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A 3-D chemistry-transport model has been applied to the Mexico Citymetropolitan area to investigate the origin of elevated levels of non-fossil(NF) carbonaceous aerosols observed in this highly urbanized region. Hightime resolution measurements of the fine aerosol concentration andcomposition, and 12 or 24 h integrated 14C measurements of aerosolmodern carbon have been performed in and near Mexico City during the March2006 MILAGRO field experiment. The non-fossil carbon fraction (fNF),which is lower than the measured modern fraction (fM) due to theelevated 14C in the atmosphere caused by nuclear bomb testing, isestimated from the measured fM and the source-dependent information onmodern carbon enrichment. The fNF contained in PM1 total carbonanalyzed by a US team (fNFTC) ranged from 0.37 to 0.67 at thedowntown location, and from 0.50 to 0.86 at the suburban site. Substantiallylower values (i.e. 0.24–0.49) were found for PM10 filters downtown byan independent set of measurements (Swiss team), which are inconsistentwith the modeled and known differences between the size ranges, suggestinghigher than expected uncertainties in the measurement techniques of14C. An increase in the non-fossil organic carbon (OC) fraction(fNFOC) by 0.10–0.15 was observed for both sets of filters duringperiods with enhanced wildfire activity in comparison to periods when fireswere suppressed by rain, which is consistent with the wildfire impactsestimated with other methods. Model results show that the relatively highfraction of non-fossil carbon found in Mexico City seems to arise from thecombination in about equal proportions of regional biogenic SOA, biomassburning POA and SOA, as well as non-fossil urban POA and SOA. Predictedspatial and temporal variations for fNFOCare similar to those inthe measurements between the urban vs. suburban sites, and high-fire vs.low-fire periods. The absolute modeled values of fNFOC areconsistent with the Swiss dataset but lower than the US dataset. Resolvingthe 14C measurement discrepancies is necessary for further progress inmodel evaluation. The model simulations that included secondary organicaerosol (SOA) formation from semi-volatile and intermediate volatility(S/IVOC) vapors showed improved closure for the total OA mass compared tosimulations which only included SOA from VOCs, providing a more realisticbasis to evaluate the fNF predictions. fNFOC urban sources ofmodern carbon are important in reducing or removing the difference infNF between model and measurements, even though they are oftenneglected on the interpretation of 14C datasets. An underprediction of biomass burning POA by the model duringsome mornings also explains a part of the model-measurement differences. The fNF of urban POA and SOAprecursors is an important parameter that needs to be better constrained bymeasurements. Performing faster (≤3 h) 14C measurements in futurecampaigns is critical to further progress in this area. To our knowledgethis is the first time that radiocarbon measurements are used together withaerosol mass spectrometer (AMS) organic components to assess the performanceof a regional model for organic aerosols

Can 3-D models explain the observed fractions of fossil and non-fossil carbon in and near Mexico City?