Global mean OH concentration

Introduction Conclusions References Tables Figures Back Close Print Version Interactive Discussion Abstract Introduction Conclusions References Tables Figures Back Close Print Version Interactive Discussion Abstract The global mean OH concentration ([OH] GM) has been used as an indicator of the atmospheric oxidizing efficiency or 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 5 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 CH 4 or CH 3 CCl 3. 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 10 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 15 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 or trends 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 20 have been used in both studies. Furthermore, we find that the only direct indicator of the global atmospheric oxidizing efficiency of OH with respect to a particular gas (e.g. CH 4 or CH 3 CCl 3) is the [OH] GM value weighted by the reaction with that gas; the mass-weighted and volume-weighted [OH] GM values, in contrast, are generally poor indicators of the atmospheric oxidizing efficiency on a global basis (regionally they are 25 better). We recommend that in future studies the [OH] GM value weighted by the reaction with CH 4 , along with the CH 4 turnover time, be given as the primary indicators of the atmospheric oxidizing efficiency, and that …