Correlations between fine and coarse particle concentrations at Pueblo, Colorado

Abstract

One of the considerations in the setting of the secondary (welfare) Ambient Air Quality Standards in the U.S. under the Clean Air Act Amendments of 1970 was protection of visibility; it was assumed that visibility was causally related to the total concentration of suspended particulate matter, as measured by the High-Volume sampler. The correlations on which this decision was based were significant, but limited, both in geographic coverage and in time (see, e.g., Charlson, 1968). It was subsequently shown by Whitby, et al. (1972) that atmospheric particulate matter comprised two separate populations of different origin, with a dividing point in the vicinity of 2–3 μm. Since, on a mass basis, particles below 2 μm have a far greater effect on visibility, Charlson, et al. (1978) investigated the correlation between particle mass below 2.5 μm and light scattering coefficient, which is itself closely correlated with visibility. For a large variety of environments, this relationship was found to be essentially invariant, and the correlation coefficient extremely high. However, although various authors have displayed isolated examples of cases in which concentrations of particles larger and smaller than 2 or 3 μm varied independently (see, e.g., Hidy, et al., 1975), it has been assumed that, on the average, concentrations of particles smaller than 2 or 3 μm correlated with the total suspended particulate matter. Especially with samples of long enough duration to obscure diurnal cycles, the concentrations of most pollutants are more controlled by meteorological factors than by anything else, and thus tend to vary together. In the light of these considerations, data for one year from the city of Pueblo, Colorado were studied.