Is There a Megacity Effect on SOA Production

Mexico City has a more concentrated set of emission sources than found in most, if not all, areas of the U.S. resulting in very high concentrations of gas phase pollutants. It is of interest to determine whether secondary organic aerosol (SOA) per unit precursor depends on absolute concentration. If it did, then the growth of megacities would lead to a different set of impacts compared to the situation where the same number of people and the same amount of industrial activity remain spread out over several smaller population centers. An expectation that SOA production might be greater in regions with high emissions can be justified on the basis of the absorptive/partitioning model, in which aerosol yields increase when there is a large amount of organics in the aerosol phase into which low volatility VOCs can partition. SOA production in Mexico City urban air masses is determined from the change in OA/CO as a function of photochemical age. Dilution is accounted for by normalizing results to CO, an urban tracer that is assumed to be emitted in proportion to SOA precursors. SOA production, so determined during the 2006 Milagro campaign, is compared with that determined in the eastern U.S. by ourselves and others during the 2002 and 2004 NEAQS campaigns. To provide a context for this comparison, we contrast chemical conditions in Mexico City with the NEAQS domain. Subject to several caveats, the Mexico City value for SOA per unit CO is similar to that found in the eastern U.S. __________ NOTICE: This manuscript has been authored by employees of Brookhaven Science Associates, LLC under Contract No. DE-AC0298CH10886 with the U.S. Department of Energy. The publisher by accepting the manuscript for publication acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes.