Search

The wintertime outbreaks of Saharan dust, increasing in intensity and frequency over the last decade, have become an important component of the global dust cycle and a challenging issue in elucidating its feedback to the ongoing climate change. For their adequate monitoring and characterization, systematic multi-instrument observations and multi-aspect analyses of the distribution and properties of desert aerosols are required, covering the full duration of dust events. In this paper, we present observations of Saharan dust in the atmosphere above Sofia, Bulgaria, during a strong dust episode over the whole of Europe in February 2021, conditioned by a persistent blocking weather pattern over the Mediterranean basin, providing clear skies and constant measurement conditions. This study was accomplished using different remote sensing (lidar, satellite, and radiometric), in situ (particle analyzing), and modeling/forecasting methods and resources, using real measurements and data (re)analysis. A wide range of columnar and range/time-resolved optical, microphysical, physical, topological, and dynamical characteristics of the detected aerosols dominated by desert dust are obtained and profiled with increased accuracy and reliability by combining the applied approaches and instruments in terms of complementarity, calibration, and normalization. Vertical profiles of the aerosol/dust total and mode volume concentrations are presented and analyzed using the LIRIC-2 inversion code joining lidar and sun-photometer data. The results show that interactive combining and use of various relevant approaches, instruments, and data have a significant synergistic effect and potential for verifying and improving theoretical models aimed at complete aerosol/dust characterization. [ABSTRACT FROM AUTHOR]

An analysis of the optical and microphysical characteristics of aerosol passages over Sofia City, Bulgaria, was performed on the basis of data provided by the AErosol RObotic NETwork (AERONET). The data considered are the result of two nearly complete annual cycles of passive optical remote sensing of the atmosphere above the Sofia Site using a Cimel CE318-TS9 sun/sky/lunar photometer functioning since 5 May 2020. The values of the Aerosol Optical Depth (AOD) and the Ångström Exponent (AE) measured during each annual cycle and the overall two-year cycle exhibited similar statistics. The two-year mean AODs were 0.20 (±0.11) and 0.17 (±0.10) at the wavelengths of 440 nm (AOD440) and 500 nm, respectively. The two-year mean AEs at the wavelength pairs 440/870 nm (AE440/870) and 380/500 nm were 1.45 (±0.35) and 1.32 (±0.29). The AOD values obtained reach maxima in winter-to-spring and summer and were about two times smaller than those obtained 15 years ago using a hand-held Microtops II sun photometer. The AOD440 and AE440/870 frequency distributions outline two AOD and three AE modes, i.e., 3 × 2 groups of aerosol events identifiable using AOD–AE-based aerosol classifications, additional aerosol characteristics, and aerosol migration models. The aerosol load over the city was estimated to consist most frequently of urban (63.4%) aerosols. The relative occurrences of desert dust, biomass-burning aerosols, and mixed aerosols were, respectively, 8.0%, 9.1% and 19.5%. [ABSTRACT FROM AUTHOR]

This work presents a selective overview of natural fogs in terms of fog types, forms and states of occurrence, physical, micro-physical, chemical and dynamic properties, basic characterizing parameters, etc. In focus are related achievements and contributions reported mainly during the last decade and a half, as a result of both laboratory studies and field observations. Processes of homogeneous and heterogeneous nucleation are analyzed in the aspects of condensation, nuclei diversity and specifics, as related to the activation, growth and deposition of fog droplets. The effect is highlighted of the water vapor's partial pressure on the surface tension of the liquid water-air interface and the freezing point of the water droplets. Some problems and aspects of fog modeling, parameterization, and forecasting are outlined and discussed on the examples of newly developed relevant 1D/3D theoretical models. Important issues of fog impacts on the air quality, ecosystems, water basins, societal life, and human health are also addressed and discussed, particularly in cases of anthropogenically modified (chemical, radioactive, etc.) fogs. In view of reducing the possible negative effects of fogs, conclusions are drawn concerning the new demands and challenges to fog characterization imposed by the changing natural and social environment and the needs for new data on and approaches to more adequate observations of fog-related events. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The laser radiation wavelength is one of the factors that may influence the strength of the elastic-lidar return signal from clear and hazy atmospheres, either in the presence or in the absence of specific dense aerosol objects, such as cirrus clouds, Saharan dust or volcanic ash layers, etc. At a stronger signal and, respectively, higher measurement signal-to-noise ratio (SNR), the images of characteristic features along the lidar line of sight (LOS), including dense aerosol objects, would be, in general, brighter and clearer and better detectable. To reveal purposely and systematically the role of the sensing laser wavelength, at equal other factors, in achieving good images by elastic lidar, we recently began developing an approach consisting in numerical and statistical modeling, comparison and analysis of single-scattering mean lidar and SNR profiles and the corresponding noisy profile estimates obtained at different sensing wavelengths. The signal and background fluctuations are generated as due to the prevailing Poisson shot noise.The purpose of this work is to demonstrate the performance and efficiency of the above-mentioned investigatory approach by applying it to aerosol stratification models of clear and hazy atmospheres containing simultaneously structures resembling a Saharan-dust layer and a cirrus cloud. The sensing radiation wavelengths considered belong to the UV (337.1 nm), VIS (514.5 nm), and NIR (1060 nm) spectral ranges.The results obtained show that, due to stronger backscattering and not so high extinction (attenuation), the aerosol stratification in a clear atmosphere is better imaged when using shorter (UV) wavelengths. In a hazy atmosphere, when the turbidity increases, the advantage of the shorter wavelength radiations decreases and even vanishes at some altitudes, because of the additional increase in the attenuation, which is higher as a rule. At the same time, the detectability (imaging quality) of relatively larger-particle aerosol objects (cirrus clouds and Saharan dust layers) is better when using longer-wavelength (NIR) sensing radiation. Then, because of the lower optical background, lower attenuation and backscattering outside of the object and the nearly wavelength-independent attenuation and backscattering inside the object, its image obtained using NIR radiation is the brightest, clearest and most contrasting. The SNR of detecting the object is then maximum and an acceptable image quality is achievable at a smaller statistical volume and not so severe smoothing along the LOS, thus ensuring better temporal and spatial sensing resolutions. Besides, multiple scattering could be considered negligible when NIR radiation is used and any corrections to the single-scattering lidar equation would not be necessary. The conclusions deduced here are in agreement with experimental results obtained formerly.Acknowledgements: This research is funded by the Bulgarian Academy of Sciences (BAS) Program for supporting young scientists and PhD students in BAS – 2017, under the project "Optimal effective elastic lidar sensing of aerosol atmosphere containing strongly scattering compact objects" (Contract No DFNP-17-4/24.07.2017). A partial financial support from the Bulgarian Ministry of Education and Science under grant D01-151/28.08.2018 is also acknowledged. [ABSTRACT FROM AUTHOR]