Modeling organic aerosols during MILAGRO : importance of biogenic secondary organic aerosols

The meso-scale chemistry-transport model CHIMERE is used to investigate major sources and formation processes leading to a fairly large amount of organic aerosols (OA including primary OA (POA) and secondary OA (SOA)) observed in Mexico City during the MILAGRO field project (March 2006). Comparison of near-surface model predictions with aerosol mass spectrometer data shows that predicted OA correlates reasonably well with measurements during the campaign, however it remains a factor of 2 lower than the measured total OA. Very good agreement is found between observed and predicted POA within the city indicating that primary anthropogenic and biomass burning emissions are reasonably captured. Consistent with previous studies in Mexico City, large discrepancies are encountered for SOA species, with a factor of 5-10 model underestimate. When only anthropogenic SOA precursors were considered, the model was able to reproduce within a factor of two the sharp increase in SOA concentrations during the late morning at both urban and near-urban locations. However, predicted SOA concentrations were unrealistically low when photochemistry was not active, especially overnight. These discrepancies were not significantly reduced when greatly enhanced partitioning to the aerosol phase was assumed. Model sensitivity results suggest that observed night-time SOA concentrations are dominated by the regional background (~2µg/m3) from biogenic origin which is transported from the coastal regions into the Mexico City basin. The relative contribution of biogenic SOA to monthly mean SOA levels was estimated to over 20% within the city and up to 65-90% at the regional scale which is consistent with measurements of modern carbon during low biomass burning periods. Our results confirm the large underestimation of the SOA mass by traditional models in polluted regions, and emphasize for the first time the key role of biogenics in this region. Significance of newly proposed SOA formation pathways (i.e. night-time oxidation of isoprene, POA volaitilty) is also investigated in this study.