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List of Abstracts 52to improve our ability to identify the actual deposition levels and impacts, and how they can influence on ourpolicy and ecosystem management needs.P-Interfaces.24 ID:4446 15:35Gas-particle partitioning and bi-directional flux of atmospheric ammoniaJennifer Murphy 1 , Raluca Ellis 1 , Jay Slowik 1 , Rachel Chang 1 , Jon Abbatt 1 , Paul Makar 21 University of Toronto2 Environment CanadaContact: jmurphy@chem.utoronto.caGas phase ammonia mixing ratios are affected both by emissions from and deposition to the land surface andby uptake to and volatilization from atmospheric particles. The partitioning between gas phase ammonia andsolid or aqueous ammonium is strongly influenced by temperature, humidity, pH and other physical andchemical variables. During the Border Air Quality and Meteorology Study (BAQS-Met) in the summer of2007, measurements of both gas phase ammonia and particulate ammonium were made for three weeks at aground site near Harrow Ontario. The sampling location was in an agricultural region in southwesternOntario, frequently downwind of industrial and transportation emission sources. Gas phase ammonia wasmeasured by quantum cascade tunable infrared laser differential absorption spectroscopy (QC-TILDAS) andthe chemical composition of particles with diameters less than 1 µm (PM1) was measured by time of flightaerosol mass spectrometry (C-ToF-AMS). The gas-particle partitioning of ammonia is investigated as afunction of meteorological parameters and aerosol composition. While the fraction of total ammonia (NHx)observed in the gas phase peaks between 0.2 and 0.8, the chemical transport model AURAMS (A UnifiedRegional Air-quality Modelling System), tends to predict fractions of either less than 0.1 or greater than 0.8.The model frequently predicted acidic aerosol, however observations of NHx always exceeded the observedequivalents of sulphate. One explanation for our observations is that the net flux of ammonia from the landsurface increases when aerosol sulphate is present, effectively buffering the mixing ratio of gas phaseammonia. We explore the impact of a bi-directional flux parameterization on the predicted gas-particlepartitioning of atmospheric ammonia.P-Interfaces.25 ID:4587 15:35Understanding aerosol mixing state impacts on CCN activity using particle-resolved aerosolsimulationsNicole Riemer 1 , Joseph Ching 1 , Matthew West 2 , Rahul Zaveri 3 , Richard Easter 31 Department of Atmospheric Sciences, University of Illinois at Urbana-Champaign2 Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign3 Atmospheric Sciences & Global Change Division, Pacific Northwest National LaboratoryContact: nriemer@illinois.eduThe hygroscopic behavior of aerosol particles depends on their size, and also on their chemical composition.Observations have shown that initially hydrophobic particles, e.g. soot, can be turned hydrophilic by addinga coating of soluble substances. Therefore the composition of individual particles, i.e. their mixing state, isan important factor for determining whether an aerosol particle can act as cloud condensation nucleus(CCN). However, tracking the mixing state in conventional aerosol models requires treating amultidimensional size distribution, which is computationally prohibitive. Thus current models usuallyassume an internal mixture within one mode or size section. The uncertainties associated with thisassumption regarding CCN properties are not well quantified.In this study we address this problem and present a new approach for aerosol modeling, the stochasticiCACGP-IGAC 2010 12 July, 2010