What does the global mean OH concentration tell us?

[摘要] The global mean OH concentration([OH]_GM) has been used as an indicator of the atmospheric oxidizing efficiency and its changes overtime.It is also used for evaluating the performance of atmospheric chemistry models by comparing with other models or withobservationally-based reference [OH]_GM levels.We contend that the treatment of this quantity in the recent literature renders itproblematic for either of these purposes.Several different methods have historically been used to compute[OH]_GM: weighting byatmospheric mass or volume, or by the reaction with CH₄ or CH₃CCl₃. In addition, these have been applied over differentdomains to represent the troposphere.While it is clear that this can lead to inconsistent[OH]_GMvalues, to date there has been no careful assessment of the differences expected when[OH]_GMis computed using various weightings and domains.Here these differences are considered using four different 3D OH distributions, along withthe weightings mentioned above applied over various atmospheric domains.We find that the[OH]_GMvalues computed based on agiven distribution but using different domains for the troposphere can result in differences of10% or more, while different weightings can lead to differences of up to30%, 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 areactually similar or not, except in the few cases where the same weighting and domain have been used in both studies.We define theatmospheric oxidizing efficiency of OH with respect to a given gas as the inverse of the lifetime of that gas, and show that this isdirectly proportional to the [OH]_GM value weighted by the reaction with that gas, where the proportionality constant depends onthe temperature distribution and the domain. We find that the airmass-weighted and volume-weighted[OH]_GMvalues, in contrast, are generally poor indicators of the global atmospheric oxidizingefficiency with respect to gases such as CH₄ and CH₃CCl₃ witha strong temperature dependence in their reaction with OH.We recommend that future studies provide both the airmass-weighted andthe CH₄-reaction-weighted [OH]_GM values, over the domain from the surface to a climatological tropopause. The combination of thesevalues helps to reduce the chance of coincidental agreement between very different OH distributions.Serious evaluations of modeled OHconcentrations would best be done with airmass-weighted [OH]_GM broken down into atmospheric sub-compartments, especially focusing onthe tropics, where the atmospheric oxidizing efficiency is the greatest for most gases.

[学科分类] 大气科学