Southern Ocean latitudinal gradients of cloud condensation nuclei

Abstract

Abstract. The Southern Ocean region is one of the most pristine in the world and serves as an important proxy for the pre-industrial atmosphere. Improving our understanding of the natural processes in this region is likely to result in the largest reductions in the uncertainty of climate and earth system models. While remoteness from anthropogenic and continental sources is responsible for its clean atmosphere, this also results in the dearth of atmospheric observations in the region. Here we present a statistical summary of the latitudinal gradient of aerosol (condensation nuclei larger than 10 nm, CN10) and cloud condensation nuclei (CCN at various supersaturations) concentrations obtained from five voyages spanning the Southern Ocean between Australia and Antarctica from late spring to early autumn (October to March) of the 2017/18 austral seasons. Three main regions of influence were identified: the northern sector (40–45∘ S), where continental and anthropogenic sources coexisted with background marine aerosol populations; the mid-latitude sector (45–65∘ S), where the aerosol populations reflected a mixture of biogenic and sea-salt aerosol; and the southern sector (65–70∘ S), south of the atmospheric polar front, where sea-salt aerosol concentrations were greatly reduced and aerosol populations were primarily biologically derived sulfur species with a significant history in the Antarctic free troposphere. The northern sector showed the highest number concentrations with median (25th to 75th percentiles) CN10 and CCN0.5 concentrations of 681 (388–839) cm−3 and 322 (105–443) cm−3, respectively. Concentrations in the mid-latitudes were typically around 350 cm−3 and 160 cm−3 for CN10 and CCN0.5, respectively. In the southern sector, concentrations rose markedly, reaching 447 (298–446) cm−3 and 232 (186–271) cm−3 for CN10 and CCN0.5, respectively. The aerosol composition in this sector was marked by a distinct drop in sea salt and increase in both sulfate fraction and absolute concentrations, resulting in a substantially higher CCN0.5/CN10 activation ratio of 0.8 compared to around 0.4 for mid-latitudes. Long-term measurements at land-based research stations surrounding the Southern Ocean were found to be good representations at their respective latitudes; however this study highlighted the need for more long-term measurements in the region. CCN observations at Cape Grim (40∘39′ S) corresponded with CCN measurements from northern and mid-latitude sectors, while CN10 observations only corresponded with observations from the northern sector. Measurements from a simultaneous 2-year campaign at Macquarie Island (54∘30′ S) were found to represent all aerosol species well. The southernmost latitudes differed significantly from both of these stations, and previous work suggests that Antarctic stations on the East Antarctic coastline do not represent the East Antarctic sea-ice latitudes well. Further measurements are needed to capture the long-term, seasonal and longitudinal variability in aerosol processes across the Southern Ocean.

New particle formation in coastal New Zealand with a focus on open ocean air masses

“…Zealand have observed N10 of 681 (388-839) cm −3 at latitudes similar to Baring Head(Humphries et al, 2021), which is also in line with our results. Out of other coastal sites, previous work at Mace Head by Dall'Osto et al (2011) saw N10 of 327 cm −3 in open ocean air and 1469 cm −3 during open ocean nucleation.…”

Coalescence Scavenging Drives Droplet Number Concentration in Southern Ocean Low Clouds

“…I. L. McCoy et al (2021) suggest that these FT Aitken particles (diameters ≤0.1 μm), in addition to acting as the primary source of CCN, also buffer SO CCN and N d against precipitation removal during biologically active time periods. The latitudinal pattern is driven in part by the large increases in the amount of phytoplankton and thus DMS production in the summertime seasonal ice zone around Antarctica (Curran & Jones, 2000) and increases in small aerosol particles south of the oceanic polar front during the SO summer (Humphries et al, 2016(Humphries et al, , 2021. Across the SO, strong correlations are observed between satellite retrieved N d and sea surface chlorophyll-a concentrations (a proxy for phytoplankton biomass; D. T. McCoy et al, 2015).…”

Key challenges for tropospheric chemistry in the Southern Hemisphere

“…There is an annual cycle in observed aerosol optical depth at Cape Grim, driven by both biogenic activity and long-range transport (Mitchell et al, 2017); however, the magnitude and location dependence is poorly characterized. These data sets only measured the northern region of the Southern Ocean, which has recently been shown to be distinct from the mid-and high-latitude regions (Humphries et al, 2021). There are also a number of portable sun photometers measurements that have been made within the AERONET Maritime Aerosol Network (Smirnov et al, 2009;Smirnov et al, 2011) and used in validation and model development of important aerosol components such as oceanic sea salt (Bian et al, 2019).…”