1
                                      17

                                     2.03
                                      13
NH2, NH, N
                                       1
                                      14

                                       1
                                      14

                                      [b]

[a]	Notes for derivations of the group rate constants and group correction factors are as follows:
   1	Same as used in SAPRC-99
   2	Kwok and Atkinson (1995)
   3	Based on IUPAC (1997) recommendations for acetaldehyde and propionaldehyde
   4	Reaction at formate group assumed to be negligible based on tabulated formate rate constants.
   5	Derived to fit the NASA (2006) recommended rate constant and branching ratio for reaction of OH with methyl hydroperoxide. This results in an unusually large group substituent factor.
   6	Average of estimated rate constants per methyl group for the alkylbenzenes for which aromatic aldehyde yields have been derived. Reaction at the methyl group is estimated based on the aromatic aldehyde yield and the total rate constant.
   7	Derived from the difference for the total rate constants for ethylbenzene and toluene, assuming the same rate constant for addition to the aromatic ring and the estimated rate constant at the methyl group in ethylbenzene.
   8	Derived from the difference for the total rate constants for isopropyl benzene and toluene, and p-cymene and p-xylene, assuming the same rate constant for addition to the aromatic ring and the estimated rate constants for reactions at the methyl groups in isopropyl benzene and p-cymene.
   9	From Atkinson (1991). Derived from the correlation between NO3 and OH radical rate constants.
   10	Assumed to be negligible.
   11	Derived from the IUPAC (1997) recommended rate constant for acetaldehyde.
   12	No explicit assignment made. By default, the system uses the same assignment as for standard CH3, CH2, or CH groups.
   13	Derived in this work from measured chlorine + VOC rate constants. See text and Table 4
   14	Assigned.
   15	From Kwok et al (1996).
   16	Adjusted to fit OH + ethyl and propyl formate rate constants (Wallington et al, 1988b)
   17	No assignment made. This is the default value used by the mechanism generation system, and may not be appropriate.
   18	This is the assignment incorporated into the current system. It may not be appropriate. It is not used as the basis for estimating NO3 rate constants for any detailed model species.
   19	Based on chloroacetaldehyde only
   20	Estimated to be approximately the same as the factor for OH.
   21	Estimated to be approximately the same as the factor for Cl.
   22	Derived to give the best fits to the available kinetic data for simple amines and alcohol amines for which kinetic data are available, as indicated in Table 3c. In order to fit the data for tertiary amines, it is necessary to assume that OH reacts with an enhanced rate at groups next to the amino group, and in order to fit the data for amines, such as AMP and t-butyl amine, that do not have abstractable hydrogens next to the amino group it is necessary to assume that reaction also occurs at a significant rate at the NH or NH2 groups.
   23	No data are available for which to derive group-additivity estimates for this type of group. The group rate constant is estimated by linear interpolation from those used for CH3(NHx) and CH2(NHx).
   24	The only kinetic data concerning the reactions of NO3 radicals with amines is for N-methyl-2-pyrrolidinone (NMP), where a relatively high rate constant of 1.26 x 10[-13] cm[3] molec-1 s-1 is measured (Aschmann and Atkinson, 1999). This suggests that this reaction is also rapid for other amines. We assume that this is an H-atom abstraction reaction, forming HNO3. For lack of other data, use the rate constant for NMP as the basis for estimating the group rate constants for reaction at the -CH2(N) or CH3(N) groups. The group rate constants for reactions at other amine groups, including NH and NH2, are estimated assuming that the ratios of rate constants for reactions at the various groups are the same for the NO3 reaction as derived for the OH reaction. This is highly uncertain.
[b]	Group rate constants for reactions of chlorine with amines were not derived

Table 3c.	Experimental and best fit estimated rate constants for the reactions of OH radicals with amines that were used as the basis for deriving structure-reactivity estimates
Compound
               OH Rate Constant (10[-11] cm[3] molec[-1] s[-1])

                           Measured value, reference
                                      Avg
                                     Calc
                                     Diff
Methyl amine
                                                                           1.73
                                       1
                                                                           2.20
                                       2
                                     1.97
                                     3.08
                                      36%
Dimethyl amine
                                                                           6.49
                                       1
                                                                           6.54
                                       3
                                     6.52
                                     6.51
                                      0%
Trimethyl amine
                                                                           3.58
                                       1
                                                                           6.09
                                       3
                                     4.84
                                     5.01
                                      4%
Ethyl amine
                                                                           2.38
                                       1
                                                                           2.77
                                       3
                                     2.58
                                     3.59
                                      28%
N,N-Dimethylethanolamine (DMAE)
                                                                           9.00
                                       4
                                                                           4.70
                                       5
                                     6.85
                                     6.02
                                     -14%
t-Butyl amine
                                                                           1.18
                                       6

                                     1.18
                                     1.79
                                      34%
2-Amino-2-methyl-1-propanol (AMP)
                                                                           2.80
                                       5

                                     2.80
                                     2.19
                                     -28%
                      References for measured rate constants:
	1	Carl and Crowley (1998)	4	Anderson and Stephens (1988)
	2	Atkinson et al. (1977)	5	Harris and Pitts (1983)
	3	Atkinson et al. (1978)	6	Koch et al (1996)
For addition to double bonds, we assume that the rate constant is determined only by the number of non-hydrogen substituents about the double bond, with correction factors used for some non-alkyl substituents such as halogens. Although this doesn't affect the rate constant, for the purpose of estimating mechanisms it is also necessary to assign factors for the fractions that react at each position around the double bond. This has to be estimated because we are aware of no data available concerning this.
The organic + Cl atom rate constants that were used as the basis for deriving group-additivity estimates for the mechanism generation system are given in Table 4, with footnotes indicating the sources of the rate constants used. (Note that the rate constants in Table 4 are restricted to those used for deriving the group-additivity methods, and does not include compounds for which the estimation method was not developed to support. See Table B-7 for the chlorine atom rate constants for all the VOCs in the mechanism.) Group additivity parameters found to give the best fits to the data on Table 4 are shown on Table 3, above, for the abstraction reactions, and on Table 5 for reactions at double bonds. The "Est'n error" column on Table 4 shows the extent to which the estimated rate constant agrees with the measured value, with positive numbers indicating overprediction, and vise-versa. If there is no entry in this column it means that the current estimation method is not applicable to those compounds. The group additivity parameters were determined by minimizing the sum-of-square relative errors in for the compounds listed on Table 4. In most cases the estimated rate constants agree with the measured values to better than 25%. However, cases with prefect agreement usually indicate that a parameter was determined only by the data for a single compound, so perfect agreement is not always evidence for the success of the method.
The Cl abstraction reactions form the same radicals as the corresponding reactions with OH, so the procedures for generating the subsequent reactions of those radicals formed have already been developed and described (Carter, 2000a). Therefore, for VOCs for which only the abstraction reaction is important, which include all saturated VOCs and also simple aldehydes, ketones, and esters, the derived mechanisms for the chlorine reactions are the same as those for the reaction with OH, except for the different branching ratios for the initial reactions because of the differences in the group additivity parameters, discussed above.
Although in principle the system could also derive mechanisms for the reactions of chlorine atoms with alkenes, in practice this proved difficult because at present the system does not contain the necessary thermochemical or kinetic assignments to estimate reaction rates for many of the chlorine-substituted alkoxy radicals predicted to be formed. The only way to deal with this is to make explicit assignments of branching ratios for the Cl-substituted radicals that cannot presently be handled by the system, which can be time consuming for large molecules because of the number of radicals that can be formed. For that reason, assignments were made only to permit the generation of mechanisms for the explicitly represented alkenes (ethylene and isoprene), those chosen as representative of the lumped model species used in airshed models (discussed later in this report), or those present in the chamber experiments relevant to evaluating mechanisms for the chlorine-containing compounds for which chamber data are available.
The alkoxy radical branching ratio assignments used to generate the chlorine + alkene mechanisms that were derived for this project are summarized on Table 6, with footnotes giving a brief indication of the basis of the assignments. In addition, except as indicated on Table 6, it is assumed that -chloro-alkoxy radicals predominantly decompose to form chlorine atoms, e.g.,
	ClCH2O·  HCHO + Cl·
	RCH(Cl)O·  RCHO + Cl·
	RC(R')(Cl)O·  RC(O)R' + Cl·

Table 4.	Rate constants for reactions with chlorine atoms for organic compounds used to derive parameters for group-additivity estimates.
Compound
                         Rate Constant Assignment [a]
                                     Note
                                      [b]
                               Est'n Error [c]

                                    k(298)
                                       A
                                  Ea (deg K)

Methane
                                   1.03e-13
                                   6.60e-12
                                     1240
                                       1

Ethane
                                   5.93e-11
                                   8.30e-11
                                      100
                                       1
                                      15%
Propane
                                   1.37e-10
                                   1.20e-10
                                      -40
                                       2
                                      -3%
n-Butane
                                   2.05e-10
                                   2.05e-10
                                       0
                                       1
                                      -5%
n-Pentane
                                   2.80e-10

                                       2
                                      -8%
n-Hexane
                                   3.40e-10

                                       2
                                      -6%
n-Heptane
                                   3.90e-10

                                       2
                                      -3%
n-Octane
                                   4.60e-10

                                       2
                                      -4%
n-Nonane
                                   4.80e-10

                                       2
                                      5%
n-Decane
                                   5.50e-10

                                       2
                                      3%
Isobutane
                                   1.43e-10

                                       2
                                      0%
Neopentane
                                   1.11e-10
                                   1.11e-10
                                       0
                                       2
                                      18%
Iso-Pentane
                                   2.20e-10

                                       2
                                      -7%
2,3-Dimethyl Butane
                                   2.30e-10

                                       2
                                      -6%
2-Methyl Pentane
                                   2.90e-10

                                       2
                                      -8%
3-Methylpentane
                                   2.80e-10

                                       2
                                      -4%
2,2,3-Trimethyl Butane
                                   2.90e-10

                                       2
                                     -29%
2,4-Dimethyl Pentane
                                   2.90e-10

                                       2
                                      -4%
2-Methyl Hexane
                                   3.50e-10

                                       2
                                      -6%
2,2,3,3-Tetramethyl Butane
                                   1.75e-10

                                       2
                                      12%
2,2,4-Trimethyl Pentane
                                   2.60e-10

                                       2
                                      3%
Cyclohexane
                                   3.50e-10

                                       2
                                      6%
Methylcyclohexane
                                   3.90e-10

                                       2
                                      -2%
Propene
                                   2.67e-10

                                       4
                                      -1%
1-Butene
                                   3.39e-10

                                       5
                                      -3%
1-Pentene
                                   4.05e-10

                                       5
                                      -4%
3-Methyl-1-Butene
                                   3.52e-10

                                       6
                                      -4%
3-Methyl-1-Pentene
                                   3.78e-10

                                       6
                                      6%
Isobutene
                                   3.25e-10

                                       6
                                      9%
2-Methyl-1-Butene
                                   3.82e-10

                                       6
                                      10%
cis-2-Butene
                                   3.88e-10

                                       6
                                     -15%
trans-2-Butene
                                   3.55e-10

                                       4
                                      -7%
2-Methyl-2-Butene
                                   3.23e-10

                                       6
                                      7%
Cis 4-Methyl-2-Pentene
                                   4.04e-10

