Thesis Size Distribution Measurements of Wildfire Smoke-influenced Aerosol at Yosemite National Park

The main aim of the study was to determine chemical, optical and physical characteristics of aerosol particles responsible for visibility degradation in Yosemite National Park. The park, located in the central Sierra Nevada range in eastern California, was impacted by smoke from wildfire activity in the western United States. Determining the contribution made by smoke to visibility degradation is important in order to adequately understand the impacts of anthropogenic emissions and regulate them accordingly to meet Environmental Protection Agency requirements. Measurements of dry aerosol size distributions were made with a differential mobility analyzer (DMA) and an optical particle counter (OPC). The DMA sizes spherical aerosol particles by their electrical mobility diameter, close to the true diameter, while the OPC sizes particles by their optical diameter, a function of the particle refractive index. An iterative alignment method assuming a range of refractive indices was applied to OPC size distributions to match them to DMA size distributions, returning iv the real refractive index that yielded the best fit. Best-fit OPC and DMA size distributions were then merged to give a complete size distribution for 0.038 < D p < 2.1 µm. Concentrations of commonly used wood smoke tracers and backward trajectory analyses were used to determine periods of smoke influence at the park. Volume geometric mean diameters ranged from ~0.2 µm during non-smoke periods to ~0.4 µm during periods of highest fine aerosol mass concentrations associated with smoke-impacted times. PM 2.5 composition was determined to be dominated by organic carbon for most of the study period. Composition data and volume-weighted mixing rules were used to calculate aerosol refractive index, both real (1.570 ± 0.006) and imaginary (– 0.015i ± 0.003i) components, and bulk density (1.58 ± 0.09 g cm-3). Calculated refractive index and densities were most sensitive to the values assumed for organic carbon refractive index and density, since this species dominated the composition. The average retrieved refractive index was 1.577 ± 0.008, in relatively good agreement with calculated values. Imaginary (absorbing) components of the complex refractive index, calculated from aerosol composition, were within the range seen for aged biomass burning smoke in previous studies. Dry aerosol scattering coefficients, b sca , were calculated from Mie theory using the retrieved refractive index and the calculated imaginary component for each 15-minute sample interval, at a wavelength of 530 nm and assuming homogeneous spherical particles. Scattering coefficients were roughly 50 Mm-1 on average, though …