gas-phase system. Therefore, the amount of amine initially present in the gas phase had to be treated as an adjustable parameter in the model simulations of the experiments, in order to obtain the fits shown on Figure C-108 and Figure C-109. The amount of initial amine that best fit the reactivity data ranged from ~5% to ~60% of the calculated amount of amine injected, with no apparent dependence on reaction conditions or type of experiment (Carter, 2008a). Because of this, the use of these experiments to evaluate amine mechanisms is highly uncertain. Nevertheless, the estimated mechanisms correctly predict that AMP and t-butyl amine significantly inhibit NO oxidation O3 formation, while they correctly predict that the other two amines have positive effects on initial NO oxidation and O3 formation rates.
When the amounts of initially present amines are appropriately adjusted to simulate the effects on initial NO oxidation rates, Figure C-109 shows that the data for AMP and t-butyl amine are reasonably well simulated throughout the experiments. On the other hand, as shown on Figure C-108, the chamber data indicated that the two reactive amines studied, aminoethanol and isopropyl amine, tended to inhibit O3 formation under low NOx conditions, which was not predicted by the mechanism. The reason for this inhibition is unknown, and this discrepancy could not be resolved by reasonable adjustments to the mechanisms for these compounds and the representations of the reactions of their major products (Carter, 2008a). Therefore, it must be concluded that the mechanisms for the positively reactive amines probably overpredict the reactivities of the compounds under low NOx conditions, if the uncertainties regarding availability are not considered.
Methyl Iodide. Methyl iodide is another compound that was not represented in SAPRC-99 but whose mechanism has been derived for SAPRC-07. The mechanism for this compound, and its evaluation against chamber data, are described in detail by Carter (2007), and that discussion is not duplicated here. Figure C-110 and Figure C-111 show that the mechanism simulates most of the data of Carter (2007) reasonably well.
Evaluation Experiments for Mixtures
Figure 9 through Figure 13 show distribution plots for the various types of mixture - NOx experiments in the current evaluation set. For most types of mixtures the model performance is generally satisfactory in terms of overall biases, but there are cases where the updated mechanism does not perform quite as well as SAPRC-99.  These are discussed below for the various types of experiments.
Simple mixture experiments. These are runs with mixtures that are not considered to be complete surrogates because they do not contain at least one representative each of alkanes, alkenes, and aromatics. Figure 9 shows distributions of model errors for these runs, grouped by the chamber employed. It shows that the average biases and distribution of errors are similar for both mechanisms. The mixture experiments in the TVA and UCR EC chamber are reasonably well simulated, while the reactivity experiments in the mixture experiments in the UNC chamber are overpredicted by about the same amount in both mechanisms. Given the good performance for the quite different EC and TVA chambers, the poorer performance in simulating the UNC chamber data is considered to be likely due to characterization rather than mechanism problems.
Miscellaneous Surrogate Experiments. These are experiments with mixtures that contain at least one alkane, alkene and aromatic that are designed to represent, albeit in a simplified fashion, reactive organic gas mixtures that are emitted into ambient atmospheres. The more recent experiments in the UCR EPA chamber are considered separately because they were carried out at generally lower pollutant levels than the earlier chamber experiments, and conditions in that chamber are considered to be somewhat better characterized. Figure 10 shows the distribution of model errors in the earlier surrogate - NOx experiments carried out in various UCR and the TVA chamber, and Figure 11 shows the results for various surrogate mixtures in the UNC chamber. Because of the large number of the many standard surrogate experiments used as base cases in incremental reactivity studies in the UCR chambers, these are excluded from Figure 10 and are shown separately in Figure 12. Overall the model performance for the updated mechanism is similar to SAPRC-99 for these experiments, though the biases tend to be somewhat more (either positive or negative, depending on the type of experiment) for the miscellaneous mixtures shown on Figure 10. The results with both mechanisms for the UNC or UCR standard surrogate runs, shown on Figure 11 and Figure 12, respectively, are similar for both mechanisms, with the reactivity in the UNC experiments again being consistently overpredicted (though not as much so as for the simple mixture runs shown on Figure 9), and the earlier UCR standard base case surrogate runs being well simulated.
New Surrogate Experiments in UCR EPA Chamber. A large number of surrogate - NOx experiments have been carried out in the new UCR EPA chamber, most based on a similar 7- or 8-compound mixture designed to represent ambient VOC mixtures (Carter et al, 1995c, Carter and Malkina, 2005). As discussed by Carter (2004), a large number of such experiments were carried out at a variety of NOx and ROG levels, including NOx levels as low as 2 ppb, giving a large and reasonably well characterized database for testing model predictions of how O3 changes with ROG and NOx levels. The results indicated a bias in the SAPRC-99 mechanism in underpredicting O3 formation and NO oxidation at low ROG/NOx levels, which have not been seen in simulating the earlier surrogate experiments carried out at higher NOx levels (Carter, 2004; Carter et al, 2005b). Since then, a number of other experiments have been carried out in this chamber to further investigate this, including experiments using the blacklight light source and experiments using a "non-aromatic" surrogate with the aromatic constituents of the 7- or 8- compound surrogate mixture removed. The latter are useful to evaluate whether the bias at low ROG/NOx is due to problems with the aromatics mechanisms, which are considered to be the most uncertain component of the overall mechanism for mixtures.
The performance of the mechanisms in simulating these experiments is shown in Figure 13 and Figure 14.  Figure 13 shows distribution plots of model errors similar to those shown above for other mixtures, and shows that overall the updated mechanism simulates the full surrogate (i.e., the surrogates with the aromatics) approximately as well as SAPRC-99, but that the updated mechanism has a slight positive bias in overpredicting O3 formation and NO oxidation rates in the non-aromatic surrogate runs. Figure 14 shows plots of model errors against the ROG/NOx ratio, similar to those shown previously (Carter, 2004; Carter et al, 2005b). This shows that the problem with the model underpredicting O3.

Figure 9.	Plots of model errors in simulations of miscellaneous mixture - NOx experiments carried out in various chambers.

Figure 10.	Plots of model errors in simulations of miscellaneous surrogate - NOx experiments carried out in various UCR chambers and in the TVA chamber.

Figure 11.	Plots of model errors in simulations of surrogate - NOx experiments carried out in the UNC chamber.

