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Multi-wavelength Raman lidar, sun photometric 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, P. Kokkalis, S. Rapsomanikis, E. T. Karageorgos, G. Tsaknakis, A. Nenes, S. Kazadzis,E. Remoundaki
Atmospheric Measurement Techniques (AMT) & Discussions (AMTD) , 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 that 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 0.14–0.4 (±0.14) μm, ω was 0.63–0.88 (±0.08) (at 532 nm) and m ranged from 1.44 (±0.10) + 0.01 (±0.01)i to 1.55 (±0.12) + 0.06 (±0.02)i, in good agreement (only for the reff values) with in situ aircraft measurements. The water vapor and temperature profiles were incorporated into the ISORROPIA II model to propose a possible 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; 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 sun photometer CIMEL data.
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, H. D. Kambezidis, R. E. Mamouri, G. Georgoussis,G. Avdikos
Atmospheric Measurement Techniques (AMT) & Discussions (AMTD) , 2011,
Abstract: This study presents an inter-comparison of two active remote sensors (lidar and ceilometer) to determine the mixing layer height and structure of the Planetary Boundary Layer (PBL) and to retrieve tropospheric aerosol vertical profiles over Athens, Greece. This inter-comparison was performed under various strongly different aerosol loads/types (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). Spectral conversions of the ceilometer's data were performed using the ngstr m exponent estimated by ultraviolet multi-filter radiometer (UV-MFR) measurements. The inter-comparison was based on two parameters: the mixing layer height determined by the presence of the suspended aerosols and the attenuated backscatter coefficient. Additionally, radiosonde data were used to derive the PBL height. In general, a good agreement was 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.
Systematic lidar observations of Saharan dust layers over Athens, Greece in the frame of EARLINET project (2004–2006)
A. Papayannis, R. E. Mamouri, V. Amiridis, S. Kazadzis, C. Pérez, G. Tsaknakis, P. Kokkalis,J. M. Baldasano
Annales Geophysicae (ANGEO) , 2009,
Abstract: In this paper we present a statistical analysis on the geometrical and optical properties of Saharan dust layers observed over Athens, Greece, in a three-year period from 1 January 2004 up to 31 December 2006. The observations of the vertical aerosol profile were performed by the multi-wavelength (355-532-1064-387-607 nm) Raman lidar system of the National Technical University of Athens (NTUA) operated in the city of Athens (37°98' N, 23°77' E), Greece, in the frame of the European Aerosol Research Lidar Network (EARLINET-ASOS) project. The number of dust events was greatest in late spring, summer, and early autumn periods. This was evident also by aerosol observations during dust outbreaks obtained from the Moderate Resolution Imaging Spectroradiometer (MODIS). In our lidar measurements, multiple aerosol dust layers of variable thickness (680–4800 m) were observed. The center of mass of these layers was located in altitudes between 1600 and 5800 m. However, the mean thickness of the dust layer typically stayed around 2700 m and the corresponding mean center of mass was of the order of 2900 m. The top of the dust layer ranged from 2000 to 8000 m, with a mean value of the order of 4700 m. MODIS observations during dust outbreaks showed that the AOD values at 550 nm ranged between 0.3–0.6, while the corresponding Angstr m exponent (AE) values were of the order of 0.5–0.65, indicating the presence of rather large particles.
Inter-comparison of lidar and ceilometer retrievals for aerosol and Planetary Boundary Layer profiling over Athens, Greece  [PDF]
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.
Validation of CALIPSO space-borne-derived aerosol vertical structures using a ground-based lidar in Athens, Greece  [PDF]
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.
Validation of CALIPSO space-borne-derived attenuated backscatter coefficient profiles using a ground-based lidar in Athens, Greece
R. E. Mamouri, V. Amiridis, A. Papayannis, E. Giannakaki, G. Tsaknakis,D. S. Balis
Atmospheric Measurement Techniques (AMT) & Discussions (AMTD) , 2009,
Abstract: We present initial aerosol validation results of the space-borne lidar CALIOP -onboard the CALIPSO satellite- Level 1 attenuated backscatter coefficient profiles, 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 ground-based measurements were performed each time CALIPSO overpasses the station location within a maximum distance of 100 km. The duration of the ground–based lidar measurements was approximately two hours, centred on the satellite overpass time. From the 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 increase of aerosol horizontal inhomogeneity within the Planetary Boundary Layer, resulting to the observation of possibly different air masses by the two instruments. In cases of aerosol layers underlying Cirrus clouds, comparison results for aerosol tropospheric profiles become worse. This is attributed to the significant multiple scattering effects in Cirrus clouds experienced by CALIPSO which result in an attenuation which is less than that measured by the ground-based lidar.
