**5.2 Lidar observations on Saharan dust loadings in the atmosphere**

Numerous observations on Saharan dust presence in the atmosphere over Sofia are carried out by both lidars of LRL-IE, in order to follow the concentration, spreading, and temporal

LIDAR Atmospheric Sensing by Metal Vapor and Nd:YAG Lasers 361

Fig. 6. Evolution diagrams of the aerosol backscatter coefficient at 510.6 nm (a) and 532 nm

The eruption of Eyjafjallajokull volcano in Island on 14 April 2010 offered an opportunity lidar stations, participating in the European Lidar Network, to demonstrate the effectiveness of the lidar sensing for 4-dimensional characterization of the volcanic ash transport. The lidar monitoring of Eyjafjallajokull plumes spreading by the Sofia lidar station started on 18 April 2010 and finished on 25 May 2010 (Grigorov et al., 2011). Results of lidar measurements, performed on 22 April 2010, by using a CuBr lidar at a wavelength of 510.6 nm are given in Fig.7, showing presence of volcanic ash layer positioned at ~2.2 and 3 km altitudes AGL. The observed low limit of the layer frequently remains mixed with the

The lidar observations are presented in two formats: as a single averaged vertical profile of the retrieved backscattering coefficient (Fig.7a) and as a map of the time evolution of the range-corrected lidar signal (RCS) (Fig.7b). The corresponding BSC-DREAM forecast map and the calculated HYSPLIT backward trajectories, proving the origin of the detected

On the plot of the backscattering coefficient, two peaks appear just at the top of the PBL, indicating the presence of aerosol layers at about 2.2 and 3 km altitude AGL. As it can be seen in Fig.7b, where the denser aerosol layers are color coded by orange-red colors, these two layers do not disappear during the whole period of measurement. The forecast map of BSC-DREAM concerning the Sahara dust transport (Fig.7c) shows an atmosphere free of desert dust over the Balkans at that time. In addition, the HYSPLIT backward trajectories (see Fig.7d), corresponding to altitudes of 1.5 km and 3 km AGL, cross the volcano site and/or European countries with volcanic ash atmospheric contamination. So, a conclusion can be drawn, that the detected two aerosol layers are due to the transport of the volcanic ash. The aerosol layers appearing at heights of about 8 km AGL are identified as cirrus

(a) (b)

**5.3 Detection of volcanic ashes** 

PBL, at about 2-2.5 km altitude AGL.

clouds.

aerosols, are presented in Figs.7c and 7d, respectively.

(b) as measured by the CuBr and Nd:YAG lidars, respectively, on 04.11.2010.

evolution of Saharan dust transported over European continent (Papayannis et al., 2008; Grigorov et al., 2009). Measurements are synchronized in time with the BCS-DREAM model forecasts for dust loadings and transport.

As an illustration, vertical profiles of the aerosol backscattering coefficient measured by the Nd:YAG lidar at 1064 nm and 532 nm on 4 November 2010, during a dust-transport event, are presented in Fig.5a. The altitude range 1-5 km above sea level (ASL) is only shown in order to zoom the profile part containing the Saharan dust layer. The latter is located in the range 2.8-4 km ASL, just above the PBL as typically. The color-coded DREAM dust loading forecast map for a time preceding the measurements is displayed in Fig.5c. As one can see on the map, a dust layer with density of about 0.2 g/m2 has covered the lidar station region, in good correlation with the intense peaks of dust backscattering coefficient (exceeding 1x10 -6 m-1sr-1 at 532 nm) observed in Fig.5a.

Fig. 5. Vertical profiles of the aerosol backscatter coefficient at the two lidar wavelengths (a), corresponding profile of BAE (b), and BSC-DREAM forecast map of dust loading (c).

The vertical profile of backscatter-related Ångström exponent (BAE) is shown in Fig.5b, corresponding to profiles in Fig.5a. The BAE values are nearly constant (~1.2) in the underlying PBL. They increase with height reaching 1.3-1.4 just in the dust layer. Such values (1-1.4) are typical for Saharan dust, implying sub-micron dust particle size domination.

Height-time-coordinate diagrams of the backscatter coefficient evolution are presented in Fig.6, as measured in successive time intervals by the CuBr and Nd:YAG lidars at 510.6 nm (a) and 532 nm (b), respectively. As obvious from both diagrams in Fig.6, the Saharan dust layer is well expressed, intense, and relatively stable in terms of height and thickness.

Nevertheless, one can perceive specific internal structure of density distribution evolving over time. The aerosol layer at 5-6 km ASL, observed by the upward-looking CuBr lidar (Fig.6a), is absent on the other diagram because the Nd:YAG lidar is operated at a slope angle of 58 degrees with respect to the zenith, receiving signals from different spatial domains.

