Poster Session Abstract

1. Organic Aerosol Composition in Yosemite National Park-Results from the 2002 Yosemite Special Study

Graham Bench
Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, California

Physicochemical properties of the ambient aerosol in Yosemite National Park were measured for two months during the summer of 2002 in order to investigate sources of haze and the resulting visibility impact. In particular, the influence of wildfires on the regional haze was to be determined. The effect of atmospheric particles on visibility depends strongly on their physical and chemical characteristics. Depending on location, season, and sources, atmospheric aerosols contain organic carbon in a variety of forms and concentrations. Particulate organic matter (POM) is of special interest due to its complex nature and the limited understanding of its composition and formation mechanisms.

Sampling was conducted with multiple collectors, including PM 2.5, PM 10 and 6-stage cascade-impactor high-volume collectors. The POM composition was determined for weekly composite samples as well as daily samples. Shorter sampling (12 hours) was performed during intensive periods and special occurrences such as local fires. Size distributions were determined for all POM components as well. Speciation of the organic aerosol fraction was accomplished by solvent extraction, followed by separation, identification and quantification using various instrumental techniques, including gas chromatography with mass spectrometric detection (GC-MS), high-performance liquid chromatography (HPLC) and capillary electrophoresis with pulsed amperometric detection (CE-PAD).

Organic carbon (OC) dominated the PM 2.5 mass during the study period, representing more than 80% of the total PM 2.5 mass. A large spectrum of POM constituents was identified and quantified, including n-alkanes, alkanols, carbonyls, carboxylic acids, PAHs, and a variety of molecular tracer compounds. Wood smoke tracers and marker compounds for secondary organic aerosols (SOA) were investigated as well.

The investigation of particle size distributions of a variety of organic compounds revealed interesting patterns. While some compounds, such as n-alkanes and n-alkanoic acids, were present in particles of a wider size range (super- and sub-micron particles), others, including hopanes, dicarboxylic acids, and selected pinene oxidation products, were detected only in sub-micrometer particles. Compounds of secondary origin, such as pinic acid, are expected to be present in the sub-micron mode due to their formation mechanism by nucleation and condensation. Many size-segregated organic aerosol species showed a mono-modal pattern, while some, such as dehydroabietic acid, had a bi-modal distribution.

Carbon isotope measurements indicated that more than 85% of the fine OC was from contemporary sources. Consistent with this finding, low concentrations of tracers for vehicular emissions, such as hopanes and steranes, indicated less than a 10% contribution to the PM 2.5 mass from traffic on average. A significant influence of wildfires on PM 2.5 mass in Yosemite was illustrated by high concentrations of wood smoke markers during certain periods. Contributions of smoke from biomass burning to the total fine POM during these periods was estimated based on selected markers, including resin acids, anhydro sugars and methoxyphenols. Secondary biogenic compounds constituted an additional important source of contemporary carbon, as indicated by relatively high concentrations of pinene oxidation products such as pinonaldehyde, pinic and pinonic acids. These biogenic SOA tracers also showed good correlation with wood smoke tracers. This phenomenon may be partly explained by higher emission rates of biogenic VOCs due to elevated temperatures during a wildfire. Thus, POM in Yosemite during summer 2002 seemed to be dominated by natural sources, in particular by wild fire smoke and secondary organic species.