10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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The Concentrations, Spatial Distribution, and Compositions of Ice Nucleating Particles in and around Stratiform Clouds over the Southern Ocean
PAUL DEMOTT, Christina McCluskey, Kathryn Moore, Thomas Hill, Ezra Levin, Cynthia Twohy, Lynn Russell, Darin Toohey, Bryan Rainwater, Greg McFarquhar, Alain Protat, Ruhi Humphries, Gerald Mace, Melita Keywood, Roger Marchand, Cory Wolff, Jeffrey Stith, Sonia Kreidenweis, Colorado State University
Abstract Number: 334 Working Group: Unraveling the Many Facets of Ice Nucleating Particles and Their Interactions with Clouds
Abstract The Southern Ocean is one of the stormiest places on Earth, and is blanketed in frequent clouds whose properties, phase and lifetime are not well represented in climate models, leading to a high bias in predicted radiation reaching the ocean surface (compared to satellite measurements) over a broad latitude range south of about 55° S latitude. These model biases in turn lead to potentially incorrect predictions of ocean and atmospheric feedbacks. The Southern Ocean Clouds, Radiation, Aerosol Transport Experimental Study (SOCRATES), conducted in January and February 2018 in the region between Hobart, Tasmania and 62° S, provided the most comprehensive assessment of aerosol interactions with supercooled and mixed-phase clouds ever compiled over the Southern Ocean. Studies were specially designed to document primary ice nucleation and its influence on cloud phase. These studies provide constraints for follow on numerical modeling studies to explore primary and secondary (ice “multiplication”) ice formation processes in these clouds.
The NSF/NCAR G-V payload for SOCRATES included two types of ice nucleating particle (INP) measurements, a continuous flow diffusion chamber (CFDC) for real-time INP measurements and bulk volume filter collections for offline immersion freezing temperature spectra measurements. Flight plans were designed to quantify the INP budget around clouds and its potential influence on ice formation in clouds. Typically, measurements were first made of aerosols in ambient air just above supercooled cloud layers (sometimes multiple layers were sampled), and then a level leg was flown within the peak cloud liquid water layer to observe cloud microphysics. Cloud residual particles were sampled with a counterflow virtual impactor (CVI) inlet and the residuals collected for further analysis. Finally, a level leg was flown below clouds in the marine boundary layer (MBL) to sample aerosols coming primarily from sea spray emissions. Both ambient aerosols and cloud residual particles were analyzed to determine INP number concentrations. Aerosol collections for compositional analyses were made from each region around clouds, and from activated INPs from the CFDC. Offline treatments on bulk aerosol samples are also planned to investigate contributions from heat labile INPs and organic INPs. These aircraft data were supported by the same INP measurement suite operated on the Australian Marine National Facility ship, the RV Investigator, leading to comprehensive spatial data on INPs in the Southern Ocean MBL.
INP concentrations over the Southern Ocean region were found to be some of the lowest on Earth, both in the free troposphere and in the MBL. CFDC INP measurements focused at water supersaturated conditions were often at detection limits, well below 1 per liter at -30°C. The particle number concentration enhancement (20-40 times) of the CVI inlet thus permitted best INP detection, as did use of an aerosol particle concentrator on the RV Investigator. INP concentration and compositional data will be summarized, relationships to aerosol properties including bioaerosols will be discussed, and a first comparative inspection of INP versus cloud ice particle concentrations and evolution will be presented.