                                       6
                                      0%
Toluene
                                   6.20e-11

                                       7
                                      12%
m-Xylene
                                   1.35e-10

                                       7
                                      3%
o-Xylene
                                   1.40e-10

                                       7
                                      -1%
p-Xylene
                                   1.44e-10

                                       7
                                      -4%
1,3,5-Trimethyl Benzene
                                   2.42e-10

                                       8
                                     -14%
Methanol
                                   5.50e-11
                                   5.50e-11
                                       0
                                       1
                                     -34%
Ethanol
                                   1.00e-10
                                   8.60e-11
                                      -45
                                       1
                                      5%
Isopropyl Alcohol
                                   8.60e-11

                                       1
                                      31%
n-Propyl Alcohol
                                   1.62e-10
                                   2.50e-10
                                      130
                                       1
                                      3%
Acetic Acid
                                   2.65e-14

                                       1
                                      0%
Acetaldehyde
                                   8.00e-11
                                   8.00e-11
                                       0
                                       1
                                      0%
Propionaldehyde
                                   1.23e-10

                                       9
                                      0%
Acetone
                                   2.69e-12
                                   7.70e-11
                                     1000
                                      10
                                      0%
Methyl Ethyl Ketone
                                   3.60e-11

                                       1
                                      2%
Methyl Chloride
                                   4.89e-13
                                   2.17e-11
                                     1130
                                      10
                                      -7%
Dichloromethane
                                   3.49e-13
                                   7.40e-12
                                      910
                                      10
                                      11%
Chloroform
                                   1.19e-13
                                   3.30e-12
                                      990
                                      10
                                      -7%
Vinyl Chloride
                                   1.27e-10

                                      11
                                      4%
1,1-Dichloroethene
                                   1.40e-10

                                      11
                                     -19%
Trans-1,2-Dichloroethene
                                   9.58e-11

                                      11
                                      8%
Cis-1,2-Dichloroethene
                                   9.65e-11

                                      11
                                      7%
Trichloroethylene
                                   8.08e-11

                                      11
                                     -18%
Perchloroethylene
                                   4.13e-11

                                      11
                                      9%
3-Chloropropene
                                   1.30e-10

                                      12
                                      0%
Chloroacetaldehyde
                                   1.29e-11

                                      13
                                      0%
            [a]	Rate constants and A factors in units of cm[3] molec-1 s-1. If no A factor or activation energy is given, rate constant is given only for 298K. Otherwise, 298K rate constant is calculated from A factor and activation energy.
            [b]	Notes:
               1	IUPAC (2006) recommendation.
               2	Atkinson (1997) recommendation.
               3	This reaction is in the pressure falloff region under atmospheric conditions.
               4	Average of values tabulated by Wang et al (2002). Value of Wang et al (2002) placed on an absolute basis using the Atkinson (1997)-recommended rate constant for n-heptane.
               5	Average of value of Coquet et al (2000), placed on an absolute basis using the Atkinson (1997)-recommended n-hexane rate constant, and the value of Wang et al (2002), placed on an absolute basis using the Atkinson (1997)-recommended rate constant for n-heptane.
               6	Value of Wang et al (2002), placed on an absolute basis using the Atkinson (1997)-recommended rate constant for n-heptane.
               7	Average of values tabulated by Wang et al (2005).
               8	Wang et al (2005).
               9	Average of values listed by Le Crane et al (2005)
               10	NASA (2006) recommendation.
               11	From rate constants relative to n-butane from Atkinson and Aschmann (1987), placed on an absolute basis using the n-butane rate constant recommended by IUPAC (2006).
               12	Average of values tabulated by Albaladejo et al (2003).
               13	Average of values tabulated by Scollard et al (1993)
            [c]	(Estimated rate constant - measured rate constant) / measured rate constant.

Table 5.	Group additivity rate constants and factors used for estimating rates of addition of Cl atoms to double bonds.
Group
                                    k(add)
                         Add'n to most substituted end
                                     Note

Group
                            Substituent Correction
                                     Note
CH2=CH-
                                   2.30e-10
                                      35%
                                      1,2

Alkyl
                                       1
                                       3
CH2=C<
                                   2.89e-10
                                      25%
                                      1,4

-Cl
                                     0.58
                                       1
-CH=CH-
                                   2.63e-10
                                      50%
                                      1,5

2 -Cl's
                                     0.68
                                       1
-CH=C<
                                   2.47e-10
                                      25%
                                      1,4

-CH2Cl
                                     0.56
                                      1,6
>C=C<
                                   2.47e-10
                                      50%
                                      1,5

         1	Addition rate constant and substituent correction factor derived to minimize sum of squares error in predictions of Cl + alkene rate constants. (Optimization for group additivity parameters for abstraction reaction carried out at the same time.)
         2	Assume same terminal bond addition fraction as used for the reaction of OH with propene (Carter, 2000a).
         3	All alkyl substituents assumed to have the same factor. Unit factor assigned.
         4	No information available concerning relative addition rates at the different positions. Assume addition at terminal position occurs about 25% of the time
         5	Assume equal probability of addition, regardless of substituents.
         6.	Based on rate constant for 3-chloropropene.

These assignments, and therefore the mechanisms derived for the chlorine + alkene reactions that are based on them, are uncertain. However, reactions of alkenes with chlorine atoms are generally not major sinks either for chlorine or alkenes under most conditions, so these assumptions are probably not major sources of uncertainty in terms of overall model predictions.
Updates to Rate Constants and Mechanisms
Because of time and resource limitations, we were not able to update most of the estimation methods incorporated in the SAPRC-99 mechanism generation system used for this project, so most of the various estimation assignments given by Carter (2000a) are still applicable for this version. However, we did update the assignments for the rate constants for the initial reactions of the individual VOCs in those cases where rate constant measurement data are available, and these were used when generating the mechanisms for the applicable VOCs. Table B-2, and Table B-4 through Table B-6 and their footnotes indicate the rate constants used and the sources of those rate constants, where applicable.
Based on results of recent evaluations and also results the evaluation against chamber data, some updates were also made to assigned reactions of excited Criegee radicals involved in the ozone + alkene reactions. The current assignments for Criegee biradical reactions are summarized on Table 7. Note that the amount of radical formation in the decomposition of the primary biradicals, which was adjusted based on simulations of chamber data, is somewhat higher than used in the SAPRC-99 mechanism, but still lower than currently recommended radical yields for these reactions. This inconsistency, which is also seen in evaluations of the MCM, is discussed further by Pinho et al (2006).
Additional updates or modifications were made for mechanisms of some individual VOCs, based primarily on results of model simulations of chamber experiments. For example, the mechanism for the reaction of O[3]P with ethene was updated based on the Calvert et al. (2000)
Table 6.	Branching ratio assignments for chlorine-substituted alkoxy radicals made to generate mechanisms for the reactions of chlorine with representative alkenes.
Used for
Reaction
                                    Fract.
                                     Notes
Propene
CH3CH(O·)CH2Cl + O2  CH3C(O)CH2Cl + HO2·
                                     100%
                                       1

CH3CH(CH2O·)Cl + O2  CH3CH(CHO)Cl + HO2·
                                     100%
                                       2
2-Butenes
CH3CH(O·)CH(CH3)Cl + O2  CH3C(O)CH(CH3)Cl + HO2·
                                     100%
                                       3
1-Pentene
CH3CH2CH2CH(O·)CH2Cl  ·CH2CH2CH2CH(OH)CH2Cl
                                     100%
                                       4

CH3CH2CH2CH(CH2O·)Cl  CH3CH(·)CH2CH(Cl)CH2OH
                                     100%
                                       4

CH3CH(O·)CH2CH(Cl)CH2OH  CH3CH(OH)CH2CH(Cl)CH(·)OH
                                     100%
                                       4
2-Pentenes
CH3CH2CH(O·)CH(CH3)Cl  CH3CH2CHO + CH3CH(·)Cl
                                     100%
                                       5

CH3CH2CH(Cl)CH(O·)CH3  CH3CHO + CH3CH2CH(·)Cl
                                     100%
                                       5
Isoprene
CH2=C(CH3)CH(·)CH2Cl + O2  CH2=C(CH3)CH(OO·)CH2Cl
                                      60%
                                       6

                                                    CH3C(CH2OO·)=CHCH2Cl
                                      41%

CH2=CH-C(·)(CH3)CH2Cl + O2  CH2=CH-C(OO·)(CH3)CH2Cl
                                      44%
                                       6

                                                     CH3C(=CH-CH2OO·)CH2Cl
                                      56%

CH2=C(CH3)CH(O·)CH2Cl + O2  CH2=C(CH3)C(O)CH2Cl + HO2·
                                     100%
                                       7

CH3C(=CH-CH2O·)CH2Cl + O2  CH3C(CH2Cl )=CHCHO + HO2·
                                     100%
                                       8

CH3C(CH2O·)=CH-CH2Cl + O2  CH3C(CHO)=CHCH2Cl + HO2·
                                      50%
                                       9

                                            CH3C(CH2OH)=CH-CH(·)Cl
                                      50%

CH2=CH-C(O·)(CH3)CH2Cl  CH2=CHC(O)CH3 + .CH2Cl
                                     100%
                                       5

CH2=C(CHO)CH2O· + O2  CH2=C(CHO)CHO + HO2·
                                     100%
                                      10

CH2=C(CH3)CH(CH2O·)Cl + O2  CH2=C(CH3)CH(CHO)Cl + HO2·
                                      50%
                                       9

                                             CH2=C(CH2·)CH(Cl)CH2OH
                                      50%

CH2=C(CH2O·)CH(Cl)CH2OH  CH2=C(CH2OH)CH(Cl)CH(·)OH
                                      50%
                                       9

                                                 + O2  CH2=C(CHO)CH(Cl)CH2OH + HO2·
                                      50%

CH2=CH-C(CH3)(CH2O·)Cl + O2  CH2=CH-C(CH3)(CHO)Cl + HO2·
                                     100%
                                      10
-Pinene
CH3C(O·)*[1]CH(Cl)CH2CH*[2]CH2CH*[1]C*[2](CH3)CH3  CH3C(O)CH(Cl)-CH2CH*[1]CH2CH(·)C*[1](CH3)CH3
                                     100%
                                     4,11

CH3C(O)CH(Cl)CH2CH(O·)CH2CHO  HCOCH2CHO + CH3C(O)-CH(CH2·)Cl
                                     100%
                                       5

CH3C(O)CH(CH2O·)Cl  ·CH2C(O)CH(Cl)CH2OH
                                     100%
                                       4

.OCH2C(O)CH(Cl)CH2OH  HOCH2C(O)CH(Cl)CH(·)OH
                                     100%
                                       4

CH3C*[1](Cl)CH(O·)CH2CH*[2]CH2CH*[1]C*[2](CH3)CH3  CH3C(·)(Cl)CH*[1]-CH2CH(CH2CHO)C*[1](CH3)CH3
                                     100%
                                     4,11
2-(Chloromethyl)-3-chlororopene

ClCH2C(O·)(CH2Cl)CH2Cl  ClCH2C(O)CH2Cl + .CH2Cl
                                     100%
                                      12

·OCH2C(Cl)(CH2Cl)CH2Cl + O2  HCOC(Cl)(CH2Cl)CH2Cl + HO2·
                                     100%
                                      10
1,3-Dichloropropenes