Figure 12.	Plots of model errors in simulations of the standard base case surrogate - NOx irradiations carried out in conjunction with the incremental reactivity experiments in the ETC and DTC (blacklight), and CTC and XTC (arc light)  chambers.

Figure 13.	Plots of model errors in simulations of the surrogate - NOx experiments carried out in the UCR EPA chamber.

Figure 14.	Plots of model errors in simulations of 6-hour ([O3]-[NO]) against initial ROG surrogate /NOx ratio.

formation and NO oxidation rates at the lower ROG/NOx ratios has not been eliminated, and that the extent to which the bias is affected by ROG/NOx ratios is essentially the same. This is observed in the blacklight as well as the arc light chamber experiments.
On the other hand, Figure 14 shows that if the aromatics are removed, the dependence of the bias on ROG/NOx is in the opposite direction as seen with the full surrogate, with the model tending to overpredict reactivity more at lower ROG/NOx ratios, and giving better predictions of the data at the higher ROG/NOx conditions. This is true for both versions of the mechanism, but has been somewhat exacerbated as a result of the mechanism update
Overall, the evaluation with the experiments with mixtures indicate a slight degradation of model performance for the updated mechanism compared to SAPRC-99, though this is not seen for all types of mixtures. Figure 7, above shows a very slight (~5%) bias in the updated mechanism in simulating the mixture experiments when all are lumped together, with the bias being greater (~10%) in simulating initial NO oxidation and O3 formation rates. However, when examining run types separately, one sees the biases vary with type of experiment, with some types of experiments having greater positive biases and some having greater negative biases, compared to SAPRC-99.
The difference for the UCR EPA full surrogate vs. non-aromatic surrogate at low ROG/NOx conditions is suggestive, with the experiments with the aromatics having the distinct negative bias while those with aromatics removed having the bias in the opposite direction. But the evaluations of the single compound or incremental reactivity experiments with the individual alkanes, alkenes, and aromatics, shown in figures in Appendix C and summarized on Table 23 above, do not indicate trends in biases with individual compounds that is consistent with this. The cause and significance of this problem is uncertain, and clearly this needs further study. It is important to note, however, the biases are not large compared to overall run-to-run variability and characterization uncertainties in model simulations of environmental chamber experiments.
Mechanism Performance for Direct Reactivity Experiments
Figure 15 shows the model performance in simulating the direct reactivity experiments of Carter and Malkina (2002) for all the compounds for which useful direct reactivity data were obtained. The results for SAPRC-99 are also shown. Except for benzene and toluene the results were very similar for the two mechanisms, with generally good fits being obtained for most compounds except for isooctane (2,2,4-trimethylpentane) and ethyl acetate. This is as expected since the mechanisms for these compounds were not changed significantly. The reasons for the discrepancies for isooctane and ethyl acetate are unknown, and have not been resolved.
Figure 15 shows that the new aromatics mechanisms do perform somewhat better in simulating the direct reactivities for benzene and toluene, though the direct reactivities of these compounds, and to a lesser extent 1,3,5-trimethylbenzene, are still overpredicted. This can be attributed to the updated mechanism assuming direct OH formation, without NO to NO2 conversions, in the ring opening pathways that do not involve -dicarbonyl formation. However, the results suggest that actual total amount of NO to NO2 conversion may still be less than assumed in this mechanism.

Figure 15.	Model performance in simulating the direct reactivity data of Carter and Malkina (2002).
Updated Reactivity Scales
One of the major applications of the SAPRC mechanisms is calculation of ozone reactivity scales for VOCs, including the MIR scale that is used or being considered for use in regulatory applications (CARB 1993, 2000, 2006, 2007), and a major reason behind this mechanism update is to update these reactivity scales. The SAPRC-99 mechanism was used to calculate MIR and other reactivity scales for many types of VOCs, with the latest update being that of Carter (2003a). That update includes reactivities for almost 780 types of VOCs, based on explicit or assigned mechanisms for ~560, and lumped molecule or mixture assignments for the rest. The listing included uncertainty codes for the reactivity estimates that may be potentially useful for regulatory applications or determining priorities for research. These scales, and their corresponding uncertainty classifications, have been updated as part of this project.
Methods
The methods, scenarios, and reactivity scales that were used in this reactivity scale update are the same as employed previously for the SAPRC-99 scales (Carter 2000a, 2003), and those references should be consulted for detail. Briefly this is based on the methods and scenarios originally developed by Carter (1994a,b), with slight modifications in the averaging methods as described by Carter (2000a). These are based on 39 single-day "base case" EKMA box model scenarios (EPA, 1984) derived by the EPA for assessing how various ROG and NOx control strategies would affect ozone nonattainment in various areas of the country (Baugues, 1990). The conditions of these scenarios are summarized on Table 24, and more details concerning the modeling inputs are given by Carter (1994b).
The scenario conditions include specifications of initial concentrations and emissions schedules for NOx and total anthropogenic VOC emissions and also background O3, aloft VOCs, and initial and emitted biogenic VOCs (Bauges, 1990; Carter, 1994a,b). The scenarios as originally developed by Bauges (1990) do not specify the composition of the anthropogenic VOCs, and Carter (1994a) used a standard mixture of hydrocarbons derived by Jeffries (Jeffries et al (1989) from analysis of air quality data, with minor modifications as discussed by Carter (1994a,b). This is referred to as the "Base ROG mixture" in the subsequent discussion. This base ROG mixture was not modified for the SAPRC-99 reactivity scale updates (Carter, 2000a, 2003a), and was also used in this update. Note that same base ROG mixture was used as the basis for deriving the "fixed parameter" version of the SAPRC-07 mechanism for airshed models, as discussed above. The composition of this mixture in terms of fixed parameter SAPRC-07 lumped model species in given in Table 17, above.
The base case scenarios with the NOx inputs as specified by Bauges (1990) were used to derive the updated "base case" reactivity scales, comparable to those given by Carter (1995a, 2000a) for SAPRC-90 or SAPRC-99. Because absolute and even relative impacts of VOCs on O3 formation are highly dependent on NOx conditions that are highly variable in the base case scenarios, scenarios with adjusted NOx inputs were derived to obtain scales that are more representative of standard conditions of conditions of NOx availability. These are as follows:
The Maximum Incremental Reactivity (MIR) scale is derived from the scenarios where the NOx inputs are adjusted to yield highest incremental reactivities (changes in O3 caused by small VOC additions, divided by the amount of VOC added) of VOCs. Although the NOx conditions yielding highest incremental reactivities tend to be the same for most VOCs, the sensitivity of O3 formation to changes in total base ROG inputs was used to determine the NOx levels corresponding to MIR conditions.  This represents relatively high NOx conditions where, by definition, O3 is most sensitive to changes in VOC emissions.

