A machine learning examination of hydroxyl radical differences among model simulations for CCMI-1

The hydroxyl radical (OH) plays critical roles within the troposphere, such as determining the lifetime of methane (CH4), yet is challenging to model due to its fast cycling and dependence on a multitude of sources and sinks. As a result, the reasons for variations in OH and the resulting methane lifetime (tau CH4), both between models and in time, are difficult to diagnose. We apply a neural network (NN) approach to address this issue within a group of models that participated in the Chemistry-Climate Model Initiative (CCMI). Analysis of the historical specified dynamics simulations performed for CCMI indicates that the primary drivers of tau CH4 differences among 10 models are the flux of UV light to the troposphere (indicated by the photolysis frequency JO1D), the mixing ratio of tropospheric ozone (O-3), the abundance of nitrogen oxides (NOx equivalent to NO+NO2), and details of the various chemical mechanisms that drive OH. Water vapour, carbon monoxide (CO), the ratio of NO:NOx, and formaldehyde (HCHO) explain moderate differences in tau CH4, while isoprene, methane, the photolysis frequency of NO2 by visible light (JNO(2)), overhead ozone column, and temperature account for little to no model variation in tau CH4. We also apply the NNs to analysis of temporal trends in OH from 1980 to 2015. All models that participated in the specified dynamics historical simulation for CCMI demonstrate a decline in tau CH4 during the analysed timeframe. The significant contributors to this trend, in order of importance, are tropospheric O-3, JO(1)D, NOx, and H2O, with CO also causing substantial interannual variability in OH burden. Finally, the identified trends in tau CH4 are compared to calculated trends in the tropospheric mean OH concentration from previous work, based on analysis of observations. The comparison reveals a robust result for the effect of rising water vapour on OH and tau CH4, imparting an increasing and decreasing trend of about 0.5 % decade(-1), respectively. The responses due to NOx, ozone column, and temperature are also in reasonably good agreement between the two studies.