Southern Ocean (50°S-68°S, 63°E-150°E) Boundary Layer CCN-active Aerosols Latitudinal and Seasonal Distribution during the MARCUS
QING NIU, Greg McFarquhar, Connor Flynn,
University of Oklahoma Abstract Number: 558
Working Group: Aerosols, Clouds and Climate
AbstractThe Southern Ocean (SO), for its remoteness, providing a natural laboratory to study aerosol-clouds interactions in varying meteorological conditions. The Atmospheric Radiation Measurement Program’s Mobile Facility-2 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. After the analysis of meteorological regimes, the SO has been divided into NSO (50°S-60°S) and SSO (62°S-68°S).
The negative Aerosol Scattering Angstrom Exponent (AE) curvature indicates the aerosol size distribution in both NSO and SSO are monomodal and dominated by the fine mode aerosols with diameter D < 250 nm. However, histogram of Condensation Nuclei (CN) shows high fine mode aerosols events (CN > 1000 cm
-3) is significantly more frequent in SSO (P<0.05), with limited seasonal variation. This may be related to more frequent new particle formation events over the SSO. The AE at wavelengths 450, 550, and 700 nm measured by the Nephelometer show AE modes (α) of 0.5 (α
450-550) and 0.8 (α
450-700) in NSO, and of 1.1 (α
450-550) and 1.4 (α
450-700) in SSO. This implies aerosols have higher effective radii in NSO. 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 N
60-1000nm (similar notation below), is examined by latitude and season because of their potential influence on cloud microphysical properties. N
60-1000nm shows larger seasonal variation in SSO than NSO. Besides, the NSO N
500-1000nm is 41% larger and aerosol concentrations for 60 nm < D < 200 nm, N
60-200nm, 32% less, and N
CCN,0.2 (N
CCN,0.5) is 60 cm
-3 (79 cm
-3) less compared to the SSO.
Further, aerosol hygroscopicity Growth Factor (GF) measured by the Hygroscopic Tandem Differential Mobility Analyzer (HTDMA) stayed close to 1.4 for N
60-1000nm with D < 250 nm in SSO, but with mode changes from 1.3 to 1.67 in NSO as D increases. This implies the chemical compositions of aerosols with 50 nm < D < 250 nm are relatively uniform in SSO, while in NSO 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 in NSO might be driven by inorganic salts composition). The modality of aerosols size distribtuion (60 nm < D < 250 nm) shifts when non-precipitating low clouds cloud-base-height decreases, consistent with clouds acting as sources.