THE EFFECT OF THE ORGANIC AEROSOL FRACTION ON ICE NUCLEATION

Aerosols can affect the radiative properties of clouds by acting as cloud condensation nuclei (CCN) or ice nuclei (IN). Ice particles affect the global radiative budget and also the atmospheric water vapor distribution, both representing large uncertainties in predicting climate. In the atmosphere the formation of ice can proceed via homogeneous or heterogeneous ice nucleation. The latter mechanism includes contact, immersion, and deposition freezing. We report laboratory data on homogeneous ice nucleation and heterogeneous ice formation by immersion and deposition nucleation induced by organics containing particles generated in the laboratory and sampled in the field. These include: homogeneous freezing temperatures and nucleation rates of pure and mixed particles composed of aqueous (NH4)2SO4 and levoglucosan serving as proxies for biomass burning aerosol; heterogeneous ice nucleation via immersion mode of marine particle proxies consisting of aqueous NaCl solutions coated with organic monolayers and aqueous NaCl and sea salt solutions containing water insoluble organic particles including phytoplankton cells and empty frustules. Results indicate that aqueous NaCl droplets coated by a monolayer of nonadecanol can significantly increase freezing temperatures by about 10 K compared to uncoated droplets. Ice nucleation from immersed diatom frustules occur at temperatures about 10 K higher than expected homogeneous freezing. The ice formation efficiency via deposition mode nucleation of particles consisting of humic acid like substances is significantly less than by mineral dust particles. Urban particles sampled at the MILAGRO T0 supersite indicate that the ice nucleation efficiency does not depend on the amount of the organic material as inferred from chemical single particle analysis. The experimental approach is based on previously developed ice nucleation cells coupled to optical microscopes in which particles are deposited on hydrophobically coated substrates. Homogeneous and heterogeneous ice nucleation via the immersion mode is studied in the temperature range of 180 - 273 K with a novel apparatus allowing determination of particle water activity instead of particle composition at particle preparation conditions. Heterogeneous nucleation via deposition mode is investigated using an improved version of a previously developed nucleation cell which now allows control of relative humidity with respect to ice for particle temperatures as low as 205 K.