Microphysics and interactions of aerosols and fog on the <scp>northwest Atlantic Ocean</scp>

Abstract Atmospheric aerosols, such as sea salt particles and those derived from air pollutants, provide a surface for water condensation leading to fog droplet formation in the lower atmosphere. Fog formation depends on synoptic to microphysical scale processes and this range involved in fog formation presents a challenge for forecasting. This study used aerosol (10–500 nm) and droplet (2–50 μm) data collected on the northwest Atlantic Ocean during July 2022 as part of the FATIMA (Fog And Turbulence Interactions in the Marine Atmosphere) project to study marine fog. The objective was to evaluate the effect of atmospheric aerosols on fog properties such as droplet size distribution. The study was split into two regimes. While sampling during fog on the Grand Banks of Newfoundland, aerosol size distributions were bimodal, aerosol and fog droplet concentrations were low, and fogs were shorter (&lt;12 hours). In contrast, near Sable Island, aerosol size distributions had a single broad mode that was likely influenced by continental emissions, corresponding with higher aerosol and droplet concentrations as well as longer (&gt;40 hours) fog events. Droplet size distributions were bimodal for all fog events, with a predominant peak at small sizes of 3–5 μm and a secondary peak around 20–30 μm. The mean concentration of droplets smaller than 12 μm ( N d,2–12 ) correlated better with the mean accumulation mode (100–500 nm) aerosol number concentration ( N 100–500 ) during the individual case studies with lower aerosol concentrations, consistent with an aerosol‐limited regime. The correlation disappears at higher N 100–500 , suggesting that other processes were more important for droplet formation during those case studies. It should be noted that the small number of case studies limits our interpretation and additional numerical analysis would aid our understanding of fog droplets in this region.