Observation of aerosol in the mixing layer by a ground-based lidar ceilometer

The Vaisala ceilometer CT25K is an eye-safe commercial lidar mainly used to report cloud base heights and vertical visibility for aviation safety purposes. Compared to ceilometers with bi-axial optics, its single-lens design provides a higher signal-to-noise ratio for lidar return signals from distances below about 600 in, thus increasing its abilities to examine the mixing layer. A CT25K ceilometer takes part in the environmental measuring campaign VALIUM at the Lower Saxony State Agency for Ecology (NLO) in Hannover, Germany, investigating the air pollution in an urban surrounding with various sensors. Lidar return signals are reported every 15 s with a height resolution of 15 in. This paper concentrates on the interpretation of these signals in respect of the aerosol backscatter of the atmosphere up to 30 m. Every 30 minutes the NLO reports PM10 and PM2.5 concentrations measured with in situ sensors installed 20 in above the ceilometer. Humidity and precipitation monitor sensors help ruling out weather situations with water droplets contributing mainly to the ceilometer. backscatter signal. Data collected between 01. 03. 2002 and 31. 07. 2002 show that during dry weather situations there is a correlation of more than 80% between the dust concentration and the aerosol backscatter, allowing a quantitative analysis of the atmospheric dust contents with a standard ceilometer. The ratio PM10/PM2.5 of in situ measurements is investigated also giving a regression function and a correlation coefficient.