One year of Raman lidar observations of free-tropospheric aerosol layers over South Africa

Abstract

Abstract. Raman lidar data obtained over a 1 year period has been analysed in relation to aerosol layers in the free troposphere over the Highveld in South Africa. In total, 375 layers were observed above the boundary layer during the period 30 January 2010 to 31 January 2011. The seasonal behaviour of aerosol layer geometrical characteristics, as well as intensive and extensive optical properties were studied. The highest centre heights of freetropospheric layers were observed during the South African spring (2520 ± 970 m a.g.l., also elsewhere). The geometrical layer depth was found to be maximum during spring, while it did not show any significant difference for the rest of the seasons. The variability of the analysed intensive and extensive optical properties was high during all seasons. Layers were observed at a mean centre height of 2100 ± 1000 m with an average lidar ratio of 67 ± 25 sr (mean value with 1 standard deviation) at 355 nm and a mean extinctionrelated Ångström exponent of 1.9 ± 0.8 between 355 and 532 nm during the period under study. Except for the intensive biomass burning period from August to October, the lidar ratios and Ångström exponents are within the range of previous observations for urban/industrial aerosols. During Southern Hemispheric spring, the biomass burning activity is clearly reflected in the optical properties of the observed free-tropospheric layers. Specifically, lidar ratios at 355 nm were 89 ± 21, 57 ± 20, 59 ± 22 and 65 ± 23 sr during spring (September-November), summer (December-February), autumn (March-May) and winter (June-August), respectively. The extinction-related Ångström exponents between 355 and 532 nm measured during spring, summer, autumn and winter were 1.8 ± 0.6, 2.4 ± 0.9, 1.8 ± 0.9 and 1.8 ± 0.6, respectively. The mean columnar aerosol optical depth (AOD) obtained from lidar measurements was found to be 0.46 ± 0.35 at 355 nm and 0.25 ± 0.2 at 532 nm. The contribution of freetropospheric aerosols on the AOD had a wide range of values with a mean contribution of 46 %.

Detection and characterization of birch pollen in the atmosphere using a multiwavelength Raman polarization lidar and Hirst-type pollen sampler in Finland

“…The local minimum was considered to be the layer top. To verify the determined layers, the layer boundaries identified by the gradient method were compared with the bottom and top heights of coherent structures in the height-time illustration of the range-corrected signal (Giannakaki et al, 2015). The layer identification was based on the assumption that the optical properties should be relatively homogeneous, which means that within one layer the variability of the optical properties should be lower than the statistical uncertainty of the individual data points.…”

Optical and microphysical characterization of aerosol layers over South Africa by means of multi-wavelength depolarization and Raman lidar measurements

“…In addition, emissions from biomass burning (wild fires) contribute significantly to regional emission loads (e.g. Giannakaki et al, 2015). Both natural phenomena (lightning) and human-induced activities are responsible for biomass burning (Edwards et al, 2006).…”

“…Groundbased Raman lidars provide vertically resolved information on the distribution and optical properties of aerosols. Giannakaki et al (2015) used Raman lidar data obtained over a 1-year period at Elandsfontein in South Africa (26 • 15 S, 29 • 26 E, 1745 m above sea level, a.s.l.) to study the geometrical characteristics and intensive and extensive optical properties of free-tropospheric aerosol layers.…”

Validation of LIRIC aerosol concentration retrievals using airborne measurements during a biomass burning episode over Athens

“…upper troposphere), and a linear relationship between aerosol backscatter and extinction coefficient, the so called lidar ratio ( ), constant in the laser-telescope path. A variety of studies performed in the framework of EARLINET, revealed a wide range for the lidar ratios, covering values from 20 to 100 sr (Ackermann, 1998;Mattis et al, 2004;Amiridis et al, 2005;Müller et al, 2007;Papayannis et al, 2008;Amiridis et al, 10 2009;Groß et al, 2011;Groß et al, 2013;Groß et al, 2015;Giannakaki et al, 2015). In elastic backscatter lidar technique, the assumption of a constant lidar ratio value throughout the laser sounding range, is the most critical for solving the lidar equation, while the overall uncertainty, including both statistical and systematic errors, on the retrieved values, is of the order of 20-30% (Bösenberg et al, 1997;Comerón et al, 2004;Rocadenbosch et al, 2010).…”