The paper presents a model for the photochemistry of the global troposphere, constrained by observed concentrations of H₂O, O₃, CO, CH₄, NO, NO₂, and HNO₃. The model calculates concentrations of various reactive species as functions of altitude, latitude, and season. Results suggest that the source of 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 yr⁻¹ and 4.5 × 10⁸ tons C yr⁻¹, respectively. CO is derived from combustion of fossil fuels (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. The model suggests 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. The hydroxyl radical plays an important role in the photochemistry of the troposphere. Reaction with OH provides the dominant path for removal of a variety of atmospheric species. The chemistry of tropospheric OH is complex. Hydroxyl is produced by reaction of O(¹D) with H₂O. OH may be regenerated by a suite of reactions involving HO₂ and H₂O₂. Rates for these reactions vary appreciably in both time and space. A comprehensive test of photochemical models for OH would require simultaneous measurement of these and a number of other species. The concentration of OH has been measured in selected environments by a number of investigators. The measurements are difficult, and concentrations reported so far are subject to considerable uncertainty. Analysis of data for atmospheric CH₃CCl₃ may provide the best current check on photochemical models. Methylchloroform is used extensively as a solvent. It is released to the atmosphere at a known rate. The concentration of the gas in the atmosphere has increased by more than a factor of 3 since it was first measured. Methylchloroform is removed from the atmosphere primarily by reaction with tropospheric OH. To the extent that the source is determined, measurements of atmospheric CH₃CCl₃ may be used to check calculations for the global distribution of OH. Preliminary studies suggested that the concentration of OH was overestimated in early models by aThe paper presents a model for the photochemistry of the global troposphere, constrained by observed concentrations of H₂O, O₃, CO, CH₄, NO, NO₂, and HNO₃. The model calculates concentrations of various reactive species as functions of altitude, latitude, and season. Results suggest that the source of 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 yr⁻¹ and 4.5 × 10⁸ tons C yr⁻¹, respectively. CO is derived from combustion of fossil fuels (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. The model suggests 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. The hydroxyl radical plays an important role in the photochemistry of the troposphere. Reaction with OH provides the dominant path for removal of a variety of atmospheric species. The chemistry of tropospheric OH is complex. Hydroxyl is produced by reaction of O(¹D) with H₂O. OH may be regenerated by a suite of reactions involving HO₂ and H₂O₂. Rates for these reactions vary appreciably in both time and space. A comprehensive test of photochemical models for OH would require simultaneous measurement of these and a number of other species. The concentration of OH has been measured in selected environments by a number of investigators. The measurements are difficult, and concentrations reported so far are subject to considerable uncertainty. Analysis of data for atmospheric CH₃CCl₃ may provide the best current check on photochemical models. Methylchloroform is used extensively as a solvent. It is released to the atmosphere at a known rate. The concentration of the gas in the atmosphere has increased by more than a factor of 3 since it was first measured. Methylchloroform is removed from the atmosphere primarily by reaction with tropospheric OH. To the extent that the source is determined, measurements of atmospheric CH₃CCl₃ may be used to check calculations for the global distribution of OH. Preliminary studies suggested that the concentration of OH was overestimated in early models by a