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Search Results: 1 - 10 of 130398 matches for " V. Amiridis "
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The effect of the global UV irradiance measurement accuracy on the single scattering albedo retrieval
S. Kazadzis, J. Gr bner, A. Arola,V. Amiridis
Atmospheric Measurement Techniques (AMT) & Discussions (AMTD) , 2010,
Abstract: The possibility of measuring aerosol optical absorption properties in the UV spectral range such as single scattering albedo (SSA), using remote sensing techniques, is currently an open scientific issue. We investigate the limitations on calculating column average SSA using a combination of global UV spectral measurements (that are comon in various UV monitoring stations worldwide) with radiative transfer modeling. To point out the difficulties in such a retrieval we have used the travelling reference spectroradiometer QASUME (Quality Assurance of Spectral Ultraviolet Measurements in Europe) results from 27 visits to UV monitoring stations around Europe. We have used the QASUME instrument as relative reference, analyzing absolute differences and also temporal and spectral deviations of UV irraidances, that are used as basic input for the SSA retrieval. The results comparing the mean SSA derived by all instruments, measuring synchronous UV spectra, showed that 5 were within ± 0.02 difference from the SSA calculated from the QASUME instrument, while 17 were within ± 0.04, for the Solar zenith angle of 60 degrees. As for the uncertainty that has been calculated using the 2σ standard deviation of the spectral measurements, a mean 0.072 and 0.10 (2σ) uncertainties have been calculated for 60° and 30°, respectively. Based on the fact that additional uncertainties would be introduced in the SSA retrieval from AOD model input accuracy, assymetry parameter assumptions, we show that only very few instrumnents could be able to detect long term SSA changes. However, such measurements/results ar useful in order to retrieve SSA at UV wavelengths, a product needed for various applications such as, inputs for modeling radiative forcing studies and satellite retrieval algorithms.
Optical and geometrical characteristics of cirrus clouds over a Southern European lidar station
E. Giannakaki, D. S. Balis, V. Amiridis,S. Kazadzis
Atmospheric Chemistry and Physics (ACP) & Discussions (ACPD) , 2007,
Abstract: Optical and geometrical characteristics of cirrus clouds over Thessaloniki, Greece (40.6° N, 22.9° E) have been determined from the analysis of lidar and radiosonde measurements performed during the period from 2000 to 2006. Cirrus clouds are generally observed in a mid-altitude region ranging from 8.6 to 13 km, with mid-cloud temperatures in the range from 65° to 38°C. The cloud thickness generally ranges from 1 to 5 km and 38{%} of the cases studied have thickness between 2 and 3 km. The retrieval of optical depth and lidar ratio of cirrus clouds is performed using three different methods, taking into account multiple scattering effect. The mean optical depth is found to be 0.31±0.24 and the corresponding mean lidar ratio is 30±17 sr following the scheme of Klett-Fernald method. Sub-visual, thin and opaque cirrus clouds are observed at 3%, 57% and 40% of the measured cases, respectively. A comparison of the results obtained between the three methods shows good agreement. The multiple scattering errors of the measured effective extinction coefficients range from 20 to 60%, depending on cloud optical depth. The temperature and thickness dependencies on optical properties have also been studied in detail. A maximum mid-cloud depth of ~3.5 km is found at temperatures around ~ 47.5°C, while there is an indication that optical depth and mean extinction coefficient increases with increasing mid-cloud temperature. A correlation between optical depth and thickness was also found. However, no clear dependence of the lidar ratio values on the cloud temperature and thickness was found.
