Influences of Recent Particle Formation on Southern Ocean Aerosol
Variability and Low Cloud Properties
Abstract
Controls on pristine aerosol over the Southern Ocean (SO) are critical
for constraining the strength of global aerosol indirect forcing.
Observations of summertime SO clouds and aerosols in synoptically varied
conditions during the 2018 SOCRATES aircraft campaign reveal novel
mechanisms influencing pristine aerosol-cloud interactions. The SO free
troposphere (3-6 km) is characterized by widespread, frequent new
particle formation events contributing to much larger concentrations (≥
1000 mg-1) of condensation nuclei (diameters > 0.01 μm)
than in typical sub-tropical regions. Synoptic-scale uplift in warm
conveyor belts and sub-polar vortices lifts marine biogenic
sulfur-containing gases to free-tropospheric environments favorable for
generating Aitken-mode aerosol particles (0.01-0.1 μm).
Free-tropospheric Aitken particles subside into the boundary layer,
where they grow in size to dominate the sulfur-based cloud condensation
nuclei (CCN) driving SO cloud droplet number concentrations (Nd
~ 60-100 cm-3). Evidence is presented for a hypothesized
Aitken-buffering mechanism which maintains persistently high summertime
SO Nd against precipitation removal through CCN replenishment from
activation and growth of boundary layer Aitken particles. Nudged
hindcasts from the Community Atmosphere Model (CAM6) are found to
underpredict Aitken and accumulation mode aerosols and Nd, impacting
summertime cloud brightness and aerosol-cloud interactions and
indicating incomplete representations of aerosol mechanisms associated
with ocean biology.