Aerosol composition and hygroscopicity studies: Instrument development/characterization, ambient and laboratory measurements, and modeling

Abstract

Aerosols influence climate by altering the global energy balance via scattering and absorbing solar radiation (direct effects), and by their effect on the reflectance of clouds and occurrence of precipitation (indirect effects). Aerosols also influence biogeochemical cycles, lead to diminished environmental visibility, and harm human health. This thesis focuses on advancing knowledge of critical properties, specifically composition and hygroscopicity, which govern the role of aerosols in climatic and environmental change. The methods used in this work include a combination of instrument development/characterization, ambient and laboratory measurements, and modeling. An instrument was developed to quantify the water-soluble composition of aerosols. The particle-into-liquid sampler (PILS) grows ambient particles into droplets that grow sufficiently large to be collected by inertial impaction. After being collected in vials, the liquid sample can be analyzed with a variety of analytical methods including ion chromatography. Results from characterization tests are presented, which summarize instrument accuracy, precision, size and time resolution, and uncertainties. An instrument model was developed to simulate operation of the PILS; the model considers plumbing transmission efficiencies, droplet growth, mixing effects, and volatilization losses. Model predictions and measurements are compared and are shown to exhibit good agreement. A second instrument, termed the differential aerosol sizing and hygroscopicity spectrometer probe (DASH-SP), was developed to quantify aerosol hygroscopic growth and refractive index. This technique employs size classification of dry aerosol particles, equilibrates the classified particles to multiple relative humidities, and then measures the sizes of the grown particles using optical particle counters. Similar to the PILS, results from an extensive set of characterization test are presented. DASH-SP measurements of growth factor for various inorganic and organic acid salts are reported and compared to thermodynamic predictions. Airborne aerosol measurements from four separate field campaigns are presented. The main topics of investigation from the ambient experiments include the following: (1) in-cloud production of secondary organic aerosol; (2) a characterization of the sources and character of water-soluble aerosol composition during the 2006 Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS); and (3) a comprehensive airborne characterization of aerosol from a massive bovine source.