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It has been established that observed local and regional levels of secondaryorganic aerosols (SOA) in polluted areas cannot be explained by theoxidation and partitioning of anthropogenic and biogenic VOC precursors, atleast using current mechanisms and parameterizations. In this study, the 3-Dregional air quality model CHIMERE is applied to estimate the potentialcontribution to SOA formation of recently identified semi-volatile andintermediate volatility organic precursors (S/IVOC) in and around MexicoCity for the MILAGRO field experiment during March 2006. The model has beenupdated to include explicitly the volatility distribution of primary organicaerosols (POA), their gas-particle partitioning and the gas-phase oxidationof the vapors. Two recently proposed parameterizations, those of Robinson et al. (2007) ("ROB") and Grieshop et al. (2009) ("GRI") are compared andevaluated against surface and aircraft measurements. The 3-D model resultsare assessed by comparing with the concentrations of OA components fromPositive Matrix Factorization of Aerosol Mass Spectrometer (AMS) data, andfor the first time also with oxygen-to-carbon ratios derived fromhigh-resolution AMS measurements. The results show a substantial enhancementin predicted SOA concentrations (2–4 times) with respect to the previouslypublished base case without S/IVOCs (Hodzic et al., 2009), both within anddownwind of the city leading to much reduced discrepancies with the total OAmeasurements. Model improvements in OA predictions are associated with thebetter-captured SOA magnitude and diurnal variability. The predictedproduction from anthropogenic and biomass burning S/IVOC represents40–60% of the total measured SOA at the surface during the day and issomewhat larger than that from commonly measured aromatic VOCs, especiallyat the T1 site at the edge of the city. The SOA production from thecontinued multi-generation S/IVOC oxidation products continues activelydownwind. Similar to aircraft observations, the predicted OA/ΔCOratio for the ROB case increases from 20–30 μg sm−3 ppm−1 upto 60–70 μg sm−3 ppm−1 between a fresh and 1-day aged airmass, while the GRI case produces a 30% higher OA growth than observed.The predicted average O/C ratio of total OA for the ROB case is 0.16 at T0,substantially below observed value of 0.5. A much better agreement for O/Cratios and temporal variability (R2=0.63) is achieved with the updatedGRI treatment. Both treatments show a deficiency in regard to POA ageingwith a tendency to over-evaporate POA upon dilution of the urban plumesuggesting that atmospheric HOA may be less volatile than assumed in theseparameterizations. This study highlights the important potential role ofS/IVOC chemistry in the SOA budget in this region, and highlights the needfor further improvements in available parameterizations. The agreementobserved in this study is not sufficient evidence to conclude that S/IVOCare the major missing SOA source in megacity environments. The model isstill very underconstrained, and other possible pathways such as formationfrom very volatile species like glyoxal may explain some of the mass andespecially increase the O/C ratio

atmospheric chemistry and physics

Modeling organic aerosols in a megacity: potential contribution of semi-volatile and intermediate volatility primary organic compounds to secondary organic aerosol formation