Explicit Modeling of Organic Chemistry and SOA Partitioning in Mexico City

We use the GECKO-A (Generator of Explicit Chemistry and Kinetics of Organics in the Atmosphere) model to simulate the composition of organic aerosol in Mexico City's urban atmosphere and outflow plume during the MILAGRO campaign. The mechanism is extremely detailed, treating 5.9 million gas phase reactions among > 1 million species. Gas/aerosol partitioning is treated by Raoult's law-type condensation for > 275 000 non-radical species. We are able to speciate the simulated primary and secondary organic aerosol (POA and SOA) and find that it is distributed somewhat equally between species of different carbon number (with C>5), despite large disparities in emissions and gas-phase abundance. Over 90% of the total burden of C23+ carbon is present as POA. Close to the source region, the simulated SOA is composed predominantly of delta-hydroxy-ketones and hydroxy-nitrates. SOA functionality becomes more diverse with age downwind, reflecting a shift from high-NOx to low-NOx chemistry within the plume. In addition, the degree of substitution increases as SOA ages, resulting in greater relative contributions from lower carbon-number species. Compared to observations, the modeled O/C ratio of the aerosol is lower and the N/C ratio is higher, suggesting that additional important reaction pathways either in the gas or the particle phase remain to be elucidated.