Lidar and Sun photometer observations of atmospheric boundary-layer characteristics over an urban area in a mountain valley

Abstract

We present measurements of the vertical aerosol structure and the aerosol optical depth in the lower troposphere performed above the city of Sofia (an urban area situated in a mountain valley), western Bulgaria by means of a ground-based aerosol lidar operating continuously for a number of years. The lidar measurements were accompanied by measurements of the aerosol optical depth (AOD) in the visible and near infrared regions of the spectrum performed in October 2004 using Microtops II radiometers. The maximum values of the AOD were found to occur 1-2 h before the complete development of the atmospheric boundary layer, i.e. during the residual layer destruction, which confirms our hypothesis concerning the slope circulation effect on the processes taking place in the atmospheric boundary layer. The AOD values obtained by the lidar are lower than those taken by the sun photometer. Further, the AOD exhibits two different types of behaviour. In the case of a 'clear atmosphere' (i.e. in the absence of volcanic eruptions and/or dust transport from the Sahara) most of the aerosol accumulated within the atmospheric boundary layer over the urban area considered. The combined use of the two instruments allows the comparison between the optical characteristics of the atmospheric aerosol (e.g. aerosol extinction coefficient, etc.) obtained by the lidar and through an independent method (sun photometer).Keywords Aerosol optical depth · Atmospheric boundary layer · Lidar · Mixing layer · Residual layer · Sun photometer N. Kolev · I. Grigorov · I. Kolev (B)

Three-point observation in the troposphere over Sofia-Plana Mountain, Bulgaria

“…This project is based on the idea of comparing various air quality aspects pertaining to the Sofia city 'basin' with those pertaining to surrounding mountain areas (Kolev et al 2007, Evgenieva et al 2009). An overview of the Sofia city plus Plana region is shown in figure 1.…”

“…The S-function is used instead of the lidar return itself since it reveals the aerosol structure of the atmosphere much better. The S-function is commonly defined as S = r 2 ln P, where P is the received backscattered light power and r is the distance from the lidar along the sounding path (it is well known that the lidar return decreases as the inverse square of the distance r) (Kolev et al 2007). Figure 2 shows results of the lidar experiment that was commenced on 5 September 2008 at 08:05 LST and finished at 13:00 LST.…”

“…Email: blteam@ie.bas.bg; evgenieva@mail.bg Ts. T. Evgenieva et al The ABL height is often identified with the height of the mixing layer (ML), one of the most important parameters governing the concentration and dispersion of trace gases and aerosol particles , Katsambas et al 1997, Varotsos et al 2001a, Gerasopoulos et al 2005, Kolev et al 2007, Evgenieva et al 2009). Hence, assessments of the ABL height are fundamental to environmental monitoring, weather forecasting and air pollution research (Jacovides et al 1994, Seibert et al 2000, Jacob and Winner 2009.…”

“…Substances emitted into the ABL are gradually dispersed horizontally and vertically through the action of the turbulence, and finally become completely mixed across this layer (Jacovides et al 1994, Ferm et al 2005, 2006, Alfoldy et al 2007, Chou et al 2007, Kolev et al 2007.…”

“…Over an urban area, most of the aerosol resides within the first 2-3 km (by height) of the atmosphere (Kolev et al 2007). The atmospheric aerosol is of fundamental importance to our understanding of climate change (e.g.…”