Microphysics of aerosol, fog and droplet residuals on the Canadian Atlantic coast

Fog has a wide range of impacts that can include transportation disruptions and climate effects. Despite being a common phenomenon, models struggle to incorporate fog microphysical properties which can affect the visibility, formation and dissipation of fog. Results are presented from a fog microphysics study conducted in June and July of 2016 near Halifax, on the eastern coast of Canada, where droplet and aerosol size distributions were measured, as well as fog droplet residuals using a ground-based counterflow virtual impactor. In the ten distinct events that were analyzed, fog never formed when the dew-point temperature was lower than the sea-surface temperature, suggesting that advection fog was primarily observed. During fog events, the concentration of particles larger than 500 nm in diameter was observed to decrease up to 90%, suggesting that nucleation scavenging was an important loss process during fog. Maximum droplet concentration was correlated (r = 0.61) to the total pre-fog aerosol concentration, despite a large (423 nm) estimated activation diameter. From our measurements, the peak supersaturation (SS) was estimated to range from 0.015–0.046%, including uncertainties due to chemical composition, which is lower than previous estimates for continental fog, including mountain sites, and would be consistent with advection fog as well as the presence of sea salt aerosol. Using basic assumptions, we estimate that for each added aerosol per cm³ in the air mass, the maximum fog droplet concentration increases by 0.011 per cm³ and the fog albedo increases by (0.55–3.8) × 10⁻⁴. This in-situ dataset will help evaluate models to ultimately improve fog forecasts for coastal areas and further our understanding of aerosol-cloud interactions.