Critical evaluation of emissions of potential new gases for OH estimation

Abstract

Reactions with OH in the troposphere provide the dominant removal path for a wide variety of greenhouse and ozone-depleting gases. Accurate determination of global and regional tropospheric OH concentrations is therefore very important and can be achieved by measuring gases that react with OH and whose emissions are well known. CH 3 CCl 3 has been used for this purpose. Prior studies have shown that three of the new chlorofluorocarbon (CFC) substitutes, HFC-134a (CH 2 FCF 3), HCFC-141b (CH 3 CCl 2 F), and HCFC-142b (CH 3 CClF 2), could be used potentially to derive accurate global-average OH concentrations in the future provided that the industrial emissions of these gases can be reliably estimated. As a test of their currently available emission estimates, we optimally determine the monthly emissions for these three gases using global measurements from the NOAA-CMDL and Advanced Global Atmospheric Gases Experiment (AGAGE) networks. Our study uses two distinct OH scenarios (constant OH, quadratically varying OH) derived from CH 3 CCl 3 observations. We conclude that current published industrial estimates [Alternative Fluorocarbon Environmental Assessment Study (AFEAS), 2002] of HCFC-142b and HCFC-141b need to be increased by 18 ± 15% over the 1992-2000 period and 10 -13% 15% over the 1993-2000 period, respectively, while the emissions of HFC-134a (from 1993 to 2000) are only 4 ± 10% more than those yielding the best agreement with atmospheric observations. Estimates of global-average OH concentrations using measurements and AFEAS [2002] emissions differ statistically from the average OH derived from CH 3 CCl 3 for HCFC-141b and HCFC-142b but not for HFC-134a. On the other hand, OH trends deduced from all three HCFC/HFC gases are implausibly large implying problems with the AFEAS [2002] estimates of the temporal variations in their emissions. As the viability of CH 3 CCl 3 for estimating OH declines in the future, additional evaluations of the emissions and OH kinetics of HCFC-141b, HCFC-142b, and HFC-134a therefore have high priority. Copyright 2002 by the American Geophysical Union.