Reaction with the hydroxyl (OH) radical is the dominant removal process for
volatile organic compounds (VOCs) in the atmosphere. Rate coefficients for
the reactions of OH with VOCs are therefore essential parameters for chemical
mechanisms used in chemistry transport
models, and are required more generally for impact assessments involving
estimation of atmospheric lifetimes or oxidation rates for VOCs. A
structure–activity relationship (SAR) method is presented for the reactions
of OH with aromatic organic compounds, with the reactions of aliphatic
organic compounds considered in the preceding companion paper. The SAR is
optimized using a preferred set of data including reactions of OH with 67
monocyclic aromatic hydrocarbons and oxygenated organic compounds. In each
case, the rate coefficient is defined in terms of a summation of partial rate
coefficients for H abstraction or OH addition at each relevant site in the
given organic compound, so that the attack distribution is defined. The SAR
can therefore guide the representation of the OH reactions in the next
generation of explicit detailed chemical mechanisms. Rules governing the
representation of the reactions of the product radicals under tropospheric
conditions are also summarized, specifically the rapid reaction sequences
initiated by their reactions with
Aromatic hydrocarbons make a significant contribution to anthropogenic
emissions of volatile organic compounds (VOCs), representing an important
component of vehicle exhaust and other combustion emissions, and evaporative
emissions of petroleum and from industrial processes and solvent usage (e.g.
Calvert et al., 2002; Passant, 2002). They are also emitted from sources
that are either partially or wholly natural. They represent a significant
proportion of VOC emissions from biomass burning sources (e.g. Hays et al.,
2002; Lewis et al., 2013), and are emitted substantially from vegetation
(e.g. Misztal et al., 2015). An important contributor to these natural
emissions is
The complete gas-phase oxidation of aromatic hydrocarbons proceeds via highly detailed mechanisms, producing a variety of intermediate oxidized organic products, some of which retain the aromatic ring (e.g. Calvert et al., 2002; Jenkin et al., 2003; Bloss et al., 2005). Reaction with the hydroxyl (OH) radical is generally the dominant or exclusive removal process for aromatic hydrocarbons, and makes a major contribution to the removal of aromatic oxygenates. Quantified rate coefficients for these reactions are therefore essential parameters for chemical mechanisms used in chemistry transport models, and are required more generally for environmental assessments of their impacts, e.g. to estimate the kinetic component of ozone formation potentials (Jenkin et al., 2017). In addition to the total rate coefficient, quantification of the branching ratio for attack of OH at each site within a given compound is required for explicit representation of the subsequent oxidation pathways in chemical mechanisms.
In the present paper, a structure–activity relationship (SAR) method is presented for the reactions of OH with aromatic organic compounds, with the reactions of aliphatic organic compounds considered in the preceding companion paper (Jenkin et al., 2018a). In each case, the rate coefficient is defined in terms of a summation of partial rate coefficients for H-atom abstraction or OH addition at each relevant site in the given organic compound, so that the attack distribution is also defined. This is therefore the first generalizable SAR for reactions of OH with aromatic compounds that aims to capture observed trends in rate coefficients and the site-specificity of attack. Application of the methods is illustrated with examples in the Supplement.
The information is currently being used to guide the representation of the
OH-initiation reactions in the next generation of explicit detailed chemical
mechanisms, based on the Generator for Explicit Chemistry and Kinetics of
Organics in the Atmosphere (GECKO-A; Aumont et al., 2005), and the Master
Chemical Mechanism (MCM; Saunders et al., 2003). It therefore contributes to
a revised and updated set of rules that can be used in automated mechanism
construction, and provides formal documentation of the methods. To
facilitate this, rules governing the representation of the reactions of the
product radicals under tropospheric conditions are also summarized,
specifically the rapid reaction sequences initiated by their reactions with
A set of preferred kinetic data has been assembled from which to develop and
validate the estimation methods for the OH rate coefficients, as described in
the companion paper (Jenkin et al., 2018a). The subset relevant to the
present paper comprises 298 K data for 25 monocyclic aromatic hydrocarbons
(with temperature dependences also defined in 13 cases) and 42 aromatic
oxygenated organic compounds (with temperature dependences also defined in
7 cases). In one case (1,2-diacetylbenzene), the preferred rate coefficient
is an upper-limit value. The information is provided as a part of the
Supplement (as identified in spreadsheets SI_6 and SI_7). As described
in more detail in Sect. 3.2, the oxygenates include compounds containing a
variety of oxygenated substituent groups that are prevalent in both emitted
VOCs and their degradation products, namely -OH, -C(OH)<,
-C(
Neighbouring group factors,
Comments:
Group rate coefficients for OH addition to carbon atoms in
monocyclic aromatic rings, and their temperature dependences described by
Comments:
The reaction of OH with a given aromatic compound can occur by both addition
of OH to the aromatic ring and by abstraction of an H atom from a C-H or O-H
bond in a substituent group. The estimated rate coefficient is therefore
given by
A method for estimating rate coefficients for OH addition to the aromatic
ring (
For aromatic compounds containing an unsaturated substituent, the addition of
OH to C
The set of preferred kinetic data contains rate coefficients for the reactions of OH with 12 methyl-substituted aromatic hydrocarbons possessing between one and six methyl substituents. This class is the most comprehensively studied, with room temperature data covering all possible methyl-substituted isomers. Although rate coefficients for this class of compound do not therefore need to be estimated, the SAR described below aims to rationalize the variation of reactivity from one compound to another, and to provide a method of estimating the OH attack distributions that can be applied in automated mechanism generation.