Optical properties of different aerosol types: seven years of combined Raman-elastic backscatter lidar measurements in Thessaloniki, Greece
E. Giannakaki, D. S. Balis, V. Amiridis,C. Zerefos
Atmospheric Measurement Techniques (AMT) & Discussions (AMTD) , 2010,
Abstract: We present our combined Raman/elastic backscatter lidar observations which were carried out at the EARLINET station of Thessaloniki, Greece, during the period 2001–2007. The largest optical depths are observed for Saharan dust and smoke aerosol particles. For local and continental polluted aerosols the measurements indicate high aerosol loads. However, measurements associated with the local path indicate enhanced aerosol load within the Planetary Boundary Layer. The lowest value of aerosol optical depth is observed for continental aerosols, from West directions with less free tropospheric contribution. The largest lidar ratios, of the order of 70 sr, are found for biomass burning aerosols. A significant and distinct correlation between lidar ratio and backscatter related ngstr m exponent values were estimated for different aerosol categories. Scatter plot between lidar ratio values and ngstr m exponent values for local and continental polluted aerosols does not show a significant correlation, with a large variation in both parameters possibly due to variable absorption characteristics of these aerosols. Finally for continental aerosols with west and northwest directions that follow downward movement when arriving at our site constantly low lidar ratios almost independent of size are found.
Multi-wavelength Raman lidar, sunphotometric and aircraft measurements in combination with inversion models for the estimation of the aerosol optical and physico-chemical properties over Athens, Greece
R. E. Mamouri,A. Papayannis,V. Amiridis,D. Müller
Atmospheric Measurement Techniques Discussions , 2012, DOI: 10.5194/amtd-5-589-2012
Abstract: A novel procedure has been developed to retrieve, simultaneously, the optical, microphysical and chemical properties of tropospheric aerosols with a multi-wavelength Raman lidar system in the troposphere over an urban site (Athens, Greece: 37.9° N, 23.6° E, 200 m a.s.l.) using data obtained during the European Space Agency (ESA) THERMOPOLIS project which took place between 15–31 July 2009 over the Greater Athens Area (GAA). We selected to apply our procedure for a case study of intense aerosol layers occurred on 20–21 July 2009. The National Technical University of Athens (NTUA) EOLE 6-wavelength Raman lidar system has been used to provide the vertical profiles of the optical properties of aerosols (extinction and backscatter coefficients, lidar ratio) and the water vapor mixing ratio. An inversion algorithm was used to derive the mean aerosol microphysical properties (mean effective radius – reff), single-scattering albedo (ω) and mean complex refractive index (m) at selected heights in the 2–3 km height region. We found that reff was 0.3–0.4 μm, ω at 532 nm ranged from 0.63 to 0.88 and m ranged from 1.45 + 0.015i to 1.56 + 0.05i, in good accordance with in situ aircraft measurements. The final data set of the aerosol microphysical properties along with the water vapor and temperature profiles were incorporated into the ISORROPIA model to infer an in situ aerosol composition consistent with the retrieved m and ω values. The retrieved aerosol chemical composition in the 2–3 km height region gave a variable range of sulfate (0–60%) and organic carbon (OC) content (0–50%), although the OC content increased (up to 50%) and the sulfate content dropped (up to 30%) around 3 km height; in connection with the retrieved low ω value (0.63), indicates the presence of absorbing biomass burning smoke mixed with urban haze. Finally, the retrieved aerosol microphysical properties were compared with column-integrated sunphotometer data.
Optical properties of different aerosol types: seven years of combined Raman- elastic backscatter lidar measurements in Thessaloniki, Greece
E. Giannakaki,D. S. Balis,V. Amiridis,C. Zerefos
Atmospheric Measurement Techniques Discussions , 2009,
Abstract: We present our combined Raman/elastic backscatter lidar observations which were carried out at the EARLINET station of Thessaloniki, Greece, during the period 2001–2007. The largest optical depths are observed for Saharan dust and smoke aerosol loads. For "local" and "continental polluted" aerosols the measurements indicate moderate aerosol loads. However, measurements associated with the "local" path show lower values of free tropospheric contribution (37% versus 46% for "continental polluted") and thus, enhanced aerosol load within the Planetary Boundary Layer. The lowest value of aerosol optical depth is observed for "continental clean" aerosols. The largest lidar ratios, of the order of 70 sr are found for biomass burning aerosols. A significant and distinct correlation between lidar ratio and backscatter related ngstr m exponent values was estimated for well defined aerosol categories, which provides a statistical measure of the lidar ratio's dependency on aerosol-size, which is a useful tool for elastic lidar systems. Scatter plot between lidar ratio values and ngstr m exponent values for "local" and "continental polluted" aerosols does not show a significant correlation, with a large variation in both parameters possibly due to variable absorption characteristics of these aerosols. Finally for "clean continental" aerosols we found constantly low lidar ratios almost independent of size.
