Measurement report: Cloud processes and the transport of biological emissions affect southern ocean particle and cloud condensation nuclei concentrations
Authors: Sanchez Kevin J. , Roberts Gregory C. , Saliba Georges , Russell Lynn M. , Twohy Cynthia , Reeves J. Michael , Humphries Ruhi S. , Keywood Melita D. , Ward Jason P. , McRobert Ian M. 
Published: March 2021

Abstract. Long-range transport of biogenic emissions from the coast
of Antarctica, precipitation scavenging, and cloud processing are the main
processes that influence the observed variability in Southern Ocean (SO)
marine boundary layer (MBL) condensation nuclei (CN) and cloud condensation
nuclei (CCN) concentrations during the austral summer. Airborne particle
measurements on the HIAPER GV from north–south transects between Hobart,
Tasmania, and 62∘ S during the Southern Ocean Clouds, Radiation
Aerosol Transport Experimental Study (SOCRATES) were separated into four
regimes comprising combinations of high and low concentrations of CCN and
CN. In 5 d HYSPLIT back trajectories, air parcels with elevated CCN
concentrations were almost always shown to have crossed the Antarctic coast,
a location with elevated phytoplankton emissions relative to the rest of the
SO in the region south of Australia. The presence of high CCN concentrations
was also consistent with high cloud fractions over their trajectory,
suggesting there was substantial growth of biogenically formed particles
through cloud processing. Cases with low cloud fraction, due to the presence
of cumulus clouds, had high CN concentrations, consistent with previously
reported new particle formation in cumulus outflow regions. Measurements
associated with elevated precipitation during the previous 1.5 d of their
trajectory had low CCN concentrations indicating CCN were effectively
scavenged by precipitation. A coarse-mode fitting algorithm was used to
determine the primary marine aerosol (PMA) contribution, which accounted for
<20 % of CCN (at 0.3 % supersaturation) and cloud droplet
number concentrations. Vertical profiles of CN and large particle
concentrations (Dp>0.07 µm) 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. 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 the
SOCRATES study. The R/V Investigator observed elevated CCN concentrations near Australia,
likely due to continental and coastal biogenic emissions. The Antarctic
coastal source of CCN from the south, CCN sources from the midlatitudes, and
enhanced precipitation sink in the cyclonic circulation between the Ferrel
and polar cells (around 60∘ S) create opposing latitudinal
gradients in the CCN concentration with an observed minimum in the SO
between 55 and 60∘ S. The SOCRATES airborne
measurements are not influenced by Australian continental emissions but
still show evidence of elevated CCN concentrations to the south of
60∘ S, consistent with biogenic coastal emissions. In addition, a
latitudinal gradient in the particle composition, south of the Australian
and Tasmanian coasts, is apparent in aerosol hygroscopicity derived from CCN
spectra and aerosol particle size distribution. The particles are more
hygroscopic to the north, consistent with a greater fraction of sea salt
from PMA, and less hygroscopic to the south as there is more sulfate and
organic particles originating from biogenic sources in coastal Antarctica.