3. Experimental results and discussion

Results of lidar measurements and mapping of the near-surface atmospheric aerosol fields over the city of Sofia, suburbs, and surrounding villages, obtained during the experimental campaign in 2015, are shown and discussed below. Lidar maps are presented from vertical and horizontal scanning of the areas investigated.

#### 3.1. Mapping of aerosol fields by vertical scanning

precision and reliability, which are accordingly transferred to the colormaps based on them of

Figure 3. Range profiles of the aerosol backscattering coefficient at three different azimuth angles (a) and aerosol distribution lidar map based on a series of BCS profiles (b) as measured in the time interval 20:35–21:28 LT on 5 November

Generally, the aerosol field could be described as a distribution of the aerosol mass concentra-

extinction and backscattering coefficients of the aerosol particles are determined that are

The mass concentration could be retrieved from the lidar data combining different experimental and numerical approaches [31]. So, obtaining data about the distribution of the aerosol backscattering coefficient could be regarded as representative for the aerosol mass concentra-

Figure 3 shows an example of the stages of formation of an aerosol lidar map using measurements performed on 5 November 2015, in the time interval 20:35–21:28 local time (LT). Aerosol backscattering profiles obtained at different azimuth angles along a fixed elevation angle are presented in Figure 3(a). On the basis of a series of such profiles, 2D color-coded sector maps of the near-surface aerosol density could be created. In Figure 3(b), an aerosol lidar map is displayed in Cartesian coordinates, based on the entire set of BSC profiles in the azimuth sector 190–220°, including the ones in Figure 3(a). Finally, the sector maps so-obtained are

) defined as the mass of PM per unit volume. From the lidar measurements, the

M ¼ kα<sup>a</sup> ¼ kβaSa: (5)

the near-surface aerosol density distribution.

directly proportional to the aerosol mass concentration:

superposed on the satellite maps of the corresponding city region.

2.4. Lidar mapping of aerosol fields

tion M (μg/m<sup>3</sup>

2015.

92 Aerosols - Science and Case Studies

tion distribution.

In order to acquire detailed information about the vertical structure of the aerosol concentration, lidars perform vertical slice scans. The lidar data shown on Figure 4 represent a twodimensional color-coded sector map of the aerosol density distribution within the scanned volume of the atmosphere. The map is constructed using lidar profiles (averaged over five individual scans) obtained along a fixed azimuth in NW-direction at different elevation angles (0–10°), with an increment of 1°. The horizontal direction of the lidar scanning, covering

Figure 4. Color-coded sector map of the vertical aerosol density distribution obtained along a fixed azimuth in NWdirection (326° with respect to the north clockwise).

Figure 5. Colormaps of the near-surface aerosol density distribution demonstrating the maximum achievable operational distances in NW (a) and SW (b) directions, performed by the lidar systems shown in the insets.

distances of about 12 km was close to a thoroughfare with intense traffic. The vertical structure of the aerosol density of the atmosphere is clearly visible on the map. A well-pronounced vertical layer near ground surface was observed at a height in the range of 500–700 m, located above the city away from the lidar station until beyond the city center. At a height of ~1 km above ground, aerosol formations were observed with a density exceeding that of the ambient atmosphere, probably low clouds. Thus, this vertical map demonstrates the capability of such a type of lidar measurements to determine quickly and efficiently the location of the sources of anthropogenic PM emission in the atmosphere. On the other hand, it is clear that the horizontal scanning lidar measurements, made at a low altitude in the range of 500–700 m, provide sufficient information about the air pollution and near-ground surface aerosol fields.

#### 3.2. Range limits of lidar measurements

Figure 5 illustrates the range limits of lidar measurements in NW (Figure 5(a)) and SW (Figure 5(b)) directions, performed by lidars with a Cu-vapor laser and a Nd:YAG laser, respectively. In the first case, the operational distance was from 900 m to 25–28 km in nighttime and decreased to about 10–15 km in daytime, due to intensive sky illumination. The maximum distance was limited by the high laser pulse repetition rate, because of an overlap of the laser pulse scattered from far away with the next pulse scattered from a close distance. In SWdirection to the Vitosha Mountain, the assessed maximum distance was longer than 20 km as determined by the surface topology in the observation area.

Figure 6. Colormap of the aerosol density distribution as measured on 27 July 2015 in the time interval 21:33–22:10 LT, at distances of up to 15 km.