Validation of CALIPSO space-borne-derived aerosol vertical structures using a ground-based lidar in Athens, Greece
R. E. Mamouri,V. Amiridis,A. Papayannis,E. Giannakaki
Atmospheric Measurement Techniques Discussions , 2009,
Abstract: We present initial aerosol validation results of the space-borne lidar CALIOP retrievals -onboard the CALIPSO satellite-, using coincident observations performed with a ground-based lidar in Athens, Greece (37.9° N, 23.6° E). A multi-wavelength ground-based backscatter/Raman lidar system is operating since 2000 at the National Technical University of Athens (NTUA) in the framework of the European Aerosol Research LIdar NETwork (EARLINET), the first lidar network for tropospheric aerosol studies on a continental scale. Since July 2006, a total of 40 coincidental aerosol ground-based lidar measurements were performed over Athens during CALIPSO overpasses. The duration of the ground-based lidar measurements was approximately two hours, centred on the satellite overpass time. From the statistical analysis of the ground-based/satellite correlative lidar measurements, a mean bias of the order of 22% for daytime measurements and of 8% for nighttime measurements with respect to the CALIPSO profiles was found for altitudes between 3 and 10 km. The mean bias becomes much larger for altitudes lower that 3 km (of the order of 60%) which is attributed to the decrease of the CALIOP signal-to-noise ratio, as well as to the incomplete overlap height region of the ground based lidar and finally to the distance between the two instruments, resulting to the observation of possibly different air masses. In cases of aerosols layers underlying cirrus clouds, comparison results for aerosol tropospheric profiles become worst, illustrating the limitations of space-borne downward-looking lidar measurements due to strong signal attenuations.
Optical characteristics of biomass burning aerosols over Southeastern Europe determined from UV-Raman lidar measurements
V. Amiridis,D. S. Balis,E. Giannakaki,A. Stohl
Atmospheric Chemistry and Physics Discussions , 2008,
Abstract: The influence of smoke on the aerosol loading in the free troposphere over Thessaloniki, Greece is examined in this paper. Ten cases during 2001–2005 were identified when very high aerosol optical depth values in the free troposphere were observed with a UV-Raman lidar. Particle dispersion modeling (FLEXPART) and satellite hot spot fire detection (ATSR) showed that these high free tropospheric aerosol optical depths are mainly attributed to the advection of smoke plumes from biomass burning regions over Thessaloniki. The biomass burning regions were found to extend across Russia in the latitudinal belt between 45° N–55° N, as well as in Eastern Europe (Baltic countries, Western Russia, Belarus, and the Ukraine). The highest frequency of agricultural fires occurred during the summer season (mainly in August). The data collected allowed the optical characterization of the smoke aerosols that arrived over Greece, where limited information has so far been available. Two-wavelength backscatter lidar measurements showed that the backscatter-related ngstr m exponent ranged between 0.5 and 2.4 indicating a variety of particle sizes. UV-Raman lidar measurements showed that for smoke particles the extinction to backscatter ratios varied between 40 sr for small particles to 100 sr for large particles. Dispersion model estimations of the carbon monoxide tracer concentration profiles for smoke particles indicate that the variability of the optical parameters is a function of the age of the smoke plumes.