Table 24.	Scenarios used for reactivity assessment, with updated calculated maximum O3, Integrated OH, and MIR, MOIR, and EBIR NOx inputs.
Scenario
                                 Max O3
(ppb)
                             Max 8-Hr Avg O3 (ppb)
                                 ROG / NOx [a]
                                Max Height (kM)
                                 ROG input [b]
                                O3 aloft (ppb)
                              Int'd OH
(ppt-min)

                                     Base
                                      MIR
                                     MOIR
                                     EBIR

 Averaged Conditions (Used for screening calculations and sensitivity studies)
Max React (MIR)
                                      179
                                      117

                                     3.70

                                      1.8
                                      15
                                      70
                                      121

Max O3 (MOIR)
                                      229
                                      161

                                     5.63

                                      1.8
                                      15
                                      70
                                      200

Eq. Benefit (EBIR)
                                      213
                                      166

                                     8.86
                                      1.8
                                      15
                                      70
                                      200

                Base Case (Used for MIR, MOIR, and EBIR scales)
Atlanta, GA
                                      173
                                      127
                                     7.25
                                     3.54
                                     5.37
                                     7.66
                                      2.1
                                      12
                                      63
                                      198

Austin, TX
                                      171
                                      141
                                     9.30
                                     3.36
                                     5.08
                                     8.00
                                      2.1
                                      11
                                      85
                                      180

Baltimore, MD
                                      318
                                      196
                                     5.15
                                     3.90
                                     5.92
                                     10.04
                                      1.2
                                      17
                                      84
                                      171

Baton Rouge, LA
                                      237
                                      168
                                     6.83
                                     4.31
                                     6.36
                                     8.46
                                      1.0
                                      11
                                      62
                                      186

Birmingham, AL
                                      239
                                      198
                                     6.94
                                     2.71
                                     4.09
                                     6.26
                                      1.8
                                      13
                                      81
                                      204

Boston, MA
                                      191
                                      161
                                     6.50
                                     2.69
                                     4.13
                                     6.53
                                      2.6
                                      14
                                      105
                                      250

Charlotte, NC
                                      139
                                      123
                                     7.79
                                     1.80
                                     2.78
                                     3.86
                                      3.0
                                       7
                                      92
                                      207

Chicago, IL
                                      286
                                      230
                                     11.63
                                     4.29
                                     6.39
                                     9.75
                                      1.4
                                      25
                                      40
                                      176

Cincinnati, OH
                                      196
                                      145
                                     6.38
                                     3.29
                                     5.05
                                     9.01
                                      2.8
                                      17
                                      70
                                      208

Cleveland, OH
                                      245
                                      170
                                     6.62
                                     4.24
                                     6.64
                                     10.09
                                      1.7
                                      16
                                      89
                                      180

Dallas, TX
                                      194
                                      128
                                     4.74
                                     3.99
                                     6.10
                                     8.86
                                      2.3
                                      18
                                      75
                                      160

Denver, CO
                                      197
                                      132
                                     6.33
                                     4.88
                                     7.37
                                     11.37
                                      3.4
                                      29
                                      57
                                      140

Detroit, MI
                                      237
                                      168
                                     6.82
                                     3.64
                                     5.55
                                     9.85
                                      1.8
                                      17
                                      68
                                      224

El Paso, TX
                                      178
                                      130
                                     6.59
                                     4.49
                                     6.96
                                     9.70
                                      2.0
                                      12
                                      65
                                      136

Hartford, CT
                                      167
                                      140
                                     8.39
                                     2.74
                                     4.26
                                     7.09
                                      2.3
                                      11
                                      78
                                      213

Houston, TX
                                      303
                                      206
                                     6.08
                                     3.94
                                     5.96
                                     9.36
                                      1.7
                                      25
                                      65
                                      217

Indianapolis, IN
                                      205
                                      141
                                     6.64
                                     3.87
                                     6.22
                                     9.61
                                      1.7
                                      12
                                      52
                                      203

Jacksonville, FL
                                      150
                                      111
                                     7.62
                                     3.44
                                     5.18
                                     7.22
                                      1.5
                                       8
                                      40
                                      202

Kansas City, MO
                                      152
                                      120
                                     7.09
                                     2.95
                                     4.59
                                     8.34
                                      2.2
                                       9
                                      65
                                      221

Lake Charles, LA
                                      289
                                      211
                                     7.42
                                     3.46
                                     5.01
                                     6.91
                                      0.5
                                       7
                                      40
                                      235

Los Angeles, CA
                                      566
                                      401
                                     7.59
                                     5.11
                                     7.75
                                     11.08
                                      0.5
                                      23
                                      100
                                      136

Louisville, KY
                                      204
                                      148
                                     5.53
                                     3.10
                                     4.77
                                     7.17
                                      2.5
                                      14
                                      75
                                      248

Memphis, TN
                                      224
                                      174
                                     6.78
                                     3.21
                                     4.79
                                     7.59
                                      1.8
                                      15
                                      58
                                      240

Miami, FL
                                      129
                                      108
                                     9.63
                                     2.71
                                     4.19
                                     6.09
                                      2.7
                                       9
                                      57
                                      178

Nashville, TN
                                      162
                                      134
                                     8.05
                                     2.46
                                     3.74
                                     5.79
                                      1.6
                                       7
                                      50
                                      224

New York, NY
                                      371
                                      293
                                     8.09
                                     4.59
                                     6.41
                                     9.54
                                      1.5
                                      39
                                      103
                                      161

Philadelphia, PA
                                      237
                                      157
                                     6.18
                                     3.98
                                     6.00
                                     9.44
                                      1.8
                                      19
                                      53
                                      213

Phoenix, AZ
                                      271
                                      189
                                     7.58
                                     4.87
                                     7.61
                                     12.98
                                      3.3
                                      40
                                      60
                                      155

Portland, OR
                                      160
                                      121
                                     6.46
                                     2.90
                                     4.66
                                     6.68
                                      1.6
                                       6
                                      66
                                      223

