Atmospheric methane, CO and the gaseous OH radical are interdependent: If CH4, CO or OH is perturbed, background concentrations of the other two constituents are affected. Perturbations to OH alter photo-oxidation rates of numerous natural and anthropogenic trace gases and affect lifetimes of those species that pass from the Earth's surface to the free troposphere and stratosphere. It is now known that global atmospheric methane concentrations are increasing1-6; less definite data suggest that carbon monoxide is also increasing1,7-9. Even before the measurements of refs 1-9 were made, modelling studies of CH4-CO-OH coupling had led to predictions10-12 of future temporal increases of CH4 and CO. Here we look backwards in time, using a photochemical model to simulate the trace-gas composition of the unpolluted troposphere at the start of the industrial era (taken as 1860), and at intervals up to 1985. We find that the OH concentration in the background troposphere has decreased significantly and O3 has increased due to increases of CH4 and CO; calculated changes depend on temporal trends of NOx (NOx = NO + NO2), for which no historical data are available. The calculations allow recent trace-gas trends affecting background chemistry and climate to be viewed in a longer-term context.
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