Homogenous and Heterogeneous Ice Nucleation from Biomass Burning Aerosol: Freezing Temperatures and Nucleation Rates
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Biomass burning is a major source of water soluble and water insoluble organic compounds in the atmosphere. These compounds have the potential to form ice via homogeneous and heterogeneous mechanisms, both of which are involved in the formation of cirrus clouds with subsequent effects on the global radiative budget. Aqueous levoglucosan/(NH4)2SO4 droplets of mass ratio 10:1, 1:1, 1:5, and 1:10 and aqueous multi component organic droplets with and without (NH4)2SO4 serve as surrogates of water soluble organic biomass compounds to be examined for homogeneous ice nucleation. Pahokee Peat and Leonardite serve as sur- rogates of aerosols composed of HULIS, employed for immersion mode heterogeneous ice nucleation. Homogeneous and heterogeneous freezing and melting temperatures were experi- mentally obtained for all particle systems over a water activity range of 1.0 - 0.8. Subsequent homogeneous and heterogeneous ice nucleation rates were derived and analyzed based on water activity based theory. Contact angles were derived for the two ice nuclei at atmospher- ically relevant conditions. Cumulative and differential freezing spectra are calculated from freezing data for the two ice nuclei. Atmospheric implications are also discussed.