Singlet molecular oxygen, O2(1Δg) has been speculated to play a role in the formation of photochemical smog. It is mainly produced from the photolysis of ozone in the troposphere and lost in the troposphere due to the non-reactive collisions with third bodies (e.g. N2, O2, H2O) and by reacting with ozone. The mean annual flux from sources to sinks, seasonal variation and global distribution of O2(1Δg) were studied for the first time using a three-dimensional global chemistry transport model, STOCHEM-CRI. The global burden and the atmospheric lifetime of O2(1Δg) were found to be 0.58 Gg and 0.29 s, respectively. Up to 0.6 ppt of O2(1Δg) was modelled at the surface over anthropogenic emission regions (e.g. North America, central Europe and South East Asia) and forested regions (e.g. Africa and south America). The model O2(1Δg) could have a modest effect on the production of hydroperoxides (up to 3.0 ppt) through reaction with olefins.
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