Abstract
Ozone concentration data measured in 1977-1979 and 1981-1983 at rural sites in northwestern England have been analyzed in relation to elevated concentrations. Overall, concentrations exceed 60 p.p.b. on 11.2% of days, 80 p.p.b. on 4.2% of days, and reach levels >100 p.p.b. on 1.5% of days monitored. It is concluded that photochemical pollution is the most frequent cause of elevated concentrations, and that both long-range (continental) and middle-distance (U.K. urban) sources contribute. On a smaller percentage of days, elevated ozone levels arise from enhanced intrusions of stratospheric air associated with vigorous frontal activity. The meteorological situations associated with tropospheric photochemical ozone formation are summer anticyclonic conditions, in common with other observations in the U.K. and other parts of the world.