The Role of Organics in Heterogeneous Ice Nucleation – Anthropogenic, HULIS, and Marine Biological Particles

DANIEL A. KNOPF (1), BingBing Wang (1), Peter A. Alpert (1), Josephine Y. Aller (1), Alexander Laskin (2), Ryan C. Moffet (3), Mary K. Gilles (3)

(1) Stony Brook University, Stony Brook, NY, (2) W.R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA, (3) Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA

     Abstract Number: 386
     Last modified: May 11, 2010

Abstract
Ice crystal formation by heterogeneous nucleation plays a crucial role in determination of the atmospheric global radiative and water vapor budget and the hydrological cycle. Organic material constitutes a significant fraction of aerosol mass and thus affects the ability of particles to serve as ice nuclei (IN). Here we report on ice nucleation via deposition, immersion, and condensation mode of organic and organics containing particles in the temperature (T) and relative humidity (RH) range typical for cirrus and mixed phase clouds. Ice nucleation is reported as a function of T and RH and corresponding ice nucleation rates for estimation of ice particle production are presented. Anthropogenic particles dominated by organic material collected in and around Mexico City and impacted by photochemical aging, nucleate ice at T and RH for cirrus onset conditions typical for the northern hemisphere. Particle analyses were conducted using CCSEM/EDX and STXM/NEXAFS. These results are in stark contrast to previous measurements employing laboratory generated organic particles. Humic acid like substances (HULIS), such as Leonardite (humic acid) and Suwannee River Fulvic Acid (fulvic acid), are employed as surrogates of biomass burning particles. Their ability to nucleate ice heterogeneously before and after exposure to ozone is investigated. The humic acid and fulvic acid particles are efficient IN via the deposition mode. Oxidation by ozone decreases the IN activity of Leonardite particles. For the first time, a marine diatom species is reported as efficient IN. Airborne diatoms have been identified previously and suggested to potentially affect cloud formation processes particularly in polar regions. Diatoms immersed in aqueous NaCl solution droplets induced freezing at temperatures up to 20 K higher than aqueous NaCl droplets without diatoms. We observed that heterogeneous ice nucleation did not depend on diatom surface area. Atmospheric implications of our findings are discussed.