Atmospheric Sciences Division Seminar

Aerosols can affect the radiative properties of clouds by acting as cloud condensation nuclei or ice nuclei (IN). Ice particles impact the global radiative budget and atmospheric water vapor distribution, both representing large uncertainties in predicting climate. In the atmosphere, the formation of ice can proceed via homogeneous or heterogeneous nucleation. Various organic compounds constitute a significant fraction of aerosol mass and thus affect the ability of particles to serve as IN. Here we report on homogeneous and heterogeneous ice nucleation via deposition, immersion, and condensation modes of organic and organics containing particles in the temperature (T) and relative humidity (RH) range typical for cirrus and mixed phase clouds. Ice nucleation as a function of T and RH and corresponding ice nucleation rates for estimation of ice particle production rates are presented. Laboratory generated particles serving as surrogates of atmospheric organics containing particles and field collected aerosol are employed in the studies. Homogeneous and heterogeneous ice nucleation via the immersion mode are studied from pure and multi-component (inorganic/organic) aqueous particles composed of major species typical of marine and biomass burning aerosol including levoglucosan, (NH4)2SO4, NaCl containing solid humic and fulvic acids, and surface-active organic monolayers. Of particular interest is how the organic material affects particle water activity in the supercooled temperature range. Measured freezing temperatures and nucleation rates are discussed with respect to the water-activity based nucleation theory. Anthropogenic particles dominated by organic material collected in and around Mexico City and impacted by photochemical aging nucleate ice heterogeneously at T and RH for cirrus onset conditions typical for the northern hemisphere. Particle analyses conducted using CCSEM/EDX and STXM/NEXAFS. These results are in stark contrast to previous measurements employin