Tropospheric chemistry: A global perspective

A model for the photochemistry of the global troposphere constrained by observed concentrations of H 2 O, O 3 , CO, CH 4 , NO, NO 2 , and HNO 3 is presented. Data for NO and NO 2 are insufficient to define the global distribution of these gases but are nonetheless useful in limiting several of the more uncertain parameters of the model. Concentrations of OH, HO 2 , H 2 O 2 , NO, NO 2 , NO 3 , N 2 O 5 , HNO 2 , HO 2 NO 2 , CH 3 O 2 , CH 3 OOH, CH 2 O, and CH 3 CCl 3 are calculated as functions of altitude, latitude, and season. Results imply that the source for nitrogen oxides in the remote troposphere is geographically dispersed and surprisingly small, less than 10 7 tons N yr −1 . Global sources for CO and CH 4 are 1.5 × 10 9 tons C yr −1 and 4.5 × 10 8 tons C yr −1 , respectively. Carbon monoxide is derived from combustion of fossil fuel (15%) and oxidation of atmospheric CH 4 (25%), with the balance from burning of vegetation and oxidation of biospheric hydrocarbons. Production of CO in the northern hemisphere exceeds that in the southern hemisphere by about a factor of 2. Industrial and agricultural activities provide approximately half the global source of CO. Oxidation of CO and CH 4 provides sources of tropospheric O 3 similar in magnitude to loss by in situ photochemistry. Observations of CH 3 CCl 3 could offer an important check of the tropospheric model and results shown here suggest that computed concentrations of OH should be reliable within a factor of 2. A more definitive test requires better definition of release rates for CH 3 CCl 3 and improved measurements for its distribution in the atmosphere.