What does the global mean OH concentration tell us?

Abstract

Abstract. 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 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 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

Earth System Chemistry integrated Modelling (ESCiMo) with the Modular Earth Submodel System (MESSy) version 2.51

“…Prather et al, 2012;Prinn et al, 2005, with 11.2 ± 1.3 and 10.2 (+0.9/−0.7) years, respectively). The wide range of lifetime estimates is mainly caused by different methods of calculation and applied weighting (Lawrence et al, 2001), whereas varying included vertical layers due to different tropopause heights have a minor impact (see also O'Connor et al, 2014). The global mean temperature nudged RC1SD-base-08 simulation (royalblue dashed) predicts the shortest lifetime.…”

Global modelling of the total OH reactivity: investigations on the missing OH sink and its atmospheric implications

“…was calculated in accordance with Lawrence et al (2001), with the troposphere defined as the domain below 250 hPa. This is also consistent with the convention used in the Atmospheric Chemistry and Climate Model…”

Global atmospheric chemistry – which air matters

“…For L-CH4, the only general agreement is the lesser importance of parcels at altitudes above 500 hPa. For this August test, most models find that the 20-40 • N dominates (note that plots are ppb day −1 and not area weighted), and the 60-40 and 40-20 • S domains are least important (similar to OH structures in Spivakovsky et al, 2000;Lawrence et al, 2001). Most models show increasing L-CH4 in the first few km above the ocean because of low-level clouds shifting photolysis to the middle troposphere The results for L-O3 show similar patterns of agreement and disagreement among models but emphasize the dominant role of the middle troposphere (500-800 hPa) for O 3 loss.…”