Possible perturbations to atmospheric CO, CH4, and OH

A photochemical model is used to predict temporal trends in OH, CH 4 , and CO over the next 50 years and to assess possible past changes in these trace gases from 1860 to 1985. Various scenarios of perturbed CH 4 and CO levels based on recently reported CH 4 and CO data are simulated at several levels of background NO x . With low NO x conditions (NO + NO 2 = 25 pptv (parts per trillion by volume)), typical of the nonpolluted troposphere, we compute a monotonic loss of tropospheric OH from 1860 to 2035, with the magnitude of the decrease dependent on CO and CH 4 increases during the period. If current trends continue (ground‐level mixing ratios (mole fractions) of CH 4 rising about 1% per year), by 2035 northern latitude CH 4 will increase from 1.6 to 2.9 ppmv (parts per million by volume) and CO will double or triple its present day level (to about 250–350 ppbv (parts per billion by volume) in the nonpolluted northern hemisphere). The column abundance of OH in the background troposphere will decrease 25–35%, depending on the magnitude of CH 4 and CO increases and assuming that global temperature increases do not raise water vapor levels during that time. Calculations with increased H 2 O show a 17–30% decrease in OH. Under higher‐NO x conditions (1 ppbv), OH shows a decline from 1860 to 2035, but it shows only half as much decline as with low‐NO x conditions. In the case of NO x in a transitional zone (i.e., NO x increasing from 20 pptv in 1860 to 0.5 ppbv in 2035), CO and CH 4 increases accompany a rise in OH, followed by a small decline. The turning point in OH depends on the rate of change in NO x and O 3 . Recently observed upward trends in CO and CH 4 are probably due to increasing emissions of both CH 4 and CO. We always compute a temporal increase in tropospheric O 3 when CH 4 and CO increase. Typical 1860 values for surface O 3 are 25 ppbv, compared to 30 ppbv (NO x = 25 pptv)and 40 ppbv (NO x = 1 ppbv) in 1985. In a transition NO x zone we find surface O 3 increasing from 10 ppbv in 1860 to 27 ppbv in 1985.