#### 3.3. Lidar mapping of aerosol fields over Sofia's central parts

distances of about 12 km was close to a thoroughfare with intense traffic. The vertical structure of the aerosol density of the atmosphere is clearly visible on the map. A well-pronounced vertical layer near ground surface was observed at a height in the range of 500–700 m, located above the city away from the lidar station until beyond the city center. At a height of ~1 km above ground, aerosol formations were observed with a density exceeding that of the ambient atmosphere, probably low clouds. Thus, this vertical map demonstrates the capability of such a type of lidar measurements to determine quickly and efficiently the location of the sources of anthropogenic PM emission in the atmosphere. On the other hand, it is clear that the horizontal scanning lidar measurements, made at a low altitude in the range of 500–700 m, provide

Figure 5. Colormaps of the near-surface aerosol density distribution demonstrating the maximum achievable operational

distances in NW (a) and SW (b) directions, performed by the lidar systems shown in the insets.

sufficient information about the air pollution and near-ground surface aerosol fields.

Figure 5 illustrates the range limits of lidar measurements in NW (Figure 5(a)) and SW (Figure 5(b)) directions, performed by lidars with a Cu-vapor laser and a Nd:YAG laser, respectively. In the first case, the operational distance was from 900 m to 25–28 km in nighttime and decreased to about 10–15 km in daytime, due to intensive sky illumination. The maximum distance was limited by the high laser pulse repetition rate, because of an overlap of the laser pulse scattered from far away with the next pulse scattered from a close distance. In SWdirection to the Vitosha Mountain, the assessed maximum distance was longer than 20 km as

3.2. Range limits of lidar measurements

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determined by the surface topology in the observation area.

Lidar monitoring and mapping of the near-surface aerosols in the atmosphere above the central parts of Sofia were performed by the lidar equipped with a Cu-vapor laser at the wavelength of 510.6 nm.

Figure 6 presents the results of lidar measurements carried out on 27 July 2015, at 21:33–22:10 LT, when relatively strong air pollution was observed. The distance covered by the lidar sounding was 15 km in an azimuth sector of 8.5°. A dust cloud was observed in the atmosphere near ground surface over most of the observation zone. Only the blue-colored areas, at 1.5–2 km away from the two large boulevards, showed a lower concentration of dust particles in the air. The specific movement of air masses, from SW-to-NE-direction, causes a mixing of dust pollutants into the larger part of the area over the city observed by the lidar. The values measured of the aerosol BSC are in the order of 0.5–8 × 10−<sup>6</sup> m−<sup>1</sup> sr−<sup>1</sup> .

Figure 7 presents results of lidar mapping in the two main sectors of scanning over the central parts of Sofia, performed in different time periods during the measurement campaign and exhibiting similar features of the aerosol distribution, in particular, the influence of populated areas on the aerosol density. The near-surface aerosol distribution, resulting from lidar

Figure 7. Color-coded maps of the near-surface aerosol density distribution as measured on 5 August 2015, at 22:10–22:50 LT (a) and on 7 October 2015 in the time interval 19:35–20:30 LT (b).

measurements performed on 5 August 2015, at 22:10–22:50 LT, is shown in Figure 7(a), covering a distance of 25 km. Due to the heavy city traffic, relatively higher values of the aerosol BSC were observed close to the busy streets and over the entire central parts reaching the city ring road. At distances beyond the ring road, the aerosol pollution concentration dropped rapidly, except for some areas near two local villages. We, therefore, assumed that the aerosol fields observed by the lidar were of anthropogenic origin. The values measured of the aerosol BSC were in order of 0.3–4.3 × 10−<sup>6</sup> m−<sup>1</sup> sr−<sup>1</sup> . Figure 7(b) presents a map of a lidar scanning conducted on 7 October 2015 within the NW sector, in the time interval 19:35–20:30 LT. The sounding comprised 11 successive scans in an angular sector of 17° by an angle step of 1.7°, covering a distance of 16 km. Well-defined areas of higher aerosol pollution were visible over the city areas, as well as over some residential districts in the far measurement zone. The values calculated of the atmospheric BSC were from 0.5 to 9.8 × 10−<sup>6</sup> m−<sup>1</sup> sr−<sup>1</sup> .

