Qing Niu

NWC Seminar 

Observations and Simulations of the Marine Boundary Layer Cloud Condensation Nuclei over the Southern Ocean 

Qing Niu

Tuesday, July 2nd, 2024

Skype / 4:00 PM Central

https://join.skype.com/ZtXooVnUoqD8

Abstract:

Ship-borne observations acquired 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 show that the CCN number concentration (NCCN) at 0.2% and 0.5% supersaturation were 28% and 49% less over the North Southern Ocean (NSO, 50S-60S) compared to over the South Southern Ocean (SSO, 62°S-68°S). This is related to the increased populated small accumulation mode aerosols (small Ac) with 60 nm < D < 200 nm, which have different chemical compositions indicated by the aerosol hygroscopicity growth factor than that over the NSO. Both CCN and Ice Nucleating Particles (INPs) showed a stronger variation with season than with latitude. The variation of heat-labile and presumably proteinaceous INPs suggests an increase of ice-nucleating-active microbes in summer. 

Comparisons between CCN observations from field campaigns during the 2017-2018 Austral summer over the Australasian sector of the SO and simulated CCN from the Community Atmospheric Model 6 (CAM6) are presented. Modeled Marine Boundary Layer (MBL) NCCN is underestimated in general, by close to 100% at latitudes south of 56°S. Further, the CCN bias in CAM6 is 1) the largest close to the Antarctic peninsula during summer, implying the biased CCN type has seasonal and latitudinal variation and, 2) 3 times larger over the sea ice than over the open water, implying sea spray CCN are better simulated compared to the secondary Marine Biogenic CCN (MB-CCN). Chemical mass concentrations indicate that the sulfate in Ac mode is underestimated by a factor of 3-7 over the SSO. With secondary aerosols including sulfate being less hygroscopic than sea salt CCN, the CCN-activation-ratio derived using bulk hygroscopicity kappa in Abdul-Razzak function is underestimated for the overestimation of critical supersaturation. 

With the sea-ice oriented less convective atmospheric boundary layer over the Antarctic peninsula showing the higher possibility of clouds with high cloud droplet number concentration, the meteorological control of MBL CCN is investigated. Data show that the activation ratio of aerosols to become CCN is ~23% higher over the SSO during the summer for the amount of substantial small-Ac compensates the less hygroscopic nature of secondary MB-CCN and thus results in the higher value of NCCN. Accumulated rain amounts in 35-hour back trajectories, current wind, and current precipitation are respectively analyzed as potential meteorological controls. Wet scavenging is identified as the most frequent scheme over the SO in general, while the current wind dominates the NCCN over the 55°S-60°S.