The Aerosol Size Distribution of Los Angeles Smog

A total of 342 atmospheric size distributions have been measured at the California Institute of Technology in Pasadena, CA during August and September 1969 over the size range 0.003–6.8 μm using the Minnesota Aerosol Analyzing System in collaboration with other investigators making chemical, meteorological, mass, and optical measurements. This paper presents characteristics of the aerosol size distributions and discusses the significance of these findings. It also presents a new computer-compatible system of nomenclature and a new system for separating and analyzing the size distribution in one decade ranges. In addition to the usual log ΔN/ΔDp versus log Dp plot, it was found that a plot of ΔV/Δ log Dp versus log Dp was very useful for characterizing the distribution of particles larger than 0. 05 μm. From the later volume distribution plot, it was found that the smog aerosol was universally bimodal with the saddle point in the 1- to 2-μm size range and with the volume fraction for the individual size distributions below 1 μm being characterized with a grand correlation coefficient of 0.9719 with log normal distributions. The grand log normal distribution had a geometric mean of 0.302 μm and a geometric standard deviation of 2.25. The grand average volume below 1.05 μm was 34 μm3/cm3 and the volume above 1.05 μm was 24.1 μm3/cm3. For sizes above 1 μm, the volume distribution increases steadily up to 6.8 μm, the largest size measured by the MAAS. Comparisons with Lundgren impactor data suggest that this larger mode peaks at 7–10 μm and then drops sharply at about 15 μm. The grand average number distribution log ΔN/ΔDp vs. log Dp was very similar to the grand average measured by Clark and Whitby in Minneapolis in 1966. The number distribution can be characterized reasonably well in the size range from 0. 2 to 7 μm by ΔN/ΔDp = 0. 4 VTDp−4, where VT = 58. 1 μm3/cm3, the total volume fraction. The volume fraction smaller than 1. 05 μm was found to correlate well with light scattering, solar radiation, and, to a lesser degree, with ozone. Analysis of the data and comparisons with laboratory studies of coagulating aerosols suggest strongly that, during the daytime when there is even light smog, most of the aerosol mass, in the size range smaller than 1 μm, is contributed by condensation of photochemical reaction products on particles larger than 0. 1 μm rather than from coagulation of smaller particles. There is considerable evidence that, while most of the mass of particles smaller than 1 μm is contributed by photochemical reactions during smog periods, most of the mass of particles larger than a few microns comes from other sources. While the aerosol mass of particles smaller than 1 μm peaks at about noon, the maximum in the condensation nuclei count peaked at about 1500 at values on the order of 1. 8 × 105/cm3. On the other hand, in Minneapolis, when there was no volume peak at noon, the condensation nuclei count peaked at 1. 8 × 106/cm3. The third paper in this series shows that, because the coagulation coefficient between 0. 01-μm particles and the 0.3-μm particles of the lower volume mode is much higher than for 0.01-μm particles with themselves, the presence of the relatively large volume below 1 μm in Los Angeles smog decreases the maximum condensation nuclei count observed compared to cities with less severe air pollution.