Measurement of CCl 3F and CCl 4 at Harwell over the period January 1975–November 1977

Abstract

Measurements of CCl 3F and CCl 4 in the ambient air have been made at Harwell since October 1974. The measurements are taken every 2 h on average and this data is used to produce monthly histograms. Over the period January 1975–November 1977 the mean and standard deviations of these histograms for both F11 and CCl 4 have varied considerably. Months dominated by easterly winds show larger values than months dominated by westerly winds, reflecting no doubt the difference in anthropogenic activity under the two classes of trajectories. The average CCl 3F concentration at Harwell increased with time and the data have been used to determine the growth in the background CCl 3F concentration over the period in question. This was occurring at approximately 1.1 pptv per month, in very good agreement with our calculated value of 1.0 pptv per month, for removal entirely by photolysis in the stratosphere. The data is of sufficient quality to show that a substantial tropospheric sink for CCl 3F is most unlikely. There is a good correlation between the monthly mean concentrations of CCl 3F and CCl 4, which is also increasing with time. A value for the rate of CCl 4 increase in the background atmosphere has been derived (0.35 pptv per month) which is significantly different from our calculated estimate (0.2 pptv per month). The reason for this could either be errors made in estimating emissions, or possibly formation in the atmosphere from C 2Cl 4. In any case the data fits the idea of an anthropogenic source much better than a natural one.