What does the global mean OH concentration tell us?

The global mean OH concentration ([ OH](GM)) has been used as an indicator of the atmospheric oxidizing efficiency and its changes over time. It is also used for evaluating the performance of atmospheric chemistry models by comparing with other models or with observationally-based reference [OH](GM) levels. We contend that the treatment of this quantity in the recent literature renders it problematic for either of these purposes. Several different methods have historically been used to compute [OH](GM) : weighting by atmospheric mass or volume, or by the reaction with CH4 or CH3CCl3. In addition, these have been applied over different domains to represent the troposphere. While it is clear that this can lead to inconsistent [OH](GM) values, to date there has been no careful assessment of the differences expected when [OH](GM) is computed using various weightings and domains. Here these differences are considered using four different 3D OH distributions, along with the weightings mentioned above applied over various atmospheric domains. We find that the [OH](GM) values computed based on a given distribution but using different domains for the troposphere can result in differences of 10% or more, while different weightings can lead to differences of up to 30%, comparable to the uncertainty which is commonly stated for [OH](GM) or its trend. Thus, at present comparing [OH](GM) values from different studies does not provide clearly interpretable information about whether the OH amounts are actually similar or not, except in the few cases where the same weighting and domain have been used in both studies. We define the atmospheric oxidizing efficiency of OH with respect to a given gas as the inverse of the lifetime of that gas, and show that this is directly proportional to the [OH](GM) value weighted by the reaction with that gas, where the proportionality constant depends on the temperature distribution and the domain. We find that the airmass-weighted and volume-weighted [OH](GM) values, in contrast, are generally poor indicators of the global atmospheric oxidizing efficiency with respect to gases such as CH4 and CH3CCl3 with a strong temperature dependence in their reaction with OH. We recommend that future studies provide both the airmass-weighted and the CH4-reaction-weighted [OH](GM) values, over the domain from the surface to a climatological tropopause. The combination of these values helps to reduce the chance of coincidental agreement between very different OH distributions. Serious evaluations of modeled OH concentrations would best be done with airmass-weighted [OH](GM) broken down into atmospheric sub-compartments, especially focusing on the tropics, where the atmospheric oxidizing efficiency is the greatest for most gases.