62°S Witnesses the Transition of Boundary Layer Marine Aerosol Pattern Over the Southern Ocean (50°S–68°S, 63°E–150°E) During the Spring and Summer: Results From MARCUS (I).

The Atmospheric Radiation Measurement Mobile Facility‐2 was installed onboard the research vessel Aurora Australis to measure aerosol properties during the 2017–2018 Measurement of Aerosols, Radiation, and CloUds over the pristine Southern ocean (MARCUS) Experiment, providing unique data on aerosols latitudinal and seasonal variation, including south of 60°S where previous observations are scarce. Data from a Cloud Condensation Nuclei (CCN) counter and Ultra‐High‐Sensitivity Aerosol Spectrometer show that both the number concentration (NCCN) and size distribution of CCN‐active aerosols, with diameters (D) between 60 nm < D < 1,000 nm are different over the North Southern Ocean (NSO) (50°S–60°S) and the South Southern Ocean (SSO) (62°S–68°S). The average NSO NCCN at 0.2% and 0.5% supersaturation were 28% and 49% less than that over the SSO. This increase of CCN over the SSO is caused by the increase of aerosols with 60 nm < D < 200 nm, consistent with calculations of Aerosol Scattering Angstrom Exponents derived from a nephelometer. Aerosol hygroscopicity growth factor measured by the Hygroscopic Tandem Differential Mobility Analyzer stayed close to 1.41 for aerosols with 50 nm < D < 250 nm over the SSO, but increased from 1.30 to 1.67 over the NSO, indicating different chemical compositions. Both CCN and Ice Nucleating Particles (INPs) showed a stronger variation with season than with latitude. The variation of heat‐labile and presumably proteinacous INPs suggests an increase of ice nucleating‐active microbes in summer. Plain Language Summary: The Atmospheric Radiation Measurement Mobile Facility‐2 was installed onboard an ice breaker to measure small particles suspended in the air (aerosols), particularly those with potential influences on cloud formation and evolution. The 2017–2018 Measurement of Aerosols, Radiation, and CloUds over the pristine Southern ocean (MARCUS) measurement campaign provides unique data on the latitudinal and seasonal variation of the suspended particles, including south of 60°S where previous observations are scarce. Data show that both the number concentration and size distribution of particles that serve as embryos of cloud droplets are different over the North Southern Ocean (NSO) (50°S–60°S) and the South Southern Ocean (SSO) (62°S–68°S). There are greater concentrations of these embryos over the SSO for increased total amount of suspended particles there. These observations are consistent with data collected by other instruments that show differences over the NSO and SSO in how these suspended particles scatter radiation and how they absorb water vapor, which shows the particles have different sizes and chemical compositions respectively. The concentrations of both particles that serve as embryos for cloud drops and ice crystals both vary more with season than latitude, which has an implication for the energy balance of the Southern Ocean. Key Points: Machine Learning is applied to identify ship stack contamination of ship‐borne aerosol measurementsSurface Cloud Condensation Nuclei number concentration and its seasonal variation over 62°S–68°S is higher than over 50°S–60°SIce Nucleating Particles over the Southern Ocean originate from primarily organic and biological sources during MARCUS [ABSTRACT FROM AUTHOR]