ClCH2CH(O·)CH(Cl)Cl + O2  Cl-CH2C(O)CH(Cl)Cl + HO2·
                                     100%
                                      13
1	Assumed to be favored over decomposition on the basis of estimates for CH3C(O·)CH3 and the expectation that Cl-substitution makes radicals less stable
2	Assumed to be favored over decomposition on the basis of estimates for CH3CH2CH2O· and the expectation that Cl-substitution makes radicals less stable
3	Assumed to be favored over decomposition on the basis of estimates for the radical where H- replaces Cl- and the expectation that Cl-substitution makes radicals less stable
4.	Isomerization is assumed to dominate over competing processes for this radical.
5	This decomposition is assumed to dominate based on estimates for similar radicals.
6	Branching ratios based on the mechanism of Fan and Zhang (2004)
7	Assumed to be fast on the basis of observation of this compound as a product in the isoprene - chlorine reactions.
8	Although isomerizations might be expected to be more important than O2 reaction, this is assumed to dominate to account for the observed formation of CMBA as a product in the isoprene + chlorine reactions.
9	Reaction with O2 is assumed to be the dominant product when the radical is in the trans configuration, and isomerization is assumed to be the dominant process when it is in the cis configuration. Equal possibility of cis and trans configuration is assumed.
10	It is expected that the competing decomposition isn't favorable, so the reaction with O2 is assumed to dominate. However, this is uncertain.
11	The "*[1]" and "*[2]" symbols indicate join points for these cyclic radicals.
12	This appears to be the only available reaction.
13	Assumed to be favored over decomposition on the basis of estimates for CH3C(O·)CH3 and the expectation that Cl-substitution makes radicals less stable

recommendation, but it was still necessary to assume lower fragmentation yields than recommended in order to simulate the chamber data, as was the case for the reactions of O[3]P with other alkenes (Carter, 2000a; Pinho et al, 2006). Overall nitrate yields that were derived based on simulations of chamber data had to be readjusted for a number of compounds because changes to the base mechanism apparently caused the values of the nitrate yields that best fit the data to change somewhat. Assignments are made to the mechanism generation system to implement these adjustments where applicable.
Adjusted Product Mechanisms
Although the mechanism generation system derives fully explicit mechanisms, by default it lumps all the reactive products predicted to be formed into the set of organic product model species in the base mechanism, as indicated on Table A-1 in Appendix A. The mechanisms so derived are given in Table B-3 in Appendix B. Because the mechanisms used for the product species are fixed regardless of what VOC is being represented, these are referred to as "fixed product" mechanisms in the subsequent discussion. However, lumping the many types of reactive products into a few model species is an approximation, and some VOCs may form products that are not particularly well represented by any of the generic product model species. This could introduce inaccuracies to calculations of atmospheric reactivity scales. For this reason the mechanism generation system is also capable of deriving mechanisms for the major reactive products of the VOCs that could be used "adjustable product" mechanisms where the more reactive product species are represented by lumped (or explicit) product species whose mechanisms were derived based on the distribution of products they are being used to represent. Note that these adjustable mechanism products are only used on the mechanism for the particular VOC in the reactivity calculations, not for mechanisms of other VOCs in the base mechanism. The derivation and implementation of these adjustable product mechanisms is discussed by Carter et al (2000a). Since the same procedures were used for deriving the adjustable product mechanisms in this work, they are not discussed further here.

Table 7.	Assigned mechanisms for Crigiee Biradicals used for deriving mechanisms for reactions of O3 with alkenes.
Reaction
                                   Fraction
Discussion
H2COO[excited]

Branching ratios based on recommendations by Atkinson (1997), modified to be consistent with the OH yield in the IUPAC (2006) recommendations
         H2COO[stab]
                                      37%

         HCO + OH
                                      16%

         CO2 + H2
                                      12%

         CO + H2O
                                      35%

CH3CHOO[excited]

Atkinson (1997) recommendation.
         CH3CHOO[stab]
                                      15%

         CH3· + CO + OH
                                      54%

         CH3· + CO2 + H
                                      17%

         CH4 + CO2
                                      14%

RCX(X')CXOO[excited]

(R=Alkyl other than CH3; X, X' = H or alkyl): Available data concerning stabilization and decompositions of RCHOO Criegee biradicals are limited and inconsistent. In order to fit chamber data to fit results of 1-butene and 1-hexene experiments, it is necessary to assume that stabilization occurs ~85% of the time for 4+-carbon Criegee biradicals.
         RCX(X')CXOO[stab]
                                      85%

         RCX(X')· + CO + OH
                                     9.5%

         RCX(X')· + CO2 + H
                                     3.0%

         RCHXX' + CO2
                                     2.5%

XCX'H-COO[excited]-R

(X, X' = H or alkyl; R = alkyl): OH formation after hydroperoxide rearrangement assumed to be dominant process for disubstituted Criegee biradicals based on high OH yields, as discussed by Atkinson (1997). Relative importance of competing rearrangements estimated to be approximately proportional to estimated OH abstracting rate constant from H-donating group. (Unchanged from SAPRC-99)
         ·CXX'-CO-R + OH
                                     100%

Rt-COO[excited]-Rt'

(Rt = tertiary alkyl): Most reasonable decomposition mode is formation of CO2 and radicals. No information about reactions of disubstituted Criegee biradicals that cannot undergo the hydroperoxide rearrangement.  Roughly estimate that most is stabilized. The decomposition fraction is a guess. It probably depends on size of the molecule, but this is not taken into account. (Unchanged from SAPRC-99)
         Rt-COO[stab]-Rt'
                                      90%

         Rt· + Rt' + CO2
                                      10%

Adjustable product mechanisms were derived for all VOCs whose reactions and product reactions could be completely generated using the mechanism generation system. To assess the effects of using adjustable product mechanisms on calculated atmospheric incremental reactivities, Figure 1 shows a comparison of incremental reactivities calculated for the "averaged conditions MIR scale" calculated using the two methods for all VOCs for which adjustable product mechanisms could be derived. [Incremental reactivity scales are discussed in the "Updated Reactivity Scales" section, below. Briefly, the Averaged Conditions MIR scale gives a good approximation of the MIR scale used in CARB regulations (CARB 1993, 2000) using a single scenario rather than the average of 39 MIR scenarios.] It can be seen that for most VOCs the adjustable mechanism reactivities are very close to the fixed mechanism values, with the average bias being -1% and the average error being 6%. However, there were a number of compounds with non-negligible differences where use of the adjustable product mechanisms for reactivity calculations is appropriate. The compounds where the magnitude of the change was greater than 20% are listed in Table 8, with the reasons for the differences indicated for the top three compounds.
 For the purpose of calculating reactivity scales, we used the adjustable product mechanism if the calculated reactivity difference in the averaged conditions MIR scale was greater than 8%, and the fixed product mechanism otherwise. The use of 8% is somewhat arbitrary, but given the uncertainties in mechanisms in general, and product mechanisms derived using the mechanism generation system in particular, we did not consider the added complexity to be appropriate if the effect of less than approximately this amount. Table B-3 in Appendix B gives the adjustable product mechanisms that were employed in the reactivity scale calculations as determined by this criterion. Note that the primary VOC rate constants and methods for deriving the mechanisms are the same as indicated in Table B-2 for the fixed parameter mechanism, so this documentation is not duplicated on this table. If the mechanism for a

Figure 1.	Effect of using the adjustable product mechanisms vs. the standard fixed product mechanisms on incremental reactivities in the "Averaged Conditions" MIR scale for those VOCs for which adjustable product mechanisms could be derived.
Table 8.	Compounds whose Averaged Conditions MIR reactivities changed by more than 20% when adjustable product mechanisms were employed.
Compound
                   Averaged Conditions MIR (gm O3 / gm VOC)
Discussion

                                     Fixed
                                     Adj'd
                                     Diff

2,3-Butanediol
                                     2.51
                                     4.41
                                      75%
High yield product reacts to form biacetyl
1,2-Dihydroxy hexane
                                     2.01
                                     2.54
                                      26%
Products react to form methyl glyoxal
Dimethoxy methane
                                     1.25
                                     0.93
                                     -26%
Product represented by PROD2 has a relatively low rate constant for this group
cis-5-Decene
                                     2.96
                                     3.73
                                      26%

Glycerol
                                     2.53
                                     3.17
                                      25%

4-Methyl-2-pentanol
                                     2.15
                                     2.61
                                      21%

cis-4-Octene
                                     3.94
                                     4.79
                                      22%

