Subsaturated aerosol hygroscopicity over the northwest Atlantic: Impacts of seasonal factors, offshore location, and clouds

Aerosol water-uptake behavior affects aerosol remote sensing and prediction of visibility, cloud processes, and phenomena sensitive to aerosol liquid water such as heterogeneous chemistry. Aerosol hygroscopic properties are not well characterized seasonally over marine regions, including the northwest Atlantic. The NASA Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment (ACTIVATE) conducted aircraft measurements over the northwest Atlantic Ocean (NWAO) with two spatially coordinated aircraft to better understand aerosol-cloud interactions in the region. A statistically dense set of observations of aerosol particle hygroscopicity, composition, size, and optical properties were strategically made below, in, and above boundary layer clouds with instruments on board an HU-25 Falcon aircraft from 2021 to 2022. Results show that median submicron aerosol hygroscopicity f(RH), comparing light scattering at <40 % and 82 % relative humidity, is highest in the winter (1.40 with lower to upper quartiles of 1.21–1.63) over the NWAO, and generally higher farther offshore around clouds as compared to closer to the coast where it is more cloud-free. Higher f(RH) coincides with enhanced inorganic mass fraction, which is dominated by accumulation mode sulfate. In Bermuda, median submicron f(RH) (1.20) is lower than typical clean marine environments, coinciding with transported dust particles. Though f(RH) generally decreases with altitude, there is a slight increase at cloud level presumed to be due to cloud processes related to sulfate, nitrate, and oxidized organics. Our study shows the need to further characterize the variety of processes affecting aerosol hygroscopicity in the NWAO and their impacts on clouds.