Cloud‐Nucleating Particles Over the Southern Ocean in a Changing Climate

Abstract

Stratocumulus clouds over the Southern Ocean have fewer droplets and are more likely to exist in the predominately supercooled phase than clouds at similar temperatures over northern oceans. One likely reason is that this region has few continental and anthropogenic sources of cloud-nucleating particles that can form droplets and ice. In this work, we present an overview of aerosol particle types over the Southern Ocean, including new measurements made below, in and above clouds in this region. These measurements and others indicate that biogenic sulfur-based particles >0.1 μm diameter contribute the majority of cloud condensation nuclei number concentrations in summer. Ice nucleating particles tend to have more organic components, likely from sea-spray. Both types of cloud nucleating particles may increase in a warming climate likely to have less sea ice, more phytoplankton activity, and stronger winds over the Southern Ocean near Antarctica. Taken together, clouds over the Southern Ocean may become more reflective and partially counter the region's expected albedo decrease due to diminishing sea ice. However, detailed modeling studies are needed to test this hypothesis due to the complexity of oceancloud-climate feedbacks in the region. Plain Language Summary Clouds over the Southern Ocean tend to have less droplets and ice crystals than similar clouds over northern oceans due to fewer sources of cloud-nucleating aerosol particles in the region. In this work, we present an overview of aerosol particle types over the Southern Ocean, including new measurements made below, in and above clouds. These measurements indicate that while sea-spray-derived salts do provide cloud nuclei, the majority of aerosol particles that influence summertime clouds in this region are biogenic-that is, derived from ocean microorganisms, with the ocean region near Antarctica being a large summertime source. These cloud-nucleating particles may increase in a warming climate likely to have less sea ice and more phytoplankton activity near Antarctica. These additional particles could make low clouds reflect more light and offset a portion of the warming expected due to diminishing sea ice in a future climate. TWOHY ET AL.

Influences of Recent Particle Formation on Southern Ocean Aerosol Variability and Low Cloud Properties

“…If new particle formation generated through synoptic uplift (Figure 6) is a major source of FT Aitken aerosols, we expect to find signatures of DMS oxidation products in FT particles sampled during SOCRATES. Ambient aerosol measurements from behind the CVI (Section 2.1) support this speculation and provide additional insights into the size and composition of SO aerosol in the summertime: i) Aitken and the few, small accumulation mode (≤0.2 μm) particles occurring in high number concentration samples in the FT are composed primarily of H 2 SO 4 with possible contributions from more volatile DMS oxidation products like MSA; ii) the majority of these Aitken particles in the FT are smaller than 0.02 μm in diameter; and iii) accumulation‐mode particles (∼0.1–0.5 μm) sampled from AC, IC, and SC flight levels are primarily sulfur‐based with limited sea spray influence (Twohy et al., 2021).…”

“…First, Twohy et al. (2021) use a scanning transmission electron microscope (STEM) and X‐ray analysis of particles impacted on formvar carbon grids for examining particles (diameters 0.1–1 μm) AC, IC, and SC (Twohy et al., 2013). Specific examples for particles with diameters 0.1–0.5 μm are highlighted here, see Twohy et al.…”

“…Specific examples for particles with diameters 0.1–0.5 μm are highlighted here, see Twohy et al. (2021) for complete examination. Second, particle volatility estimates are reconstructed for FT particles from comparing CVI heated CN (using a TSI‐3010 condensation nucleus counter, CN CVI ) and UHSAS100 (using a UHSAS‐G, S/N 15 as in Kupc et al., 2018, UHSAS100 CVI ) measurements to un‐heated CN and UHSAS100 measurements.…”

“…Aerosol measurements behind a counterflow virtual impactor (CVI) (Noone et al, 1988;Twohy et al, 1997) are used to interpret aerosol composition in two ways. First, Twohy et al (2021) use a scanning transmission electron microscope (STEM) and X-ray analysis of particles impacted on formvar carbon grids for examining particles (diameters 0.1-1 μm) AC, IC, and SC (Twohy et al, 2013). Specific examples for particles with diameters 0.1-0.5 μm are highlighted here, see Twohy et al (2021) for complete examination.…”

Observations of Clouds, Aerosols, Precipitation, and Surface Radiation over the Southern Ocean: An Overview of CAPRICORN, MARCUS, MICRE, and SOCRATES

“…Figure 4 shows STEM results for six flights after grouping particles into different types based on elemental composition and morphology (Twohy and Anderson 2008). Figure 4a shows results for each flight, while Fig.…”