In Figure 8, results are presented of lidar measurements in the two main sectors of scanning as in Figure 7, performed consecutively in the time intervals 19:22–19:48 and 19:54–20:54 LT on 4 November 2015. The first measurement (shown at the left angular sector of Figure 8) directed northwestward covered the central city zones including and being nearly parallel to one of the main city thoroughfare, reaching distances of up to 12 km. The second lidar sounding (shown at the right angular sector of Figure 8) was directed north-northwestward to distances of 13 km, covering densely populated residential districts in the part of the map near the lidar.

Figure 8. Color-coded maps of the near-surface aerosol density distribution as measured on 4 November 2015, in the time intervals 19:22–19:48 LT (left angular sector) and 19:54–20:54 LT (right angular sector).

measurements performed on 5 August 2015, at 22:10–22:50 LT, is shown in Figure 7(a), covering a distance of 25 km. Due to the heavy city traffic, relatively higher values of the aerosol BSC were observed close to the busy streets and over the entire central parts reaching the city ring road. At distances beyond the ring road, the aerosol pollution concentration dropped rapidly, except for some areas near two local villages. We, therefore, assumed that the aerosol fields observed by the lidar were of anthropogenic origin. The values measured of

Figure 7. Color-coded maps of the near-surface aerosol density distribution as measured on 5 August 2015, at 22:10–22:50

scanning conducted on 7 October 2015 within the NW sector, in the time interval 19:35–20:30 LT. The sounding comprised 11 successive scans in an angular sector of 17° by an angle step of 1.7°, covering a distance of 16 km. Well-defined areas of higher aerosol pollution were visible over the city areas, as well as over some residential districts in the far measurement zone. The

In Figure 8, results are presented of lidar measurements in the two main sectors of scanning as in Figure 7, performed consecutively in the time intervals 19:22–19:48 and 19:54–20:54 LT on 4 November 2015. The first measurement (shown at the left angular sector of Figure 8) directed northwestward covered the central city zones including and being nearly parallel to one of the main city thoroughfare, reaching distances of up to 12 km. The second lidar sounding (shown at the right angular sector of Figure 8) was directed north-northwestward to distances of 13 km, covering densely populated residential districts in the part of the map near the lidar.

values calculated of the atmospheric BSC were from 0.5 to 9.8 × 10−<sup>6</sup> m−<sup>1</sup>

sr−<sup>1</sup>

. Figure 7(b) presents a map of a lidar

sr−<sup>1</sup> .

the aerosol BSC were in order of 0.3–4.3 × 10−<sup>6</sup> m−<sup>1</sup>

LT (a) and on 7 October 2015 in the time interval 19:35–20:30 LT (b).

96 Aerosols - Science and Case Studies

Along this second direction, an area of high aerosol concentration was observed, which extended to a distance of 5 km with respect to the lidar. Another area of high concentration of the near-surface aerosols was observed at a distance of 7–8 km, at the end of the urban area. At greater distances, the aerosol air pollution observed was negligible, as shown by the greenblue colors in the figure. The BSC values calculated ranged from 0.5 to 6.2 × 10−<sup>6</sup> m−<sup>1</sup> sr−<sup>1</sup> .

In order to demonstrate the capability of the lidar aerosol mapping technology applied to follow the temporal evolution of the near-surface aerosol density distribution, a series of successive lidar scans over the same areas were carried out. Figure 9 presents four lidar maps resulting from measurements conducted on 18 November 2015, as averaged over 30 min intervals. The start times of each measurement are marked in the upper right corner of the corresponding figure panels. The measurements were implemented in the NW angular sector, reaching distances of up to 9.5 km. These maps illustrate the changes occurring in the near-surface aerosol fields measured over the city area in the observation zones. As can be seen, the areas located near the main city thoroughfare with the most intense traffic are colored in red-brown, indicating strong aerosol pollution, probably due to the car exhaust emissions. Inspecting the four pictures

Figure 9. Color-coded maps of the aerosol density distribution as measured on 18 November 2015 in the time intervals 18:2718:52 LT (a), 18:55–19:20 LT (b), 19:22–19:47 LT (c), and 19:49–19:14 LT (d).

presented in their chronological order, one can perceive a progressive shrinking of the part of the maps polluted by aerosols. This peculiarity can be ascribed to the progressively diminishing traffic intensity in the evening hours, resulting in less car aerosol emissions.