The contribution of H-atom abstraction to the total rate coefficient is known
to be minor at temperatures relevant to the atmosphere for methyl-substituted
aromatics (e.g. Calvert et al., 2002; Loison et al., 2012; Aschmann et al.,
2013). The temperature-dependent reference substituent factor for a phenyl
group,
Substituent factors
Comments: given parameter contributes to the calculation of
The current estimation method defines site-specific parameters for addition
of OH to each carbon atom in the aromatic ring. As shown in Table 2,
Comparison of estimated and reported branching ratios for H-atom
abstraction,
Comments: sources of observed values:
As shown in Table 3, the dataset was described in terms of 11 substituent
factors, representing the effects of between one and five methyl
substituents. Based on the results of previous assessments (e.g. see Calvert
et al., 2002), the number of parameters was limited by assuming that
The values of the
Comparison of estimated branching ratios for OH addition to
alkyl-substituted aromatic hydrocarbons at 298 K with those reported in
density functional theory (DFT) studies. Displayed values are presented relative to
Comments:
The estimated contributions of H-atom abstraction from the methyl substituents in the series of aromatic hydrocarbons are compared with those reported in Table 4. The values confirm that rate coefficients assigned to these reactions in Table 1 provide a reasonable description for the complete dataset of methyl-substituted aromatics.
There have been no direct experimental determinations of the branching ratios
for OH addition to methyl-substituted aromatic rings, although a number of
density functional theory (DFT) studies have been reported for toluene,
Substituent adjustment factors,
Temperature-dependent recommendations are available for benzene and 10
methyl-substituted aromatics in Arrhenius format
The set of preferred kinetic data contains rate coefficients for a further
eight alkyl-substituted aromatic hydrocarbons, namely ethylbenzene,
The methyl group substituent factors in Table 3 provide a reasonable first
approximation for the effects of the higher alkyl groups on OH addition rate
coefficients, and use of those factors leads to a set of estimated rate
coefficients that are all within 30 % of the observed values for the
current set of eight higher alkyl-substituted aromatic hydrocarbons. On the
whole, however, this results in a slight overestimation of the rate
coefficients. Table 6 shows a set of adjustment factors for non-methyl
substituents,
These result in a generally improved agreement, with deviations from the
observed rate coefficients of
Similarly to above, there have been no direct experimental determinations of
branching ratios for OH addition to higher alkyl-substituted aromatics,
although Huang et al. (2010) have reported a DFT study for ethylbenzene, and
Alarcón et al. (2014) for
Temperature-dependent studies are only available for
The set of preferred kinetic data contains rate coefficients for the
reactions of OH with four alk-1-enyl (or vinyl) substituted aromatic
hydrocarbons, namely styrene (ethenylbenzene),
The addition of OH to more remote C
The preferred 298 K data include rate coefficients for reactions of OH with
42 aromatics containing a variety of oxygenated substituent groups, which
were used to extend the methods described above for estimating rate
coefficients for aromatic hydrocarbons. Rate coefficients for H-atom
abstraction from the oxygenated groups are generally represented using the
methods applied to aliphatic oxygenates (Jenkin et al., 2018a), in
conjunction with the values of
A log–log correlation of
The contribution of H-atom abstraction from the -OH substituent in phenolic
compounds has generally been inferred from the measured yields of
nitrophenolic products, under conditions when the intermediate phenoxy
radicals are expected to react predominantly with
The values of
The attack distributions predicted by the optimized parameters recreate some
of the features inferred from reported experimental studies for phenol and
cresols (e.g. Olariu et al., 2002), initiating routes to the observed
formation of catechols (1,2-dihydroxyarenes), benzoquinones and nitrophenols
(see Sect. 4.2). As shown in Table 6, comparable values of
The set of preferred kinetic data contains rate coefficients for
benzaldehyde, three methyl-substituted benzaldehydes and six
dimethyl-substituted benzaldehydes. In addition, preferred data are included
for phthaldialdehyde (1,2-diformylbenzene) and 2-acetylbenzaldehyde, and an
upper-limit rate coefficient for the related compound 1,2-diacetylbenzene,
based on Wang et al. (2006). The data show that the presence of methyl
substituents in the benzaldehydes increases the OH reactivity systematically.
