Summertime Primary and Secondary Contributions to Southern Ocean Cloud Condensation Nuclei

Abstract

Atmospheric aerosols in clean remote oceanic regions contribute significantly to the global albedo through the formation of haze and cloud layers; however, the relative importance of ‘primary’ wind-produced sea-spray over secondary (gas-to-particle conversion) sulphate in forming marine clouds remains unclear. Here we report on marine aerosols (PM1) over the Southern Ocean around Antarctica, in terms of their physical, chemical, and cloud droplet activation properties. Two predominant pristine air masses and aerosol populations were encountered: modified continental Antarctic (cAA) comprising predominantly sulphate with minimal sea-salt contribution and maritime Polar (mP) comprising sulphate plus sea-salt. We estimate that in cAA air, 75% of the CCN are activated into cloud droplets while in mP air, 37% are activated into droplets, for corresponding peak supersaturation ranges of 0.37–0.45% and 0.19–0.31%, respectively. When realistic marine boundary layer cloud supersaturations are considered (e.g. ~0.2–0.3%), sea-salt CCN contributed 2–13% of the activated nuclei in the cAA air and 8–51% for the marine air for surface-level wind speed < 16 m s−1. At higher wind speeds, primary marine aerosol can even contribute up to 100% of the activated CCN, for corresponding peak supersaturations as high as 0.32%.

On the annual variability of Antarctic aerosol size distributions at Halley Research Station

“…Udisti et al, 2012). The results presented here suggest that, in coastal Antarctica, aerosol composition is a strong function of wind speed and that the mechanisms determining aerosol composition are likely linked to blowing snow (Giordano et al, 2018;Yang et al, 2019;Frey et al, 2020). We note that Legrand et al (2016) suggested that on average, the sea ice and open-ocean emissions equally contribute to the sea salt aerosol load of the inland Antarctic atmosphere.…”

“…It is interesting to note that the recent, spatially extensive study of the concentration of sea salt aerosol throughout most of the depth of the troposphere and over a wide range of latitudes (Murphy et al, 2019) reported a source of sea salt aerosol over pack ice that is distinct from that over open water, likely produced by blowing snow over sea ice (Huang et al, 2018;Giordano et al, 2018;Frey et al, 2020). In recent years, a number of long-term aerosol size distribution datasets have been discussed (Järvinen et al, 2013;, but these types of datasets are still scarce.…”

Sea-spray regulates sulfate cloud droplet activation over oceans

“…The general properties of the submicron aerosol representative of the Austral summertime Southern Ocean region were reported in an earlier study 19 where aerosol properties were classified in terms of air mass origin and subsequent advection. The area was dominated by maritime polar (mP) and marine modified continental Antarctic (cAA) air masses ( Supplementary Fig.…”

“…As presented in a related study 19 , we can regard the number concentration of cloud droplets, or the number concentration of activated CCN, as the number of particles larger than the intermodal minimum, which has been shown to be reasonable from direct measurements of in-cloud and out-of-cloud aerosol 21,24 . In doing so, we find that only 45% of the total measured particles were apparently activated in mP air masses while 71% were apparently activated into cloud droplets in cAA air masses, leading to a similar average CDNC in both air masses (176 cm −3 versus 187 cm −3 , respectively).…”

“…Approaches to quantify the less complex scenario of the relative contribution of sulfate and sea-salt to CCN have typically concluded that non-sea-salt sulfate (nss-SO 2À 4 ) dominates marine boundary layer CCN at higher supersaturations while sea-salt dominates at low supersaturations [16][17][18][19] . In this study, we aim to not only quantify the first order effect of sea-salt addition (or subtraction) to a sulfate-rich CCN population, but also to elucidate any possible higher order effects such as modification of the supersaturation which can indirectly impact on the sulfate CCN concentration.…”