Street level versus rooftop concentrations of submicron aerosol particles and gaseous pollutants in an urban street canyon

Gaseous air pollutants and aerosol particle concentrations were monitored in an urban street canyon for two weeks. The measurements were performed simultaneously at two different heights: at street level (gases 3 m, aerosol particles 1.5 m) and at a rooftop 25 m above the ground. The main objective of the study was to investigate the vertical changes in concentrations of pollutants and the factors leading to the formation of the differences. The physical parameters controlling the concentration gradients (e.g. the flow and micrometeorology) were not directly measured and the conclusions of the study rely mostly on the high time resolution concentration measurements. It was concluded that dilution and dispersion decreases the concentrations of pollutants emitted at street level by a factor of roughly 5 between the two sampling heights. However, for some compounds the chemical reactions were seen to be of more importance when the vertical gradient is formed. In order to determine the processes leading to gradients in aerosol particle concentrations the photochemical formation of submicrometer aerosol particles was investigated using a theoretical expression based on the measured data. It was clearly seen that most of the particles originate from traffic in the vicinity of the measurement site. Also a few events were detected which might have been due to local gas-to-particle conversion. (C) 1999 Elsevier Science Ltd. All rights reserved.