Plamen B. Savov

Lidar Investigation of the Temporal and Spatial Distribution of Atmospheric Aerosols in Mountain Valleys

Lidar experiments were conducted in the mountainous region of Bulgaria to determine the spatial and temporal distribution of major aerosol sources and the zones of aerosol accumulation. When these lidar data are combined with conventional meteorological observations of temperature and wind profiles they provide a clear picture of the physical processes that lead to the accumulation and subsequent dispersion of aerosols and other pollutants in the valleys. The observations showed that the valley gradually fills with cool air after sunset, producing an inversion that traps aerosols and other pollutants emitted at night. After sunrise a convective boundary layer develops in the valley; its evolution is delayed by the confining valley walls. Insolation causes airflow up the slope, producing divergence near the surface and subsidence of the inversion core. The one winter experiment conducted suggests that weaker winter insolation delays the process until much later than in the summer, sometimes to the extent that the inversion persists throughout the day, or even for several days. The findings described here are in good agreement, qualitatively and quantitatively, with the model described by Whiteman and McKee. The results also demonstrate the power of combining conventional meteorological observations with lidar techniques for determining the nature of boundary layer processes in a valley.

Influence of the boundary layer development on the ozone concentration over an urban area

Results obtained during two campaigns (summer 2004 and autumn 2005) of observation of the planetary boundary layer dynamics over the Sofia city urban area are presented. An EARLINET scanning aerosol lidar, an ozone analyser and a ground meteorological station were used during the observations. The stable boundary layer height varied from 200 m to 600 m during the different seasons. The residual layer was found to be at 700–1200 m, being destroyed between 10:30 and 12:30 LST. The mixing layer developed up to heights of about 800–1300 m. The ground level ozone concentration was measured to be from 10 µg/m3 to 90 µg/m3. The convective boundary layer formation in three case studies (a clear sunny day, a partial solar eclipse, and in the presence of internal atmospheric gravity waves) are presented. In particular, the mixing layer development and the residual layer destruction are studied and considered, along with the relevant ground level ozone concentration variation.

Boundary layer development and meteorological parameters impact on the ground level ozone concentration over an urban area in a mountain valley Sofia, Bulgaria

The ecological problems caused by the increasing ozone concentration are not easily solved because ozone is not directly emitted by certain sources Its concentration depends on numerous dynamical and chemical processes. Stratosphere–troposphere exchange and subsequent ozone penetration into the boundary layer determine the contribution of so-called ‘natural’ ozone to ozone pollution near the ground. However, the main contribution to the concentration of this pollution is that of the anthropogenic ozone, which is generated as a result of complex photochemical reactions. The purpose of this research is the ground level ozone concentration behaviour to be studied during the stable boundary layer (SBL) and the residual layer (RL) destruction and the convective boundary layer (CBL) formation, so the influence of the temperature, the relative humidity and the height of the mixing layer (ML) as well as that of the ML formation in different areas of Sofia (42° 39′ N, 23° 23′ E, 591 m above sea level), Bulgaria, have to be determined. The ground level ozone concentration in the area of the Institute of Electronics changes synchronously with the development of the ML. The maximum values of the ground level ozone concentration are reached when the height of the ML reached its maximum and afterwards. The maximum growth of the ground level ozone concentration is around 11:00–12:30 h LST when a fast growth of the ML begins and the complete destruction of the RL is observed, that is, the two processes of ML growth and entrainment of aerosol and ozone from the higher layers of the atmospheric boundary layer are observed. The values of the ground level ozone concentration during the summer months are higher than those during the fall.

Experimental study of polarization characteristics of lidar signal in case of occlusion front

In this work, experimental data of a light detection and ranging (lidar) polarization study of cloud formations in a case of warm occlusion front in winter are presented. The changes in the low clouds at the different stages of the front advection are followed: before, during and after the cold air mass settles down. The experiment was carried out using a polarization lidar with variable viewing angle of the receiver, which allows the influence of the multiple scattering on the signal depolarization to be estimated. The data are acquired by simultaneously recording two polarized components of the lidar return: parallel and perpendicular with respect to that of the sounding radiation. The depolarization coefficient of the signals from various clouds types (stratus, stratocumulus, nimbus stratus, etc.) is determined by receiving and rejecting the multiply scattered lidar returns. The depolarization of the lidar returns is determined also in the space between the ground and the clouds base during different stages of the front advection including wet snowfall and no precipitation; the typical values obtained are: 3–5% before precipitation, 5–7% during rain, 10–40% during snowfall and 1–2% after precipitation. Conclusions are drawn about the phase composition of the clouds formations and the heights of the ice crystals nucleation during snowfall. So the evolution of the atmospheric formations is followed during the different stages of the warm occlusion front advection.

