Are Marine Low Cloud Droplet Concentrations Buffered by Entrained Aitken-Mode Aerosol

Are Marine Low Cloud Droplet Concentrations Buffered by Entrained Aitken-mode Aerosol?
Christopher S. Bretherton, University of Washington (Principal Investigator)

During the summertime, the high-latitude oceans come to life with green phytoplankton, which gain their energy from sunlight and are food for sea creatures small and large. When the phytoplankton are eaten or die, sulfur-rich gases are released under the ocean surface and mix into the air. Recent observations suggest that over the Southern Ocean, frequent storms lift this air high into the atmosphere while raining out particulates like salt. As a result, the sulfur-rich air then spawns high concentrations of small ‘Aitken-mode’ aerosol

aerosols

Small particles or droplets, such as wildfire smoke, volcanic gases, or salty sea spray, that float in the air. They are emitted by both natural events and human activities.

Source: NASA Global Climate Change Source

particles. We hypothesize that these particles work their way down into the marine boundary layer, where they can replenish the supply of cloud-condensation nuclei scavenged by frequent precipitation. Further, this process maintains high concentrations of liquid cloud droplets in austral summer, promoting more sunlight to be reflected to space. We call this ‘Aitken buffering’. We expect this process to be much less active in winter, when there is much less daylight to sustain the phytoplankton.
The objective of this project is to better document and test whether this Aitken-mode buffering process is actually important for Southern Ocean clouds by documenting the range of variability in surface aerosol, cloud and precipitation observations from the 2017-2018 Measurements of Aerosols, Radiation, and Clouds over the Southern Ocean (MARCUS) deployment of an ARM Mobile Facility on service cruises of an Australian icebreaker across the Southern Ocean between Tasmania and Antarctica during early summer to early fall, These observations will be compared with surface and airborne observations from the Atmospheric Radiation Measurement (ARM) facility's Eastern North Atlantic (ENA) site, where we expect summertime free-tropospheric new particle production to be less common, and where there are year-round observations. A new computer model called AA-SAM that realistically simulates the aerosol processes, and the small-scale turbulent air motions that move aerosols around and create the clouds will be used to interpret and extend these observations for process understanding, by allowing different factors that contribute to the aerosol budget, such as surface wind speed, precipitation, surface gas exchange, etc. to be separated.