Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 7 2 2007 10.5194/acpd-7-5365-2007 http://www.atmos-chem-phys-discuss.net/7/5365/2007/ http://www.atmos-chem-phys-discuss.net/7/5365/2007/acpd-7-5365-2007.html http://www.atmos-chem-phys-discuss.net/7/5365/2007/acpd-7-5365-2007.pdf 5365 5412 2007-04-19 Oxidative capacity of the Mexico City atmosphere – Part 1: A radical source perspective R. Volkamer rainer@alum.mit.edu P. M. Sheehy L. T. Molina M. J. Molina Department of Earth, Atmospheric and Planetary Sciences, Massachussetts Institute of Technology, Cambridge, MA, USA Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA, USA Molina Center for Energy and the Environment (MCE2), La Jolla, CA, USA A detailed analysis of OH, HO2 and RO2 radical sources is presented for the near field photochemical regime inside the Mexico City Metropolitan Area (MCMA). During spring of 2003 (MCMA-2003 field campaign) an extensive set of measurements was collected to quantify time resolved ROx (sum of OH, HO2, RO2) radical production rates from day- and nighttime radical sources. The Master Chemical Mechanism (MCMv3.1) was constrained by measurements of (1) concentration time-profiles of photosensitive radical precursors, i.e., nitrous acid (HONO), formaldehyde (HCHO), ozone (O3), glyoxal (CHOCHO), and other oxygenated volatile organic compounds (OVOCs); (2) respective photolysis-frequencies (J-values); (3) concentration time-profiles of alkanes, alkenes, and aromatic VOCs (103 compound are treated) and oxidants, i.e., OH- and NO3 radicals, O3; and (4) NO, NO2, meteorological and other parameters. The ROx production rate was calculated directly from these observations; MCM was used to estimate further ROx production from unconstrained sources, and express overall ROx production as OH-equivalents (i.e., taking into account the propagation efficiencies of RO2 and HO2 radicals into OH radicals). Daytime radical production is found to be about 10-25 times higher than at night; it does not track the abundance of sunlight. 12-h average daytime contributions of individual sources are: HCHO and O3 photolysis, each about 20%; O3/alkene reactions and HONO photolysis, each about 15%; unmeasured sources about 30%. While the direct contribution of O3/alkene reactions appears to be moderately small, source-apportionment of ambient HCHO and HONO identifies O3/alkene reactions as being largely responsible for jump-starting photochemistry about one hour after sunrise. 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