Evolution of atmospheric aerosols along trajectories crossing the Los Angeles basin

[摘要] A primary concern in the regulation, control and monitoring of particulate air pollution is its detrimental effect on human health. One objective of this thesis is to determine the concentration and chemical composition of urban atmospheric fine (diameter < 2.2 µm) and ultrafine (diameter < 0.1 µm) particulate matter (PM). Laboratory research into ultrafine particles suggests that these particles may be especially damaging to health. Size-segregated fine PM was collected in Pasadena, California, using micro orifice impactors. Ultrafine particle mass concentrations ranged from 0.88-1.58 µg m^(-3), small in comparison to fine PM, but ultrafine particle number concentrations in the size range 0.017-0.1 µm particle diameter were relatively large at 1.3x10^4 ± 8.9x10^3 cm^(-3). This study provides the first detailed data published on urban ultrafine particle chemical composition including organics and elemental carbon, trace metals, and ionic species. Organic compounds were the largest component of ultrafine particle mass.Development of effective particulate control policies requires a thorough understanding of the chemical and physical processes affecting primary particle emissions as they are transported downwind from emissions sources. Mathematical models which calculate the chemical evolution of airborne particles as they are affected by gas-to-particle conversion processes can provide the needed insight when combined with experimental data on particle evolution at the single particle level. Three field studies were conducted in the Los Angeles area for the purpose of gathering the necessary experimental data by sampling singleair parcels at multiple points along an air parcel trajectory. Cascade impactors, particle size distribution monitors, filter-based PM samplers, and aerosol time-of-flight mass spectrometry instruments capable of making single-particle size and composition measurements were employed at three urban monitoring sites chosen along typical wind trajectories. The three studies focused on different aspects of particle evolution: particle transformation during transport inland over urban areas from the ocean; evolution of particle populations dominated by motor vehicle emissions from Central Los Angeles; and formation of particulate ammonium nitrate during transport over large ammonia sources. Particle populationstransported over the Los Angeles area showed depletion and chemical alteration of sea salt particles, addition of fine carbon-containing particles, and secondary aerosol nitrate formation. Measurements made at the single particlelevel show that particles of the same size can differ substantially in chemical composition and that single particle composition becomes more complex overtime as particles are transformed through a chemically reactive atmosphere.

[发布机构] University:California Institute of Technology;Department:Engineering and Applied Science

[关键词] Environmental Engineering