Richmond, VA
                                      233
                                      162
                                     6.18
                                     3.44
                                     5.25
                                     9.30
                                      1.9
                                      16
                                      64
                                      204

Sacramento, CA
                                      198
                                      136
                                     6.59
                                     3.73
                                     5.75
                                     8.89
                                      1.1
                                       7
                                      60
                                      200

St Louis, MO
                                      311
                                      194
                                     6.08
                                     4.55
                                     6.93
                                     11.51
                                      1.6
                                      26
                                      82
                                      167

Salt Lake City, UT
                                      181
                                      147
                                     8.47
                                     3.36
                                     5.30
                                     8.96
                                      2.2
                                      11
                                      85
                                      182

San Antonio, TX
                                      124
                                      90
                                     3.92
                                     2.85
                                     4.42
                                     6.11
                                      2.3
                                       6
                                      60
                                      173

San Diego, CA
                                      188
                                      142
                                     7.09
                                     4.54
                                     6.86
                                     9.63
                                      0.9
                                       8
                                      90
                                      141

San Francisco, CA
                                      231
                                      124
                                     4.77
                                     5.99
                                     8.86
                                     11.87
                                      0.7
                                      25
                                      70
                                      62

Tampa, FL
                                      219
                                      141
                                     4.36
                                     3.32
                                     4.93
                                     6.71
                                      1.0
                                       8
                                      68
                                      192

Tulsa, OK
                                      221
                                      149
                                     5.31
                                     3.08
                                     5.06
                                     8.57
                                      1.8
                                      15
                                      70
                                      242

Washington, DC
                                      274
                                      198
                                     5.32
                                     3.06
                                     4.59
                                     7.22
                                      1.4
                                      13
                                      99
                                      225

[a]	Ratio of initial + emitted anthropogenic reactive organic gas (ROG) input to initial + emitted NOx. Biogenic VOC input not included.
[b]	Initial + emitted anthropogenic VOC input, in units of millimoles m[-2].

The Maximum Ozone Incremental Reactivity (MOIR) scale is derived from the scenarios where NOx inputs are adjusted to yield highest maximum O3 concentrations. This represents NOx conditions that are most favorable to O3 formation. MOIR NOx levels are generally about a factor of 1.5 lower than those yielding maximum incremental reactivities.
The Equal Benefits Incremental Reactivity (EBIR) scale is derived from scenarios where NOx inputs are adjusted so that the reduction in O3 caused by reducing base ROG inputs are the same as those caused by changing total NOx inputs by the same percentage. This represents the lowest NOx conditions where controls of VOCs are at least as effective as controlling NOx; since for lower NOx levels NOx controls are always more effective for reducing O3. EBIR NOx levels are generally about a factor of 1.5 lower than those yielding maximum O3 concentrations, or 2.3 times lower than MIR levels.
Table 24 gives the NOx levels that correspond to these various conditions of NOx availability that were used to derive the MIR, MOIR, or EBIR scales. The incremental reactivities for those scales were averages of the incremental reactivities calculated for the 39 scenarios of the various types.
The fixed parameter version of SAPRC-07 with the base ROG mixture composition given in Table 17 was used to simulate the base cases for the reactivity assessment scenarios used in this work. For the purpose of calculating incremental reactivities of individual compounds, model species were added to explicitly represent the compound, and also to represent the more reactive oxidation products if the "adjustable products" mechanisms were employed. For simulating reactivities of complex mixtures, lumped model species were added as indicated on Table 17, above, with the mechanisms for the adjustable parameter species (ALKn, OLEn, and AROn) being derived based on the particular mixture being assessed.
Note that as discussed by Carter (2008a) the impacts of the reactions of amines with HNO3 under atmospheric conditions are very uncertain. This is because they are sensitive to atmospheric sources and other sinks of HNO3, which are highly variable and not determined for the reactivity assessment scenarios. For the purpose of deriving reactivity scales for regulatory applications, it is probably appropriate to use upper limit estimates of the ozone impacts of these compounds under conditions where they will have their highest impacts. For that reason, the atmospheric reactivities calculated for the amines are based on assuming that loss by reaction with HNO3 is negligible. As discussed by Carter (2008a) if this reaction is important the magnitudes of the O3 impacts of the amines could be lower by more than an order of magnitude.
The incremental reactivity calculations were carried out by adding the amount of test compound or mixture such that the estimated amount reacted would be 0.05% the mole carbon of the base ROG input. The incremental reactivities were calculated change in final (i.e., maximum) O3 concentrations in terms of total moles formed, divided by the moles of test compound or mixture added in the calculations. The incremental reactivities are then converted from mole to mass basis by using the molecular weights for O3 and the test VOCs.
Results
Predicted Ozone Formation in the Scenarios
Before presenting the incremental reactivity results, it may be of interest to show the model predictions of O3 formation in the various scenarios used for reactivity assessment. The maximum O3 formed in the base case (unadjusted NOx) scenarios are tabulated in Table 24, and Figure 16 shows predicted maximum O3 levels for all scenarios used, plotted against total ROG inputs, which gives a

Figure 16.	Plots of maximum O3 concentrations calculated for the various scenarios used for reactivity assessment against the initial ROG levels in the scenarios.

measure of the total amount of pollution in the scenario. As expected the O3 levels correlate with the total amount of ROG input, though the correlation is not perfect, indicating the importance of other factors such as dilution, etc. As also expected (and by definition) the O3 levels are highest for the MOIR scenarios, and in all cases are the lowest for the MIR scenarios.
Note that some scenarios have O3 levels as high as 500 ppb or more, which are much higher than now occurs in urban areas in the United States. This suggests that the scenarios should be updated to reflect present conditions. However, updating the scenarios for reactivity assessment was beyond the scope of this project. Figure 17 shows the changes in O3 formation in the various scenarios predicted using this updated version of the mechanism, compared to those calculated previously using SAPRC-99. Figure 17a shows the differences when the NOx levels in the MIR, MOIR, and EBIR scenarios are adjusted separately for each mechanisms to yield the corresponding reactivity characteristics, while Figure 17a shows the differences when the NOx levels are not adjusted, i.e., when the SAPRC-99 MIR, MOIR or EBIR NOx levels are used in the SAPRC-07 calculations. (The NOx levels, and therefore the mechanisms differences, are the same in the base case calculations shown on both plots.) It can be seen that if the NOx levels are adjusted to yield the same reactivity conditions, the changes in O3 predictions resulting from the mechanism update varies from scenario to scenario, ranging from a ~10% increase to a ~5% increase, though for most scenarios the update causes a decrease in predicted O3, by about 5% on average.  For a given type of scenario the decrease tends to be correlated with the ROG/NOx ratio, with the decrease being largest at the lowest ROG/NOx ratios.
However, the mechanism change caused a ~5 - ~15% decrease in the NOx levels that yielded MIR, MOIR, or EBIR levels, so the results shown on Figure 17a do not reflect the same NOx levels.  If the NOx levels are held constant, the changes caused by the update have a much more consistent dependence on ROG/NOx ratio, and also larger changes for the MIR scenarios and smaller changes for the EBIR scenarios. This is consistent with the fact that O3 sensitivities to mechanism differences are

