Can we use ground-based measurements of HCFCs and HFCs to derive their emissions, lifetimes, and the global OH abundance?

Abstract:

The hydroxyl radical (OH) is the major oxidant in the atmosphere. Reaction with OH is the primary removal process for many non-CO₂greenhouse gases (GHGs), ozone-depleting substances (ODSs) and their replacements, e.g. hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs). Traditionally, the global OH abundance is inferred using the observed atmospheric rate of change for methyl chloroform (MCF). Due to the Montreal Protocol regulation, the atmospheric abundance of MCF has been decreasing rapidly to near-zero values. It is becoming critical to find an alternative reference compound to continue to provide quantitative information for the global OH abundance.

Our model analysis using the NASA 3-D GEOS-5 Chemistry Climate Model suggests that the inter-hemispheric gradients (IHG) of the HCFCs and HFCs show a strong linear correlation with their global emissions. Therefore it is possible to use (i) the observed IHGs of HCFCs and HFCs to estimate their global emissions, and (ii) use the derived emissions and the observed long-term trend to calculate their lifetimes and to infer the global OH abundance. Preliminary analysis using a simple global two-box model (one box for each hemisphere) and information from the global 3-D model suggests that the quantitative relationship between IHG and global emissions varies slightly among individual compounds depending on their lifetime, their emissions history and emission fractions from the two hemispheres. While each compound shows different sensitivity to the above quantities, the combined suite of the HCFCs and HFCs provides a means to derive global OH abundance and the corresponding atmospheric lifetimes of long-lived gases with respect to OH (t_OH). The fact that the OH partial lifetimes of these compounds are highly correlated, with the ratio of t_OH equal to the reverse ratio of their OH thermal reaction rates at 272K, provides an additional constraint that can greatly reduce the uncertainty in the OH abundance and t_OH estimates. We will use the observed IHGs and long-term trends of three major HCFCs and six major HFCs in the two-box model to derive their global emissions and atmospheric lifetimes as well as the global OH abundance. The derived global OH abundance between 2000 and 2014 will be compared with that derived using MCF for consistency.