Contrasting Effects of Fine and Coarse Aerosols in Modulating Marine Lightning under Clean and Polluted Conditions

The mechanisms by which aerosols influence marine lightning remain poorly understood, particularly regarding the role of aerosol sizes. This study addresses this question using a novel WRF-based aerosol-cloud-lightning modeling framework to investigate contrasting effects of fine (diameter < 1 μm) and coarse (diameter > 1 μm) aerosols on marine lightning. Two baseline aerosol conditions are considered: a polluted condition constrained with an observed aerosol size distribution and a clean condition that has 10 times fewer cloud condensation nuclei. The results demonstrate that the influence of fine and coarse aerosols on lightning activity varies with the aerosol baseline conditions. Under clean conditions, increasing fine aerosol concentration causes more numerous but smaller cloud droplets, promoting the upward transport of cloud liquid water and its subsequent conversion to cloud ice. This enhances graupel-ice collision and charge separation, thereby boosting lightning activity. Conversely, increasing coarse aerosols favors larger cloud droplets that accelerate warm rain formation, suppressing convection and lightning. Under polluted conditions, increasing fine aerosols has negligible effects due to high cloud droplet concentration, while increasing coarse aerosols reduces the average cloud droplet size, bolstering the upward transport of cloud water and the formation of cloud ice, intensifying lightning activity. These results indicate that the impact of increasing aerosols on lightning depends not only on concentration but also on the aerosol size distribution and baseline conditions, highlighting the need for more detailed aerosol treatment in studies of aerosol–lightning interactions.