Atmospheric trend and lifetime of chlorodifluoromethane (HCFC-22) and the global tropospheric OH concentration

Abstract

Concentrations of CHCIF2 (HCFC-22) in clean background air collected at Cape Grim, Tasmania, over the period 1978-1996, and at La Jolla, California, over the period 1992-1997, have been measured by oxygen-doped electron capture detection gas chromatography. The mid-1996 dry-air mole fractions and trends were 116.7 parts per trillion (ppt) and 6.0 ppt yr-1 in Cape Grim and 132.4 ppt and 5.5 ppt yr-1 in California, respectively. These observations, together with estimates of industrial 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 CHCIF2 of 9.1+4.4-2.8 years and 10.0+4.4-3.0 years, respectively. These lifetimes correspond to a temperature- and density-weighed global tropospheric OH abundance of 11.0+5.0-3.6 x 105 radical cm-3, which is in statistical agreement with our recent more accurate estimate of OH abundance based on measurements of CH3CCl3. Our analysis suggests that, compared to current industrial estimates, southern hemisphere emissions are higher, global emissions are larger in earlier years and smaller in later years, and, finally, production by nonreporting companies is less.