Unexpectedly high ultrafine aerosol concentrations above East Antarctic sea ice

Abstract

Abstract. Better characterisation of aerosol processes in pristine, natural environments, such as Antarctica, have recently been shown to lead to the largest reduction in uncertainties in our understanding of radiative forcing. Our understanding of aerosols in the Antarctic region is currently based on measurements that are often limited to boundary layer air masses at spatially sparse coastal and continental research stations, with only a handful of studies in the vast sea-ice region. In this paper, the first observational study of sub-micron aerosols in the East Antarctic sea ice region is presented. Measurements were conducted aboard the icebreaker Aurora Australis in spring 2012 and found that boundary layer condensation nuclei (CN 3 ) concentrations exhibited a fivefold increase moving across the polar front, with mean polar cell concentrations of 1130 cm −3 -higher than any observed elsewhere in the Antarctic and Southern Ocean region. The absence of evidence for aerosol growth suggested that nucleation was unlikely to be local. Air parcel trajectories indicated significant influence from the free troposphere above the Antarctic continent, implicating this as the likely nucleation region for surface aerosol, a similar conclusion to previous Antarctic aerosol studies. The highest aerosol concentrations were found to correlate with low-pressure systems, suggesting that the passage of cyclones provided an accelerated pathway, delivering air masses quickly from the free troposphere to the surface. After descent from the Antarctic free troposphere, trajectories suggest that sea-ice boundary layer air masses travelled equatorward into the low-albedo Southern Ocean region, transporting with them emissions and these aerosol nuclei which, after growth, may potentially impact on the region's radiative balance. The high aerosol concentrations and their transport pathways described here, could help reduce the discrepancy currently present between simulations and observations of cloud and aerosol over the Southern Ocean.

New particle formation events observed at King Sejong Station, Antarctic Peninsula – Part 1: Physical characteristics and contribution to cloud condensation nuclei

“…The CN 2.5−10 /CN 10 ratio is the parameter used to distinguish between particles newly formed from gas-to-particle conversion and background particles (Warren and Seinfeld, 1985;Humphries et al, 2015). Humphries et al (2016) also used the CN 2.5−10 /CN 10 ratio to distinguish the NPF days during a 52 d voyage in the East Antarctic sea-ice region because the number concentration data were more reliable than the size distribution data.…”

“…In order to understand the characteristics of the NPF, studies have been conducted in various regions including coastal, forest, mountainous, rural and urban sites (O'Dowd et al, 2002;Komppula et al, 2003;Yoon et al, 2006;Park et al, 2009;Kim et al, 2011;Rose et al, 2015;Bianchi et al, 2016;Kontkanen et al, 2017). In addition, studies on the NPF phenomenon have recently been conducted at various sites in the polar regions (Asmi et al, 2010;Järvinen et al, 2013;Kyrö et al, 2013;Park et al, 2004;Weller et al, 2015;Humphries et al, 2016;Nguyen et al, 2016;Willis et al, 2016;Barbaro et al, 2017;Dall'Osto et al, 2017). A NPF event occurring in the period between December 1998 and December 2000 at the South Pole was reported by Park et al (2004).…”

Size distribution and ionic composition of marine summer aerosol at the continental Antarctic site Kohnen

“…In a seminal review, Carslaw et al (2013) concluded that uncertainties in cloud radiative forcing are inter alia caused by uncertainties in natural emissions of dimethylsulfide (DMS) producing biogenic sulfur aerosol (i.e., MS − and nss-SO 2− 4 ) and sea spray. The radiation budget over the Southern Ocean is a particular challenge for current climate models, as they disturbingly underpredict the aerosol optical depth, pointing to a missing source of aerosols influencing cloud properties in this region (Bodas-Salcedo et al, 2014;Humphries et al, 2016). The competing role of biogenic sulfur (linked with phytoplankton presence) and otherwise sea salt aerosol (linked with stormy sea) in controlling cloudiness above the Southern Ocean is still up for debate (Meskhidze and Nenes, 2006;Korhonen et al, 2008;Quinn and Bates, 2011;Gras and Keywood, 2017).…”

“…In addition, dedicated aerosol investigations within EPICA at Kohnen have demonstrated the conspicuous impact of a cyclone on aerosol transport: in the aftermath of an intense low-pressure system over the eastern Weddell Sea region in combination with a blocking highpressure ridge to the east in early January 2002 (Birnbaum et al, 2006), MS − and nss-SO 2− 4 aerosol entry showed an exceptional maximum . So far, a distinct impact of cyclones on aerosol concentrations has been frequently observed at coastal sites (e.g., Ito andIwai, 1981, for Syowa andPant et al, 2010, for Maitri), but only rarely on the Antarctic Plateau region (Hogan and Barnard, 1978, for the South Pole). In the most elaborate study about this topic, Pant et al (2010) reported an almost constant background aerosol concentration under calm conditions, while during the passage of cyclones coarse-mode sea salt aerosol increased by an order of magnitude and occasionally new particle formation could be detected in the aftermath of the storms.…”

Size distribution and ionic composition of marine summer aerosol at the continental Antarctic site Kohnen

“…Ocean is a challenge for current climate models, as they disturbingly under predict the aerosol optical depth, pointing to a missing source of aerosols influencing cloud properties in this region (Bodas-Salcedo et al, 2014;Humphries et al, 2016).…”