Relating Fog Droplets, Sources, and Aerosols in the Atlantic and the Central Arctic

Fog droplet size distributions on or near the coast of Atlantic Canada are often bimodal, with modes present at approximately 5 and 25 microns. The origin of these two modes, as well as their ability to coexist, is still under investigation. As part of the Halifax Fog and Air Quality Study (HaliFAQS), measurements of fog droplet size distribution were conducted in the eastern Canadian coastal city Halifax, Nova Scotia from May to August 2019 using a fog droplet monitor (FM-120, Droplet Measurement Technologies). During this study, a strong seasonal difference was observed in the droplet size distribution: during spring (May & June) fog events, the smaller droplet mode dominated resulting in lower liquid water content; while during summer (July & August), the larger droplet mode dominated, resulting in a higher liquid water content and lower visibility. Footprint analysis using the Lagrangian particle dispersion model FLEXPART revealed that fog events when the larger droplet mode dominated originated from local (continental) sources and were likely strongly influenced by radiative fog formation processes. By contrast, the fog events in which the smaller droplet mode dominated were more influenced by marine sources and coincided with local precipitation. These results will be compared to fog droplet and aerosol size distributions and chemical composition measured off the coast of Atlantic Canada as part of the Fog and Turbulence Interactions in the Marine Atmosphere (FATIMA) that took place in July 2022. These measurements will provide additional constraints on the contribution of aerosols to fog droplet size distributions and provide primarily marine-influenced fog cases compared to the results from HaliFAQS, where the urban setting and emissions appear to contribute strongly to the observed droplet distribution. Overall, our findings suggest that the visibility in coastal urban fogs is highly dependent on air mass history and require different considerations in models.