Evidence for a Biological Control on Emissions of Marine Ice Nucleating Particles: Laboratory, Field and Modeling Results
October 24, 2017
Christina McCluskey
Committee: Sonia Kreidenweis (Advisor), Paul DeMott (Co-Advisor), Jeffrey Pierce, Jeffrey Collett, Donald Mykles (Biology)
Abstract
Numerical models poorly represent cloud phase (liquid or ice) partitioning in high latitude regions, resulting in uncertainties in climate sensitivity estimates. Over the last decade, new observations have revealed that supercooled liquid clouds are unexpectedly abundant over the Southern Ocean, compared with midlatitudes, leading to large hemispheric biases in radiation budget estimates in models that assume the incorrect phase is present. One hypothesis for the persistence of liquid clouds at low temperatures is a relative absence of sources of ice nucleating particles (INPs), particles required for the first freezing of supercooled liquid water droplets at temperatures between 0 ËšC and -38 ËšC. Sea spray aerosol (SSA) generated by bubble bursting at the ocean surface is a main source of particles in the remote marine boundary layer over oceans. Since soluble sea salt is not active as an INP, it is hypothesized that organic components arising from oceanic biological activity (i.e., phytoplankton blooms) serve as the only local potential source of INPs in regions such as the Southern Ocean. Although this hypothesis was first proposed in 1976, advancements in knowledge regarding marine INP abundance, composition, and production mechanisms have been minimal. This work provides an extensive evaluation of the role of marine INPs in atmospheric INP populations over oceans.Findings from the laboratory, field, and numerical modeling studies provide evidence that 1) marine INPs are important contributors to remote ambient INP populations, 2) emissions of marine INPs are modulated by ocean biological activity, and 3) use of a SSA-specific INP parameterization improves model estimates of INPs for remote marine boundary layer locations evaluated in this work. Laboratory simulated phytoplankton bloom experiments demonstrated that marine INP emissions are in fact linked to biological activity and comprised ice nucleation active molecules and microbes. Field observations, including two remote coastal sites and six research voyages over the Pacific and Southern Oceans, spanning 70 ËšS to 60 ËšN over various seasons, significantly contributed to the first surveys of oceanic INPs in more than 30 years. These data revealed that marine INPs are important contributors to INP populations present in remote regions. The ice nucleation ability of SSA is significantly lower (3 orders of magnitude) than that of mineral dust, motivating implementation of aerosol-specific INP parameterizations in numerical representations of primary ice nucleation. Data also indicated that ambient marine INPs were composed of organic aerosol and the highest INP number concentrations observed in pristine SSA were associated with organic aerosol arising from offshore biological activity. A subset of data from this work was used to develop a parameterization for estimating INP number concentrations in pristine sea spray organic aerosol over the North Atlantic Ocean. Potential utility of the recently proposed marine INP parameterizations were examined using the atmospheric component of the Community Earth System Model with implementation of a physically-based parameterization for sea spray organic aerosol emissions. Preliminary results indicate that the majority of simulated INP number concentrations were within a factor of 10 of those observed at a remote coastal site and during a research voyage over the Southern Ocean.