VOC is not given in Table B-3, the mechanism used in Table B-2 was used to calculate its reactivity. The reactivity listings on Table B-1 in Appendix B indicate those cases where adjustable products mechanisms were used.
Updated Aromatics Mechanisms
Although significant progress has been made in recent years concerning the atmospheric reactions of aromatics and their reactive products (e.g., Calvert et al, 2002, Barnes, 2006, and references therein), the aromatics continue to represent the major class of compounds for which insufficient information exists for deriving predictive and explicit mechanisms based on mechanistic considerations alone. The most important uncertainties concern the reactions of the aromatic ring fragmentation products, which include highly photoreactive compounds that make significant contributions to overall aromatic reactivity. SAPRC-99 mechanism incorporates a highly simplified and parameterized set of model species to represent these products, whose yields and photolysis rates are adjusted to fit chamber data. In addition, the mechanisms for the reactions of ring-retaining products such as phenols, cresols, and nitrophenols are also highly uncertain, and SAPRC-99 uses simplified and parameterized representations for these reactions as well. Other mechanisms currently used in airshed models (Gery et al, 1988, Yarwood et al, 2005; Stockwell et al, 1990, 1997) also incorporate simplified and parameterized representations of aromatics, and although the MCM incorporates an attempt at a more explicit aromatics representation, its performance in simulating available environmental chamber data is so poor that it is not suitable for airshed modeling (Bloss et al, 2005).
The uncertain parameters in the SAPRC-99 aromatics mechanisms were optimized to give reasonably good simulations of results of aromatics - NOx and environmental chamber experiments, and the mechanisms so derived also gave generally satisfactory results of incremental reactivity experiments with aromatics (Carter, 2000a), including some experiments carried out after the SAPRC-99 mechanism was developed at lower NOx concentrations in the new UCR EPA chamber (Carter, 2004). However, Carter (2004) also found that the SAPRC-99 mechanism consistently underpredicts the extent of ozone enhancement caused by adding CO to aromatics - NOx irradiations, Carter and Malkina (2002) found that the mechanism overpredicts "direct reactivity" in experiments designed to be sensitive to the direct effects of the primary reactions of aromatics on NO to NO2 conversions. This suggests that there may be compensating errors in the SAPRC-99 aromatics mechanisms. In addition Carter (2004) found that SAPRC-99 (and also CB4) tends to underpredict ozone formation in at low ROG/NOx ratios and low NOx concentrations in Surrogate - NOx experiments when the surrogate contains aromatics, but this is not seen in surrogates where the aromatics have been removed (unpublished results from this laboratory  -  also see "Mechanism Evaluation" section, below). Because of these problems, updating and improving the aromatics mechanisms was an important priority for this project.
The approach that was initially attempted to derive updated aromatics mechanisms was to derive fully (or at least nearly) explicit mechanisms for representative aromatic hydrocarbons and their oxidation products, and use these as a basis for deriving more condensed mechanisms that can be used for representing aromatics in general. However, this turned out to be unworkable in practice, at least within the time and resource constraints for this project. Developing fully explicit mechanisms that are entirely consistent with available data as given by Calvert et al (2002) is not possible because the low and inconsistent measured product yields can not account for complete reaction pathways, and mechanisms based on some of the data could not be made to give predictions that were consistent with the available environmental chamber reactivity data. This was found to be the case both for the initial aromatic ring-opening reactions, where yields of the expected unsaturated dicarbonyl ring-opening products were found to be low, and the photolysis reactions of the unsaturated dicarbonyl products, where assuming relatively high yields of non-radical products gave mechanisms that significantly underpredicted aromatic reactivities in chamber experiments. Also, mechanism incorporating new product data for phenols and cresols (Barnes, 2006; Berndt and Boge, 2003, Olariu et al, 2002) gave predictions that were inconsistent with the limited chamber data and therefore could not be used.
The approach that was therefore adopted for this version of the mechanism is to use a simplified but consistent and chemically reasonable model for the initial ring fragmentation reactions that give improved simulations of the direct reactivity data of Carter and Malkina (2002), and derive new mechanisms for the model species used to represent the unknown or uncertain aromatic ring fragmentation products based on estimated mechanisms for representatives of the unsaturated 1,4- and 1,6-dicarbonyls expected to be formed. The only adjustable parameters employed concerned the ratio of photoreactive to un-photoreactive model species representing the unsaturated 1,4-dicarbonyls, which were adjusted to fit the data for the aromatics - NOx experiments for the individual compounds. This yielded mechanisms that performed as well as SAPRC-99 in simulating aromatics - NOx and aromatics incremental reactivity experiments, and that performed somewhat better in simulating the direct reactivity results of Carter and Malkina (2002). However, the problem of underpredicting the effects of adding CO to aromatics - NOx irradiations and underpredicting O3 formation in surrogate - NOx experiments at low NOx and low ROG/NOx conditions remained. Therefore, although the updated mechanisms are probably more chemically reasonable and consistent with somewhat more of the available data than was the case previously, not all of the problems could be resolved.
Reactions of Alkylbenzenes with OH
The major initial atmospheric reaction of aromatics is with OH radicals. Table 9 gives the measured or estimated product yields for the known products for the OH radical reaction that are used as the bases for the aromatics mechanisms developed in this work, with footnotes indicating the sources of these yields. The initial reactions can either be abstraction of H from side groups off the ring, or OH addition to the aromatic ring. The fractions reacted at the various positions, and their subsequent mechanisms, are derived as described below.
Reactions off the Aromatic Ring. The reactions following abstraction from the groups off the ring are exactly analogous to those in the alkane photo-oxidation system, and are assumed to proceed as shown below for toluene

Table 9.	Measured or estimated yields of known products for benzene and methylbenzene reactions that were incorporated in the SAPRC-07 aromatics mechanisms
Compound
                             Yields and Notes [a]

                               Aromatic Aldehyde
                               Phenol or Cresol
                                    Glyoxal
                                Methyl Glyoxal
                                   Biacetyl
                              Nitrate yield [b]
Benzene

                                                                            57%
 [1]
                                                                            29%
 [2]

                                    7.7% [3]
Toluene
                                                                           6.5%
 [4]
                                                                            18%
 [5]
                                                                            24%
 [6]
                                                                            15%
 [7]

                                   10.6% [8]
Xylenes

                                   13.6% [3]
  o-
                                                                           4.7%
 [9]
                                                                            16%
 [10]
                                                                           8.4%
 [11]
                                                                            24%
 [11]
                                                                            19%
 [12]

  m-
                                                                           4.1%
 [13]
                                                                            16%
 [14]
                                                                            10%
 [15]
                                                                            38%
 [16]

  p-
                                                                           8.8%
 [17]
                                                                            16%
 [18]
                                                                            29%
 [19]
                                                                            11%
 [20]

Trimethylbenzenes
                                   16.2% [3]
  1,2,3-
                                                                           4.5%
 [21]
                                                                           3.1%
 [22]
                                                                           7.2%
 [23]
                                                                            18%
 [23]
                                                                            45%
 [24]

  1,2,4-
                                                                           3.6%
 [25]
                                                                           2.2%
 [26]
                                                                           7.4%
 [27]
                                                                            41%
 [28]
                                                                            11%
 [29]

  1,3,5-
                                                                           3.0%
 [30]
                                                                           4.0%
 [31]

                                                                            64%
 [34]

[a]	Footnotes giving the sources of the assignments are as follows. Italic indicates estimated values.
   1	Average of values of Berndt and Boge (2006) and Volkamer et al (2002).
   2	The yields of Berndt and Boge (2006), which are reasonably consistent with previous studies (Calvert et al, 2002), are used.
   3	Estimated based on carbon number and the nitrate yield derived for toluene as discussed in the text and shown on Figure 3.
   4	Yields tabulated by Calvert et al (2002) range from 5-12%. Value used is average of data from studies published since 1989, which tend to be reasonably consistent as a group. This is lower than the 8.5% used in SAPRC-99
   5	Total of yields for individual isomers from Calvert et al (2002)
   6	Data tabulated by Calvert et al (2002) indicate a range of yields, from 4-24%, with Volkamer and co-workers (Volkamer, personal communication) reporting 35%. We tentatively use the value of Smith et al (1998), which is the highest tabulated by Calvert et al (2002) but still lower than the Volkamer value. The methyl glyoxal yield reported by Smith et al (1998) are in good agreement with the value used, which tends to support use of their data. Also, the mechanism estimation system estimates higher yields of glyoxal than methyl glyoxal will be formed from toluene.
   7	Data tabulated by Calvert et al (2002) indicate a range of yields, but a number of recent studies are reasonably consistent in indicating yields of ~15%. The average of the data from the studies with the higher yields is used.
   8	Derived from the ratio of the benzyl nitrate to the benzaldehyde yield as discussed in the text. Assumed that the same ratio is applicable to the other peroxy radicals in the toluene photo-oxidation system.
   9	Most of the recent o-tolualdehyde yield data tabulated by Calvert et al (2002) are around 5%, and the value used is the average of those studies. A few studies indicate higher yields, but these are not used in computing the average.
   10	The total dimethylphenol yields of Atkinson et al (1991) are used because they were conducted at the lowest NOx levels.
   11	The data of Bandow and Washida (1985a) and Tuazon et al (1986) are in good agreement, and the average is used. The low value of Shepson et al (1984) is not used.
   12	The low NOx limit value of Atkinson and Aschmann (1994) is used. It is reasonably consistent with data from other studies at lower NOx levels tabulated by Calvert et al (2002).
   13	Most of the recent o-tolualdehyde yield data tabulated by Calvert et al (2002) are around 5%, and the value used is the average of those studies. A few studies indicate higher yields, but these are not used in computing the average.
   14	Average of the total dimethylphenol data reported by Atkinson et al (1991) and Smith et al (1999). However, these two studies are not in particularly good agreement on yields for the individual isomers. The total is consistent with the value used for o-xylene.
   15	Average of the data tabulated by Calvert et al (2002). There is not particularly agreement among the studies, but there are no obvious outliers.
   16	Average of the data tabulated by Calvert et al (2002), excluding the low value of Tuazon and co-workers which is superceded by a more recent measurement from the same group.
   17	Average of the various measurements tabulated by Calvert et al (2002). There is not particularly good agreement, but no obvious outliers to exclude from the average.
   18	Average of 2,5-dimethylphenol yields of Atkinson et al (1991) and Smith et al (1999). Yields reported by Becker et al (1997) are lower and not used.
   19	Average of yields reported by Bandow and Washida (1985a), Tuazon et al (1986) and Smith et al (1999).
   20	The data tabulated by Calvert et al (2002) show some variation, but three studies give good agreement and indicate yields of about 10%. The value used is the average from those studies.
   21	No data available. Estimated from the average rate constant per methyl group for the other methylbenzenes and the total OH rate constant.
   22	No data available. The average of the assigned phenolic product yield for the other trimethylbenzenes is used.
   23	The yields determined by Bandow and Washida (1985b) are used for consistency with assignment for biacetyl.
   24	Average of Bandow and Washida (1985b) and Atkinson and Aschmann (1994). The slightly lower yield of Tuazon et al (1986) not used but may not be outside of range of uncertainty.
   25	Sum of yields of 2,4-, 2,5-, and 3,4-dimethylbenzaldehyde from Smith et al (1999)
   26	Sum of 2,4,5-, 2,3,5-, and 2,3,6-trimethylphenol from Smith et al (1999)
   27	Average of glyoxal yields of Bandow and Washida (1985b) and Smith et al (1999) and 3-methyl-3-hexene-2,5-dione of Smith et al (1999). The lower value of Tuazon et al (1986) is not used. Yields of glyoxal and 3-methyl-3-hexene-2,5-dione are assumed to be equal.
   28	Average of Bandow and Washida (1985b) and Smith et al (1999). The value of Tuazon et al (1986) is not used because of the higher NOx levels and for consistency with other assignments.
   29	Average of Bandow and Washida (1985b) and Atkinson and Aschmann (1994). The lower yield of Tuazon et al (1986) is not used but may not be outside of range of uncertainty.
   30	The 3,5-dimethylbenzaldehyde yield from Smith et al (1999) is used.
   31	The 2,4,6-trimethylphenol yield from Smith et al (1999) is used.
   34	The yield from Bandow and Washida (1985b) is used. The data of Smith et al (1999) appear to be high and the data from Tuazon et al (1986) was not used for consistency with assignments for the other trimethylbenzenes.
[b]	Nitrate yield for each peroxy reaction. Total nitrate yield is less because not all pathways involve peroxy reactions.

