A missing source of aerosols in Antarctica – beyond long-range transport, phytoplankton, and photochemistry

Abstract

Abstract. Understanding the sources and evolution of aerosols is crucial for constraining the impacts that aerosols have on a global scale. An unanswered question in atmospheric science is the source and evolution of the Antarctic aerosol population. Previous work over the continent has primarily utilized low temporal resolution aerosol filters to answer questions about the chemical composition of Antarctic aerosols. Bulk aerosol sampling has been useful in identifying seasonal cycles in the aerosol populations, especially in populations that have been attributed to Southern Ocean phytoplankton emissions. However, real-time, high-resolution chemical composition data are necessary to identify the mechanisms and exact timing of changes in the Antarctic aerosol. The recent 2ODIAC (2-Season Ozone Depletion and Interaction with Aerosols Campaign) field campaign saw the first ever deployment of a real-time, high-resolution aerosol mass spectrometer (SP-AMS – soot particle aerosol mass spectrometer – or AMS) to the continent. Data obtained from the AMS, and a suite of other aerosol, gas-phase, and meteorological instruments, are presented here. In particular, this paper focuses on the aerosol population over coastal Antarctica and the evolution of that population in austral spring. Results indicate that there exists a sulfate mode in Antarctica that is externally mixed with a mass mode vacuum aerodynamic diameter of 250 nm. Springtime increases in sulfate aerosol are observed and attributed to biogenic sources, in agreement with previous research identifying phytoplankton activity as the source of the aerosol. Furthermore, the total Antarctic aerosol population is shown to undergo three distinct phases during the winter to summer transition. The first phase is dominated by highly aged sulfate particles comprising the majority of the aerosol mass at low wind speed. The second phase, previously unidentified, is the generation of a sub-250 nm aerosol population of unknown composition. The second phase appears as a transitional phase during the extended polar sunrise. The third phase is marked by an increased importance of biogenically derived sulfate to the total aerosol population (photolysis of dimethyl sulfate and methanesulfonic acid (DMS and MSA)). The increased importance of MSA is identified both through the direct, real-time measurement of aerosol MSA and through the use of positive matrix factorization on the sulfur-containing ions in the high-resolution mass-spectral data. Given the importance of sub-250 nm particles, the aforementioned second phase suggests that early austral spring is the season where new particle formation mechanisms are likely to have the largest contribution to the aerosol population in Antarctica.

High summertime aerosol organic functional group concentrations from marine and seabird sources at Ross Island, Antarctica, during AWARE

“…One such cause would be additional CN that are too small to contribute to CCN. Previous observations at a site 10 km from McMurdo Station showed an increase in the fraction of CN smaller than 250 nm at polar sunrise (September-October), although a specific cause was not clear (Giordano et al, 2017). The higher CCN / CN ratio in the summer (Table 1) is consistent with both the higher biogenic sulfate contributions during the highest productivity season (summer) and the slightly larger diameter of the accumulation mode particles observed in previous summers (Kim et al, 2017).…”

“…Furthermore, the station is unique in that McMurdo Station is one of the two sites that have published aerosol measurements starting in 1968, with the other one being the Amundsen Scott Station at the South Pole. The site has at least 10 publications describing aerosol measurements over the past 50 years, most of which were limited to summer (Cadle et al, 1968;Warburton, 1973;Ondov et al, 1973a;Hogan, 1975;Hofmann, 1988;Hansen et al, 2001;Mazzera et al, 2001a, b;Giordano et al, 2017;Kalnajs et al, 2013;Khan et al, 2018). No stations in Antarctica measured inorganic chemical composition year-round until 1978 (Parungo et al, 1981), and none have measured year-round organic components.…”

“…Kalnajs et al (2013) showed that ozone depletion is correlated to aerosol concentrations because halogencontaining aerosol consumed ozone. An aerosol mass spectrometer (AMS) at a site 20 km northeast from McMurdo Station during October to December 2014 and August to October 2015 (Giordano et al, 2017) found sulfate accounted for more than 50 % of nonrefractory composition. Many measurement campaigns were limited to austral summer months because of restrictions on access (Cadle et al, 1968;Ondov et al, 1973a;Warburton, 1973) and so lack information on seasonal changes.…”

In situ measurements of cloud microphysics and aerosol over coastal Antarctica during the MAC campaign

“…Although aerosol composition in summer is more variable, sea salt still accounts for a significant fraction (50 % by mass) but now with a large contribution from non-sea-salt sulfate (27 % by mass; Weller et al, 2008). Measurements at the coastal Antarctic McMurdo station show the persistent presence of sulfate aerosol throughout the year (Giordano et al, 2017). In the winter, these particles are highly aged.…”

“…In the winter, these particles are highly aged. Sulfate aerosol then increases through the austral spring/summer due to enhanced emissions of dimethyl sulfide (DMS) and methanesulfonic acid (MSA) from phytoplankton in the Southern Ocean (Gibson et al, 1990;Giordano et al, 2017). Giordano et al (2017) also report the presence of a sub-250 nm aerosol population of unknown composition during the winter-to-summer transition.…”