Figure 17.	Change in maximum O3 concentrations in the scenarios used for reactivity assessment, calculated using SAPRC-07 compared to SAPRC-99.

greatest at the lowest ROG/NOx ratios (e.g., MIR), and simulations become much less sensitive as the higher ROG/NOx ratios (EBIR). O3 reductions of up to ~20% are seen for the MIR scenarios with the lowest ROG/NOx ratios, while the changes for most of the EBIR scenarios are less than 5%.
Updated Ozone Reactivity Scales
Table B-1 in Appendix B gives the results of the MIR, MOIR, EBIR, and average base case reactivity scale calculation for all the VOCs currently represented in this version of the mechanism. This includes a total of 1029 types of VOCs, of which 722 have explicit mechanism assignments and the remainder are represented using the "lumped molecule" approach, and a total of 52 complex mixtures. (The latter include the 32 complex mixtures used in previous reactivity tabulations, using the same compositions as employed previously [Carter, 2000a and references therein, Carter et al, 1997e, 2000a, 2000e; Carter and Malkina, 2005, 2007], plus new or revised unspeciated alkane and aromatic mixture categories. The Unspeciated alkane mixtures assuming equal amounts of normal, branched, and cyclic alkanes, and unspeciated aromatic mixtures derived as discussed by Carter and Malkina [2005].) The table indicates whether the VOC is represented explicitly, and if so whether its reactivity was calculated using adjusted product mechanisms, whether the VOC was represented using the lumped molecule method, or whether the reactivities are given for complex mixtures. Molecular weights, carbon numbers, and (where applicable) CAS numbers are also given in the table.
The incremental reactivities in the MIR scale are of greatest interest because this is the scale that is most often used or proposed for use in regulatory and other applications. Figure 18 shows a plot of the incremental reactivities in the updated scale against those in the latest SAPRC-99 scale provided by Carter (2003a). The 1:1 and 30% lines are also shown. The average change (excluding the 13 outliers where the change was greater than 50%) in the MIR's was -8%, and the average absolute magnitude of the change (again excluding the outliers) was 11%. The change was less than 30% for approximately 96% of these VOCs. The MIR for the base ROG mixture, which is used as the standard for relative reactivity scales, decreased by about 3%. If this is taken into account, the average change in relative MIR values would be approximately -5%, excluding the outliers.
The compounds whose incremental reactivities in the MIR scale changed by more than 30% are listed in Table 25. The table also indicates the probable reasons for the changes for those with the greatest changes. Excluding 3-methoxy-1-butanol, where the SAPRC-99 mechanism had a factor of 10 error in the OH rate constant, the greatest changes were for halogenated compounds because of the higher estimated photolysis rates for the chloroaldehyde and chloroketone products predicted to be formed, and for amines, for which new mechanisms were derived based on new chamber data by Carter (2008a).
Updated Uncertainty Estimates
The reactivity tabulation in Table B-1 in Appendix B gives updated uncertainty classifications for the various types of VOCs and other information that may be useful in this regard. This includes an indication of the availability of environmental chamber data to evaluate the mechanism for the compound, and the comprehensiveness of the data for this purpose, and also estimates of likely biases in the reactivity estimates, if known. The uncertainty and other codes that are used for this purpose are given and described in footnotes to Table B-1.

Figure 18.	Plots of incremental reactivities in the MIR scale computed using the updated mechanism against those reported previously by Carter (2003a).

Table 25.	List of compounds and mixtures whose reactivities in the MIR scale changed by more than 30%.
Compound
                             MIR (gm O3 / gm VOC)
                              Probable Reason [a]

                                      Old
                                      New
                                    Change

3-Methoxy-1-butanol
                                     0.97
                                     3.87
                                     300%
                                       1
2-(Chloromethyl)-3-chloropropene
                                     3.13
                                     7.00
                                     123%
                                       2
1,2-Dibromoethane
                                     0.046
                                     0.102
                                     119%
                                       3
1,2-Dichloroethane
                                     0.098
                                     0.21
                                     119%
                                       2
trans-1,2-Dichloroethene
                                     0.81
                                     1.70
                                     110%
                                       2
2-Amino-2-methyl-1-propanol
                                     4.75
                                     0.25
                                     -95%
                                       4
Dimethyl amine
                                     9.37
                                     3.17
                                     -66%
                                       4
Mesityl oxide (2-methyl-2-penten-4-one)
                                     17.37
                                     6.51
                                     -63%
                                       5
3,5,5-trimethyl-2-cyclohexenone
                                     10.58
                                     4.63
                                     -56%
                                       5
Propionic acid
                                     0.79
                                     1.22
                                      55%

Triethanolamine
                                     2.75
                                     4.21
                                      53%
                                       4
2,4-Toluene diisocyanate
                                    -0.133
                                    -0.065
                                     -52%

Phenol
                                     1.82
                                     2.76
                                      51%
                                       6
1,1,2-Trichloroethane
                                     0.058
                                     0.086
                                      48%
                                       4
Furan
                                     16.54
                                     9.15
                                     -45%
                                       7
2-Methyl-2,4-pentanediol
                                     1.03
                                     1.45
                                      41%

CARB Hydrocarbon Bin 5
                                     2.56
                                     1.56
                                     -39%
                                       8
Diethanol amine
                                     4.05
                                     2.47
                                     -39%
                                       4
p-Xylene
                                     4.24
                                     5.84
                                      38%