	BzCH3 + OH  H2O + BzCH2·
	BzCH2· + O2  BzCH2OO·
	BzCH2OO· + NO  NO2 + BzCH2O·	(Fraction = n7)
	                         BzCH2ONO2 	(Fraction = 1-n7)
	BzCH2O· + O2  BzCHO + HO2
or ethylbenzene
	BzCH2CH3 + OH  H2O + BzCH(·)CH3
	BzCH(·)CH3 + O2  BzCH(OO·)CH3
	BzCH(OO·)CH3 + NO  NO2 + BzCH(O·)CH3	(Fraction = n8)
	                                 BzCH(ONO2)CH3	(Fraction = 1-n8)
	BzCH(O·)CH3 + O2  BzC(O)CH3 + HO2
Here, "Bz" refers to the aromatic ring, and n7 and n8 refer to the nitrate yields assumed for aromatics with 7 or 8 carbons, which are derived as discussed below. In terms of SAPRC-07 model species, these overall processes are represented as
	TOLUENE + OH  (1-n7) {RO2C + xBALD} + (n7) zRNO3 + yR6OOH
	C2-BENZ + OH  (1-n8) {RO2C + PROD2} + (n8) Zrno3 + yR6OOH
In general, the model species "BALD" is used to represent aromatic aldehydes such as benzaldehyde and tolualdehyde, PROD2 is used to represent aromatic ketones such as methyl phenyl ketone and methyl benzyl ketone, RNO3 is used to represent the organic nitrates formed from peroxy + NO reactions in the aromatic and other systems, and R6OOH is used to represent the hydroperoxides formed in the reactions off the aromatic ring.
Reactions on the Aromatic Ring. The major uncertainty concerning the reactions of aromatics with OH radicals concern the subsequent reactions after OH adds to the aromatic ring. Information and data concerning what is known about these processes is discussed by Calvert et al (2002), and that discussion is not duplicated here. The general model we assume for these reactions, which is probably an oversimplification of what really happens, is shown on Figure 2, using benzene as an example. The processes are assumed to be analogous for the substituted benzenes such as toluene, xylenes, etc., except that the number of reactions and products are much greater because of various possible positions of the substituents on the intermediates and the products.
The products that are known to be formed and whose yields have been quantified include the phenols and cresols, and the -dicarbonyls (phenol and glyoxal in the case or benzene). The formation of phenols is generally assumed to occur by OH abstraction of the initial OH - aromatic adduct, as shown on Figure 2 as "A". The observed yields of these phenolic products are then used to derive the branching ratio (a)/(1-a) as shown on the figure. The co-product is HO2, formed without the intermediacy of any peroxy radicals and without any NO to NO2 conversions.
The processes forming the -dicarbonyls are more uncertain, but the most chemically reasonably process involves O2 addition to form a peroxy radical that then internally adds to the double bonds to form an allylic-stabilized bicyclic radical, which subsequently reacts to form the -dicarbonyl(s) as shown on Figure 2 as the processes leading to product set "B". The predicted co-products are the monounsaturated 1,4-dicarbonyls, such as the 2-butene 1,4-dial predicted for the benzene system. These products are indeed observed (Calvert et al, 2002, and references therein), though the total yields are generally much less than the total -dicarbonyl yields. This could be due to other, unknown, processes leading to the -dicarbonyls, or with problems with quantifying the unsaturated 1,4-dicarbonyls, which are highly reactive. In any case, because of lack of available information and chemically reasonable

Figure 2.	Reactions of the OH - aromatic ring adduct that are assumed in the current mechanism. The example shown is for benzene, but analogous reactions are assumed for the alkylbenzenes.

alternatives to explain the products that are observed, we assume that the processes leading to "B" as indicated on Figure 2 are the only processes in the forming the -dicarbonyls in the primary OH + aromatic reactions, and that the unsaturated 1,4-dicarbonyls are the corresponding co-products. The yields of the model species used to represent the unsaturated 1,4-dicarbonyls (discussed below) are then set as the total yields of the measured -dicarbonyl products. The total -dicarbonyl and phenolic product yields are used to determine the branching ratio indicated as (b)/(1-b) on Figure 2.
Note that this process that are assumed to account for the -dicarbonyl formation involve the intermediacy of a peroxy radical that converts NO to NO2 in the processes of forming these products, but that can also react with NO to form an alkyl nitrate ("C" on Figure 2), react with HO2 to form a highly oxygenated bicyclic hydroperoxide ("D" on Figure 2), or react with other peroxy radicals to form other products (not shown on the figure). The hydroperoxides formed in the HO2 reaction (e.g., "D") are represented by a separate model species RAOOH because they are expected to have higher PM formation potential than other hydroperoxides, though they could be lumped with R6OOH if SOA predictions are not important. The general procedures used for representing products formed from peroxy reactions in this mechanism have been discussed previously, and these procedures are employed for these -dicarbonyl and mono-unsaturated 1,4-dicarbonyl products.
The sum of the measured yields of the phenolic and -dicarbonyl products from the OH addition reactions, the aromatic aldehydes from the abstraction reactions off the aromatic ring, and estimated nitrate yields from the peroxy + NO reactions (discussed below) is not sufficient to account for all the processes involved in the OH reactions of toluene, xylene, and trimethylbenzenes (Calvert et al, 2002), so additional processes must also occur. The results of aromatic - NOx environmental chamber experiments at various NOx levels cannot be simulated by mechanisms that assume that yields of the photoreactive aromatic products such as -dicarbonyls or the monounsaturated 1,4-dicarbonyls are affected by total NOx levels, and most of the available product data for the -dicarbonyls from methylbenzenes do not indicate a NOx -dependence on -dicarbonyl yields at the NOx levels relevant to the atmosphere and the chamber experiments used for mechanism evaluation. This rules out mechanisms involving competitions between unimolecular reactions of peroxy radicals and reactions of peroxy radicals with NO, such as between (b) and (x) shown on Figure 2.
An additional consideration concerning potential competing processes is the fact that the direct reactivity experiments of Carter and Malkina (2002) indicate that the SAPRC-99 mechanism has too many NO to NO2 conversions in the reactions of aromatics with OH. We were unable to come up with chemically reasonable mechanisms for -dicarbonyl, phenol or cresol, or aromatic aldehyde formation that involved fewer NO to NO2 conversions, which leaves only the remaining process(es) forming the remaining products. Because of this, and also to avoid introducing competitions between unimolecular and peroxy + NO reactions, we assume that the additional process involves a 6-member ring H-shift of the aromatic - OH - O2 adduct, giving rise to formation of a di-unsaturated 1,6-dicarbonyl and OH radicals, shown as process (1-b) and "E" on Figure 2. This is a competition between two unimolecular reactions and thus the photoreactive product yields are predicted to be independent of NOx, and the direct formation of OH radicals in this set of reactions involves two fewer NO to NO2 conversions than assumed for these unknown processes in the SAPRC-99 mechanism. Therefore, this is used in the current mechanism to represent all the processes that are not otherwise accounted for by the observed formation of aromatic aldehyde, phenolic, and -dicarbonyl products and the estimated formation of organic nitrates. The model species used to represent these di-unsaturated 1,6-dicarbonyls is discussed below.
Estimation of Nitrate Formation Reactions. The mechanisms involving OH abstractions off the aromatic ring, and also the mechanism involving the formation of the -dicarbonyls and the monounsaturated 1,4-dicarbonyls all involve the intermediacy of peroxy radicals that can react with NO to form the corresponding nitrate. The only data available concerning nitrate formation from peroxy + NO reactions in alkylbenzene systems concerns the observed formation of benzyl nitrate in the toluene system, for which the average of the yields tabulated by Calvert et al (2002) is ~0.8%. This is attributed to the reaction of benzyl peroxy radicals with NO (shown above), for which the competing process is formation of the alkoxy radical that reacts with O2 to form benzaldehyde. Based on this and the benzaldehyde yield used in our toluene mechanism (discussed below), we derive an overall nitrate yield for the benzyl peroxy radical to be 10.6%. This can be compared with the 13.5% nitrate yield predicted by the mechanism generation system for 7 carbons for standard secondary alkylperoxy radicals formed in the alkane photooxidation system (Carter, 2000a). Based on this, we assume that nitrate yields for all peroxy radicals in aromatic systems, whether resulting from abstractions by OH off the ring or formation of -dicarbonyls following OH + aromatic ring additions, are 78.5% those estimated for secondary alkyl peroxy radicals. Figure 3 shows the dependence of the nitrate yields on carbon number that is used when deriving aromatic mechanisms for SAPRC-07. The nitrate yields are assumed to range from ~8% for benzene (n6) to ~21% for aromatics with carbon numbers greater than ~13 (n13+).
Assignments of Branching Ratios and Product Yields. Table 9 gives the measured or estimated yields from the known products from benzene, toluene, the xylenes, and the trimethylbenzenes that are used as the basis for the aromatic mechanisms derived for this work. Footnotes to the table

Figure 3.	Nitrate yield factors used for estimating aromatic oxidation mechanisms

indicate the basis for the assignments or estimates used. These were used, in conjunction with the nitrate yield estimates derived above, as the basis for deriving the branching ratios for all the reactions in the mechanisms for these aromatics. Note that, as discussed above, the total yields of the model species used for the mono-unsaturated 1,4-dicarbonyls were set to the sum of the glyoxal, methyl glyoxal, and biacetyl yields shown on Table 9, and the yields of the model species representing the di-unsaturated 1,6- dicarbonyls was set at 1 - the sum of the aromatic aldehyde, phenol or cresol, -dicarbonyl, and estimated nitrate yields.
For the other alkylbenzenes or model species used to represent groups of alkylbenzenes with similar structures and carbon numbers, the mechanisms were derived based on estimates for reactions at various positions on the molecule and other estimates, as follows. The total rate constants for abstraction reactions off the aromatic ring were derived using structure-reactivity estimates, using the OH abstraction rate parameters given on Table 3, above. These total non-ring rate constants are given for the various aromatic model species on Table 10, along with the substituents assumed for the various model species used to represent groups of compounds. The total rates for additions to the aromatic ring were estimated by assuming they were the same as to those derived for the methylbenzene with the most similar structure, as indicated on Table 10. The ring addition reactions for those compounds were derived from the total rate constant and the estimated extent of abstraction off the aromatic ring derived from the aromatic aldehyde and estimated off-ring abstraction rate constant derived from the aromatic aldehyde and estimated nitrate yield as indicated on Table 9. The fractions of ring additions vs. off-ring abstractions were derived from the ratio of the estimated rate constants for the two processes, and are shown on Table 10. Note that for the compounds listed on Table 9 the mechanisms used the values derived based on the measured or estimated product yields shown on that table, not based on the estimated rate constants shown on Table 10. Table 10 shows the differences between the off-ring abstraction fractions derived using these two methods for these compounds.

Table 10.	Rate constants and assumptions used to estimate fractions of reaction of on and off the aromatic ring for the reactions of OH with aromatic compounds or groups of compounds for which mechanistic parameters have been derived.
Compound or detailed model species group
                               Rate Constant [a]
Ring k Based on
                                 Non-Ring Rxn.
Substituents [c]

                                     Total
                                   Non-Ring
                                     Ring

                                   Expt [b]
                                     Est'd

Toluene
                                   5.58e-12
                                   4.92e-13
                                   5.18e-12
Toluene
                                      7%
                                      9%
methyl
o-Xylene
                                   1.36e-11
                                   9.85e-13
                                   1.29e-11
o-Xyl.
                                      5%
                                      7%
dimethyl
m-Xylene
                                   2.31e-11
                                   9.85e-13
                                   2.20e-11
m-Xyl.
                                      5%
                                      4%
dimethyl
p-Xylene
                                   1.43e-11
                                   9.85e-13
                                   1.28e-11
p-Xyl.
                                      10%
                                      7%
dimethyl
1,2,3-Trimethyl benzene
                                   3.27e-11
                                   1.48e-12
                                   3.11e-11
124-TMB

                                      5%
trimethyl
1,2,4-Trimethyl benzene
                                   3.25e-11
                                   1.48e-12
                                   3.11e-11
124-TMB
                                      4%
                                      5%
trimethyl
1,3,5-Trimethyl benzene
                                   5.67e-11
                                   1.48e-12
                                   5.47e-11
135-TMB
                                      4%
                                      3%
trimethyl
Ethylbenzene
                                   7.00e-12
                                   2.05e-12
                                   5.18e-12
Toluene

                                      28%
ethyl
n-Propyl benzene
                                   5.80e-12
                                   3.64e-12
                                   5.18e-12
Toluene

                                      41%
1-propyl
o-Ethyl toluene
                                   1.19e-11
                                   2.54e-12
                                   1.29e-11
o-Xyl.

