Composition of the fine organic aerosol in Yosemite National Park during the 2002 Yosemite Aerosol Characterization Study

The Yosemite Aerosol Characterization Study (YACS) was conducted during the summer of 2002 to investigate regional haze in Yosemite National Park by characterizing the chemical, physical and optical properties of the ambient aerosol. Previous analyses reveal that the composition of PM2.5 during YACS was dominated by carbonaceous material derived primarily from contemporary carbon sources rather than fossil fuel combustion. In addition to several local wildfires and prescribed burns, two regional haze episodes during YACS were strongly influenced by smoke from biomass burning that was subject to long-range transport. Several classes of biomass burning smoke tracers, including anhydrosugars, methoxyphenols, and resin acids, were used to determine contributions of primary biomass burning smoke to PM2.5. Levoglucosan was measured with peak concentrations of 234 ng/m3 during periods with smoke influence from local fires, and primary biomass burning smoke contributions to fine particle organic carbon were estimated to be as high as 100% on individual days during that period. Relatively high concentrations of monoterpene oxidation products and other organic compounds of secondary origin, such as dicarboxylic acids, indicated secondary organic aerosol (SOA) to be an important contributor to contemporary carbon. Biomass combustion plumes impacting the measurement site are likely a significant contributor to the observed SOA. Low concentrations of organic compounds of anthropogenic origin, such as hopanes and steranes, indicated contributions from automobile exhaust to organic carbon of approximately 10% on average. Overall, the fine aerosol in Yosemite National Park during the summer of 2002 was dominated by natural sources, in particular by smoke from wildfires and by secondary organic aerosol of biogenic origin.