Influences of Recent Particle Formation on Southern Ocean Aerosol Variability and Low Cloud Properties

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 (>= 1,000 mg(-1)) of condensation nuclei (diameters > 0.01 mu m) 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 mu m). 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 (N-d similar to 60-100 cm(-3)). Evidence is presented for a hypothesized Aitken-buffering mechanism which maintains persistently high summertime SO N-d 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 N-d, impacting summertime cloud brightness and aerosol-cloud interactions and indicating incomplete representations of aerosol mechanisms associated with ocean biology. Plain Language Summary The remote Southern Ocean (SO) is a unique analog to pre-industrial environments due to limited continental and anthropogenic influences. Understanding how aerosols are produced in this region and their influence on cloud droplet concentrations is vital for understanding how much sunlight these clouds reflect to space, which affects ocean temperatures and global climate. This is a key uncertainty in modeling past and future climate change due to anthropogenic emissions of carbon dioxide and other pollutants. To understand this pristine environment, we analyze novel observations of SO clouds and aerosols from a summertime aircraft campaign. We present evidence for an aerosol production mechanism driven by synoptic storms and sourced from emissions of ocean biology. This mechanism produces a reservoir of small aerosols above cloud that subside into the marine boundary layer, where they grow into cloud-affecting sizes and control cloud droplet number. In addition to acting as a source of boundary layer aerosol, these small particles help SO clouds to resist precipitation depletion of cloud-affecting aerosol, maintaining surprisingly high cloud droplet number concentrations that help to keep present-day SO clouds persistently bright. This mechanism has important implications for understanding pre-industrial and other pristine environments and their response to anthropogenic aerosol. Key Points Summertime Southern Ocean free tropospheric aerosol number is dominated by Aitken particles recently generated through synoptic uplift Entrained Aitken aerosols buffer Southern Ocean boundary layer cloud condensation nuclei and cloud droplets against precipitation removal Southern Ocean cloud droplet number is too low in the CAM6 climate model due to inadequate free tropospheric production of Aitken aerosols

Automated summary

The study reveals that Southern Ocean free tropospheric aerosol is dominated by Aitken particles generated through synoptic uplift, which buffer the boundary layer cloud condensation nuclei and cloud droplets against precipitation removal. The Community Atmosphere Model (CAM6) underpredicts Aitken and accumulation mode aerosols and cloud droplet number, indicating incomplete representations of aerosol mechanisms associated with ocean biology.