The influence of human activities on the distribution of hydroxyl radicals in the troposphere

The free radical reactions occuring in the sunlit troposphere involving methane, carbon monoxide, oxides of nitrogen, ozone and water vapour, generate a distribution of hydroxyl radicals which play a major role in removing many of the trace gases emitted by human activities. The United Kingdom Meteorological Office two-dimensional (altitude and latitude) chemistry transport model is used to describe the trace gas life cycles and the fast photochemistry and hence to calculate the distribution of hydroxyl radicals. The mean concentration of hydroxyl radicals estimated in the lowest 0-12 km of the atmosphere is 1.2 $\times $ 10$^{6}$ molecules cm$^{-3}$ (0.74 $\times $ 10$^{6}$ molecules cm$^{-3}$ over 0-24 km). This concentration is in good agreement with estimates based on the OH radical concentrations required to balance the budgets of methyl chloroform and $^{14}$CO. The two-dimensional model was used to investigate the likely future methane concentrations that would build up in the Intergovernmental Panel on Climate Change IS92a emission scenario over the period up to the year 2050. Because of the decrease in likely future OH radical concentrations due to the influence of human activities, methane concentrations build up significantly faster than emissions. There are therefore important consequences for global warming from human interference in the oxidizing capacity of the troposphere.