Measurement report: Cloud processes and the transport of biological emissions affect southern ocean particle and cloud condensation nuclei concentrations

Abstract

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.

Cloud‐Nucleating Particles Over the Southern Ocean in a Changing Climate

“…This is similar to the latitudinal dependence of chlorophyll‐a (Figure 1), and is consistent with biological sources of particles to the north and south of ∼50–60°S. CCN concentrations (Sanchez et al., 2021) also peaked in air to the north and south and were highly correlated with sulfate concentrations ( r 2 = 0.77 for 1 h averages). Ammonium and nitrate mass were both below their detection limits along the track, even for areas with higher sulfate values, consistent with low oceanic sources of ammonia and nitrate (Jokinen et al., 2018; O'Dowd, Lowe, et al., 1997).…”

Summer aerosol measurements over the East Antarctic seasonal ice zone

“…These particle concentration characteristics contrast with observations made by Schmale et al (2019) in the West Antarctic sector of the Southern Ocean. Using k-means clustering, they observed two clusters of aerosol size distribution clusters with different air mass histories.…”

“…The low concentration observed in the present study suggests sampled air masses are free from recent terrestrial influence and are representative of Southern Ocean boundary layer conditions. Overall particle number concentration and CCN concentrations reported here are also similar to those reported by Schmale et al (2019) as CN 7 (leg 1 median: 470 cm −3 ) and CCN 0.2 (leg 1 median: 114 cm −3 ) during the Antarctic Circumnavigation Expedition (ACE-SPACE), though the activation ratio observed during ACE-SPACE was lower than that observed during PCAN.…”

“…The same is true when comparing the cluster 2 distribution to the present polar distribution. This information, coupled with the more concentrated accumulation mode observed by Schmale et al (2019), suggests the particle populations observed during PCAN may have different air mass histories than those observed in the West Antarctic. There is evidence for differing aerosol regimes near the East and West Antarctic coasts in the differences in observed particle concentrations and activation ratios during the 2016-2017 austral summer (Schmale et al, 2019).…”

“…These results suggest aerosol populations of the seasonal ice zones are distinct from those of the Antarctic continent and the mid-latitudes and upper latitudes of Southern Ocean. Campaigns in the last decade, such as SOAP (Law et al, 2017), SIPEXII (Humphries et al, 2015), ACE-SPACE (Schmale et al, 2019), CAPRICORN , MAR-CUS, and SOCRATES (Sanchez et al, 2021), have measured aerosol properties in the Antarctic seasonal ice zone from ship-based and aircraft platforms.…”