Lidar observations of the diurnal variations in the depth of urban mixing layer: A case study on the air quality deterioration in Taipei, Taiwan Charles C.-K. Chou a, ⁎ , C.-T. Lee b , W.-N. Chen a , S.-Y. Chang a , T.-K. Chen a , C.-Y. Lin a , J.-P. Chen c a Research Center for Environmental Changes, Academia Sinica, Taipei 115, Taiwan, ROC b Graduate Institute of Environmental Engineering, National Central University, ChungLi 320, Taiwan, ROC c Department of Atmospheric Sciences, National Taiwan University, Taipei 106, Taiwan, ROC Received 28 April 2006; received in revised form 1 November 2006; accepted 23 November 2006 Available online 31 January 2007 Abstract An aerosol light detection and ranging (LIDAR) system was used to measure the depth of the atmospheric mixing layer over Taipei, Taiwan in the spring of 2005. This paper presents the variations of the mixing height and the mixing ratios of air pollutants during an episode of air quality deterioration (March 7–10, 2005), when Taipei was under an anti-cyclonic outflow of a traveling high- pressure system. It was found that, during those days, the urban mixing height reached its daily maximum of 1.0–1.5 km around noon and declined to 0.3–0.5 km around 18:00 (LST). In terms of hourly averages, the mixing height increased with the ambient temperature linearly by a slope of 166 m/°C in daytime. The consistency between the changes in the mixing height and in the ambient temperature implied that the mixing layer dynamics were dominated by solar thermal forcing. As the cap of the mixing layer descended substantially in the afternoon, reduced dispersion in the shallow mixing layer caused the concentrations of primary air pollutants to increase sharply. Consequently, the pollutant concentration exhibited an anti-correlation with the mixing height. While attentions are usually focused on the pollution problems occurring in a morning inversion layer, the results of this study indicate that the air pollution and its health impacts could be even more severe as the mixing layer is getting shallow in the afternoon. © 2006 Elsevier B.V. All rights reserved. Keywords: Urban mixing layer; LIDAR; Urban air quality; Tropospheric aerosols 1. Introduction The mixing layer is the lowest part of the atmosphere where the constituents of air are mixed due to convection and mechanical turbulence over the ground (Seibert et al., 2000). Because the emission of air pollutants due to human activities are mostly from ground-level sources, mixing layer is usually the most polluted part of the atmosphere, particularly over urban and industrial areas. Considering that the air in the mixing layer is what people actually breathe, its composition is therefore of great public concern, and the air quality is generally defined by the concentrations of air pollutants in the mixing layer. Given that the dispersion of air pollutants is often confined within the mixing layer, the depth of the Science of the Total Environment 374 (2007) 156 – 166 www.elsevier.com/locate/scitotenv ⁎ Corresponding author. Tel.: +886 2 2653 9885; fax: +886 2 2783 3584. E-mail address: ckchou@rcec.sinica.edu.tw (C.C.-K. Chou). 0048-9697/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2006.11.049