Historical sulfate aerosol formation in earth system model with interactive-chemistry: interplay between emission location, seasonality, meteorology and available oxidants

Understanding the link between emissions, atmospheric chemistry and the Earth’s radiative budget remains a challenge in climate research. Such linkage arises from the fact some aerosols are produced chemically in the atmosphere. Unlike well-mixed greenhouse gases, anthropogenic aerosols are heterogeneously distributed because of localised emissions and the short atmospheric residence time. Over the historical period emissions of greenhouse gases, and near-term climate forcers (NTCFs) including aerosol precursors, O₃ precursors and CH₄ have broadly increased. We ask how changes in anthropogenic emissions over the historical period feed through aerosol and cloud radiative forcing.  This is important because a lack of understanding of regionally heterogeneous aerosol-climate effects is hampering our understanding of historical climate change. It also limits our confidence in future climate projections and the assessment of their impacts, as aerosol emissions are expected to decline in many regions over the coming decades.

Using the UK Earth System Model 1 (UKESM1), we investigate how sulfate aerosols form under emission and oxidant changes between 1850 and 2014. We analyse simulation output from the Aerosol Chemistry Model Intercomparison Project (AerChemMIP) atmosphere-only transient experiment which was designed to evaluate NTCF transient effective radiative forcing. These simulations target each NTCF thus suitable for isolating the effects of NTCF on the Earth system responses such as aerosol and cloud formation. First, we investigate the effect of emission location on oxidation, aiming to characterise regional sulfate aerosol formation. Two regions, Europe and Eastern Asia region, were chosen to allow comparison between two regions with different emission profiles in different periods. In the UKESM1, SO₂ reacts with OH in the gas phase and O₃ and H₂O₂ in the aqueous phase. We show that emissions location and timing determine oxidation tendency via the available oxidant and meteorological properties such as clouds. Both regions see up to 80% of total sulfate production via gas phase oxidation in summer when high OH and low cloud cover are observed. The opposite is true for wintertime when aqueous phase reactions with O₃ and H₂O₂ form up to 90% of aerosol. Each region also shows distinct characteristics, for example, H₂O₂ oxidation in the European region is generally lower than that of the Eastern Asia region but it is more variable with bimodal features showing peaks in spring and autumn. Second, we investigate the effects of O₃ precursors and CH₄ on SO₂ oxidation to quantify the regional contribution of NTCFs. Influence from O₃ precursors is localised while CH₄ affect SO₂-OH oxidation on a more global scale. This work shows that the same amount of SO₂ emitted at different regions does not form aerosol at the same amount or with the same aerosol size distribution.

We present an analysis of monthly changes of oxidants and emissions to sulfur oxidation, aerosol and cloud properties. 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.