Aerosol Extinction at 500 nm in Urban and Rural Air at Milwaukee in April 1976

Abstract

Aircraft measurements of aerosol size distributions were used to calculate aerosol extinction coefficients at 12 flight altitudes over an urban and a rural site at Milwaukee. Analysis of vertical-horizontal cross sections from four days of measurement in April 1976 indicated a well-defined influence of the urban area, creating higher aerosol extinction coefficients depending on the meteorological situation. On three of the four days, extinction coefficients decreased markedly above an inversion, where the air was much drier and cleaner. On 12 April, onshore flow below 625 m altitude prevented aerosols from moving over Lake Michigan, and the shoreline was sharply delineated. Westerly flow on the other three days created no such lake-city distinction. Comparison of the most polluted day with the other three days indicated that day-to-day variations in aerosol extinction may be greater than urban-rural differences.