Fig. 6. Evolution diagrams of the aerosol backscatter coefficient at 510.6 nm (a) and 532 nm (b) as measured by the CuBr and Nd:YAG lidars, respectively, on 04.11.2010.

#### **5.3 Detection of volcanic ashes**

360 Advanced Photonic Sciences

evolution of Saharan dust transported over European continent (Papayannis et al., 2008; Grigorov et al., 2009). Measurements are synchronized in time with the BCS-DREAM model

As an illustration, vertical profiles of the aerosol backscattering coefficient measured by the Nd:YAG lidar at 1064 nm and 532 nm on 4 November 2010, during a dust-transport event, are presented in Fig.5a. The altitude range 1-5 km above sea level (ASL) is only shown in order to zoom the profile part containing the Saharan dust layer. The latter is located in the range 2.8-4 km ASL, just above the PBL as typically. The color-coded DREAM dust loading forecast map for a time preceding the measurements is displayed in Fig.5c. As one can see on the map, a dust layer with density of about 0.2 g/m2 has covered the lidar station region, in good correlation with the intense peaks of dust backscattering coefficient (exceeding

forecasts for dust loadings and transport.

1x10 -6 m-1sr-1 at 532 nm) observed in Fig.5a.

domination.

domains.

(a) (b) (c)

Fig. 5. Vertical profiles of the aerosol backscatter coefficient at the two lidar wavelengths (a), corresponding profile of BAE (b), and BSC-DREAM forecast map of dust loading (c).

The vertical profile of backscatter-related Ångström exponent (BAE) is shown in Fig.5b, corresponding to profiles in Fig.5a. The BAE values are nearly constant (~1.2) in the underlying PBL. They increase with height reaching 1.3-1.4 just in the dust layer. Such values (1-1.4) are typical for Saharan dust, implying sub-micron dust particle size

Height-time-coordinate diagrams of the backscatter coefficient evolution are presented in Fig.6, as measured in successive time intervals by the CuBr and Nd:YAG lidars at 510.6 nm (a) and 532 nm (b), respectively. As obvious from both diagrams in Fig.6, the Saharan dust layer is well expressed, intense, and relatively stable in terms of height and thickness.

Nevertheless, one can perceive specific internal structure of density distribution evolving over time. The aerosol layer at 5-6 km ASL, observed by the upward-looking CuBr lidar (Fig.6a), is absent on the other diagram because the Nd:YAG lidar is operated at a slope angle of 58 degrees with respect to the zenith, receiving signals from different spatial The eruption of Eyjafjallajokull volcano in Island on 14 April 2010 offered an opportunity lidar stations, participating in the European Lidar Network, to demonstrate the effectiveness of the lidar sensing for 4-dimensional characterization of the volcanic ash transport. The lidar monitoring of Eyjafjallajokull plumes spreading by the Sofia lidar station started on 18 April 2010 and finished on 25 May 2010 (Grigorov et al., 2011). Results of lidar measurements, performed on 22 April 2010, by using a CuBr lidar at a wavelength of 510.6 nm are given in Fig.7, showing presence of volcanic ash layer positioned at ~2.2 and 3 km altitudes AGL. The observed low limit of the layer frequently remains mixed with the PBL, at about 2-2.5 km altitude AGL.

The lidar observations are presented in two formats: as a single averaged vertical profile of the retrieved backscattering coefficient (Fig.7a) and as a map of the time evolution of the range-corrected lidar signal (RCS) (Fig.7b). The corresponding BSC-DREAM forecast map and the calculated HYSPLIT backward trajectories, proving the origin of the detected aerosols, are presented in Figs.7c and 7d, respectively.

On the plot of the backscattering coefficient, two peaks appear just at the top of the PBL, indicating the presence of aerosol layers at about 2.2 and 3 km altitude AGL. As it can be seen in Fig.7b, where the denser aerosol layers are color coded by orange-red colors, these two layers do not disappear during the whole period of measurement. The forecast map of BSC-DREAM concerning the Sahara dust transport (Fig.7c) shows an atmosphere free of desert dust over the Balkans at that time. In addition, the HYSPLIT backward trajectories (see Fig.7d), corresponding to altitudes of 1.5 km and 3 km AGL, cross the volcano site and/or European countries with volcanic ash atmospheric contamination. So, a conclusion can be drawn, that the detected two aerosol layers are due to the transport of the volcanic ash. The aerosol layers appearing at heights of about 8 km AGL are identified as cirrus clouds.

