A model for studies of tropospheric photochemistry: Description, global distributions, and evaluation

A model of atmospheric photochemistry and transport has been developed and applied toward investigating global tropospheric chemistry. The Model of At mospheric Transport and Chemistry Mas-Planck-Institute for Chemistry versio n (MATCH-MPIC) is described and key characteristics of its global simulatio n are presented and compared to available observations. MATCH-MPIC is an "o ffline" model which reads in gridded time-dependent values for the most bas ic meteorological parameters (e.g., temperature, surface pressure, horizont al winds), then uses these to compute further meteorological parameters req uired for atmospheric chemistry simulations (convective transport, cloud mi crophysics, etc.). The meteorology component of MATCH-MPIC simulates transp ort, by advection, convection, and dry turbulent mixing, as well as the ful l tropospheric hydrological cycle (water vapor transport, condensation, eva poration, and precipitation). The photochemistry component of MATCH-MPIC re presents the major known sources (e.g., industry, biomass burning), transfo rmations (chemical reactions and photolysis), and sinks (e.g., wet and dry deposition) which affect the O-3-HOx-NOy-CH4-CO photochemical framework of the "background" troposphere. The results of two versions of the model are considered, focusing on the more recent version. O-3 is in relatively good agreement with observed soundings, although it is generally underestimated at low levels and overestimated at high levels, particularly for the more r ecent version of the model. We conclude that the simulated stratosphere-tro posphere flux of O-3 is too large, despite the fact that the total flux is 1100 Tg(O-3)/yr, whereas the upper limit estimated in recent literature is over 1400 Tg(O-3)/yr. The OH distribution yields a tropospheric CH4 lifetim e of 10.1 years, in contrast to the Lifetime of 7.8 years in the earlier mo del version, which nearly spans the range of current estimates in the liter ature (7.5-10.2 years). Surface CO mixing ratios are in good agreement with observations. NO is generally underestimated, a problem similar to what ha s also been found in several other recent model studies. HNO3 is also consi derably underestimated. H2O2 and CH3OOH, on the other hand, are in relative ly good agreement with available observations: though both tend to be under estimated at high concentrations and overestimated at low concentrations. P ossible reasons for these differences are considered.