#### 3.4. Lidar mapping of aerosol fields toward Vitosha Mountain

Series of both daytime and nighttime lidar measurements of the near-surface aerosol density distribution were carried out in the period 3–9 November 2015. The meteorological conditions during the measurements were as follows: a relatively high temperature for the season (18–20° C); a weak wind; a stable temperature inversion within the atmospheric boundary layer (at altitudes 880–1200 m above ground level); atmospheric pressure: 970–920 hPa. These stable

Figure 10. Colormap of the near-surface aerosol density distribution, as measured in the time interval 12:00–14:50 LT on 3 November 2015, in an azimuth range of 40° and distances of up to 3 km.

conditions, in combination with the absence of specific aerosol loadings (e.g. fire smoke, desert dust, etc.) in that period, resulted in aerosol distribution pictures generally similar to those obtained from the individual measurements conducted. Still, the lidar data exhibit particular patterns of the aerosol fields above the city, determined by various local horizontal and vertical air circulations in the close-to-the surface atmospheric layer.

presented in their chronological order, one can perceive a progressive shrinking of the part of the maps polluted by aerosols. This peculiarity can be ascribed to the progressively diminishing

Figure 9. Color-coded maps of the aerosol density distribution as measured on 18 November 2015 in the time intervals

Series of both daytime and nighttime lidar measurements of the near-surface aerosol density distribution were carried out in the period 3–9 November 2015. The meteorological conditions during the measurements were as follows: a relatively high temperature for the season (18–20° C); a weak wind; a stable temperature inversion within the atmospheric boundary layer (at altitudes 880–1200 m above ground level); atmospheric pressure: 970–920 hPa. These stable

traffic intensity in the evening hours, resulting in less car aerosol emissions.

3.4. Lidar mapping of aerosol fields toward Vitosha Mountain

18:2718:52 LT (a), 18:55–19:20 LT (b), 19:22–19:47 LT (c), and 19:49–19:14 LT (d).

98 Aerosols - Science and Case Studies

Three separate lidar measurements were carried out on 3 November 2015—one in the first half of the day and two successive ones in the evening. During the daytime measurement, the lidar scanning was performed in south-southwest directions within a horizontal angle range of 40° to distances of up to 3 km. The results are shown in Figure 10. The correspondence between the aerosol BCS values and the lidar map colors is given by the color bar in the upper left corner. Inhomogeneous distribution of the aerosol concentration was registered, according to the spotted colormap pattern. In the left-hand upper part of the map, a dark-red colored area can be seen, extending to about 1 km and corresponding to the highest aerosol loading. This

Figure 11. Colormap of the aerosol density distribution as measured on 3 November 2015 in the time interval 19:05–19:41 LT at distances of up to 8 km (a) and in the interval 19:56–20:40 LT at distances up to 12 km (b).

observation is reasonable, taking into account the fact that in this part of the city densely populated residential districts are located, with intense daytime street traffic.

In the evening of 3 November 2015, two successive lidar soundings were performed, the results of which are presented in Figure 11. Juxtaposing data of such successive measurements conducted in the same angular sector allows one to follow temporal variations of the aerosol fields over the areas investigated.

The first measurement was carried out by horizontal lidar scanning in an angular sector of 14°, reaching distances of up to 8 km (Figure 11(a)), whereas the second one, in a sector of 20° to a distance of 12 km (Figure 11(b)). The larger distances reached during the evening measurements are due to the much lower optical background than the daytime one. The comparison of the daytime (Figure 10) and nighttime (Figure 11(a)) lidar soundings showed that the relatively high concentration of aerosols measured at midday over the zone to 3 km near the lidar was preserved until the evening. At longer distances (beyond the ring road), approaching the Vitosha Mountain, the aerosol concentration decreased and remained relatively homogeneous, as indicated by the low-contrast light-bluish coloring of the corresponding map parts.

As an exception, an increased aerosol density could be observed over some remote parts of the scanned region in the distance range 4–6 km (colored in light green-yellow), where residential districts are located in the mountain skirts. This can be ascribed either to the presence of a light fog or to smoke emissions taking into account the started heating season.

