Oxidative capacity of the Mexico City atmosphere ? Part 2: A RO_x radical cycling perspective

P. M. Sheehy (1, 2) , R. Volkamer (1, 3, 4, 5) , L. T. Molina (1, 2) , M. J. Molina (1, 3)

1 MIT - Massachusetts Institute of Technology
2 MCE2 - Molina Center for the Energy and the Environment
3 UC San Diego - University of California [San Diego]
4 CIRES - Cooperative Institute for Research in Environmental Sciences
5 University of Colorado [Boulder]

A box model using measurements from the Mexico City Metropolitan Area study in the spring of 2003 (MCMA-2003) is presented to study RO_x (RO_x=OH+HO₂+RO₂+RO) radical cycling in the troposphere. Model simulations were performed with the Master Chemical Mechanism (MCMv3.1) constrained with 10 min averaged measurements of major radical sources (i.e., HCHO, HONO, O₃, CHOCHO, etc.), radical sink precursors (i.e., NO, NO₂, SO₂, CO, and 102 volatile organic compounds VOC), meteorological parameters (temperature, pressure, water vapor concentration, dilution), and photolysis frequencies.

Modeled HO_x concentrations compare favorably with measured concentrations for most of the day; however, the model under-predicts the concentrations of radicals in the early morning. This "missing reactivity" is highest during peak photochemical activity, and is least visible in a direct comparison of HO_x radical concentrations. The true uncertainty due to "missing reactivity" is apparent in parameters like chain length, and ozone production (P(O₃)). For example, the integral amount of ozone produced could be under-predicted by a factor of two. Our analysis highlights that apart from uncertainties in emissions, and meteorology, there is an additional major chemical uncertainty in current models.

Domaines: Océan, Atmosphère

P. M. Sheehy, R. Volkamer, L. T. Molina, M. J. Molina. Oxidative capacity of the Mexico City atmosphere ? Part 2: A RO_x radical cycling perspective. Atmospheric Chemistry and Physics Discussions, 2008, 8 (2), pp.5359-5412. ⟨hal-00304036⟩