Dichloromethane
                                     0.066
                                     0.041
                                     -37%

Chloroform
                                     0.034
                                     0.022
                                     -37%

Acetic acid
                                     0.50
                                     0.68
                                      37%

CARB Hydrocarbon Bin 20
                                     1.49
                                     0.95
                                     -37%
                                       8
n-butyl bromide
                                     0.60
                                     0.82
                                      36%

1,1,1-trichloroethane
                                     0.00
                                     0.00
                                      35%

CARB Hydrocarbon Bin 3
                                     2.52
                                     1.63
                                     -35%
                                       8
1,2-Propylene glycol diacetate
                                     0.94
                                     0.61
                                     -35%

CARB Hydrocarbon Bin 4
                                     2.24
                                     1.47
                                     -35%
                                       8
p-diethyl benzene
                                     3.36
                                     4.43
                                      32%

CARB Hydrocarbon Bin 1
                                     2.08
                                     1.42
                                     -32%
                                       8
1,1-Dichloroethane
                                     0.101
                                     0.069
                                     -31%

[a]	Discussion of probable reasons for MIR change:
   1	The OH radical rate constant used in SAPRC-99 was found to be low by a factor of 10.
   2	Chlorinated aldehyde and ketone products are now assumed to be much more photoreactive.
   3	The approximate method used to represent bromine-containing compounds was changed. SAPRC-99 used a highly approximate "placeholder" mechanism. SAPRC-07 represents them using the mechanism for the corresponding Cl-containing compound.
   4	New amine mechanisms developed. Note that amines without hydrogens on groups adjacent to amino groups are now estimated to be inhibitors, though their MIR values are still positive because of their NOx removal characteristics. Note also that the magnitudes of the amine MIRs could be significantly lower if loss of amines by reaction with HNO3 is important. See Carter (2008a).
   5	These compounds are now represented by the model species used to represent the lumped C5 isoprene product IPRD. The estimated mechanisms used in SAPRC-99, derived using the mechanism generation system, are now considered to be too uncertain to use.
   6	Phenol is lumped with Cresols in SAPRC-07 but was represented explicitly in SAPRC-99. However, the change in MIR is well within the relatively large uncertainty of the mechanism.
   7	Furan is now represented explicitly. Previously it was represented by m-xylene using the lumped molecule method.
   8	The "old" value is from Kwok et al (2000), and were derived from correlations between MIR values and boiling points for various types of compounds. The "new" values are calculated using compositions derived from various compounds as discussed by Carter and Malkina (2005). Some of the differences may be due to the change in methodology, though for most bins Carter and Malkina (2005) did not notice large differences between MIRs calculated using SAPRC-99.

It is important to recognize that the uncertainty and bias estimates given on Table B-1 are entirely subjective, and not based on any comprehensive sensitivity and uncertainty analysis. This also does not take into the account that reactivities of some VOCs may be sensitive to environmental conditions or changes in the base mechanism, and may change if the base mechanism or scenarios are updated even if the mechanism for the VOC itself is unchanged. An analysis of such would clearly be useful.

