Evaluation of SO2, SO42− and an updated SO2 dry deposition parameterization in the United Kingdom Earth System Model

In this study we evaluate simulated surface SO2 and sulfate (SO₄²⁻) concentrations from the United Kingdom Earth System Model (UKESM1) against observations from ground-based measurement networks in the USA and Europe for the period 1987–2014. We find that UKESM1 captures the historical trend for decreasing concentrations of atmospheric SO2 and SO₄²⁻ in both Europe and the USA over the period 1987–2014. However, in the polluted regions of the eastern USA and Europe, UKESM1 over-predicts surface SO2 concentrations by a factor of 3 while under-predicting surface SO₄²⁻ concentrations by 25 %–35 %. In the cleaner western USA, the model over-predicts both surface SO2 and SO₄²⁻ concentrations by factors of 12 and 1.5 respectively. We find that UKESM1’s bias in surface SO2 and SO₄²⁻ concentrations is variable according to region and season. We also evaluate UKESM1 against total column SO2 from the Ozone Monitoring Instrument (OMI) using an updated data product. This comparison provides information about the model's global performance, finding that UKESM1 over-predicts total column SO2 over much of the globe, including the large source regions of India, China, the USA, and Europe as well as over outflow regions. Finally, we assess the impact of a more realistic treatment of the model's SO2 dry deposition parameterization. This change increases SO2 dry deposition to the land and ocean surfaces, thus reducing the atmospheric loading of SO2 and SO₄²⁻. In comparison with the ground-based and satellite observations, we find that the modified parameterization reduces the model's over-prediction of surface SO2 concentrations and total column SO2. Relative to the ground-based observations, the simulated surface SO₄²⁻ concentrations are also reduced, while the simulated SO2 dry deposition fluxes increase.