Impact of Reduced Sulfur in Global Shipping Fuel on Cloud Properties in the North Atlantic: Insights ARM-ENA

Understanding cloud-aerosol interactions is essential for predicting Earth's climate sensitivity, as these interactions influence future climate through cloud formation and radiative effects. The reduction in sulfur content in shipping fuel, mandated by the International Maritime Organization's 2020 regulations (IMO2020), has prompted interest in examining its impact on cloud properties. The Atmospheric Radiation Measurement program in the Eastern North Atlantic (ARM-ENA) provides an ideal location for this study due to its proximity to heavy shipping routes and frequent cloud cover. We compared data collected at ENA during two periods: June to September of 2016-2018 (pre-IMO2020) and 2021-2023 (post-IMO2020). Our analysis reveals that overall meteorological conditions remained consistent between the two periods. Following the regulatory change, we find a significant decrease in cloud condensation nuclei (CCN) concentrations, which led to a decrease in cloud droplet number (N_d) and an increase in effective radius (r_e), supporting the Twomey effect. However, cloud liquid water path (LWP) increased, and drizzle occurrences declined. The increase in LWP offset the increase in cloud effective radius, resulting in only slight changes to the net radiative effect of the clouds. Examination of MODIS data in the vicinity of ENA during these periods produce similar results. These findings suggest that the increase in LWP may counterbalance the anticipated reduction in albedo due to the Twomey effect, hinting at a more complex, buffered system than current theories account for.