Inter-comparison of lidar and ceilometer retrievals for aerosol and Planetary Boundary Layer profiling over Athens, Greece
G. Tsaknakis,A. Papayannis,P. Kokkalis,V. Amiridis
Atmospheric Measurement Techniques Discussions , 2011, DOI: 10.5194/amtd-4-73-2011
Abstract: This study presents an inter-comparison of two active remote sensors (lidar and ceilometer) in determining the structure of the Planetary Boundary Layer (PBL) and in retrieving tropospheric aerosol vertical profiles over Athens, Greece. This inter-comparison was performed under various strongly different aerosol concentrations (urban air pollution, biomass burning and Saharan dust event), implementing two different lidar systems (one portable Raymetrics S.A. lidar system running at 355 nm and one multi-wavelength Raman lidar system running at 355 nm, 532 nm and 1064 nm) and one CL31 Vaisala S.A. ceilometer (running at 910 nm). To convert the ceilometer data to data having the same wavelengths as those from the lidar, the backscatter-related ngstr m exponent was estimated using ultraviolet multi-filter radiometer (UV-MFR) data. The inter-comparison was based on two parameters: the mixing layer structure and height determined by the presence of the suspended aerosols and the aerosol backscatter coefficient. Additionally, radiosonde data were used to derive the PBL height. In general a good agreement is found between the ceilometer and the lidar techniques in both inter-compared parameters in the height range from 500 m to 5000 m, while the limitations of each instrument are also examined.
Optical and geometrical characteristics of cirrus clouds over a mid-latitude lidar station
E. Giannakaki,D. S. Balis,V. Amiridis,S. Kazadzis
Atmospheric Chemistry and Physics Discussions , 2007,
Abstract: Optical and geometrical characteristics of cirrus clouds over Thessaloniki, Greece (40.6°, 22.9°) have been determined from the analysis of lidar and radiosonde measurements performed during the period from 2000 to 2006. Cirrus clouds are generally observed in a mid altitude region ranging from 7 to 12 km, with mid-cloud temperatures in the range from –65° to –25°C. A seasonality of cirrus geometrical and temperature properties is found. The cloud thickness ranges from 0.85 to 5 km and 37% of our cases have thickness between 2 and 3 km. The retrieval of cloud's optical depth and lidar ratio is performed using three different methods, taking into account multiple scattering effects. The mean optical depth is found to be 0.3±0.24 and the corresponding mean lidar ratio is 28±17 sr. Sub-visual, thin and opaque cirrus clouds are observed at 7.5%, 51% and 42.5% of the measured cases respectively. The multiple scattering errors of the measured effective extinction coefficients range from 20% to 60% depending on cloud optical depth. A comparison of the results between the three methods shows good agreement. In addition we present the advantages and limitations of each method applied. The temperature and thickness dependencies on optical properties have also been studied in detail. A maximum mid-cloud depth of ~3 km is found at temperatures around ~–45°C while there is an indication that optical depth increases with increasing thickness and mid-cloud temperature. No clear dependence of the lidar ratio values on the cloud temperature and thickness was found.
Aerosol lidar observations and model calculations of the planetary boundary layer evolution over Greece, during the March 2006 total solar eclipse
V. Amiridis,D. Melas,D. S. Balis,A. Papayannis
Atmospheric Chemistry and Physics Discussions , 2007,
Abstract: An investigation of the Planetary Boundary Layer (PBL) height evolution over Greece, during the solar eclipse of 29 March 2006, is presented. Ground based observations were carried out using lidar detection and ranging devices (Lidars) and ground meteorological instruments, to estimate the height of the Mixing Layer (ML) before, during and after the solar eclipse in Northern and Southern parts of Greece exhibiting different sun obscuration. Data demonstrate that the solar eclipse has induced a decrease of the PBL height, indicating a suppression of turbulence activity similar to that during the sunset hours. The changes in PBL height were associated with a very shallow entrainment zone, indicating a significant weakening of the penetrative convection. Heat transfer was confined to a thinner layer above ground. The thickness of the entrainment zone exhibited its minimum during the maximum of the eclipse, demonstrative of turbulence mechanisms suppression at that time. Model estimations of the PBL evolution were additionally conducted using the Comprehensive Air Quality Model with extensions (CAMx) coupled with the Weather Research and Forecasting model (WRF). Model diagnosed PBL height decrease during the solar eclipse due to vertical transport decay, in agreement with the experimental findings; vertical profiles of atmospheric particles and gaseous species showed an important vertical mixing attenuation.
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