Chemically explicit model of secondary organic aerosol (SOA) formation in Mexico

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Description

Secondary organic aerosols are ubiquitous with mass loadings comparable to and often larger than all other aerosols, and much larger than expected from traditional yields in chamber experiments. We developed a 0D model of the PBL in Mexico City, using MILAGRO measurements to prescribe a time-varying PBL height, vertical exchange with the free troposphere, horizontal ventilation based on city-averaged winds, and emissions of NOx and about 60 hydrocarbons. A nearly explicit chemical mechanism for the oxidation of hydrocarbons down to ultimate products CO2 and H2O was created using our code Generator of Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A), resulting in ~6e6 reactions among ~1e6 chemical species, including estimates of associated thermal and photolytic rate constants as well as saturation vapor pressures for all initial and intermediate compounds. Secondary organic aerosols were formed by gas-particle partitioning using Raoult’s law. We examined the contribution to SOA from different hydrocarbon classes. Aromatics provide only a small fraction of the observed SOA, via condensable intermediates such as nitro-catechols and multifunctional aldehydes. Long-chain alkanes (containing up to 20 carbon atoms in our simulations) give much more particulate mass, bringing the total into fair agreement with SOA mass and diurnal cycles observed during MILAGRO. The chemical speciation of particles generated from alkanes is largely the precursor hydrocarbons (esp. for C18 and larger alkanes), monofunctional nitrates, and delta-hydroxy carbonyls and nitrates. The latter result from 1.4 isomerization reactions of alkoxy radicals following OH abstraction of H atoms from the hydrocarbon chains, as also observed in chamber experiments. For the NOx-rich conditions of Mexico City, nitrogen is predicted to be abundant in SOA formed from both aromatics (mostly as nitro groups) and alkanes (as nitrate groups) with N/C ratios of about 0.1. Comparisons of measured and model-predicted atomic ratios (N/C, O/C, H/C) offer key opportunities for elucidating the formation mechanisms of SOA. The importance of long-chain alkanes in our model is consistent with the proposal that evaporation of primary organic aerosols, followed by gas phase chemistry, is an important source of SOA in urban-influenced air. Several modeling studies have demonstrated the potential importance of this source using simple chemical parameterizations that treat all long-chain alkanes as a generic semi-volatile organic compound (S-VOC) or intermediate-volatility organic compound (I-VOC). Our study is the first to consider the explicit chemistry of these alkanes and allows identification of the chemical nature of the nascent particles. We emphasize the nascent aspect, because we recognize that further chemistry currently not well understood may occur on the surface and interior of particles.