Aerosol Microphysical, Chemical, Optical, and Cloud-nucleating Properties Measured during ATOMIC

Abstract:

ATOMIC took place east of Barbados during January and February of 2020 to study shallow convection and low, liquid clouds in relative isolation from other cloud types. Measurements were made from the NOAA RV Ronald H. Brown to improve the understanding of the effects of aerosol particles on clouds and radiation transfer over the northwest tropical Atlantic and the related impacts on the upper ocean. During the dry season (December – June), aerosol in this region is expected to be composed primarily of sea spray aerosol and non-sea salt sulfate from the oxidation of biogenic dimethylsulfide with a minimal contribution of African dust. Surprisingly, measurements of aerosol chemical composition revealed three periods of high dust concentrations interspersed between marine periods when the aerosol was dominated by sea spray. HYSPLIT air mass back trajectories indicated a modulation of transport from Africa (dust periods) and the central Atlantic (marine periods). During the dust periods, sub-10 micrometer dust concentrations reached 27 mg m^-3 with a corresponding enhancement in elemental carbon and non sea-salt potassium concentrations suggesting a mixture of dust and biomass burning aerosol. Aerosol Optical Depth (AOD) reached 0.6 with the majority of the light extinction occurring above the marine boundary layer (MBL). During marine periods, AOD ranged between 0.05 and 0.1 with all of the light extinction occurring in the MBL due to sea spray aerosol. Cloud condensation nuclei concentrations also were measured and found to be approximately a factor of 2 higher during the dust periods.