A Biogenic Source of Ice Nucleating Marine Aerosol Particles
Résumé
A great challenge in atmospheric sciences is predicting and understanding heterogeneous ice nucleation. Atmospheric biogenic particles from terrestrial sources, including bacteria, pollen grains and fungal spores are known to be some of the most efficient ice forming particles. The ability of aerosolized marine particles which are generally associated with biogenic organic material to efficiently form ice is less known and not well understood. An investigation into the ice nucleating capability of laboratory generated marine biogenic particles and field collected particles from the Pacific coast in California as a function of temperature and relative humidity is presented. Using the single particle analytical technique, scanning transmission X-ray microscopy coupled with near-edge X-ray absorption fine structure spectroscopy (STXM/NEXAFS), the chemical composition of ice nucleating particles and particles in the general population were compared. This technique is capable of chemical imaging and mapping of organic carbon functionalities in aerosol particles with a spatial resolution of about 30 nm. A complementary technique, computer controlled scanning electron microscopy and energy dispersive X-ray analysis (CCSEM/EDX) provided information on morphology and elemental composition of particles with high statistical significance. Biogenic particles were generated in a 1000 L laboratory mesocosm containing seawater with actively growing and cohabitating phytoplankton and bacteria from either natural seawater or unialgal phytoplankton cultures of the species Thalassiosira pseudonana, Emiliania huxleyi, and Nanochloris atomus. Organic material in aerosolized particles was always observed to be associated with sea salt and dominated by the presence of the carboxyl functional group with minor contributions from hydroxyl and carbonyl. Similar functionalities were observed in particles associated with exudates in the cultures of all three species. Ice forming field collected and laboratory generated particles shared a common morphology and organic composition, again characterized by identical X-ray spectroscopic signatures. We conclude that aerosolized exudates from phytoplankton in the mesocosms nucleated ice in our laboratory experiments. This suggests that biological activity produces organic particles to seawater which when aerosolized, may impact ice formation in the atmosphere. The results presented here broaden the definition of biogenic ice nucleation and highlight the need for future studies which investigate the ocean as a potential source of atmospheric ice forming particles.