The role of sulfur oxidation on cloud and aerosol properties in UKESM1 CMIP6 historical experiments

Understanding the link between anthropogenic emissions and radiative forcing remains a grand challenge in the field of climate research. Linkages arise between emissions, atmospheric chemistry and climate through the formation of secondary aerosols such as sulfate, nitrate and organic aerosols. Sulfur dioxide (SO₂) is an important aerosol precursor with the largest sources coming from anthropogenic activity. Unlike well-mixed greenhouse gases, anthropogenic aerosols are heterogeneously distributed because of localised emissions and the short atmospheric residence time. Thus SO₂ conversion to aerosol is dictated by its emission location and the locally available oxidants; both of which are changing rapidly and disparately with time.

This work uses the UKESM1 to investigate the modelled response of sulfate aerosol properties and cloud properties to emissions increases and oxidant changes over the period 1850-2014. We compare modelled hydrogen peroxide, which is important for SO₂ oxidation, with observations. From an analysis of the CMIP6 and AerChemMIP experiments, we show that there have been significant changes in the atmospheric oxidation processes of SO₂ over this period with consequences for the calculated radiative forcing. 

In UKESM1 historical experiments, the gas-phase reaction with hydroxyl radicals dominates the oxidation pathways in most regions. This channel is the most sensitive to oxidant changes and contributes to new aerosol particle formation. In contrast, in the aqueous-phase reaction, the oxidation of SO₂ by ozone decreased in the European region in 1980 and oxidation by hydrogen peroxide increased in Eastern Asia in 2014. We present an analysis of the impacts of these sulfur oxidation changes on cloud properties and radiative forcing. Ultimately, this work contributes to the improvement of our process-level understanding of Earth system models that interactively simulate aerosol from precursors and aims to improve the accuracy of aerosol radiative forcing predictions.