Case study of the ABL height and optical parameters of the atmospheric aerosols over Sofia

A study of the atmospheric boundary layer (ABL) height and its relation to the variations in the aerosol optical depth (AOD), Ångström coefficients, water vapor column (WVC) and total ozone column (TOC) was carried out in June 2011 and June 2012 at three sites in the city of Sofia (Institute of Electronics, Astronomical Observatory in the Borisova Gradina Park and National Institute of Geophysics, Geodesy and Geography). A ceilometer CHM15k, a sun photometer Microtops II, an ozonometer Microtops II and an automatic meteorological station were used during the experiments. Measurements of the AOD, WVC and TOC were done during the development of the ABL (followed by the ceilometer). In order to access microphysical properties of the aerosols, the Ångström coefficients α and β were retrieved from the spectral AOD data by the Volz method from three wavelength pairs 500/1020nm, 500/675nm and 380/1020nm. Comparison was done between the results obtained. Daily behavior of the AOD, Ångström exponent α and turbidity coefficient β, WVC and TOC are presented. Different types of AOD and WVC behavior were observed. The AOD had maximum values 1-2 h before ABL to reach its maximum height for the day. No significant correlation is found between TOC daily behavior and that of the AOD and WVC.

Ceilometer observation of Saharan dust over mountain valley of Sofia, Bulgaria

Atmospheric aerosol is known to considerably influence the Earth’s radiative budget and to make an impact on air quality. The influence of aerosols strongly depends on their spatial distribution and optical properties. The aerosol has natural and anthropogenic origin. Aerosol types can be also classified according to their size, sources or geographical origin (desert, continental, marine etc.). Mineral dust is one of the natural aerosols presented in the atmosphere. Its main source is the Sahara desert region. Saharan aerosol layers are frequently observed in Europe by means of active and passive remote sensing devices, especially in the frame of EARLINET and ACTRIS 3, 5, 6, 7, 8, 9. In this paper, observations of vertical distribution of aerosols and assessment of their optical properties will be presented. Two-year (2013-2014) complex measurements were carried out by a ceilometer CHM-15k (Jenoptic) and two lidars in an urban area located in a mountain valley (Sofia, Bulgaria)1. The ceilometer works 24 hours in automatic mode. Part of the results is compared with results obtained by lidars operating in photon counting modes for specific periods of simultaneous work5. Supplementary data from: two meteorological stations; HYSPLIT back trajectory model4; BSCDREAM8b dust model9; and the database of atmospheric radio sounding profiles from Department of Atmospheric Engineering of Wyoming University (USA) are also used in the analysis of the obtained results.

Aerosol extinction distribution during the morning transition of breeze circulation observed by lidar

This paper presents a study of the distribution of aerosol extinction coefficient and its time and spatial variations during the morning breeze transition period. The experiment was carried out near the town of Akhtopol with a groundbased aerosol lidar. The results show an existence of characteristic extinction distribution (aerosol stratification) for each stage. In the beginning of the period, when in the lowest 200 m there is still an off-shore breeze, an aerosol layer with high extinction exists at heights of about 150 - 200 m above the sea surface. As the sunheating gets stronger this layer is gradually destroyed. At the end of the period, when the on- shore breeze is already established, in the lowest 200 m, a layer of low extinction is observed at height of about 200 m above the sea surface. A coincidence has been revealed between the heights of the wind changes and location of layers where the extinction differs from the one of the surrounding air. Besides the change in breeze pattern, the change in the relative humidity also affects the observed time and spatial variations in aerosol extinction.

Lidar study of the convective planetary boundary layer over urban area

Results of systematic investigations of the aerosol structure of the planetary boundary layer over a part of Sofia city are presented. The study aims at investigation of formation of aerosol distribution after sunrise using a lidar-kytoon system. Lidar data are interpreted combined with profiles of atmospheric thermodynamic parameters determined by conventional means.

Lidar Determination of the Mixing Layer Formation Delay in the Presence of a Fog

The lidar investigation of dense atmospheric formations such as clouds and fogs is ofgreat interest both for the fundamental and applied meteorology. This study is carried out in the region of the Sofia city in summer. The presence of two zones (a green area and a residential district) with different albedo can be considered as a peculiarity of the underlying surface along the sounding path. The study was performed using a polarization aerosol lidar.