The paper presents a model for the photochemistry of the global troposphere, constrained by observed concentrations of key gases such as H₂O, O₃, CO, CH₄, NO, NO₂, and HNO₃. The model calculates concentrations of various species like OH, HO₂, H₂O₂, NO, NO₂, NO₃, N₂O₅, HNO₃, H₂O₂NO₂, CH₂O₂, CH₂OOH, CH₂O, and CH₂CCl₃ as functions of altitude, latitude, and season. Key findings include: 1. **Source of Nitrogen Oxides**: The source of nitrogen oxides in the remote troposphere is geographically dispersed and surprisingly small, less than 10⁷ tons yr⁻¹. 2. **Global Sources of CO and CH₄**: Global sources for CO and CH₄ are 1.5 × 10⁷ tons C yr⁻¹ and 4.5 × 10⁷ tons C yr⁻¹, respectively. 3. **Carbon Monoxide**: 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. 4. **Ozone Production and Loss**: In situ production of O₃ arises from oxidation of CO and CH₄ and depends on the concentration of NO. Ozone is also consumed in the troposphere by gas-phase chemistry, independent of NO. 5. **Model Validation**: The model's predictions for tropospheric OH concentrations are reliable within a factor of 2, with a more definitive test requiring better definition of release rates for CH₃CCl₃ and improved measurements for its distribution in the atmosphere. The paper also discusses the chemistry of odd hydrogen (OH) and odd oxygen (O₃), emphasizing the importance of reactions involving H₂O, HO₂, H₂O₂, and NO. Observational data for H₂O, O₃, CO, CH₄, NO, and NO₂ are reviewed, highlighting uncertainties and the need for more comprehensive data to improve model accuracy.The paper presents a model for the photochemistry of the global troposphere, constrained by observed concentrations of key gases such as H₂O, O₃, CO, CH₄, NO, NO₂, and HNO₃. The model calculates concentrations of various species like OH, HO₂, H₂O₂, NO, NO₂, NO₃, N₂O₅, HNO₃, H₂O₂NO₂, CH₂O₂, CH₂OOH, CH₂O, and CH₂CCl₃ as functions of altitude, latitude, and season. Key findings include: 1. **Source of Nitrogen Oxides**: The source of nitrogen oxides in the remote troposphere is geographically dispersed and surprisingly small, less than 10⁷ tons yr⁻¹. 2. **Global Sources of CO and CH₄**: Global sources for CO and CH₄ are 1.5 × 10⁷ tons C yr⁻¹ and 4.5 × 10⁷ tons C yr⁻¹, respectively. 3. **Carbon Monoxide**: 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. 4. **Ozone Production and Loss**: In situ production of O₃ arises from oxidation of CO and CH₄ and depends on the concentration of NO. Ozone is also consumed in the troposphere by gas-phase chemistry, independent of NO. 5. **Model Validation**: The model's predictions for tropospheric OH concentrations are reliable within a factor of 2, with a more definitive test requiring better definition of release rates for CH₃CCl₃ and improved measurements for its distribution in the atmosphere. The paper also discusses the chemistry of odd hydrogen (OH) and odd oxygen (O₃), emphasizing the importance of reactions involving H₂O, HO₂, H₂O₂, and NO. Observational data for H₂O, O₃, CO, CH₄, NO, and NO₂ are reviewed, highlighting uncertainties and the need for more comprehensive data to improve model accuracy.