Variation of Solar Attenuation with Height over an Urbanized Area

Abstract

The information presented in this paper is directed to environmental scientists who are concerned about the quality of air over an urban area. Specifically, the purpose of this study is to show quantitatively the magnitude of solar attenuation for individual air layers in the lowest 1000 m of an urban atmosphere. This reveals dramatically the diverse variation of mass concentration with height. The information is useful in urban planning, in site selection, and in air quality control.

The data were gathered on different roof top levels and on helicopter flights using modified Volz sun photometers, psychrometers, hot-wire anemometers, and an electric thermometer. The results show that, for instance, on a "polluted" day the lowest 1000 m of air attenuate about 65% of the solar beam, whereas on a “clean” day the contribution to the total solar attenuation is still 30%. Shallow air layer increments of 45 m near the ground were found to contribute up to 21% to the total solar attenuation. Aerosols form the dominant attenuation component in polluted air layers near the ground, whereas the extinction due to air molecules becomes dominant in higher and relatively cleaner atmospheric air layers.

Significance tests show that turbidity variations with height over widely spaced locations are relatively homogeneous. This may indicate a rather uniform spread of the urban dust dome over the neighboring countryside. No significant change in the vertical turbidity structure was found between data collected in 1961–62 and 1969.

For an effective control of the ambient air quality, conclusive information on the lateral and vertical spread of pollution is necessary. Turbidimetric studies based on scattering theory can economically supply this information.