LIDAR Atmospheric Sensing by Metal Vapor and Nd:YAG Lasers 363

aerosol backscattering coefficient based on lidar profiles with 5 min time averaging in the period 17:00-18:00 UT. The observed multi-layered aerosol structure can be explained analysing the meteorological situation using the corresponding BSC-DREAM dust load forecast map (Fig.8b). It shows that Sofia remains away from the Saharan dust flow. So, we suppose that the two aerosol layers, at 8 km and 9.5 km height, represent cirrus clouds. The layer at 3 km altitude is determined to be a residual aerosol layer, due to the decomposition

Atmospheric profiling by a network of ground-based lidar stations is an optimal approach for validation of results obtained by space-borne lidars, providing supporting data to fully exploit the information from satellite lidar missions. Such a mission is the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO). The Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP), mounted on the CALIPSO satellite, is a Nd:YAGlaser-based lidar specially designed for aerosol and cloud monitoring. Several years correlative ground-based lidar measurements, performed by the EARLINET stations as synchonized with CALIPSO overpasses, contribute to the specialized database, illustrating the potential of the lidar network to provide a sustainable ground-based support for space-

The Sofia CuBr-lidar group is involved in correlative measurements for CALIPSO since June 2006 (Grigorov et al., 2007). Results of mesurements performed on 28 April 2009 by the ground-based lidar and by the CALIOP lidar are presented in Figs.9a and 9b, respectively. Vertical red lines on the plots indicate the time of satellite passage over Sofia. On the first plot two aerosol layers can be distinguished. The lower one, located at 1-1.5 km altitude, is due to air convection in the PBL. As seen on the corresponding forecast map of Sahara dust load (Fig.9c), the region covered by the dust flow is in the immediate vicinity of Sofia.

Fig. 8. One-hour evolution diagram of the retrieved aerosol backscattering coefficient corresponding to lidar measurements carried out on 6 April 2009 (a) and the BSC-DREAM

a) b)

**5.5 Correlative space-borne and ground-based lidar measurements** 

borne lidar missions (Pappalardo et al., 2009, 2010).

of the PBL in the evening.

Saharan dust forecast map (b).

Fig. 7. Results of lidar measurements performed on 22.04.2010: a) averaged vertical profile of the retrieved aerosol backscattering coefficient; b) time evolution of range-corrected lidar signal (RCS); c) BSC-DREAM forecast map of Saharan dust load in the atmosphere; d) backward HYSPLIT air mass trajectories. The two peaks marked with arrows in Fig.7a are volcanic ash layers over Sofia.

#### **5.4 Regular lidar atmospheric measurements**

The specialized EARLINET database, resulting from the longtime monitoring of atmospheric aerosols by regular lidar measurements, contains a valuable information for atmospheric processes over Europe (Papayannis et al., 2008). It gives an opportunity for further improvement and validation of atmospheric models and retrieving algorithms applied for climatologic investigations.

Results of lidar measurements carried out by Sofia lidar station on 6 April 2009 are presented in Fig.8a. The color map represents the one-hour evolution of the retrieved

(a) (b)

(c) (d)

are volcanic ash layers over Sofia.

applied for climatologic investigations.

**5.4 Regular lidar atmospheric measurements** 

Fig. 7. Results of lidar measurements performed on 22.04.2010: a) averaged vertical profile of the retrieved aerosol backscattering coefficient; b) time evolution of range-corrected lidar signal (RCS); c) BSC-DREAM forecast map of Saharan dust load in the atmosphere; d) backward HYSPLIT air mass trajectories. The two peaks marked with arrows in Fig.7a

The specialized EARLINET database, resulting from the longtime monitoring of atmospheric aerosols by regular lidar measurements, contains a valuable information for atmospheric processes over Europe (Papayannis et al., 2008). It gives an opportunity for further improvement and validation of atmospheric models and retrieving algorithms

Results of lidar measurements carried out by Sofia lidar station on 6 April 2009 are presented in Fig.8a. The color map represents the one-hour evolution of the retrieved aerosol backscattering coefficient based on lidar profiles with 5 min time averaging in the period 17:00-18:00 UT. The observed multi-layered aerosol structure can be explained analysing the meteorological situation using the corresponding BSC-DREAM dust load forecast map (Fig.8b). It shows that Sofia remains away from the Saharan dust flow. So, we suppose that the two aerosol layers, at 8 km and 9.5 km height, represent cirrus clouds. The layer at 3 km altitude is determined to be a residual aerosol layer, due to the decomposition of the PBL in the evening.

Fig. 8. One-hour evolution diagram of the retrieved aerosol backscattering coefficient corresponding to lidar measurements carried out on 6 April 2009 (a) and the BSC-DREAM Saharan dust forecast map (b).
