Radiative impact of improved global parameterisations of oceanic dry deposition of ozone and lightning-generated NO<sub>x</sub>

Abstract. We investigate the impact of recent process-based and empirical improvements to oceanic ozone dry deposition parameterisation and lightning-generated NOx (LNOx) parameterisation on radiative fluxes by conducting a 5-year simulation of the global ACCESS-UKCA chemistry-climate model with radiative feedbacks of ozone and methane included. Both parameterisations increase the global net downward top-of-the-atmosphere (TOA) radiative flux, akin to instantaneous radiative forcing, compared to the base parameterisations. The dry deposition improvement results in a relatively small increase of 4.3 mW m-2 in the net downward TOA radiative flux. But this increases to 86 mW m-2 when the improved LNOx parameterisation (which increases the LNOx production from 4.8 to 6.9 Tg N yr-1) is also used. For comparison, this estimated difference in the radiative flux is equivalent to ~18 % of the anthropogenic effective radiative forcing (ERF) due to ozone over the years 1750–2019 reported by the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report (AR6). Other global changes from the use of the two parameterisations include an increase in the downward longwave radiation and a decrease in the downward shortwave radiation at the Earth’s surface, and a decrease of 0.64 years in the methane lifetime against loss due to the hydroxyl radical, with changes being larger in magnitude in the tropics. Although the total global LNOx production may be the same, how LNOx is distributed spatially makes a difference to radiative transfer. The changes in radiation components and methane lifetime per unit change in LNOx and column ozone are also calculated. We estimate that the uncertainty range in the net downward TOA radiation due to reported uncertainty range in global estimates of LNOx could be as much as 238 mW m-2. LNOx has a significant influence on the atmospheric lifetime of CH4, and the value of LNOx used within a model will influence the ERF and global warming potential of anthropogenic methane.