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

Abstract

A model of atmospheric photochemistry and transport has been developed and applied toward investigating global tropospheric chemistry. The Model of Atmospheric Transport and Chemistry ‐ Max‐Planck‐Institute for Chemistry version (MATCH‐MPIC) is described and key characteristics of its global simulation are presented and compared to available observations. MATCH‐MPIC is an “offline” model which reads in gridded time‐dependent values for the most basic meteorological parameters (e.g., temperature, surface pressure, horizontal winds), then uses these to compute further meteorological parameters required for atmospheric chemistry simulations (convective transport, cloud microphysics, etc.). The meteorology component of MATCH‐MPIC simulates transport by advection, convection, and dry turbulent mixing, as well as the full tropospheric hydrological cycle (water vapor transport, condensation, evaporation, and precipitation). The photochemistry component of MATCH‐MPIC represents the major known sources (e.g., industry, biomass burning), transformations (chemical reactions and photolysis), and sinks (e.g., wet and dry deposition) which affect the O3hyphen;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. O3 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 recent version of the model. We conclude that the simulated stratosphere‐troposphere flux of O3 is too large, despite the fact that the total flux is 1100 Tg(O3)/yr, whereas the upper limit estimated in recent literature is over 1400 Tg(O3)/yr. The OH distribution yields a tropospheric CH4 lifetime of 10.1 years, in contrast to the lifetime of 7.8 years in the earlier model version, which nearly spans the range of current estimates in the literature (7.5–10.2 years). Surface CO mixing ratios are in good agreement with observations. NO is generally underestimated, a problem similar to what has also been found in several other recent model studies. HNO3 is also considerably underestimated. H2O2 and CH3OOH, on the other hand, are in relatively good agreement with available observations, though both tend to be underestimated at high concentrations and overestimated at low concentrations. Possible reasons for these differences are considered.