                                      16%
methyl ethyl
m-Ethyl toluene
                                   1.86e-11
                                   2.54e-12
                                   2.20e-11
m-Xyl.

                                      10%
methyl ethyl
p-Ethyl toluene
                                   1.18e-11
                                   2.54e-12
                                   1.28e-11
p-Xyl.

                                      17%
methyl ethyl
Isopropyl benzene
                                   6.30e-12
                                   1.68e-12
                                   5.18e-12
Toluene

                                      24%
2-propyl
1-Methyl-4-isopropylbenzene
                                   1.45e-11
                                   2.17e-12
                                   1.28e-11
p-Xyl.

                                      14%
methyl 2-propyl
t-Butyl benzene
                                   4.50e-12
                                   5.13e-13
                                   5.18e-12
Toluene

                                      9%
t-butyl
C10 monosubstituted benzenes
                                   9.58e-12
                                   4.40e-12
                                   5.18e-12
Toluene

                                      46%
0.33 n-butyl + 0.33 2-methyl-1-propyl + 0.34 1-methyl-1-propyl
C11 monosubstituted benzenes
                                   1.10e-11
                                   5.82e-12
                                   5.18e-12
Toluene

                                      53%
0.33 n-pentyl + 0.33 3-methyl-1-butyl + 0.34 1-methyl-1-butyl
C12 monosubstituted benzenes
                                   1.24e-11
                                   7.25e-12
                                   5.18e-12
Toluene

                                      58%
As above, but with -CH2- added
C13 monosubstituted benzenes
                                   1.38e-11
                                   8.67e-12
                                   5.18e-12
Toluene

                                      63%

C14 monosubstituted benzenes
                                   1.53e-11
                                   1.01e-11
                                   5.18e-12
Toluene

                                      66%

C15 monosubstituted benzenes
                                   1.67e-11
                                   1.15e-11
                                   5.18e-12
Toluene

                                      69%

C16 monosubstituted benzenes
                                   1.81e-11
                                   1.29e-11
                                   5.18e-12
Toluene

                                      71%

o-C10 disubstituted benzenes
                                   1.64e-11
                                   3.51e-12
                                   1.29e-11
o-Xyl.

                                      21%
0.25 methyl n-propyl + 0.25 methyl isopropyl + 0.5 diethyl
m-C10 disubstituted benzenes
                                   2.55e-11

                                   2.20e-11
m-Xyl.

                                      14%

p-C10 disubstituted benzenes
                                   1.64e-11

                                   1.28e-11
p-Xyl.

                                      21%

o-C11 disubstituted benzenes
                                   1.82e-11
                                   5.40e-12
                                   1.29e-11
o-Xyl.

                                      30%
0.34 methyl (2-metyl-1-propyl) + 0.33 ethyl isopropyl + 0.33 ethyl n-propyl
m-C11 disubstituted benzenes
                                   2.74e-11

                                   2.20e-11
m-Xyl.

                                      20%

p-C11 disubstituted benzenes
                                   1.82e-11

                                   1.28e-11
p-Xyl.

                                      30%

o-C12 disubstituted benzenes
                                   1.90e-11
                                   6.14e-12
                                   1.29e-11
o-Xyl.

                                      32%
0.25 methyl (3-methyl-1-butyl) + 0.25 ethyl (1-methyl-1-propyl) + 0.25 propyl isopropyl + 0.25 ethyl n-butyl
m-C12 disubstituted benzenes
                                   2.82e-11

                                   2.20e-11
m-Xyl.

                                      22%

p-C12 disubstituted benzenes
                                   1.90e-11

                                   1.28e-11
p-Xyl.

                                      32%

o-C13 disubstituted benzenes
                                   2.05e-11
                                   7.63e-12
                                   1.29e-11
o-Xyl.

                                      37%
0.25 ethyl (3-methyl-1-butyl) + 0.25 propyl (1-methyl-1-propyl) + 0.25 (2-methyl-1-propyl) isopropyl + 0.25 ethyl n-pentyl
m-C13 disubstituted benzenes
                                   2.96e-11

                                   2.20e-11
m-Xyl.

                                      26%

p-C13 disubstituted benzenes
                                   2.05e-11

                                   1.28e-11
p-Xyl.

                                      37%

o-C14 disubstituted benzenes
                                   2.19e-11
                                   9.05e-12
                                   1.29e-11
o-Xyl.

                                      41%
As above, but with -CH2- added
m-C14 disubstituted benzenes
                                   3.11e-11

                                   2.20e-11
m-Xyl.

                                      29%

p-C14 disubstituted benzenes
                                   2.19e-11

                                   1.28e-11
p-Xyl.

                                      41%

o-C15 disubstituted benzenes
                                   2.33e-11
                                   1.05e-11
                                   1.29e-11
o-Xyl.

                                      45%

m-C15 disubstituted benzenes
                                   3.25e-11

                                   2.20e-11
m-Xyl.

                                      32%

p-C15 disubstituted benzenes
                                   2.33e-11

                                   1.28e-11
p-Xyl.

                                      45%

o-C16 disubstituted benzenes
                                   2.48e-11
                                   1.19e-11
                                   1.29e-11
o-Xyl.

                                      48%

m-C16 disubstituted benzenes
                                   3.39e-11

                                   2.20e-11
m-Xyl.

                                      35%

p-C16 disubstituted benzenes
                                   2.47e-11

                                   1.28e-11
p-Xyl.

                                      48%

1,2,3-C10 trisubstituted benzenes
                                   3.41e-11
                                   3.03e-12
                                   3.11e-11
124-TMB

                                      9%
Dimethyl ethyl
1,2,4-C10 trisubstituted benzenes
                                   3.41e-11

                                   3.11e-11
124-TMB

                                      9%

1,3,5-C10 trisubstituted benzenes
                                   5.77e-11

                                   5.47e-11
135-TMB

                                      5%

1,2,3-C11 trisubstituted benzenes
                                   3.57e-11
                                   4.60e-12
                                   3.11e-11
124-TMB

                                      13%
0.5 dimethyl propyl + 0.5 methyl diethyl
1,2,4-C11 trisubstituted benzenes
                                   3.57e-11

                                   3.11e-11
124-TMB

                                      13%

1,3,5-C11 trisubstituted benzenes
                                   5.93e-11

                                   5.47e-11
135-TMB

                                      8%

1,2,3-C12 trisubstituted benzenes
                                   3.66e-11
                                   5.47e-12
                                   3.11e-11
124-TMB

                                      15%
0.34 dimethyl 2-methyl-1-propyl + 0.33 methyl ethyl isopropyl + 0.33 methyl ethyl n-propyl
1,2,4-C12 trisubstituted benzenes
                                   3.66e-11

                                   3.11e-11
124-TMB

                                      15%

1,3,5-C12 trisubstituted benzenes
                                   6.01e-11

                                   5.47e-11
135-TMB

                                      9%

1,2,3-C13 trisubstituted benzenes
                                   3.81e-11
                                   7.00e-12
                                   3.11e-11
124-TMB

                                      18%
0.34 methyl ethyl 2-methyl-1-propyl + 0.33 methyl ethyl n-butyl + 0.33 methyl propyl isobutyl
1,2,4-C13 trisubstituted benzenes
                                   3.81e-11

                                   3.11e-11
124-TMB

                                      18%

1,3,5-C13 trisubstituted benzenes
                                   6.17e-11

                                   5.47e-11
135-TMB

                                      11%

1,2,3-C14 trisubstituted benzenes
                                   3.93e-11
                                   8.16e-12
                                   3.11e-11
124-TMB

                                      21%
0.25 methyl ethyl 3-methyl-1-butyl + 0.25 methyl n-propyl n-butyl + 0.25 ethyl isopropyl n-propyl
1,2,4-C14 trisubstituted benzenes
                                   3.93e-11

                                   3.11e-11
124-TMB

                                      21%

1,3,5-C14 trisubstituted benzenes
                                   6.28e-11

                                   5.47e-11
135-TMB

                                      13%

1,2,3-C15 trisubstituted benzenes
                                   4.04e-11
                                   9.31e-12
                                   3.11e-11
124-TMB

                                      23%
0.25 methyl ethyl 4-methyl-1-pentyl + 0.25 ethyl n-propyl 1-methyl-1-propyl + methyl propyl 1-methyl-1-butyl + ethyl isopropyl 2-methyl-1-propyl
1,2,4-C15 trisubstituted benzenes
                                   4.04e-11

                                   3.11e-11
124-TMB

                                      23%

1,3,5-C15 trisubstituted benzenes
                                   6.40e-11

                                   5.47e-11
135-TMB

                                      15%

1,2,3-C16 trisubstituted benzenes
                                   4.17e-11
                                   1.06e-11
                                   3.11e-11
124-TMB

                                      25%
0.25 ethyl propyl 3-methyl-1-butyl + 0.25 ethyl propyl 1-methyl-1-butyl + methyl propyl 1,3-dimethyl-1-butyl + 0.25 propyl isopropyl 2-methyl-1-propyl
1,2,4-C16 trisubstituted benzenes
                                   4.17e-11

                                   3.11e-11
124-TMB

                                      25%

1,3,5-C16 trisubstituted benzenes
                                   6.52e-11

                                   5.47e-11
135-TMB

                                      16%

[a]	Rate constant in cm[3] molec-1 s-1. "Total" is total rate constant is rate constant used in the mechanism, either experimental or estimated. "Non-Ring" is total rate constant for abstraction at various positions off the aromatic ring, estimated using the group additivity factors shown in Table 4. "Ring" is estimated rate constant for addition to the aromatic ring, derived by assuming the same aromatic ring addition rate constant as the indicated compound.
[b]	Derived from the measured yields of the expected non-ring abstraction product, corrected for estimated nitrate formation, relative to the total rate constant. These can be compared with the estimated ratios of non-ring reaction, derived from the estimated ring and non-ring rate constants.
[c]	Substituents assumed when estimating rate constants for abstraction reactions off the aromatic ring.

