MEASUREMENTS OF CCL3F AND CCL4 AT HARWELL OVER THE PERIOD JANUARY 1975 JUNE 1981 AND THE ATMOSPHERIC LIFETIME OF CCL3F

Measurements of CCl3F and CCl4 in the ambient air were made at Harwell since Oct. 1974. The background CCl3F concentration at Harwell increased with time throughout the period up to June 1981, when it had reached a value of 191 ppt [parts/trillion]. The increase was .apprx. 1.1 ppt/mo. at the start of the study, but it had slowed down following the leveling off in global CCl3F emissions. This data set represented one of the longest-maintained attempts to determine the growth in CCl3F in which careful attention was paid to details of absolute calibration. Over the period that the measurements were made, the CCl3F concentration increased by over half its original value, which provided a solid basis for comparison with release estimates. Two-dimensional modeling was employed to obtain the atmospheric lifetime of CCl3F from the observed Harwell 1975-1981 series and the published CCl3F historical release. The best estimate of the CCl3F atmospheric lifetime was 75 yr with 5-95% confidence limits of 65-90 yr. The limits were tighter than those obtained from the first 3 yr of data, which would in principle have allowed a lifetime as short as 18 yr. This latest estimate of 75 yr was wholly consistent with current theoretical estimates of the atmospheric lifetime due to stratospheric photolysis alone. Since 1979, the precision and resolution of the CCl4 measurements at Harwell has been significantly improved. The background CCl4 concentration found at Harwell for 1980-1981, was 99-102 ppt. While these values were in excellent agreement with model calculations, assuming CCl4 was wholly man-made in origin, they were lower than some other published observations. Simultaneous observations of elevated concentrations of CCl3F and CCl4 supported the contention that CCl4 was largely of man-made origins.