Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 1 1 2001 10.5194/acpd-1-43-2001 http://www.atmos-chem-phys-discuss.net/1/43/2001/ http://www.atmos-chem-phys-discuss.net/1/43/2001/acpd-1-43-2001.html http://www.atmos-chem-phys-discuss.net/1/43/2001/acpd-1-43-2001.pdf 43 75 2001-09-03 What does the global mean OH concentration tell us? M. G. Lawrence P. Jöckel R. von Kuhlmann Max-Planck-Institute für Chemie, Postfach 3060, 55020 Mainz, Germany 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 of this quantity in the recent literature renders it problematic for either of these pur-poses. 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 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 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. CH₄ or CH₃CCl₃ ) 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 better). We recommend that in future studies the [OH]_GM value weighted by the reaction with CH₄ , along with the CH4 turnover time, be given as the primary indicators of the atmospheric oxidizing efficiency, and that serious evaluations of modeled OH concentrations be done with air mass weighted [OH]_GM broken down into atmospheric sub-compartments, especially focusing on the tropics, where the atmospheric oxidizing efficiency is greatest.