On the background photochemistry of tropospheric ozone

We present a largely tutorial overview of the main processes that influence the photochemistry of the background troposphere. This is mostly driven by the photolysis of ozone by solar ultraviolet radiation of wavelengths shorter than about 340 nm, resulting in production of excited O(1D) atoms, whose reaction with water vapor produces OH radicals. In the background atmosphere the OH radicals mostly react with CO, and with CH₄ and some of its oxidation products, which in turn are oxidized by OH. Depending on the availability of NO_x, catalysts, ozone may be produced or destroyed in amounts that are much greater than the downward flux of ozone from the stratosphere to the troposphere. Using the 3D chemical-transport model MATCH, global distributions and budget analyses are presented for tropospheric O₃, CH₄, CO, and the “dd hydrogen” compounds OH, HO₂ and H₂O₂. We show that OH is present in maximum concentrations in the tropics, and that most of the chemical breakdown of CO and CH₄ also occurs in equatorial regions. We also split the troposphere into continental and marine regions, and show that there is a tremendous difference in photochemical O₃ and OH production for these regions, much larger than the difference between the northern hemisphere and southern hemisphere. Finally, we show the results from a numerical simulation in which we reduced the amount of ozone in the model stratosphere by a factor of 10 (which in turn reduced the flux of O₃ into the troposphere by about the same factor). Nevertheless, for summer conditions, model calculated O₃ mixing ratios below 5 km in the mid to high latitudes were about 70–90% as high as those calculated with the full downward flux of ozone from the stratosphere. This indicates that, at least under these conditions, O₃ concentrations in the lower troposphere are largely controlled by in situ photochemistry, with only a secondary influence from stratospheric influx.