Global OH Abundance Estimate Inferred from Surface Measurements of HFCs and HCFC-22

An accurate estimate of global hydroxyl radical (OH) abundance is important for model projections of air quality, climate, and stratospheric ozone recovery. As the atmospheric mixing ratios of methyl chloroform (CH₃CCl₃, MCF) approaches zero, it has becoming increasingly difficult to use MCF to quantitatively derive atmospheric OH abundance. In our recent work, we have presented the theoretical analysis on the possibility of using the combination of HFC-32, HFC-134a, HFC-152a, and HCFC-22, as an MCF-alternative to infer global OH abundance. The primary basis of this multi-species approach is that the OH lifetimes can be estimated by scaling the thermal reaction rates of a reference gas at 272 K on global and hemispheric scales. Thus, the derived hemispheric and global OH estimates are forced to reconcile the observed trends and gradientfor all four compounds simultaneously. While HFC-32 and HFC-134a will provide accurate information on global OH, the addition of HCFC-22 and HFC-152a add constraints to hemispheric and seasonalinformation on atmospheric OH abundance.

In this work, we will use the observed surface measurements of the three HFCs and HCFC-22 from 2000 to present from the Advanced Global Atmospheric Gases Experiment (AGAGE) network in the GEOS 3-D chemistry model as observational constraints to derive (a) latitudinally resolved emissions of these gases, (b) the optimal global and hemispheric OH abundance that best reconciles the observed long-term trend and interhemispheric gradient of these trace gases. And these derived OH abundance estimates will be evaluated against those from the MCF-based analysis.