Can 3D Models Explain the Primary and Secondary, Fossil and non-Fossil Organic Aerosols during the 2006 MILAGRO Experiment?

Organic aerosols (OA) remain one of the less well characterized components of the atmosphere despite their effects on climate and human health. In the framework of MILAGRO, the predictions of a meso-scale chemistry-transport model have been compared to AMS/PMF and 14C data to identify major sources and formation processes leading to a large amount of primary (POA) and secondary (SOA) organic aerosols in Mexico City during March 2006. SOA formation was modeled by both (i) the traditional one-step oxidation of anthropogenic (eg aromatics), biogenic (monoterpenes and isoprene), and biomass-burning SOA precursors followed by their partitioning into organic and aqueous phases; and (ii) the volatility basis set-based oxidation and partitioning of semi-volatile and intermediate volatility primary organic vapors (S/IVOCs; Robinson Sci07; Grieshop ACP09). Based on a month long comparison performed at the surface within the city and aloft in the outflow region it was found that anthropogenic and biomass burning emissions of primary OA (and SOA precursors) are reasonably captured within the city, although model overestimation of BBOA can be seen downwind of large fires. In contrary, large discrepancies are encountered for SOA, with a factor of 2-10 model underestimate when only traditional anthropogenic SOA precursors and yields are considered. A large increase in the partitioning coefficients (simulating e.g. oligomerization) reduces the model-measurement discrepancy but does not close the whole gap. Accounting for S/IVOC leads to a substantial increase of the predicted SOA. Although S/IVOC runs help explain the missing SOA mass, inconsistencies in the spatial SOA distributions and O/C ratios are found for both the original (Robinson 07) and the updated (Grieshop 09) approach. Model results also indicate that SOA found in the Mexico City basin is substantially influenced by regional SOA (~1.5 µg/m3) of biogenic origin which is formed over the coastal mountain ranges and advected around Central Mexico. The presence of biogenic SOA was confirmed by tracer-derived estimates of 1.14-1.35 µg/m3 of biogenic SOA within the city, in good agreement with model predictions, and in the first such comparison to our knowledge. The presence of this biogenic SOA helps explain the significant amount of modern carbon in the OA inside the city during low biomass burning periods (35%). For the first time, the model predictions were also evaluated in terms of the concentrations of modern and fossil carbon using combined 14C and AMS measurements. Measurement-derived (line) and model-predicted (dots) daily-averaged concentrations of biogenic SOA at T0. The variability is also indicated (+/- 1 sigma).