Department of Earth System Science

A model for the photochemistry of the global troposphere constrained by observed concentrations of H₂O, O₃, CO, CH₄, NO, NO₂, and HNO₃ is presented. Data for NO and NO₂ 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, HO₂, H₂O₂, NO, NO₂, NO₃, N₂O₅, HNO₂, HO₂NO₂, CH₃O₂, CH₃OOH, CH₂O, and CH₃CCl₃ 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 10⁷ tons N yr⁻¹. Global sources for CO and CH₄ are 1.5 × 10⁹ tons C yr⁻¹ and 4.5 × 10⁸ tons C yr⁻¹, respectively. Carbon monoxide is derived from combustion of fossil fuel (15%) and oxidation of atmospheric CH₄ (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 CH₄ provides sources of tropospheric O₃ similar in magnitude to loss by in situ photochemistry. Observations of CH₃CCl₃ 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 CH₃CCl₃ and improved measurements for its distribution in the atmosphere.