The products formed from the off-ring abstraction reactions are represented by the organic nitrate model species RNO3 and either the aromatic aldehyde model species BALD or the reactive ketone model species PROD2, depending on the substituent. The RNO3 yield was derived from the carbon number as discussed above. If the substituent is a methyl group the reaction was expected to form an aromatic aldehyde and the product was represented by the benzaldehyde model species BALD. Otherwise, the products are assumed to be primarily ketones and are represented by PROD2.
The products used to represent those formed from the reactions following the addition to the aromatic ring were derived based on those derived for the corresponding reactions of ethylbenzene, a xylene, or a trimethylbenzene, depending on the structure of the compound or group of compounds being represented. The ring-reaction products for ethylbenzene were used for the ring reactions products for the monoalkylbenzenes, the ring-reaction products for o-xylene were used for all 1,2-disubstituted benzenes, those for 1,2,3-trimethylbenzene were used for all the 1,2,3-trisubstituted benzenes, and similarly for the other isomers. The ring-reaction products for ethylbenzene were derived from those derived for toluene, taking into account estimated differences in off-ring reactions and nitrate yields, and with the ratio of the photoreactive and non-photoreactive model species used for the mono-unsaturated 1,4-dicarbonyls adjusted to fit the chamber data for ethylbenzene (see discussion of these products, below). The ring reaction products for the xylene and trimethylbenzene isomers were derived from the product data for those compounds as shown on Table 9, ratio of the photoreactive and non-photoreactive model species used for the monounsaturated 1,4-dicarbonyls adjusted to fit the chamber data for those compounds. These are summarized on Table 11.
The mechanistic assignments shown on Table 9, Table 10, Table 11 and Figure 3 were used to derive overall mechanisms for the individual aromatics or groups or aromatics as listed on Table 10. The resulting overall rate constants are given on Table B-4 and the mechanisms are given in Table B-2 in Appendix B. Footnotes to Table B-4 indicate the sources of the rate constants that were used.
Representation of Unsaturated Dicarbonyl Products
As discussed above, the co-products assumed to be formed with the -dicarbonyls in the ring opening products are the monounsaturated 1,4-dicarbonyls, and the ring opening reactions not involving these products are assumed to be di-unsaturated 1,6-dicarbonyls. For the benzene system, these consist of 2-butene-1,4-dial and 2,4-hexadienedial as shown on Figure 2, while for the methylbenzenes various methyl substituted isomers can be formed, with relative yields depending on the position of OH addition to the double bond, and also where the O2 adds to the OH - aromatic adduct. Estimated distributions of these products for the various methylbenzene isomers are shown on Table 12. These estimates were derived based on estimated fractions of reaction of OH radicals at various positions of the molecules and estimated branching ratios for O2 addition at the various positions of the OH + aromatic adducts, adjusted to be consistent with the observed yields of the -dicarbonyl co-products.
This product distribution can serve as the basis for deriving explicit or semi-explicit mechanisms for the OH reactions of these aromatics. However, for airshed and reactivity simulations a more generalized mechanism, with a more limited number of model species, is needed. The model species used to represent these products in this version of the mechanism is indicated on Table 12. The reasoning behind this representation, and the derivations of their mechanisms, are discussed below. Note that this is the same number of model species used to represent uncharacterized ring fragmentation products in SAPRC-99, but in this case the mechanisms are derived based on estimations for actual compounds.

Table 11.	Model species used to represent non-nitrate products from the reactions of OH radicals at aromatic rings for various types of ring structures. Yields are normalized to OH ring addition only, excluding nitrate formation.
Ring structure
                                     A [a]
                                     B [b]
                                    B, C, D
                                       E

                                  CRES + HO2
                                     xGLY
                                     xMGLY
                                     xBACL
                                  xAFG1+2 [c]
                                  AFG1/2 [d]
                                     xAFG3
                                      [e]
                               Peroxy  Total [f]
                                 AFG3 + OH [e]
Benzene
                                      12%
                                      31%

                                      31%
                                     0.11

                                      31%
                                      31%
Toluene
                                      34%
                                      29%
                                      18%

                                      47%
                                     1.00

                                      29%
                                      29%
Ethylbenzene [g]
                              (based on toluene)
                                     0.39
                               (same as toluene)
o-Xylene
                                      21%
                                      10%
                                      29%
                                      23%
                                      62%
                                     1.00

                                      10%
                                      10%
m-Xylene
                                      25%
                                      12%
                                      46%

                                      58%
                                     2.33

                                      12%
                                      12%
p-Xylene
                                      31%
                                      37%
                                      14%

                                      14%
                                     0.67
                                      37%
                                      37%
                                      37%
1,2,3-Trimethylbenzene
                                      9%
                                      9%
                                      23%
                                      56%
                                      88%
                                     1.00

                                      9%
                                      9%
1,2,4-Trimethylbenzene
                                      24%
                                      9%
                                      50%
                                      14%
                                      55%
                                     0.25
                                      19%
                                      9%
                                      9%
1,3,5-Trimethylbenzene
                                      17%

                                      79%

                                      79%
                                     1.00

[a]	Pathway on Figure 2 that these model species are being used to represent.
[b]	Yields depend on nitrate yield in peroxy + NO reactions, and are shown for nitrate yields of zero. Actual yields are (tabulated yields) x (1-nx), where "nx" refers to the nitrate yield for carbon number "x", determined as shown on Figure 3. The prefix "x" is used to indicate that they are formed following reactions of peroxy radicals with NO (see discussion of the base mechanism).
[c]	This is the total yield of monounsaturated 1,4 dialdehydes or aldehyde-ketones, represented by xAFG1 + xAFG2.
[d]	This is the ratio of the xAFG1 to xAFG2 yield, which is adjusted to aromatic - NOx chamber data for the indicated compound
[e]	AFG3 is the model species used to represent both monounsaturated 1,4-diketones and di-unsaturated 1,6-dicarbonyls.
[f]	This is the total level of peroxy radicals in the reaction, and also the total NO to NO2 conversion + nitrate formation. The yield of RO2C, representing NO to NO2 conversion, is given by the tabulated value x (1-nx), and the nitrate yield, represented by RO2XC + zRNO3, is given by the tabulated value x (nx), where "nx" is the nitrate yield factor as indicated in Footnote [b]. The tabulated values also give the yields of yRAOOH, which is used to represent the
[g]	The yields are based on those derived for toluene, but are slightly different because of differences between the estimated and total rate constants. However, the AFG1/AFG2 yields (AFG1/2) were adjusted separately to fit the chamber experiments for ethylbenzene.

Available data and theories concerning the atmospheric reactions of these unsaturated dicarbonyl ring-opening products are discussed by Calvert et al (2002). These compounds react at significant rates with OH radicals and O3 and information is available concerning the rate constants for representative compounds. These compounds can also photolyze at significant rates under atmospheric conditions, and absorption cross-section data are also available for representative compounds. The quantum yields are more uncertain but measurements of photolysis consumption rates have been made in the Euphore outdoor chamber that can serve as a basis for estimating overall quantum yields for consumption by photolysis. The extent to which these photolysis reactions form radicals is a very important factor affecting the contribution of these products to the overall reactivity of the aromatic starting material. The limited product and environmental chamber reactivity data indicate that photolysis to form radicals is important for the mono-unsaturated 1,4-dialdehydes and aldehyde-ketones, but that the photolysis of the mono-unsaturated 1,4-diketones and the di-unsaturated 1,6-dicarbonyls is much less
Table 12.	Estimated yields for unsaturated dicarbonyl products in the OH reactions of the methylbenzenes for which -dicarbonyl yield data are available.
Ring fragmentation product compound or type of compound
                            Methylbenzene Reactant

                                    Toluene
                                    Xylenes
                               Trimethylbenzenes

                                      o-
                                      m-
                                      p-
                                    1,2,3-
                                    1,2,4-
                                    1,3,5-
Aldehydes and aldehyde-ketones (Represented by AFG1 and AFG2)
  2-Butene-1,4-dial
                                     0.15
                                     0.19

  2-Methyl-2-butene-1,4-dial
                                     0.12

                                     0.19
                                     0.11

                                     0.11

  2,3-Dimethyl-2-butene-1,4-dial

                                     0.03

                                     0.12

  4-Oxo-2-penteneal
                                     0.11
                                     0.24
                                     0.19

                                     0.45

  2-Methyl-4-oxo-2-penteneal

                                     0.10

                                     0.10
                                     0.64
  3-Methyl-4-oxo-2-penteneal

                                     0.05

                                     0.18
                                     0.11

  2,3-Dimethyl-4-oxo-2-penteneal

                                     0.07

Diketones (Represented by AFG3)
  3-Methyl-3-hexene-2,5-dione

                                     0.29

                                     0.08

Diunsaturated dicarbonyls (Represented by AFG3)
  Methyl-substituted 1,4-hexadienedials
                                     0.22
                                     0.09
                                     0.09
                                     0.12
                                     0.03
                                     0.05

  6-Oxo-2,4-hepadienedial and methyl substituted isomers
                                     0.09
                                     0.08
                                     0.15
                                     0.15
                                     0.03
                                     0.15
                                     0.16
  3,5-Octadiene-2,7-dione and methyl substituted isomers

                                     0.03

                                     0.03
                                     0.03

efficient, or forms primarily non-radical products. Calvert et al (2002), and references therein, should be consulted for details.
Based on this information, we use two model species, designated AFG1 and AFG2, to represent the highly photoreactive mono-unsaturated dialdehydes and aldehyde-ketones, and a separate model species, AFG3, to represent the less photoreactive unsaturated diketones and di-unsaturated dicarbonyls. Because the overall quantum yields for photolyses of the mono-unsaturated dialdehydes and aldehyde-ketones to form radicals are very important in affecting overall reactivity but are uncertain and may vary from compound to compound, these are treated as adjustable parameters in the mechanism. This is implemented by using two model species for these compounds that have exactly the same mechanisms, except that one (AFG1) photolyzes to form radicals, while the other (AFG2) photolyzes at the same rate but forms non-radical products. The total yields of these model species are set at the total estimated monounsaturated aldehyde yield as indicated on Table 12, while the yield of one relative to the other are adjusted to predict overall reactivities that are consistent with results of environmental chamber experiments with the various aromatics. This is equivalent to adjusting separately the overall quantum yields for formation of radical vs. non-radical products for photoreactive dicarbonyls for each compound without having to use a separate model species for each compound.
As indicated in footnotes to Table A-2 in Appendix A, the mechanisms for AFG1 and AFG2 was derived based on those estimated for the representative dialdehydes and diketones 2-butene 1,4-dial 2-methyl-2-butene-1,4-dial, 4-oxo-2-petenal, and 2-methyl-4-oxo-2-pentenal, and the mechanism for AFG3 was derived based on those estimated for the represented di-unsaturated dicarbonyl products 3-methyl 2,4-hexene-1,6-dial, 6-oxo-2,4-heptadienal, and 3,5-octadien-2,7-dione. The relative contributions of each are based on relative yields of these and similar products from the methylbenzenes, as indicated on Table 12. Use of a separate model species, based on mechanisms for 3-methyl-3-hexene-2,5-dione, to represent the mono-unsaturated diketones was examined in an initial version of this mechanism. However, but it was found that lumping it with the model species used for the di-unsaturated dicarbonyls gave essentially the same reactivity predictions for p-xylene and 1,2,4-trimethylbenzene, the only compounds for which reactivity data are available where these products are predicted to be formed. Therefore, it was determined that use of a separate model species to represent these mono-unsaturated diketones was not necessary.
The mechanisms for the reactions of these model species with OH and O3 were derived using the enhanced mechanism generation system, using assignments for the specific compounds that are given on Table 13. The reactions of these species with NO3 and O[3]P were assumed to be negligible compared to the other processes and are not included in the mechanism. The mechanism for the photolysis of AFG1 to form radicals is based on the assignments for the initial reactions that are also given on Table 13. Specific mechanisms were not generated for the photolyses of AFG2 to form stable products; the unspecified products formed are assumed to be moderately reactive and are represented by the model species PROD2. The overall mechanisms that were derived for these model species using this method are given with the listing of the base mechanism on Table A-2 in Appendix A.
The absorption cross sections and quantum yields for the photolyses of AFG1 and AFG2 were derived from those for 2-butene-1,4-dial and 4-oxo-2-pentenal, assuming those for 2-methyl-2-butene-1,4-dial and 2-methyl-4-oxo-2-pentenal are the same as those for 2-butene-1,4-dial and 4-oxo-2-pentenal, respectively. The derivations of the absorption cross sections and quantum yields used are indicated on Table 13, and plots of the action spectra (absorption cross sections x overall quantum yields) for these two compounds are shown on Figure 4, where they are compared with that derived for AFG1 and AFG2, which is the weighed average based on ratios of these compounds used to derive the rate constants and mechanisms for the other reactions (see footnotes to Table A-2 in Appendix A). The action spectrum for AFG1,2 most closely resembles that for 4-oxo-2-pentenal because of the higher absorption cross sections and quantum yields and also because of the higher weighting of the aldehyde-ketone products (68%) compared to the dialdehyde products (32%).
Note that although this representation uses the same number of model species as SAPRC-99 to represent the unspecified non--dicarbonyl aromatic ring opening products, the meaning and the mechanisms of these species is different. In SAPRC-99 the model species DCB1 is used to represent non-photoreactive product species, DCB2 represents photoreactive products with action spectra like -dicarbonyls, and DCB3 represents photoreactive products with action spectra like acrolein. The yields of DCB1 for various aromatics were set more or less arbitrarily, while those for DCB2 and DCB3 were adjusted to fit the aromatics - NOx chamber data, with species with the different action spectra being needed to simulate experiments using differing light sources. In this mechanism, the total yields are set based on other product data or estimates that are independent of the chamber data, and only the AFG1/AFG2 yield ratio is adjusted to fit the chamber data. Note that the action spectra are derived independently of the chamber data, with no adjustments to account for reactivity differences using chambers with different light sources. This resulted in some cases in the SAPRC-07 mechanism not performing as well as SAPRC-99 in simulating blacklight chamber data, but in general model performance was satisfactory, indicating that the un-adjustable action spectra used in the model may be appropriate. This is discussed further in the "Mechanism Evaluation Results" section, below.

