May 4, 2022 - 3:00 pm
May 4, 2022 - 4:00 pm
CategoriesWeather and Climate Systems
Weather and Climate Systems
Meteorological Controls of Southern Ocean (50Â°S-68Â°S, 63Â°E-150Â°E) Boundary Layer CCN-active Aerosols during the MARCUS
Wednesday, May 4
The Southern Ocean (SO) is one of the most pristine environments on Earth because of its remoteness, providing a natural laboratory to study aerosol-clouds interactions in varying meteorological conditions. The Atmospheric Radiation Measurement Programâ€™s Mobile Facility-2 (AMF2) onboard the Australian icebreaker Aurora Australis obtained ship-based cloud, precipitation, and aerosol measurements during the 2017-18 Measurement of Aerosols, Radiation, and CloUds over the SO (MARCUS) Experiment during cruises across the SO. The latitude dependence of Cloud Condensation Nuclei-active aerosols, approximated by the concentrations of aerosols with diameter 60 nm < D < 1000 nm measured by the Ultra-High-Sensitivity Aerosol Spectrometer (UHSAS), hereafter N60-1000nm, is examined in latitudinal dependent meteorological regimes between 50Â°S and the Antarctic coast because of their potential influence on cloud microphysical properties.
No significant correlation was found between the CCN number concentration measured at both 0.2% (NCCN,0.2) and 0.5% (NCCN,0.5) supersaturation and sea surface horizontal wind speed. However, there was a positive correlation (P < 0.05) for aerosol concentration with diameter D > 500 nm (N500-1000nm) and a negative correlation (P < 0.05) for N60-100nm with wind speed. North of 62Â°S N500-1000nm is 41% larger and aerosol concentrations for 60 nm < D < 200 nm, N60-200nm, 32% less, and NCCN,0.2 (NCCN,0.5) is 60 cm-3 (79 cm-3) less compared to south of 62Â°S. This increase of CCN south of 62Â°S caused by the increase of small aerosols is consistent with measurements of the Aerosol Scattering Angstrom Exponent (Î±) at wavelengths 450, 550, and 700 nm made by the Particle Soot Absorption Photometer that show Î± modes of 0.5 (Î±450-550) and 0.8 (Î± 450-700) north of 62Â°S, and of 1.1 (Î±450-550) and 1.4 (Î± 450-700) south of 62Â°S. This implies aerosols have higher effective radii north of 62Â°S. This implies aerosols have higher effective radii north of 62Â°S. Further, aerosol hygroscopicity Growth Factor measured by the Hygroscopic Tandem Differential Mobility Analyzer (HTDMA) stayed close to 1.4 for N60-1000nm with D < 250 nm south of 62Â°S, but with mode changes from 1.3 to 1.67 north of 62Â°S as D increases. This implies the chemical compositions of aerosols with 50 nm < D < 250 nm are relatively uniform south of 62Â°S, while north of 62Â°S the composition changes with aerosol size (e.g., aerosols with D ~ 50 nm and GF ~ 1.3 might be organic aerosols or non-neutralised sulphuric acid, while the larger hygroscopicity variability for aerosols north of 62S might be driven by inorganic salts composition). The mode values of aerosols with D < 250 nm increase when non-precipitating low cloud base height decreases, consistent with clouds acting as sources.
Based on the above measurements and back trajectories from the Hybrid Single-Particle Lagrangian Integrated Trajectories (HYSPLIT) model, hypotheses are made that 1) N60-1000nm over the SO is primarily dominated by N60-200nm nurtured by cloud processes in the boundary layer; 2) increased concentrations of large aerosols (e.g., N500-1000nm) north of 62Â°S are caused by wind-generated sea spray aerosols near the sea surface because of higher wind speeds observed in this latitudinal band; 3) new particle formation is more important south of 62Â°S, generating a large number of aerosols with D < 60 nm, which later on grow larger and cause higher NCCN; 4) the boundary layer north of 62Â°S is more convective, scavenging CCN and leaving relatively aged aerosols that grew during cloud processes (e.g., cloud droplet residues). These findings are consistent with the conclusions from previous research that show increased sulfate-based aerosols and decreased chloride-based aerosols south of 62Â°S. MARCUS observations showed that north of 62Â°S 53% of wind speeds were larger than 15 ms-1, with the mode of the 95 GHz cloud radar reflectivity being -37 dBZ, and 21.6% of values larger than -15 dBZ suggesting precipitating. On the other hand, south of 62Â°S only 12% of winds were greater than 15 ms-1, with the mode of radar reflectivity -41 dBZ and 15% larger than -15 dBZ. Further, total aerosol count (CN) measured by the Condensation Particle Counter shows a higher frequency of high CN events south of 62Â°S compared to north of 62Â°S. The implications of these findings for the representation of aerosol processes in atmospheric models is discussed.