Organic aerosol composition in yosemite national park during the 2002 yosemite aerosol characterization study

The Yosemite Aerosol Characterization Study (YACS) was conducted during the summer of 2002 in order to investigate the sources of regional haze in Yosemite National Park by characterizing the physicochemical properties of the ambient aerosol. In particular, the influence of wildfires on summer haze and the resulting impact on visibility were investigated. Samples were collected on quartz fiber filters with PM_2.5, PM₁₀, and multi-stage high-volume collectors. Speciation of the organic aerosol fraction was achieved by gas chromatography with mass spectrometric detection, following solvent extraction. Average concentrations of particulate organic species were determined from weekly composite samples. In addition, daily samples were analyzed to better characterize trends in organic carbon species and source contributions during intensive haze periods. Organic carbon dominated the PM_2.5 mass, accounting for more than 70% of the total fine mass on average. In addition, radiocarbon measurements indicated that on average more than 90% of the fine organic carbon mass was contributed by contemporary sources. Wildfires had a significant influence on PM_2.5 during certain episodes, as determined by high concentrations of wood smoke markers and confirmed by complementary methods, including back trajectory analysis, satellite images and black carbon signature. Secondary biogenic compounds represented an important additional source of contemporary carbon, as indicated by relatively high concentrations of pinene oxidation products. The investigation of particle size distributions of a variety of organic compounds revealed interesting patterns. While some compounds were present in particles of a wider size range (super- and sub-micron particles), others, including dicarboxylic acids and certain pinene oxidation products, were detected only in sub-micrometer particles. Overall, fine aerosol in Yosemite National Park during the summer of 2002 was dominated by natural sources, in particular by smoke from wildfires and secondary organic species of biogenic origin.