Abundances and trends of atmospheric chlorodifluoromethane and bromomethane.

摘要

Measurements of CH3Br (bromomethane) in flasks of archived air collected since 1978 at Cape Grim, Tasmania, were used with flask data from Trinidad Head, California, to determine the atmospheric abundances and trends of this ozone-depleting natural trace gas and anthropogenic fumigant. The Trinidad Head record shows a clear seasonal cycle with an amplitude of +/-0.7 ppt, with a maximum in late April. The phase of this seasonality is consistent with the seasonality in the atmospheric OH concentration. The CH3Br annual mean for the period February 1997 to July 1998 was 9.62 ppt. No significant seasonality was observed in the Cape Grim data, suggesting a strong seasonal influence from hemispheric sources and/or non-OH sinks. A linear least squares fit to the Cape Grim data shows a January 1998 CH 3Br abundance of 7.85 ppt, with a trend of 0.028 +/- 0.009 ppt yr-1. With the assumption that a linear trend in agricultural fumigation releases of CH3Br was the only significant trend in the sources and sinks of CH3Br, a simple model was used to fit this release rate to the Cape Grim data. The lifetime of 0.55 years predicted from this model is shorter than the 0.7 years calculated using the sum of presently known sinks.;Measurements of CHClF2 (chlorodifluoromethane) in the Cape Grim archived air flasks were combined with measurements from La Jolla, California, to determine the atmospheric abundances and trends of this anthropogenic hydrochlorofluorocarbon. The Cape Grim data show a mid-1996 mean abundance of CHClF2 of 116.7 ppt with a trend of 6.0 +/- 0.1 ppt yr-1. The La Jolla data show a mid-1996 mean abundance of 132.4 ppt with a trend of 5.5 +/- 0.1 ppt yr-1. These results, together with estimates of industrial emissions, have been fitted to a two-dimensional model to yield tropospheric and total atmospheric lifetimes for chemical destruction of CHClF2 of 9.1+4.4-2.8 years and 10.0+4.4-3.0 years, respectively. These lifetimes correspond to a weighted global tropospheric OH abundance of 11.0+5.0-3.6x105 radical cm-3, which is in statistical agreement with the recent widely-accepted estimate of OH abundance based on measurements of CH3CCl3.