Circum-Antarctic abundance and properties of CCN and INPs

Abstract

Abstract. Aerosol particles acting as cloud condensation nuclei (CCN) or ice-nucleating particles (INPs) play a major role in the formation and glaciation of clouds. Thereby they exert a strong impact on the radiation budget of the Earth. Data on abundance and properties of both types of particles are sparse, especially for remote areas of the world, such as the Southern Ocean (SO). In this work, we present unique results from ship-borne aerosol-particle-related in situ measurements and filter sampling in the SO region, carried out during the Antarctic Circumnavigation Expedition (ACE) in the austral summer of 2016–2017. An overview of CCN and INP concentrations over the Southern Ocean is provided and, using additional quantities, insights regarding possible CCN and INP sources and origins are presented. CCN number concentrations spanned 2 orders of magnitude, e.g. for a supersaturation of 0.3 % values ranged roughly from 3 to 590 cm−3. CCN showed variable contributions of organic and inorganic material (inter-quartile range of hygroscopicity parameter κ from 0.2 to 0.9). No distinct size dependence of κ was apparent, indicating homogeneous composition across sizes (critical dry diameter on average between 30 and 110 nm). The contribution of sea spray aerosol (SSA) to the CCN number concentration was on average small. Ambient INP number concentrations were measured in the temperature range from −5 to −27 ∘C using an immersion freezing method. Concentrations spanned up to 3 orders of magnitude, e.g. at −16 ∘C from 0.2 to 100 m−3. Elevated values (above 10 m−3 at −16 ∘C) were measured when the research vessel was in the vicinity of land (excluding Antarctica), with lower and more constant concentrations when at sea. This, along with results of backward-trajectory analyses, hints towards terrestrial and/or coastal INP sources being dominant close to ice-free (non-Antarctic) land. In pristine marine areas INPs may originate from both oceanic sources and/or long-range transport. Sampled aerosol particles (PM10) were analysed for sodium and methanesulfonic acid (MSA). Resulting mass concentrations were used as tracers for primary marine and secondary aerosol particles, respectively. Sodium, with an average mass concentration around 2.8 µg m−3, was found to dominate the sampled, identified particle mass. MSA was highly variable over the SO, with mass concentrations up to 0.5 µg m−3 near the sea ice edge. A correlation analysis yielded strong correlations between sodium mass concentration and particle number concentration in the coarse mode, unsurprisingly indicating a significant contribution of SSA to that mode. CCN number concentration was highly correlated with the number concentration of Aitken and accumulation mode particles. This, together with a lack of correlation between sodium mass and Aitken and accumulation mode number concentrations, underlines the important contribution of non-SSA, probably secondarily formed particles, to the CCN population. INP number concentrations did not significantly correlate with any other measured aerosol physico-chemical parameter.

Measurements of Aerosol Particle Size Distributions and INPs Over the Southern Ocean in the Late Austral Summer of 2017 on Board the R/V Mirai: Importance of the Marine Boundary Layer Structure

“…N INPs in the MBL over the SO presented in this study were slightly lower than those in the MBL of the northern hemisphere (DeMott et al, 2016;Tobo et al, 2019), especially in the T ranges higher than −20°C but were very similar to those in the MBL over the SO reported by McCluskey et al (2018) and Welti et al (2020). Note that McCluskey et al (2018) investigated the behavior of INPs over the SO to the south of Tasmania (44°-53°S, 142°−151°E) in the late austral summer of 2016 and that Welti et al (2020) and Tatzelt et al (2022) studied over the Pacific Ocean sector of the Antarctic circumpolar region (>65°S) in the austral summer of 2017 (the leg 2 of the Antarctic circumnavigation expedition). They are different from the region of the SO covered in this study (67°-38°S, 70°-180°W).…”

“…(2020) and Tatzelt et al. (2022) studied over the Pacific Ocean sector of the Antarctic circumpolar region (>65°S) in the austral summer of 2017 (the leg 2 of the Antarctic circumnavigation expedition). They are different from the region of the SO covered in this study (67°–38°S, 70°–180°W).…”

“…Variabilities in INPs concentrations might strongly modulate MPC over the SO where INPs concentrations are extremely low (McCoy et al, 2014). Therefore, it is obvious that the simultaneous characterization of CCN and INPs concentrations over the SO (Tatzelt et al, 2022)…”

“…Uetake et al (2020) recently found that marine microorganisms were the dominant sources of airborne microorganisms in the SO's MBL and suggested their possible contribution to the INP population. To the best of our knowledge, measurements of PSD for understanding both CCN and INPs activities over the SO are still limited despite their significance as a preindustrial proxy of aerosol concentrations and their roles in aerosol-cloud interactions (e.g., McFarquhar et al, 2021;Schmale et al, 2019;Tatzelt et al, 2022). Furthermore, the wave breaking state, which is a key to regulate the sea-spraying emissions of aerosol particles (e.g., Zhao & Toba, 2001), has rarely been analyzed in combination with CCN and INPs over the SO.…”

Marine biogenic emissions of benzene and toluene and their contribution to secondary organic aerosols over the polar oceans

“…Although benzene and toluene do not contribute as much to aerosol mass compared to DMS, they are a source of highly oxygenated organic molecules ( 58 ) that condense onto existing particles and contribute to aerosol growth ( 59 ). Oxidized aromatics alter particle properties and may help explain the lower-than-expected particle hygroscopicity over the Southern Ocean ( 60 ). Overall, our modeling results show that ocean benzene and toluene emissions enhance SOA in the polar regions, especially over the pristine Southern Ocean, where episodic localized emissions have a disproportionate contribution to SOA mass.…”