The impact of cloud processes and the transport of biological emissions on the vertical structure of cloud condensation nuclei in the Southern Ocean.

Abstract:

Long-range transport of biogenic emissions from Antarctica coastal regions, precipitation scavenging, and cloud processing are the main factors that impact the observed variability of aerosol in Southern Ocean (SO) during the austral summer. Airborne measurements were conducted on the HIAPER GV during the Southern Ocean Clouds, Radiation Aerosol Transport Experimental Study (SOCRATES). An increase in cloud condensation nuclei (CCN) concentrations were almost always shown to cross the Antarctic coast, a location with elevated phytoplankton emissions. The presence of high CCN concentrations was also consistent with high cloud fraction, suggesting that cloud processing contributes to the growth of biogenic particles. Air masses associated with cumulus clouds and low cloud fraction had high condensation nuclei (CN) concentrations, consistent with new particle formation in cloud outflow regions. Measurements associated with higher amounts of precipitation had lower CCN concentrations – indicating that CCN were effectively scavenged and that precipitation is a major sink for SO aerosols. A course-mode fitting algorithm was used to determine that primary marine aerosol (PMA) aerosol accounted for less than 20% of CCN (at 0.3% supersaturation) and cloud droplet number concentrations. Vertical profiles indicated that particle formation occurs more frequently above the MBL; however, the growth of recently formed particles typically occurs in the MBL, consistent with cloud processing and the condensation of volatile compound oxidation products on existing aerosol particles.

CCN measurements on the R/V Investigator as part of the second Clouds, Aerosols, Precipitation, Radiation and atmospheric Composition Over the southeRn Ocean (CAPRICORN-2) campaign were also conducted during the same period as SOCRATES. The Antarctic coastal source of CCN from the south as well as CCN sources from the mid-latitudes create a meridional gradient with an observed minimum in CCN concentrations between 55°S and 60°S. The SOCRATES airborne measurements also show higher CCN concentrations south of 60°S. In addition, a gradient in the particle composition is observed, with more hygroscopic particles to the north, consistent with a greater fraction of sea salt from PMA and more sulfate / organic particles to the south.