Impact on Cloud Properties of Reduced-Sulphur Shipping Fuel in the Eastern North Atlantic

Abstract. The global reduction in shipping fuel sulphur that culminated in 2020 with an ~80 % reduction has enabled an inadvertent experiment on the role of aerosol-cloud interaction (ACI) in the climate system. We compare observations collected at the Atmospheric Radiation Measurement program's (ARM) Eastern North Atlantic site (ARM-ENA, 39.1 N, 28.0 W) during two June to September periods: 2016–2018 (pre-2020) and 2021–2023 (post-2020). We find a significant (~15 %) decrease in cloud condensation nuclei concentrations post-2020, which resulted in a decrease in cloud droplet number (N_d) and an increase in effective radius (r_e) of marine boundary layer clouds. However, cloud liquid water path (LWP) increased post-2020. The increase in LWP offset the increase in r_e, resulting in insignificant changes to the optical depth distribution. MODIS and CERES data in the vicinity of ENA during these periods produce similar results also with negligible change in the albedo and optical depth distributions. Regional cloud occurrence declined in line with changes in the large-scale meteorology. Our results point to a complicated interplay among the factors that modulate cloud feedback in the Eastern North Atlantic.