Simulated impact of NOx on SOA formation from oxidation of toluene and m-xylene

Abstract

Despite the crucial role of aromatic-derived secondary organic aerosol (SOA) in deteriorating air quality, its formation mechanism is not well understood, and the dependence of aromatic SOA formation on nitrogen oxides (NOx) is not captured fully by most SOA formation models. In this study, NOx-dependent mechanisms of toluene and m-xylene SOA formation are updated using the gas-phase Caltech Atmospheric Chemistry Mechanism (CACM) coupled to a gas/aerosol partitioning model. The updated models were optimized by comparison to eighteen chamber experiments performed under both high- and low-NOx conditions at the University of California – Riverside. Correction factors for vapor pressures imply uncharacterized association chemistry, likely in the aerosol phase. Difficulty of the presented model lies in predicting O3 production well, with overprediction (typically 20–40%), particularly under low NOx conditions (up to one fold). Mean normalized biases for other species (NO2 and SOA) and conditions are smaller (up to ±50%). The newly developed model can predict strong decreases of m-xylene SOA yield with increasing NOx. Simulated SOA speciation implies the importance of ring-opening products in governing SOA formation (up to ∼40–60% for both aromatics). Speciation distributions under varied NOx levels imply that competition between hydroperoxide radical and NO for reaction with a bicyclic peroxide radical as the most commonly accepted mechanism may not be the only factor influencing SOA formation. The reaction of aromatic peroxy radicals with NO competing with self-cyclization (a new mechanism updated in this study) might also affect NOx-dependence of SOA formation. Different simulated importance of a phenolic route in governing SOA formation between toluene and m-xylene suggests that the number of methyl groups on the aromatic ring is important in SOA formation chemistry.