Table 13.	Mechanism estimation assignments made for representative unsaturated dicarbonyl compounds for the purpose of generating mechanisms for the aromatic fragmentation product model species AFG1-3.
Compound, reaction, and rate parameters
                                   Fraction

2-Butene-1,4-dial

OH
Rate Constant = 5.29e-11 cm[3] molec-1 s-1

HCOCH=CHCHO + OH  HCOCH(·)CH(CHO)OH
                                      44%

HCOCH=CHCHO + OH  H2O + HCOCH=CHC(O)·
                                      56%

Rate constant from Bierbach et al (1994). Branching ratios estimated by assuming that abstraction from CHO has a rate constant per CHO group as estimated acrolein.

O3
Rate Constant = 1.60e-18 cm[3] molec-1 s-1

HCOCH=CHCHO + O3  HCOCHOO[excited] + HCOCHO

Rate constant from Liu et al (1999).

h
Photolysis Set = BUTEDIAL, Quantum Yield = 0.723

HCOCH=CHCHO + h  HCOCH=CHC(O)· + H.
                                      50%

HCOCH=CHCHO + h  HCO· + HCOCH=CH.
                                      50%

The absorption coefficients are from Liu et al (1999) (file provided by Ken Sexton, University of North Carolina), normalized as discussed by Calvert et al (2002). The quantum yield is set to give the photolysis rate, relative to NO2, of 0.18, based on data of Sorensen and Barnes (1998). Only the radical formation pathways (used for AFG1) are shown on this table -- stable compounds formed by AFG2 are represented by PROD2 regardless of the compounds used to derive the mechanisms. The radical-producing processes are uncertain, and the two shown are assumed to be most likely and to be equally important.

2-Methyl-2-butene-1,4-dial

OH
Rate Constant = 9.63e-11 cm[3] molec-1 s-1

CH3C(CHO)=CHCHO + OH  CH3C(·)(CHO)CH(CHO)OH
                                      52%

CH3C(CHO)=CHCHO + OH  CH3C(CHO)(OH)CH(·)CHO
                                      17%

CH3C(CHO)=CHCHO + OH  H2O + CH3C(CHO)=CHC(O)·
                                      31%

The rate constant and branching ratios derived from estimated rate constants for reactions at various locations. Reaction by abstraction from CHO assumed to have same rate constant per CHO as assumed for acrolein. Addition to the double bond is calculated from OH addition rate constants for 1,4-butanedial x methacrolein / acrolein. The relative rates of addition at the various positions are assumed to be the same as used by the mechanism generation system for alkenes with 3 substituents about the double bond.

O3
Rate Constant = 6.66e-18 cm[3] molec-1 s-1

CH3C(CHO)=CHCHO + O3  CH3COO[excited]CHO + HCOCHO
                                      70%

CH3C(CHO)=CHCHO + O3  HCOCHOO[excited] + CH3C(O)CHO
                                      30%

The rate constant is estimated from the O3 rate constants for 1,4-butanedial x methacrolein / acrolein. The branching ratios were derived by the mechanism generation system based on assignments for trisubstituted alkenes.

h
Photolysis Set = BUTEDIAL, Quantum Yield = 0.723

CH3C(CHO)=CHCHO + h  H. + CH3C(CHO)=CHC(O)·
                                      25%

CH3C(CHO)=CHCHO + h  H. + CH3CH=C(CHO)C(O)·
                                      25%

CH3C(CHO)=CHCHO + h  HCO· + CH3C(CHO)=CH.
                                      25%

CH3C(CHO)=CHCHO + h  HCO· + CH3C(·)=CHCHO
                                      25%

The photolysis reaction is assumed to have the same absorption cross-section and overall quantum yield as used for 2-butene-1,4-dial. The radical formation reactions shown above are assumed to have equal probability.

4-Oxo-2-penteneal

OH
Rate Constant = 5.67e-11 cm[3] molec-1 s-1

CH3C(O)CH=CHCHO + OH  CH3C(O)CH(OH)CH(·)CHO
                                      37%

CH3C(O)CH=CHCHO + OH  CH3C(O)CH(·)CH(CHO)OH
                                      37%

CH3C(O)CH=CHCHO + OH  H2O + CH3C(O)CH=CHC(O)·
                                      26%

Rate constant from Bierbach et al (1994). Branching ratios estimated based on ratio of total rate constant relative to the rate constant for abstraction from CHO based on that derived for acrolein. Equal probability OH addition at the two positions about the bond is assumed.

O3
Rate Constant = 4.80e-18 cm[3] molec-1 s-1

CH3C(O)CH=CHCHO + O3  CH3C(O)CHOO[excited] + HCOCHO
                                      50%

CH3C(O)CH=CHCHO + O3  HCOCHOO[excited] + CH3C(O)CHO
                                      50%

Rate constant from Liu et al (1999). Equal probability of reaction assumed for the two possible routes.

h
Photolysis Set = 4OX2PEAL, Quantum yields = 1.

CH3C(O)CH=CHCHO + h  HCOCH=CHC(O)· + CH3·
                                      25%

CH3C(O)CH=CHCHO + h  H. + CH3C(O)CH=CHC(O)·
                                      25%

CH3C(O)CH=CHCHO + h  HCOCH=CH. + CH3C(O)·
                                      25%

CH3C(O)CH=CHCHO + h  HCO· + CH3C(O)CH=CH.
                                      25%

Absorption cross-sections from Calvert et al (2002). A high quantum yield is indicated by the photolysis rate measurements made in the Euphore chamber by Sørensen and Barnes (1998), and a unit quantum yield is assumed. Equal probability of reaction is assumed for the various possible radical formation routes shown above.

2-Methyl-4-oxo-2-pentenal

OH
Rate Constant = 8.61e-11 cm[3] molec-1 s-[1]

CH3C(O)CH=C(CH3)CHO + OH  CH3C(O)CH(OH)C(·)(CH3)CHO
                                      62%

CH3C(O)CH=C(CH3)CHO + OH  CH3C(O)CH(·)C(CH3)(CHO)OH
                                      21%

Rate constant and branching ratios are derived from estimated rate constants for reactions at various locations. Reaction by abstraction from CHO assumed to have same rate constant as the analogous reaction of acrolein. Addition to double bond calculated from OH addition rate constants for 4-oxo-2-pentenal x methacrolein / acrolein. The reaction shown is assumed to be the major addition process.

O3
Rate Constant = 2.00e-17 cm[3] molec-1 s-1

CH3C(O)CH=C(CH3)CHO + O3  CH3C(O)CHOO[excited] + CH3C(O)CHO
                                      30%

CH3C(O)CH=C(CH3)CHO + O3  CH3COO[excited]CHO + CH3C(O)CHO
                                      70%

The rate constant estimated from O3 rate constants for 4-oxo-2-pentenal x methacrolein / acrolein. The branching ratios were derived by the mechanism generation system based on assignments for trisubstituted alkenes.

h
Photolysis Set = 4OX2PEAL, Quantum yields = 1.

CH3C(O)CH=C(CH3)CHO + h  CH3· + CH3C(CHO)=CHC(O)·
                                      25%

CH3C(O)CH=C(CH3)CHO + h  H. + CH3C(O)CH=C(CH3)C(O)·
                                      25%

CH3C(O)CH=C(CH3)CHO + h  CH3C(O)· + CH3C(CHO)=CH.
                                      25%

CH3C(O)CH=C(CH3)CHO + h  HCO· + CH3C(·)=C(CH3)CHO
                                      25%

The photolysis reaction is assumed to have the same absorption cross-section and overall quantum yield as used for 4-oxo-2-pentenal. The radical formation reactions shown above are assumed to have equal probability.

3-methyl 2,4-hexene-1,6-Dial

OH
Rate Constant = 1.20e-10 cm[3] molec-1 s-1

CH3C(=CHCHO)CH=CHCHO + OH  H2O + CH3C(=CHC(O)·)CH=CHCHO
                                      12%

CH3C(=CHCHO)CH=CHCHO + OH  H2O + CH3C(=CHCHO)CH=CHC(O)·
                                      12%

CH3C(=CHCHO)CH=CHCHO + OH  CH3C(=CHCHO)CH(·)CH(CHO)OH
                                      75%

Rate constant and branching ratios estimated from sum of estimated rate constants for reactions at various positions. Rate constant for abstraction from CHO assumed to be same as for analogous reaction of acrolein. Rate constant for addition to double bond estimated from addition rate constant estimated from addition rate constant for 2,4-hexene-1,6-dial x rate constant for isoprene / rate constant for 1,3-butadiene. The addition rate constant for 2,4-hexene-1,6-dial is estimated from the total rate constant of 9.00e-11cm[3] molec-1 s-1 for that compound and the estimated rate constant for reaction at the CHO groups, based on the rate constant estimated for acrolein. The total rate constant for 2,4-hexene-1-6-dial is representative of values reported by Klotz et al (1995, 1999) for the trans,trans and the cis,trans isomers. The addition reaction shown is assumed to be the major process.

O3
Rate Constant = 2.01e-17 cm[3] molec-1 s-1

CH3C(=CHCHO)CH=CHCHO + O3  CH3C(O)CH=CHCHO + HCOCHOO[excited]
                                      25%

CH3C(=CHCHO)CH=CHCHO + O3  HCOCHO + CH3COO[excited]-CH=CHCHO
                                      25%

CH3C(=CHCHO)CH=CHCHO + O3  HCOCHO + CH3C(CHOO[excited])=CHCHO
                                      25%

CH3C(=CHCHO)CH=CHCHO + O3  CH3C(CHO)=CHCHO + HCOCHOO[excited]
                                      25%