Stability of the Surface Layer and Its Relation to the Dispersion of Primary Pollutants in St. Louis

The role of atmospheric stability in the lowest 30 m in characterizing the dispersion of the primary pollutants CO, NO and hydrocarbons is investigated using the 1976 air quality data set for St. Louis. Stability is determined in three separate ways—from tower measurements of the vertical temperature gradient DT, tower measurements of wind speed u, and an approximation to the bulk Richardson number B̄ based only on DT and u in the surface layer. High positive correlation coefficients were obtained between area averages of DT and each of the species for the inner urban area of St. Louis. Because the species concentrations were derived from a non-uniform area source, similar correlations at individual stations were somewhat poorer. At night, for low wind speed, the area-averaged species correlation coefficients for DT and B̄ are positive and significant, but they are poor for wind speed alone. For all other situations, correlation diagrams for wind speed versus species show appreciable scatter, and B̄, as it has been used in this study, is not a sensitive parameter for estimating surface pollutant concentrations. Use of a stability parameter that includes the mixed layer height, in addition to DT and u, will result in a quantity that exhibits a greater range for correlation studies. These findings should be considered when urban air quality models are parameterized and evaluated in terms of their ability to disperse primary pollutant distributions.