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California Institute of Technology (2011)

Studies of aerosol composition and hygroscopicity

Hersey, Scott P

Titre : Studies of aerosol composition and hygroscopicity

Auteur : Hersey, Scott P

Université de soutenance : California Institute of Technology

Grade : Doctor of Philosophy (PhD) 2011

Résumé
Atmospheric aerosols have significant impacts on human health, regional visibility, and the radiative energy balance of Earth, but there remain many uncertainties about their sources and evolution in the atmosphere, as well as the details and magnitude of their impact on climate. This thesis introduces a novel instrument for measuring aerosol hygroscopicity, an important factor in the overall climate impact of aerosols, and presents results from several field campaigns and laboratory experiments aimed at characterizing the chemical composition and hygroscopicity of atmospheric particles. Aerosol water uptake determines particle size, which thereby determines an aerosol’s scattering properties and radiative forcing. The Differential Sizing and Hygroscopicity Spectrometer Probe (DASH-SP) was designed to make rapid measurements of hygroscopicity on timescales short enough for aircraft deployment. Combined with an iterative data processing algorithm, the DASH-SP is demonstrated to accurately measure particle size, growth, and ``effective" refractive index for particles from 135 nm to over 1 μm on timescales as short as a few seconds. The DASH-SP was deployed off the coast of Central California to measure aerosol water uptake in a marine atmosphere impacted by aged anthropogenic emissions. Composition data from an Aerosol Mass Spectrometer (AMS) indicates that organics are uniformly highly oxidized (O:C ratio = 0.92 ± 0.33), and aerosol growth data from the DASH-SP indicates that in such a highly-oxidized environment, growth factor GF = Dp,wet/Dp,dry) can be accurately predicted as a simple function of relative humidity (RH) and organic volume fraction. A major ground-based sampling study was carried out in Pasadena, CA, a receptor site for transported Los Angeles pollution, and was dubbed the Pasadena Aerosol Characterization Observatory (PACO). Results indicate that organics dominate transported Los Angeles aerosols, and that they are overwhelmingly oxidized in nature. Aerosol species tend to reside in distinct size modes, with inorganics typically found in larger, accumulation-mode aerosol, while semivolatile secondary organic aerosol (SV-OOA) products appear to reside predominantly in a fine mode. Hygroscopicity was found to be a strong function of organic mass fraction (OMF). The end of PACO sampling coincided with a major forest fire in Los Angeles County. The impact of this fire on the sampling site is explored by comparing water soluble organic carbon (WSOC) and organic mass-to-charge (m/z) markers from the AMS. In the absence of fire influence, WSOC concentrations are primarily driven by concurrent photochemistry and sea breeze transport from source-rich areas. Fire periods are characterized by significant primary production of WSOC and overnight/early morning transport of fire emissions to the sampling site. Finally, DASH-SP results from the May 2010 CalNex field experiment indicate that aerosol hygroscopicity is determined primarily by the mass fraction of organics and nitrate in the aerosol. Overall hygroscopicity is very similar to that measured during PACO, though organics appear to be less hygroscopic during CalNex - likely because PACO represented transported, aged aerosol, while CalNex flights covered the entirety of the LA basin, including more source-rich areas

Mots Clés : Aerosols, hygroscopicity, urban, marine, CCN, PILS, DASH-SP, organics, kappa, PMF

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