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A 3-D chemistry-transport model has been applied to the Mexico City metropolitan area to investigate the origin of elevated levels of non-fossil (NF) carbonaceous aerosols observed in this highly urbanized region. High time resolution measurements of the fine aerosol concentration and composition, and 12 or 24 h integrated <sup>14</sup>C measurements of aerosol modern carbon have been performed in and near Mexico City during the March 2006 MILAGRO field experiment. The non-fossil carbon fraction (<i>f</i><sub>NF</sub>), which is lower than the measured modern fraction (<i>f</i><sub>M</sub>) due to the elevated <sup>14</sup>C in the atmosphere caused by nuclear bomb testing, is estimated from the measured <i>f</i><sub>M</sub> and the source-dependent information on modern carbon enrichment. The <i>f</i><sub>NF</sub> contained in PM<sub>1</sub> total carbon analyzed by a US team (<i>f</i><sub>NF</sub><sup>TC</sup>) ranged from 0.37 to 0.67 at the downtown location, and from 0.50 to 0.86 at the suburban site. Substantially lower values (i.e. 0.24–0.49) were found for PM<sub>10</sub> filters downtown by an independent set of measurements (Swiss team), which are inconsistent with the modeled and known differences between the size ranges, suggesting higher than expected uncertainties in the measurement techniques of <sup>14</sup>C. An increase in the non-fossil organic carbon (OC) fraction (<i>f</i><sub>NF</sub><sup>OC</sup>) by 0.10–0.15 was observed for both sets of filters during periods with enhanced wildfire activity in comparison to periods when fires were suppressed by rain, which is consistent with the wildfire impacts estimated with other methods. Model results show that the relatively high fraction of non-fossil carbon found in Mexico City seems to arise from the combination in about equal proportions of regional biogenic SOA, biomass burning POA and SOA, as well as non-fossil urban POA and SOA. Predicted spatial and temporal variations for <i>f</i><sub>NF</sub><sup>OC</sup>are similar to those in the measurements between the urban vs. suburban sites, and high-fire vs. low-fire periods. The absolute modeled values of <i>f</i><sub>NF</sub><sup>OC</sup> are consistent with the Swiss dataset but lower than the US dataset. Resolving the <sup>14</sup>C measurement discrepancies is necessary for further progress in model evaluation. The model simulations that included secondary organic aerosol (SOA) formation from semi-volatile and intermediate volatility (S/IVOC) vapors showed improved closure for the total OA mass compared to simulations which only included SOA from VOCs, providing a more realistic basis to evaluate the <i>f</i><sub>NF</sub> predictions. <i>f</i><sub>NF</sub><sup>OC</sup> urban sources of modern carbon are important in reducing or removing the difference in <i>f</i><sub>NF</sub> between model and measurements, even though they are often neglected on the interpretation of <sup>14</sup>C datasets. An underprediction of biomass burning POA by the model during some mornings also explains a part of the model-measurement differences. The <i>f</i><sub>NF</sub> of urban POA and SOA precursors is an important parameter that needs to be better constrained by measurements. Performing faster (≤3 h) <sup>14</sup>C measurements in future campaigns is critical to further progress in this area. To our knowledge this is the first time that radiocarbon measurements are used together with aerosol mass spectrometer (AMS) organic components to assess the performance of a regional model for organic aerosols.