Ice Nucleating Particles, Aerosols, and Clouds over the Higher Latitude Southern Ocean

Principal Investigator(s):
Paul DeMott, Colorado State University

Improved understanding of aerosol-cloud-precipitation interactions is a key goal of the DOE-ASR program. Ice Nucleating Particles (INPs) are rare aerosol particles that trigger ice formation in clouds. By doing so they may initiate precipitation in clouds, and this impacts cloud lifetime. Nowhere may the role of INPs be manifested so dramatically as in the Southern Ocean (SO). This vast region was the focus of the DOE-ARM MARCUS (Measurement of Aerosols, Radiation and CloUds over the Southern Oceans) and MICRE (Macquarie Island Cloud and Radiation Experiment) campaigns. In the SO, a deficit of INPs is a leading hypothesis for why liquid clouds persist, which underlays the bias of global climate models in predicting excess shortwave radiation reaching the ocean surface between 55°S and Antarctica.

Sea spray emissions provide INPs of much lower ice nucleation efficiency than INPs from land. However, marine INP emissions depend strongly on meteorological and biological factors, and their spatial and temporal variability over the SO are poorly defined. Our group collected data on INP concentrations and the role of biological INP emissions in the MARCUS and MICRE campaigns during 2017 - 2018. Synergy exists with the SOCRATES (Southern Ocean Clouds, Radiation, Aerosol Transport Experimental Study) campaign funded by the National Science Foundation, wherein our group collected data on the NSF/NCAR G-V aircraft and the Australian R/V Investigator in early 2018. Importantly, the DOE campaigns provide the only data from this region that give insights to the seasonal to annual cycles of INPs. To make these data usable for numerical simulations of the INP life cycle and its influences on clouds, additional analyses are being conducted as part of this proposal.

We will use the 6-month acquisition of INP data on four MARCUS cruises from Hobart, Tasmania, to Antarctica, and the single-location full annual INP cycle observed in MICRE, to gain a holistic, quantitative picture of the INP number concentrations as a function of temperature and their variability over SO latitudes from 43°S to Antarctica. Augmenting initial processing of filter collections of aerosol particles during MARCUS and MICRE, additional freezing studies following thermal treatment (to removes INPs associated with microbes) and peroxide treatment (to remove all organics), as well as ionic chemistry and total organic carbon analysis, will be applied to samples to improve time and space resolution, and differentiate marine from terrestrial contributions. We will also perform Next Generation Sequencing to determine if bacterial composition can tag periods of enhanced marine organic INPs or occurrence of land-sourced bio-particles, supplemented by real-time bio-aerosol measurements in MARCUS. Through these analyses, other in situ and remote sensing data, trajectory/meteorological analyses, and coordination with MARCUS and MICRE collaborators, INP data will be categorized into representative source types (e.g., pristine marine, mixed and aged marine, and terrestrially-influenced). The products of our analyses will be parameterizations for use in numerical modeling studies to determine if, and when, ocean-derived INPs control the microphysical composition and radiative balance of SO clouds in a manner not presently captured by global models.

The specific objectives and approaches proposed for this project include:

  1. Complete additional and value-added chemical and biological analyses of archived samples of aerosols to categorize and quantify INP types and concentrations over the SO.
  2. Perform analyses to place INP data in meteorological, cloud, and aerosol context.
  3. Segregate sea spray aerosol INPs from other influences in the SO region.
  4. Develop new parameterizations for INP sources relevant to the SO.
  5. Contribute to integrated understanding of aerosol-cloud interactions via measurements of cloud properties and collaborative modeling studies led by MARCUS and MICRE partners.

This work will advance the science of interactions of aerosols, clouds and precipitation, to improve their representation in regional and global climate models. Results will inform representation of primary ice crystal nucleation, give inference to the role of secondary processes of ice begetting more ice, and improve investigations of aerosol influences on clouds via ice nucleation over SO high latitudes and similar vast ocean regions.