DEVELOPMENT OF REVISED SAPRC AROMATICS MECHANISMS

An indication of the biases in the SAPRC-07 aromatics mechanism in simulating the new CSIRO
and UCR EPA chamber data is given in Figure 1, which gives plots of the model error in simulating
amounts of NO oxidation and ozone formation 1 in benzene, toluene, m-xylene and o-cresol - NO x
experiments against initial NO x and initial aromatic / NO x ratios. Different symbols are used for the new
CSIRO and UCR EPA experiments. Figure 1 shows that the mechanism tends to underpredict ozone
formed and NO oxidized in essentially all the new experiments with benzene, toluene and (especially) o-
cresol and indicates a general underprediction bias for m-xylene. There also appears to be a dependence
of the underprediction bias on initial NO x levels in the case of benzene and toluene, and on aromatic/NO x
ratios in the case of toluene and m-xylene.
Not shown on Figure 1 are the results of model simulations of new UCR-EPA chamber
experiments for a number of other aromatic compounds, including other xylene isomers, the
trimethylbenzenes, and compounds for which no chamber data were available when SAPRC-07 was
developed, including the ethyltoluene and propylbenzene isomers and other phenolic compounds. It was
found that SAPRC-07 had a tendency to underpredict ozone formation rates for many of these other
compounds as well.
Therefore, the new data from the CSIRO and UCR EPA chambers indicate that the SAPRC-07
aromatics mechanisms do not give the best fits to the currently available chamber dataset, and need to be
revised to take the new data into account. In addition, the data for benzene and toluene suggest a
dependence of model bias on total NO x levels that cannot be accounted for by any adjustments to the
mechanism using its current formulation. Recent laboratory data (e.g., Nishino et al, 2010; Bethel et al
2000) indicates that there is a dependence of aromatic fragmentation product yields on total NO x levels,
which is not represented in the current mechanism. This is attributed to a competition between the OHaromatic
adduct reacting with O 2 and NO 2 (Koch et al, 2007), with the reaction with O 2 forming the
expected fragmentation products and the reaction with NO 2 at high NO x levels presumably forming other
products. However, according to the laboratory data (Koch et al, 2007), the competing reaction with NO 2
only becomes significant at NO 2 levels greater than ~1 parts per million (ppm), which is much higher than
the NO x levels used in almost all of the experiments in the current evaluation dataset. Therefore, even if
the mechanism were modified to take this aromatic-OH adduct + NO 2 reaction into account it would not
give significantly different simulations of the chamber data unless other changes were made.
To address these problems, in this work we developed a revised version of the SAPRC-07
mechanism with the aromatics mechanisms that is updated and readjusted to give better fits to the
chamber data. This modified aromatics version of SAPRC-07 is referred to as “SAPRC-11” in the
subsequent discussion, though most of the non-aromatic portions of SAPRC-11 are the same as SAPRC-
07 because a full update of the mechanism is beyond the scope of this project. This SAPRC-11 aromatics
mechanism is used as the starting point for a PM-SAPRC11 mechanism developed to predict secondary
organic aerosol (SOA) formation from aromatics, as discussed by Carter et al (2012).
1 See the discussion of ∆([O 3 ]-[NO]) model error in the mechanism evaluation section for the definitions
of this and other measures used to evaluate mechanism performance.
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