Circum-Antarctic abundance and properties of CCN and INP

Abstract

Abstract. Aerosol particles acting as cloud condensation nuclei (CCN) or ice nucleating particles (INP) 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 2016/17. An overview of CCN and INP concentrations on 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 37 and 123 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. 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, with lower and more constant concentrations when at sea. This hints towards terrestrial and/or coastal INP sources being dominant close to land. In pristine marine areas INP 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 concentration around 2.8 μg m⁻³, was found to dominate the sampled particle mass. MSA was highly variable over the SO, with 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.

Significant continental source of ice-nucleating particles at the tip of Chile's southernmost Patagonia region

“…This, together with our observations, implies that air masses, at least those at lower altitudes, will quickly be enriched in INPs when coming into contact with continental or more generally terrestrial sources. A similar observation was also made by Tatzelt et al (2021) based on observations performed in the frame of a circumvention of Antarctica.…”

“…Welti et al (2020) reported on a collection of ship-based INP measurements and found that maritime N INP in the South Temperate Zone (23.5-66.5 • S) was lower than that in the North Temperate Zone (23.5-66.5 • N). In general, maritime N INP were lower than continentally influenced N INP , indicating that land is a stronger source of INP than the ocean (Welti et al, 2020;Tatzelt et al, 2021). López et al (2018) collected soil dust in northern Patagonia and confirmed its ability to be ice active, which points out that natural mineral dust particles may be an important source of INPs in the Southern Hemisphere.…”

Significant continental source of ice-nucleating particles at the tip of Chile’s southernmost Patagonia region

“…This, together with our observations, implies that air masses, at least those at lower altitudes, will quickly be enriched in INP when getting into contact with continental or more generally terrestrial sources. A similar observation was also made by Tatzelt et al (2021) based on observations performed in the frame of a circumvention of Antarctica.…”

“…Gong et al (2020) examined INPs from the ocean, the ambient environment and from cloud water at the Cape Verde islands, and found that marine aerosols from sea spray production only explained a very minor fraction of N INP in the atmosphere and in cloud water. Tatzelt et al (2021), having collected data in the Southern Ocean and Welti et al (2020), examining a broader, more global data-set found that the ship-based measurements of ambient N INP show 1 to 2 orders of magnitude lower mean concentrations for clean marine conditions than for continental observations. Therefore, INP in our study most likely originated from land and more evidence for this is discussed in the following sections.…”

“…Welti et al (2020) reported on a collection of ship-based INP measurements and found that maritime N INP in the South Temperate Zone (23.5-66.5 • S) was lower than that in the North Temperate Zone (23.5-66.5 • N). In general, maritime N INP were lower than continentally influenced N INP , indicating that land is a stronger source of INP than the ocean (Welti et al, 2020;Tatzelt et al, 2021). López et al (2018) collected soil dust in northern Patagonia and confirmed its ability to be ice active, which points out that natural mineral dust particles may be an important source of INPs in the Southern Hemisphere.…”