Recent Particle Formation and Aerosol Variability Near Southern Ocean Low Clouds
Journal Article
Overview
abstract
Controls on the pristine aerosol population over the Southern Ocean (SO); are critical for constraining the strength of global aerosol indirect; forcing. The 2018 SOCRATES aircraft campaign sampled summertime clouds; and aerosols across varied SO synoptic conditions during repeated; southerly transects (140-160 degE, 45-62 degS). The SO free troposphere; (3-6 km) is characterized by frequent recent particle formation (RPF); events contributing to large (>1000 cm-3) and widespread; concentrations of condensation nuclei (diameters >11 nm),; consistently higher than other low cloud regions (e.g. North East; Pacific). A SO synoptic uplift mechanism responsible for producing free; tropospheric Aitken-mode particles (11-100 nm) is identified using; SOCRATES measurements, air-mass trajectories, and reanalysis. SO cloud; droplet number concentrations (Nd~100 cm-3) are; controlled by sub-cloud cloud condensation nuclei (CCN, 100-1000 nm),; but direct emissions (i.e. sea spray) are not the largest contributor to; summertime CCN. Sub-micron aerosol number concentration in, above, and; sub-cloud is dominated by sulfur-based particles, consistent with growth; of free tropospheric Aitken particles with DMS-oxidation product; volatility signatures. We propose a hypothesis for SO cloud-aerosol; interactions: above-cloud Aitken mode particles grow in and sub-cloud to; dominate sub-cloud summertime CCN, acting to buffer clouds and aerosols; against precipitation removal and leading to persistently high; summertime SO Nd. Aerosol and cloud prediction capabilities in nudged; hindcasts from the Community Atmosphere Model (CAM6) are tested. CAM6; under-predicts Nd but matches observed CCN despite sea-spray dominance; in the model, indicating incomplete representations of biological; aerosol production mechanisms and associated summertime aerosol-cloud; interactions.