It is generally accepted that abstraction of the H atom from the formyl
(-C(
Initially, it was assumed that the rate coefficient for H-atom abstraction
from the formyl group,
An alternative procedure was therefore adopted in which the contribution of
H-atom abstraction from the -C(
Temperature-dependent data are only available for benzaldehyde. Within the
constraints of the approach described above, this was used to provide the
optimized temperature dependence expression,
The set of preferred kinetic data contains rate coefficients for the
reactions of OH with a number of nitro-substituted aromatics, namely
nitrobenzene, 1-methyl-3-nitrobenzene, 2-nitrophenol and four
methyl-substituted 2-nitrophenols. These data were used to optimize the
values of
Schematic representation of the reaction of OH-aromatic adducts with
Partial rate coefficients for the reactions of
Comments:
The optimized values of
A method has been developed to describe the chemistry initiated by reaction
of
The reactions of the OH-aromatic hydrocarbon adducts with
Substituent factors
Comments:
Schematic representation of the mechanism following formation of the
“peroxide-bicyclic” radical, as shown in Fig. 3, with alkyl substituents
omitted for clarity. The initial energy-rich “peroxide-bicyclic” radical is
represented to isomerize as shown with a total optimized probability of
30 % in competition with stabilization (see Sect. 4.1). Addition of
The value of
Here,
Substituent factors,
Comments:
Correlation of calculated and observed yields of hydroxyarenes
(total and specific),
As shown in Fig. 3, the two
Routes applied following OH addition at an unsubstituted carbon
Root mean square error (RMSE), mean absolute error (MAE), mean bias
error (MBE) and box plot for the error distribution in the estimated log
The (stabilized) peroxide-bicyclic radical possesses an allyl resonance, such
that addition of
Here,
Reactions represented for phenoxy and substituted phenoxy radicals,
and their assigned rate coefficients (in units
cm
Comments:
The calculated yields presented in Table S2 also take account of minor
formation of nitrate products from the peroxy
Product and mechanistic information on the reactions of adducts formed from
the addition of OH radicals to aromatic oxygenates appears to be limited to
those formed from hydroxyarene (phenolic) compounds (e.g. Olariu et al.,
2002; Berndt et al., 2003; Coeur-Tourneur et al., 2006). Those studies have
established that 1,2-dihydroxyarenes (catechols) and 1,4-benzoquinones are
formed as ring-retaining products of the OH-initiated oxidation of phenol and
cresols. On the basis of the reported information, the pathways presented in
Fig. 6 are applied in relation to hydroxy-substituted aromatic compounds.
Addition of OH at an unsubstituted carbon
For other OH-aromatic oxygenate adducts, the mechanisms applied to
OH-aromatic hydrocarbon adducts (see Sect. 4.1) are provisionally applied, in
the absence of information. Within the framework described in Sect. 4.1, some
additional assumptions are applied in relation to addition of
Carbon-centred organic radicals (R) formed from H-atom abstraction from, or
OH addition to, substituent groups in aromatic compounds generally react as
described for those formed from aliphatic organic compounds in the companion
paper (Jenkin et al., 2018a). In the majority of cases, therefore, they react
rapidly and exclusively with molecular oxygen (
A structure activity relationship (SAR) method has been developed to estimate
rate coefficients for the reactions of the OH radical with aromatic organic
species. This group contribution method was optimized using a database
including a set preferred rate coefficients for 67 species. The overall
performance of the SAR in determining log
The distribution of errors (log
The calculated log
All relevant data and supporting information have been provided in the Supplement.
All authors defined the scope of the work. MEJ developed the SAR methods and drafted the manuscript, which were both reviewed by all co-authors. RV and BA tested the SAR methods in GECKO-A and carried out the statistical analysis in Sect. 6.
The authors declare that they have no conflict of interest.
This work received funding from the Alliance of Automobile Manufacturers, and as part of the MAGNIFY project, with funding from the French National Research Agency (ANR) under project ANR-14-CE01-0010, and the UK Natural Environment Research Council (NERC) via grant NE/M013448/1. It was also partially funded by the UK National Centre for Atmospheric Sciences (NCAS) Composition Directorate. Marie Camredon (LISA, Paris) and Mike Newland (University of York) are gratefully acknowledged for helpful discussions on this work. We also thank Luc Vereecken (Forschungszentrum Jülich) for providing detailed comments during the open discussion, and two anonymous referees for review comments, that helped to improve the manuscript. Edited by: Andreas Hofzumahaus Reviewed by: two anonymous referees