Summary and recommendations
This project was successful in accomplishing most of its objectives, particularly the primary objective of developing a complete update to the SAPRC-99 mechanism that represents the current state of the science. However, some of the objectives of this project could not be accomplished or completed in the time frame for this project, and addressing these objectives need to be included among our recommendations for future research. In this section, we summarize the major accomplishments of this project and the results and conclusions that were obtained, followed by a summary of the objectives that could not be fully accomplished and our recommendations for future research.
Summary of Project Accomplishments
The major accomplishment of this project is the development of the SAPRC-07 chemical mechanism and its associated reactivity scales that are documented in this report. Specific accomplishments, results, and conclusions of this project are summarized as follows.
Rate constants updated. All the rate constants in the mechanism have been reviewed based on results of current evaluations, and updated as needed. This includes updates to the absorption cross-sections and quantum yields for the photolysis reactions, where appropriate. The most recent IUPAC (2006) and NASA (2006) evaluations were used as the primary basis for updating the rate constants in the base mechanism and common organic products, the evaluations of Calvert et al (2000, 2002) were used for reactions relevant to alkenes (Calvert et al, 2002) and aromatics (Calvert et al, 2000), and the recent evaluations of Atkinson (1997), Atkinson and Arey (2003) and Calvert et al (2000, 2002) were used for rate constants for individual VOCs. Other sources, including recent work in our laboratory on reactivity studies of specific compounds, were used as appropriate.
Most of the rate constant changes were relatively small, but a few errors were found and corrected and some potentially significant changes occurred. Various uncertainty and sensitivity studies indicate that the rate constant for the OH + NO2 reaction is particularly important, and based on the results of the recent NASA (2006) evaluation, the rate constant for ground-level conditions increased by ~18%. This could result in somewhat lower O3 predictions under some conditions (as indicated by results of box model simulations of ambient scenarios, discussed below), and probably contributed to the need to re-adjust uncertain parameters in mechanisms for some individual VOCs to be consistent with chamber data, as indicated below. Photolysis rates for some aromatic and isoprene oxidation products were increased by ~40% or more. The effects on the aromatics mechanism are compensated by other changes in the aromatics mechanisms, but the changes for isoprene may have an impact on airshed simulations, though the model performance in simulating available isoprene chamber data was not affected. However, the effects of the many changes that were made, taken as a whole, have not been fully assessed, other than the limited number of box model simulations carried out in conjunction with updating the reactivity scale.
 Aromatics mechanisms reformulated. The mechanisms for the aromatic ring fragmentation reactions were reformulated to be more consistent with estimated explicit mechanisms that were derived as part of this project and to give predictions that are somewhat more consistent with "direct reactivity" results reported by Carter and Malkina (2002). These mechanisms incorporate much of the new information contained in the review of Calvert et al (2000), particularly in the representation of the ring opening products. However, although not as simplified and parameterized as the aromatic representation used in SAPRC-99, the updated mechanisms are still highly simplified in many respects, and are not consistent with all of the available data given by Calvert et al (2000) and others, and some of the inconsistencies with available environmental chamber data still persist.
The approach that was initially employed was to develop explicit versions of the mechanisms for representative aromatic hydrocarbons, and use these to derive more condensed, but chemically reasonable, versions for airshed model calculations. Unfortunately, because of time constraints and other problems, the effort to develop explicit mechanisms that could account for reactivity differences among aromatic isomers could not be completed within the time frame available to this project. However, the explicit mechanisms were developed to the point where they did serve as the basis for the more condensed aromatics mechanism that is described in this report. The aromatic mechanism formulation developed for this project, though simplified in some respects, represents a much closer approximation to the explicit mechanisms than SAPRC-99, and serves a useful basis for future improvements in this area.
As part of this effort, environmental chamber experiments were carried out with several furans, which were studied because these compounds are expected to form the same type of highly photoreactive fragmentation products as formed in the ring opening reactions of the alkylbenzenes, only in higher yields and without the complication of uncertain competing processes. Because the development of the explicit aromatics mechanisms based on these data could not be completed, the results of these experiments could not be incorporated in the final versions of the aromatics mechanisms developed for this work, other than in the mechanisms for the furans themselves. However, these data should be useful in future explicit aromatics mechanism development efforts. This explicit mechanism development effort, and the experiments carried out to support it, will be discussed in more detail in future reports.
Chlorine chemistry added. The representation of chlorine chemistry has been added to the mechanism as an optional capability. The inorganic ClOx and Cl atom reactions with common organic products have been added as an optional module of the base mechanism, and mechanisms for chlorine atom reactions were derived for all the explicit and lumped VOCs used in the version of the mechanism for ambient simulations and in the mechanisms used to calculate reactivities of individual chlorinated VOCs. The chlorine chemistry mechanisms were based primarily on the recent IUPAC (2006) and NASA (2006) evaluations, various evaluations and studies of VOC + chlorine reactions, results obtained in our previous studies of chloropicrin (Carter et al, 1997a) and dichloropropenes (Carter and Malkina, 2007), and also various estimation methods that were developed as part of this project.
In addition to improving the ability of airshed models to simulate air quality in regions impacted by chlorine emissions, the representation of chlorine chemistry and associated updates has resulted in improved reactivity estimates for a number of VOCs of interest in California, including the pesticides chloropicrin and dichloropropenes (Carter and Malkina, 2007), and reduced uncertainties in reactivity estimates for chlorinated compounds in general.
Mechanism generation system enhanced. The mechanism estimation and generation system is an important component of SAPRC-99 that was used to generate fully explicit mechanisms for most of the non-aromatic VOCs, from which the more condensed mechanisms used in the model were derived. This system was enhanced in a number of respects for this project. Updated rate constants for the primary reactions of the individual VOCs with OH, O3, NO3, and O[3]P were incorporated in the system. The capability of estimating and generating mechanisms for reactions of VOCs with chlorine atoms was added. The types of compounds and radicals whose reactions could be generated using the system was added to include to include species with more than one ring (e.g., terpenes), species with more than one double bond, alkynes, and (to a limited extent) aromatics and unsaturated aromatic ring opening products. This was used to assist in the derivation of mechanisms for compounds that could not be processed previously, and was also useful in deriving portions of the updated aromatics and aromatic products mechanisms.
Capability for adaptation to SOA predictions improved. Mechanisms for predictions of PM require an appropriate representation of the formation of the low volatility products that contribute to SOA, and how they depend on reactant conditions such as availability of NOx. Recent environmental chamber studies (Odum et al, 1996, Hurley et al, 2001, Sato et al, 2004a, Presto et al, 2005, Doherty et al, 2005, Song et al, 2005) indicate that SOA formation can change significantly with VOC/NOx ratios. This is attributed to competing branching ratios in reactions of peroxy radicals with NOx, HO2, and other peroxy radicals. Hydroperoxides formed in the reactions of peroxy radicals with HO2 appear to be particularly important in this regard. Unfortunately, the SAPRC-90 through SAPRC-99 mechanisms, like CB4 and CB05, uses a chemical operator lumping approach to represent peroxy radical reactions that requires representing hydroperoxide formation in a highly condensed and approximate manner. This is because explicitly representing all the possible peroxy + peroxy reactions that may occur under low NOx conditions requires an excessive number of reactions to represent processes that are unimportant to ozone formation, and the SAPRC-90/CB4 chemical operator lumping approach results in a large reduction in mechanism size without significantly impacting predictions of O3 and formation of organic product species that are important when ozone formation occurs. However, this is not as satisfactory an approximation when the model is being used to predict formation of secondary PM.
