Characterization of Chemical Composition in Size-resolved Marine Aerosol during Fog Formation events over the Northwest Atlantic Ocean during FaTIMA 2022

Aerosols play an important role in regulating the Earth’s energy balance and climate change by impacting radiative forcing (directly) and cloud condensation nuclei (indirectly). Fog formation results from radiative, thermodynamic, microphysical, dynamical, chemical, surface conditions, and meteorological interactions. Aerosols can promote fog formation as condensation nuclei when the relative humidity (RH) exceeds 100%. There is an intricate relationship between aerosol and fog characteristics; physico-chemical characteristics of aerosol change by the occurrence of fog, and aerosol properties directly impact the life cycle of a fog layer. Aerosol-fog interaction is an essential process in understanding and forecasting fog formation events. Previous studies have focused on aerosol-fog interaction on land, with fewer studies in the marine environment. Sea fog can be hazardous by reducing visibility, which causes disturbance in aviation and marine transportation. Therefore, sea fog study is critical in understanding fog formation, development, and dissipation to enable accurate forecasting and improve safe marine activities and transportation. During the Fog and Turbulence Interactions in the Marine Atmosphere (FaTIMA) project, we explored the initiation and persistence of marine fog events. The current study aimed to determine the chemical composition of aerosols during fog and non-fog episodes to better understand how aerosol chemistry affects fog formation. Size-resolved aerosol samples were collected with a Micro Orifice Uniform Deposit Impactor (16 fog samples) and a nano Micro Orifice Uniform Deposit Impactor (13 non-fog samples) on the Atlantic Condor research cruise in the northwest Atlantic Ocean off the coast of Eastern Canada from July 3 to August 1, 2022. Major water-soluble cations and anions, DEAH⁺, TMAH⁺ (diethylamine, trimethylamine), and MSA^- (methane sulfonic acid) were analyzed in aerosols by ion chromatography coupled to a conductivity detector. In the coarse mode, the dominant ions were Na⁺ and Cl^- in both fog and non-fog periods, with similar contributions as expected for sea salt. In some fog events, the concentration of sea salt ions like Na⁺, Mg²⁺, and Cl^- from the coarse mode increased before the fog formation and then decreased over the fog; this suggests that sea salt aerosols may act as the fog condensation nuclei. In the fine mode, Na⁺ was the dominant cation in non-fog samples; however, NH₄⁺ was the main cation during fog events, and nss-SO₄^2- was the most abundant anion in both fog and non-fog aerosols. An increased sum of total concentration (neq m^-3) of NH₄⁺, DEAH⁺, TMAH⁺, MSA^-, and nss-SO₄^2- was observed in fine mode aerosols during the fog vs. non-fog periods. The average concentration of NH₄⁺, DEAH⁺+TMAH⁺, MSA^-, and nss-SO₄^2- during fog events were 5.47 neq m^-3, 2.14 neq m^-3, 0.81 neq m^-3, and 8.51 neq m^-3, respectively, while during non-fog periods, they deceased to 2.84 neq m^-3, 0.76 neq m^-3, 0.17 neq m^-3, and 5.73 neq m^-3, respectively. The results indicate that the aerosol chemistry varied in the fine mode between fog and non-fog events, and this will be used to better understand of how the chemical composition of size-resolved aerosol governs the presence and/or absence of fog as well as its lifetime.