Microphysical chemistry of fog–aerosol interactions over the northwest Atlantic Ocean during Fatima 2022

Abstract A comprehensive study of aerosol chemistry, microphysics, physical processing, and meteorological parameters was conducted to investigate fog–aerosol interactions over the northwest Atlantic Ocean during the Fog And Turbulence Interactions in the Marine Atmosphere (Fatima) 2022 project. Chemical analysis of size‐resolved aerosols and fog droplets revealed a reduction in coarse‐mode sea salt ions (e.g., Cl − , Na + , ss‐SO 4 2− ) compared to ambient conditions, attributed to their growth into fog droplets. In contrast, fine‐mode aerosol processed‐S (e.g., non‐sea salt [nss]‐SO 4 2− and MSA − ) and reduced‐N (e.g., NH 4 + ) increased during fog events. Three distinct fog types were identified based on size‐resolved ion mass distributions, combined with particle number concentrations and meteorological parameters, to provide insight into the drivers of aerosol chemical composition during each chemically distinct fog type. Type I fog was the most common, characterized by depleted coarse‐mode sea salt aerosols, with fog droplets initially formed from freshly emitted coarse‐mode aerosols and most likely sustained by growth of fine‐mode aerosols. Type II fog was least common, sustained by ongoing production of coarse‐mode sea salt aerosols due to elevated winds, and supplemented by fine‐mode aerosol contributions. Type III fog had the highest mass loadings of sea salt ions at fog droplet sizes, with droplet formation primarily driven by coarse‐mode growth with minor contributions from fine‐mode aerosol. Reduction of coarse sea salt mass loadings during more than a dozen fog events confirms their expected role as effective fog condensation nuclei and the physical impact of fog processing in reducing their atmospheric mass loading. This dataset shows these aerosols are scavenged (⁓70%) in a supersaturated atmosphere to form droplets. This is the most extensive marine fog dataset to date to study fog–aerosol interactions from a chemical perspective, with the potential to improve marine fog forecasting models to provide safer marine transportation.