Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 9 1 2009 10.5194/acpd-9-4417-2009 http://www.atmos-chem-phys-discuss.net/9/4417/2009/ http://www.atmos-chem-phys-discuss.net/9/4417/2009/acpd-9-4417-2009.html http://www.atmos-chem-phys-discuss.net/9/4417/2009/acpd-9-4417-2009.pdf 4417 4488 2009-02-17 Evaluation of new secondary organic aerosol models for a case study in Mexico City K. Dzepina R. M. Volkamer S. Madronich P. Tulet I. M. Ulbrich Q. Zhang C. D. Cappa P. J. Ziemann J. L. Jimenez jose.jimenez@colorado.edu Cooperative Institute for Research in the Environmental Sciences (CIRES), University of Colorado at Boulder, Boulder, CO, USA Department of Chemistry and Biochemistry, Univ. of Colorado at Boulder, Boulder, CO, USA National Center for Atmospheric Research – Atmospheric Chemistry Division, Boulder, CO, USA Meteo France/CNRM-GREI, Toulouse, France Atmospheric Sciences Research Center, University at Albany, SUNY, Albany, NY, USA Dept. of Civil and Environmental Engineering, Univ. of California at Davis, Davis, CA, USA Air Pollution Research Center, Univ. of California, Riverside, CA, USA Recent field studies have found large discrepancies in the measured vs. modeled SOA mass loadings in both urban and regional polluted atmospheres. The reasons for these large differences are unclear. Here we revisit a case study of SOA formation in Mexico City described by Volkamer et al. (2006), during a photochemically active period when the impact of regional biomass burning is minor or negligible, and show that the observed increase in OA/ΔCO is consistent with results from several groups during MILAGRO 2006. Then we use the case study to evaluate three new SOA models: 1) the update of aromatic SOA yields from recent chamber experiments (Ng et al., 2007); 2) the formation of SOA from glyoxal (Volkamer et al., 2007a); and 3) the formation of SOA from primary semivolatile and intermediate volatility species (P-S/IVOC) (Robinson et al., 2007). We also evaluate the effect of reduced partitioning of SOA into POA (Song et al., 2007). Traditional SOA precursors (mainly aromatics) by themselves still fail to produce enough SOA to match the observations by a factor of 7. The new low-NO_x aromatic pathways with very high SOA yields make a very small contribution in this high-NO_x urban environment as the RO₂^.+NO reaction dominates the fate of the RO₂^. radicals. 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