ATMOSPHERIC TREND AND LIFETIME OF CHLORODIFLUOROMETHANE (HCFC-22) ANDTHE GLOBAL TROPOSPHERIC OH CONCENTRATION

Concentrations of CHClF2 (HCFC-22) in clean background air collected a t Cape Grim, Tasmania, over the period 1978-1996, and at La Jolla, Cal ifornia, over the period 1992-1997, have been measured by oxygen-doped electron capture detection gas chromatography. The mid-1996 dry-air m ole fractions and trends were 116.7 parts per trillion (ppt) and 6.0 p pt yr(-1) in Cape Grim and 132.4 ppt and 5.5 ppt yr(-1) in California: respectively. These observations, together with estimates of industri al emissions, have been fitted to a two-dimensional global atmospheric model by an optimal estimation inversion technique to yield estimated tropospheric and total atmospheric Lifetimes for chemical destruction of CHClF2 of 9.1(-2.8)(+4.4) years and 10.0(-3.0)(+4.4) years, respec tively. These lifetimes Correspond to a temperature- and density-weigh ed global tropospheric OH abundance of 11.0(-3.6)(+5.0) x 10(5) radica l cm(-3), which is in statistical agreement with our recent more accur ate estimate of OH abundance based on measurements of CH3CCl3. Our ana lysis suggests that, compared to current industrial estimates, souther n hemisphere emissions are higher, global emissions are larger in earl ier years and smaller in later years, and, finally, production by nonr eporting companies is less.