Oxidative capacity of the Mexico City atmosphere ? Part 1: A radical source perspective

R. Volkamer (1, 2) , P. M. Sheehy (1, 3) , L. T. Molina (1, 3) , M. J. Molina (2)

1 EAPS - Department of Earth, Atmospheric and Planetary Sciences [MIT, Cambridge]
2 Department of Chemistry and Biochemistry
3 MCE2 - Molina Center for the Energy and the Environment

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 RO_x (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 RO_x production rate was calculated directly from these observations; MCM was used to estimate further RO_x production from unconstrained sources, and express overall RO_x 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. The peak radical production is found to be higher than in any other urban influenced environment studied to date; further, differences exist in the timing of radical production.

Our measurements and analysis comprise a database that enables testing of the representation of radical sources in photochemical models. Since the photochemical processing of pollutants is radical-limited in the MCMA, our analysis identifies the drivers for such processing. Three pathways are identified by which reductions in VOC emissions induce reductions in peak concentrations of secondary pollutants, such as O₃ and secondary organic aerosol (SOA).

Domaines: Océan, Atmosphère

R. Volkamer, P. M. Sheehy, L. T. Molina, M. J. Molina. Oxidative capacity of the Mexico City atmosphere ? Part 1: A radical source perspective. Atmospheric Chemistry and Physics Discussions, 2007, 7 (2), pp.5365-5412. ⟨hal-00302722⟩