Wintertime Observations of Tropical Northwest Atlantic Aerosol Properties During ATOMIC: Varying Mixtures of Dust and Biomass Burning

The Atlantic Tradewind Ocean-Atmosphere Mesoscale Campaign (ATOMIC) took place from 7 January to 11 July 2020 in the tropical North Atlantic between the east coast of Barbados and the Northwest Tropical Atlantic Station (NTAS) mooring. Measurements of aerosol chemical, microphysical, optical, and cloud-nucleating properties were made during boreal winter onboard the NOAA RV Ronald H. Brown from 7 January to 13 February 2020. Observed aerosol properties indicate there were distinct periods when the aerosol was typical of clean marine conditions (particle number concentrations <400 cm(-3), light absorption levels <1 Mm(-1), and Rn-222 concentrations <500 mBq m(-3)) and when there were intrusions of dust mixed to varying degrees with biomass burning aerosol. In comparison to the long-term dust record at the Barbados Atmospheric Chemistry Observatory, the fraction of samples collected during ATOMIC with dust concentrations greater than 20 mu g m(-3) exceeded all years of the Barbados January to February record with the exception of 1983. The degree of mixing of the dust with biomass burning aerosol was modulated by the location of the North Atlantic subtropical high. A high located to the west of Africa led to a more northerly transport route and more dust-like (MDL) aerosol arriving at the ship. A location of the high further to the east over northern Africa led to a more southerly transport route and more biomass burning aerosol mixed with the dust. Properties for the marine, MDL, and mixed dust and biomass burning aerosol are presented.