Trends of Radiatively Important Trace Gases and their Relationship to Tropospheric Photochemistry

Infrared-absorbing trace gases other than CO₂ are accumulating in the atmosphere. The climatic effect of these gases will be comparable and will add to that expected from CO₂. Tropospheric chemistry plays an important role in the budgets and trends of two radiatively important trace gases, i. e., CH₄ and tropospheric O₃.The observed trend of CH₄ indicates that there has been a factor of two increases in its concentration in the last 350 years and the rate of increase has become greater in the last century. The increase in CH₄ is most likely due to increases in its sources, such as human activities in agriculture and energy use. Changes in the photochemical sink of CH₄ may also contribute.There is strong evidence that tropospheric O₃ in industrial countries has increased significantly due to anthropogenic emissions of NO_x and hydrocarbons. However, the trend of tropospheric O₃ has not been quantitatively established because there are large spatial and temporal variations in the distribution of O₃. More extensive measurements are needed to quantify this trend.In this report we have identified the major photochemical processes that control the budgets and trends of CH₄ and tropospheric O₃. It is shown that human activities may have significantly perturbed the atmospheric distributions of CO, NO_x, O₃, nonmethane hydrocarbons, and CH₄. The observed trends of O₃ and CH₄ are qualitatively consistent with the current understanding of their photochemistry and budgets. However, models with realistic transport and photochemical processes need to be developed to evaluate and predict the trends quantitatively.