Relating the Chemical and Physical Properties of Aerosols to the Water Uptake and Ice Nucleation Potential of Particles Collected During CARES Principal Investigator: Daniel Knopf, New York State University of Stony Brook CO Investigator: Ryan Moffet, University of the Pacific This laboratory based research project will use particles collected during the Carbonaceous Aerosols and Radiative Effects Study (CARES) to address the aerosol life cycle of primary and secondary organic aerosol (SOA) particles including black carbon (BC), primary organic aerosol (OA, biological particles, biomass burning aerosol (BBA), and SOA. OA particles are ubiquitous in the atmosphere and can represent a major mass fraction (10-90%) of submicron aerosol. A significant mass of the organic particles is secondary and results from photochemical oxidation of anthropogenic and biogenic emitted volatile organic compounds (VOCs). The formation and evolution of this SOA and primary OA is still highly uncertain. Associated with even higher uncertainties is the impact of these organic containing particles on the indirect effect which poses the largest uncertainties in predicting the global radiation budget and, thus, climate. In particular, the ability of such organic dominated particles to induce ice crystal formation thereby affecting cloud radiative properties and precipitation and thus the hydrological cycle is not well understood. Furthermore, it is not well understood how particle aging will affect chemical composition and physical state. Particle aging can involve particle coagulation, condensation of low vapor pressure gases, and heterogeneous oxidation. The aging process generally results in an overall increase in organic mass. This proposal aims to systematically investigate the change in physical and chemical properties of urban aerosol particles before and after photochemical aging. The effects of aging will be investigated by applying state-of-the-art analytical tools such as computer controlled scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (CCSEM/EDX) and scanning transmission X-ray microscopy coupled with near edge X-ray fine structure spectroscopy (STXM/NEXAFS). The efficacy of these particles to take up water and induce heterogeneous ice nucleation via condensation, immersion, and deposition freezing for temperatures as low as 200 K will be determined. Applying a novel experimental approach, the chemical and physical properties of the same individual particle that took up water and/or nucleated ice will be identified by SEM/EDX and STXM/NEXAFS. A particular focus will be placed on the detection of amorphous (highly viscous) organic particles and their ability to interact with the water and ice phase. CARES provides a unique dataset that, in combination with meteorological and gas and condensed phase data acquired during the campaign, will allow a better understanding of the formation and evolution of OA. The importance of anthropogenic and biogenic OA and SOA for cold cloud formation will also be determined. The following objectives will be executed in this proposal: i) a comprehensive database on the morphology, mixing state, and chemical composition of particles encountered during CARES including the effects of physical and chemical aging will be established; ii) the ubiquity of highly viscous secondary organic material will be evaluated and corresponding glass transition temperatures will be derived; iii) particles ejected from the biosphere will be identified and quantified; iv) particles acting as most efficient ice nuclei and corresponding onsets for water uptake and ice nucleation will be determined; v) a physical description of ice nucleation and corresponding parameterizations for implementation in cloud resolving models will be derived. These objectives will significantly improve our understanding of the role of organic aerosol in the atmosphere providing data sets for evaluation of process models of organic aerosol and cloud resolving models. By accomplishing these objectives, we will obtain a better understanding of the detailed processes underlying the large scale effects that aerosols have on clouds and climate.
Posted on: Fri, 19 Dec 2014 23:21:04 +0000
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