Figure 12. Colormap of the aerosol density distribution as measured on 5 November 2015 in the time interval 12:26–13:25 LT at distances of up to 7 km.

observation is reasonable, taking into account the fact that in this part of the city densely

Figure 11. Colormap of the aerosol density distribution as measured on 3 November 2015 in the time interval 19:05–19:41

In the evening of 3 November 2015, two successive lidar soundings were performed, the results of which are presented in Figure 11. Juxtaposing data of such successive measurements conducted in the same angular sector allows one to follow temporal variations of the aerosol

The first measurement was carried out by horizontal lidar scanning in an angular sector of 14°, reaching distances of up to 8 km (Figure 11(a)), whereas the second one, in a sector of 20° to a distance of 12 km (Figure 11(b)). The larger distances reached during the evening measurements are due to the much lower optical background than the daytime one. The comparison of the daytime (Figure 10) and nighttime (Figure 11(a)) lidar soundings showed that the relatively high concentration of aerosols measured at midday over the zone to 3 km near the lidar was preserved until the evening. At longer distances (beyond the ring road), approaching the Vitosha Mountain, the aerosol concentration decreased and remained relatively homogeneous, as indicated by the low-contrast light-bluish coloring of the

As an exception, an increased aerosol density could be observed over some remote parts of the scanned region in the distance range 4–6 km (colored in light green-yellow), where residential districts are located in the mountain skirts. This can be ascribed either to the presence of a light

fog or to smoke emissions taking into account the started heating season.

populated residential districts are located, with intense daytime street traffic.

LT at distances of up to 8 km (a) and in the interval 19:56–20:40 LT at distances up to 12 km (b).

fields over the areas investigated.

100 Aerosols - Science and Case Studies

corresponding map parts.

Four measurements were conducted on 5 November 2015 – one daytime over distances of up to 4 km and three successive nighttime ones over distances of up to 11 km. The daytime measurement was carried out in an angular sector of 14° and distances of up to 7 km. The highest aerosol loading was observed above the city zone about 4 km away from the lidar station, reaching the ring road, with a relatively homogeneous aerosol density distribution (Figure 12). These results are comparable to the ones presented above obtained during the daytime lidar measurements performed on 3 and 4 November 2015. This is reasonable because of the similar meteorological conditions and the absence of unusual aerosol pollutions.

All three nighttime lidar measurements took place in successive 1 h time intervals, over the same area in an angular sector of 30° and distances up to 12 km. The results are presented as colormaps in Figure 13. The comparison of the three panels shows the disappearance of the dynamic atmospheric processes, resulting in a considerable redistribution of the near-surface aerosol density. This can be clearly seen in the figure panels as expressed by the variable color pattern of the maps, most evident in the zones near the lidar extending to 4–5 km (colored mainly in red and yellow). In addition, the extension observed of the blue-colored remote part of the colormap sectors to the city zone in the course of the measurements could be ascribed to

Figure 13. Colormap of the aerosol density distribution as measured on 5 November 2015 in the time intervals 18:37– 19:33 LT (a), 19:36–20:29 LT (b) and 20:35–21:28 LT (c), at distances of up to 12 km.

movements of deficient in aerosols air masses from the mountain areas to the city, driven by the evening mountain breeze—characteristic of the Sofia region.

On 6 November 2015, three (two daytime and one nighttime) mapping lidar measurements were carried out within an angular sector of 26° over the same area. The two daytime measurements

Figure 14. Colormap of the aerosol density distribution as measured on 6 November 2015 in the time intervals 10:24– 11:39 LT (a), 11:48–11:53 LT (b) and 18:23–19:19 LT (c), at distances of up to 4 km (a, b) and 10 km (c).

covered distances of up to about 4 km, whereas the evening one, to nearly 10 km. The corresponding results are displayed as colormaps on the three panels of Figure 14. The analysis of the two daytime lidar maps (Figure 14(a) and (b)) revealed the highest aerosol densities in the 1 km zone near the lidar station (colored in red-yellow in the map). Examining the map color pattern, one can perceive gradations of coloring from dominating red in the near zone, through yellow-green in the middle part (forming stripe-like structures), to mainly blue approaching the mountain zone. This grading pattern might be explained as resulting from the action of air currents moving from the city to the mountain, capturing and transporting urban/anthropogenic aerosols to the mountain areas. The lidar data obtained during the nighttime mapping scans also support such an explanation. The colormap in this last case (Figure 14(c)) shows a particular aerosol distribution structure dominated by a folded aerosol plume (colored mainly in red and red-yellow) extending from the close-to-the lidar city zone, through the suburbs, up to the mountain skirts, and consisting of two differentiated but connected parts—a dense aerosol field over the urban zones and a similar one at the plane-mountain interface zone. This picture illustrates the complex nature and variability of the near-surface aerosol distribution and spreading, originating from different natural and anthropogenic sources and driven in a complex manner by the local air circulation system.