Appropriately representing hydroperoxide formation under low NOx conditions, and using separate species to represent low-volatility hydroperoxides, requires that a different method be used to represent peroxy radical reactions. An explicit approach, such as used in the MCM (Jenkin et al, 2003; Saunders et al, 2003), or a semi-explicit approach such as used in RADM-2 (Stockwell et al, 1990) or RACM (Stockwell et al, 1997) mechanisms, would be satisfactory in this respect, but it requires adding a significant number of species and reactions to the mechanism, and is not compatible with the lumped parameter approach used in the SAPRC mechanisms to derived mechanisms for lumped species based on the mixtures of compounds they represent (Carter, 1990, 2000a, 2000b; Adleman et al, 2005). A more computationally efficient approach would be desirable for most comprehensive modeling applications.
The SAPRC-07 mechanism developed for this project addresses these problems by implementing an alternative chemical operator approach for peroxy radicals reactions that involves adding a much smaller number of species and reactions to the mechanism, and is compatible with the SAPRC lumped parameter methods and software that has already been developed. With this method, hydroperoxide formation can be represented more explicitly, with separate lumped species formed from different compounds based on considerations such as reactivity and volatility. It also permits separate representation of organic nitrates formed in peroxy + NO reactions based on reactivity and volatility considerations, which was also difficult under the formulation used in SAPRC-99 and CB4/05.
Because development and evaluating a mechanism for prediction of SOA precursors is a major project that was well beyond the scope of this project, the initial version of SAPRC-07 does not fully take advantage of the enhanced capabilities of this mechanism to represent formation of SOA precursors. It does include a separate model species to represent the low volatility hydroperoxides formed from aromatics, primarily as a means to illustrate this capability, but its ability to predict SOA chamber experiments has not yet been evaluated. However, this version serves as a useful starting point in this regard, and can be used in a much more straightforward way to develop models that more accurately predict how SOA formation varies with conditions of NOx availability.
Mechanisms for many types of VOCs added or improved. A major feature of the SAPRC mechanisms is the ability to separately represent the many hundreds of types of VOCs for reactivity assessments, toxics modeling or other applications, and for deriving condensed mechanisms tailored for the specific mixtures of compounds that are present. The rate constants and mechanisms for the VOCs represented in SAPRC-99 were updated for this project, and in some cases improvements were made for VOCs previously represented using more approximate methods. The number of types of VOCs with distinct mechanisms was increased by 23% from 585 to approximately 720, and the total number of VOC classes for which reactivity estimates could be derived (whether by explicit representation or using the "lumped molecule" approach) was increased by 22% from 873 to over 1050. Most of the increases in distinct mechanisms are for alkanes, alkenes, and oxygenates found in emissions inventories, whose mechanisms could be readily derived using the mechanism generation system, but also an increased number of mechanisms were derived for more appropriate representation of the higher alkylbenzenes. Improved mechanisms are incorporated for halogenated compounds incorporating chlorine chemistry as discussed above, though most of these need to be experimentally evaluated. A few errors were found in SAPRC-99 mechanism assignments that were corrected, the most significant being a factor of 10 error in the rate constant for 3-methoxy-1-butanol.
Although mechanisms and representations were changed for a large number of the compounds, the number of compounds with significant MIR changes was relatively limited. The MIR changes were less than 25% for 95% of the chemical classes, and only 12 compounds had changes of more than 50%. However, some compounds whose MIRs were not previously listed have regulatory reactivities estimated using only "upper limit" methods (Appendix D in Carter, 2000a), so the addition of new compounds to the reactivity list may in some cases result in significantly different reactivities for regulatory applications.
Updated mechanism evaluated against chamber experiments. The updated mechanism was comprehensively evaluated by comparing predictions with results of all environmental chamber experiments used for SAPRC-99 evaluation, plus the results of more recent UCR experiments, and also the TVA chamber experiments used by Carter (2004), the UNC experiments used in the RADM2, and SAPRC-90 evaluations of Carter and Lurmann (1990, 1991). The results of this evaluation can be summarized as follows:
For some compounds it was found that re-adjustments to some of the uncertain mechanistic parameters had to be made to obtain mechanism performance comparable to SAPRC-99. This is apparently because changes to the base mechanism resulted in changes in parameter values that gave best fits to the data. This included primarily re-adjustments to overall nitrate yields in peroxy + NO reactions for VOCs where these are uncertain, but also the radical yields in some ozone + alkene reactions could be increased to values that are somewhat more consistent with, but still lower than, results of laboratory studies of these reactions (e.g., see Carter, 2000a, Pinho et al, 2006). These adjustments were incorporated in the updated mechanism.
In general, the model performance of the updated mechanism in simulating experiments for individual compounds or types of VOCs was comparable to that for SAPRC-99, with the overall bias for all single compound experiments, taken as a whole, being very low. Although major changes were made to the aromatics mechanism, the model performance in simulating the single aromatic and aromatic reactivity experiments did not change significantly. This can be attributed to the fact that for both SAPRC-99 and SAPRC-07, the yields or (in the case of SAPRC-07) effective quantum yields of photoreactive aromatic ring-fragmentation products were adjusted to optimize fits to the data. However, the updated mechanism did perform somewhat better in simulating direct reactivity data or effects of added CO (Carter and Malkina, 2002; Carter, 2004), though the results were still not completely satisfactory.
The model performance for simulating data for some types of compounds changed even though no changes were made to their mechanisms. Biases that were previously seen in model simulations in propylene and ethylene glycol (Carter et al, 2005a) and synthetic isoparaffinic mixtures (Carter and Malkina, 2005) were significantly reduced. On the other hand, slightly increased positive biases were seen in simulations of effects for some types of compounds, such as the larger n-alkanes, on maximum O3 yields in lower NOx conditions. These results indicate that O3 reactivity predictions, at least for chamber experiments, can be sensitive to changes in the base mechanism even if the mechanism of the VOC is not changed.
One area of potential concern is that the mechanism update caused a slight increase in overall biases in model simulations of experiments with mixtures of VOCs, including those designed to simulate ambient conditions. For all such experiments, the average bias in the simulations of the O3 formation and NO oxidation rates in the second hour of the experiments increased from 5% to 12%, and the biases in simulations of final O3 formed and NO oxidized increased from 3% to 6%. However, when examining run types separately, one sees the biases vary with type of experiment, with some types of experiments having greater positive biases and some having greater negative biases, compared to SAPRC-99. These changes are small compared to the 30% variability of the fits overall, but because of the large number of such experiments (>1500 total) it may be statistically significant.
The mechanism update also did not solve the problem, noted by Carter (2004) and Carter et al (2005b) that SAPRC-99 had a consistent bias in underpredicting rates of O3 formation and NO oxidation in surrogate - NOx experiments carried out at relatively low NOx levels in the new UCR EPA chamber, with the bias decreasing at higher ROG/NOx ratios. This bias still exists with the updated mechanism, with the dependence of ROG/NOx ratios being approximately the same. It is interesting to note that the dependence of the bias on ROG/NOx in experiments in this chamber is in the opposite direction when the aromatics are removed from the surrogate, suggesting that the problem may be related to the aromatics mechanism, both for SAPRC-99 and SAPRC-07.
Mechanism implementation. The mechanism developed in this project was implemented for the box model calculations used for reactivity scale calculations, and the data files used in this implementation can serve as the basis for implementing in more comprehensive airshed models such as CMAQ or CAMx. Although there was insufficient time and resources remaining in this project to adapt the updated mechanism into such a model, the necessary data files and associated implementation documentation are being made available at the project web site at http://www.cert.ucr.edu/~carter/ SAPRC. The types of data and files formats provided are similar to those used when distributing SAPRC-99, which has been successfully implemented into various models. However, we will continue to provide guidance to modelers in implementing this mechanism, as needed.
An important part of mechanism implementation is providing assignments of the many chemical categories used in speciation profiles for emissions inventories to the model species used in the mechanism. As part of this project, the emissions speciation database previously developed to provide a comprehensive and consistent speciation approach for the SAPRC-99, RADM2, CB4, and CB05 were updated to include SAPRC-07 as one of the optional mechanisms (Carter, 2008b). At present, only the fixed-parameter version of SAPRC-07 is implemented. Work on updating the system for the variable parameter version is planned, and when completed will be made available at the SAPRC mechanism web site noted above.