Studies of the Los Angeles aerosol: (I) Chemical measurements and comparison to the surface microlayer of Santa Monica Bay, (II) Measurements of the fractal properties of the ultrafine aerosol (California).

摘要

This work addresses two important but little studied aspects of the behavior of the atmospheric aerosol: (1) the contributions of the atmospheric aerosol to the surface microlayer (SMIC) of natural waters (a biochemically sensitive site) and (2) the morphological properties of atmospheric aerosols.; The first part of the study involved a cooperative program for concurrent measurements of atmospheric aerosol, SMIC, and water column samples. Our group measured aerosol chemical characteristics (in terms of total concentrations and size distributions of various elements) at several locations on the west side of Los Angeles including above Santa Monica Bay. Scatter diagrams were made of SMIC concentrations for various elements vs. atmospheric aerosol concentrations of the same elements for similar time periods. The scatter diagrams identified a subset of elements in the SMIC that tended to increase with the atmospheric concentrations of the same elements. For these elements atmospheric deposition is probably a major source in the SMIC. Our scatter diagrams offer a novel approach to source resolution for the SMIC and potentially, a new method of determining dry deposition rates to natural waters.; The second part of the research describes the first systematic study of the morphological properties of atmospheric aggregates in the ultrafine particle size range (d_p ≤ 0.1 μm). These aggregates are emitted from diesel engines and other high temperature sources and have been linked to adverse effects on public health. Particles were collected from the atmospheric air on transmission electron microscope (TEM) grids fitted on the last two stages of a single-jet, eight-stage, low pressure impactor (LPI). Photomicrographs of the TEM grids were analyzed to obtain the fractal dimension (D _f) and prefactor (A) for aggregates.; Values of D_f increased from near 1 to above 2 as the number of primary particles making up the aggregates increased from 10 to 180 for the measurements made in the Los Angeles area. This trend was not observed in the rural sites (e.g. Research Triangle Park, NC and San Jacinto, CA). The count mean prefactor was ∼2.9 for all the aggregates. In one set of measurements, the fraction of the particles present as aggregates was about 60% for particles with aerodynamic diameters between 50 and 75 nm and 34% for the range 75 to 120 nm. These data will permit better estimates of atmospheric aggregate residence times, transport and deposition in the lung, optical extinction, and heterogeneous nucleation.