Influence of mixing layer height on air pollutant concentrations in an urban street canyon

Urban areas often suffer from high air pollutant concentrations. To study the influence of the mixing layer height (MLH) on concentrations of important pollutants in an urban street canyon of a busy road, a long-term measurement campaign was performed at the kerb site of an arterial road in Essen, Germany, during the winter 2011/2012 and the early spring in 2012. Considered pollutants were NO, NO2, PM10, benzene, toluene and isoprene. The MLH was detected continuously using a ceilometer. To study the impact of MLH on pollutant concentrations, the classification scheme of Sturges was applied for the first time for this purpose. It is found that even in the immediate vicinity (kerbside) to a busy road the concentrations of air pollutants are strongly affected by MLH. It is a new result that high quantiles of concentrations decrease stronger with increasing MLH than low quantiles. The strongest correlations between concentrations and MLH are obtained for maximum concentrations with coefficients r from -0.84 to -0.95. In general, maximum concentrations of air pollutants in a street canyon provide stronger correlations with MLH than mean concentrations. NO2 is the pollutant whose concentration is affected least by MLH because it is mainly a secondary pollutant. (C) 2015 Elsevier B.V. All rights reserved.