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 shor ter than about 340 nm, resulting in production of excited O(D-1) atoms, who se reaction with water vapor produces OH radicals. In the background atmosp here the OH radicals mostly react with CO, and with CH4 and some of its oxi dation products, which in turn are oxidized by OH. Depending on the availab ility of NO, catalysts, ozone may be produced or destroyed in amounts that are much greater than the downward flux of ozone from the stratosphere to t he troposphere. Using the 3D chemical-transport model MATCH, global distrib utions and budget analyses are presented for tropospheric O-3, CH4, CO, and the "odd hydrogen" compounds OH, HO2 and H2O2. We show that OH is present in maximum concentrations in the tropics, and that most of the chemical bre akdown of CO and CH4 also occurs in equatorial regions. We also split the t roposphere into continental and marine regions, and show that there is a tr emendous difference in photochemical O-3 and OH production for these region s, much larger than the difference between the northern hemisphere and sout hern hemisphere. Finally, we show the results from a numerical simulation i n which we reduced the amount of ozone in the model stratosphere by a facto r of 10 (which in turn reduced the flux of O-3 into the troposphere by abou t the same factor). Nevertheless, for summer conditions, model calculated O -3 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 s tratosphere. This indicates that, at least under these conditions, O-3 conc entrations in the lower troposphere are largely controlled by in situ photo chemistry, with only a secondary influence from stratospheric influx.