Optical, microphysical, mass and geometrical properties of aged volcanic particles observed over Athens, Greece, during the Eyjafjallaj kull eruption in April 2010 through synergy of Raman lidar and sunphotometer measurements  [PDF]
P. Kokkalis,A. Papayannis,V. Amiridis,R. E. Mamouri
Atmospheric Chemistry and Physics Discussions , 2013, DOI: 10.5194/acpd-13-5315-2013
Abstract: Vertical profiles of the optical (extinction and backscatter coefficients, lidar ratio and ngstr m exponent), microphysical (mean effective radius, mean refractive index, mean number concentration) and geometrical properties, as well as of the mass concentration of volcanic particles from the Eyjafjallaj kull eruption were retrieved at selected heights over Athens, Greece using a multi-wavelength Raman lidar system and inversion models, during 21–24 April 2010. Additionally, Aerosol Robotic Network (AERONET) particulate columnar measurements indicated the presence of volcanic particles over our area. Simulations of the volcanic partilcles dispersion, done by the FLEXPART model, confirmed the presence of these particles over Athens. Our lidar data showed volcanic particles layers, in the form of filaments after 7-day transport from the source (approximately 4000 km away from our site) between from ground levels up to nearly 10 km. Over Athens the volcanic particles layers were found to be mixed with locally produced aerosols, inside the Planetary Boundary Layer (PBL). Mean hourly-averaged lidar signals indicated that the layer thickness of volcanic particles, ranged between 1.5 and 2.2 km. The corresponding aerosol optical depth (AOD) found to vary from 0.014 to 0.184 at 355 nm and from 0.017 up to 0.174 at 532 nm. Furthermore, the corresponding lidar ratios (LR) ranged between 59.7–79.6 sr (at 355 nm) and 43.9–88.3 sr (at 532 nm). Additionally, we calculated that the mean effective radius of the volcanic particles was 0.13–0.38 μm, while their refractive index ranged from 1.39+0.009i to 1.48+0.006i. Finally, our data also allowed us to quantitatively compare, for the first time, the volcanic ash concentrations simulated by FLEXPART with those calculated by the inversion code LIRIC, using data sets derived from coincident lidar-AERONET measurements. In general, good agreement was found between simulations and observations, concerning not only the geometrical properties of the volcanic particles layers, but also the particles mass concentration, with a correlation coefficient of the order of 0.75.
Study of the rain intensity in Athens and Thessaloniki, Greece
C. M. Philandras, P. T. Nastos, A. G. Paliatsos,C. C. Repapis
Advances in Geosciences (ADGEO) , 2010,
Abstract: In this study, the mean rain intensity (mm/h) in Athens and Thessaloniki, Greece, is examined during the period 1930–2007. The daily meteorological data were acquired from the meteorological stations of the National Observatory of Athens (Thissio) and the University Campus of Thessaloniki. These stations have recorded the longest and reliable daily rain time series in Greece. The trends of the annual and seasonal mean, as well as the maximum rain intensity, are analyzed. On the one hand, all of the time series in Athens present higher figures of mean rain intensity during the period 1990–2007 compared to the period 1930–1990. On the other hand, regarding the time series in Thessaloniki, the mean rain intensity remains in lower level than the respective one in Athens without significant trend at the Confidence Level (CL) 95%. As far as the annual and seasonal maximum rain intensity is concerned, similar patterns appear. However, the summer maximum rain intensity in Thessaloniki remains at higher levels till 1970's. The atmospheric circulation patterns for the extreme rain intensities show intense negative anomalies centred over Greece and positive anomalies to the north, with centre over Scandinavia.
The potential of the synergistic use of passive and active remote sensing measurements for the validation of a regional dust model
V. Amiridis, M. Kafatos, C. Perez, S. Kazadzis, E. Gerasopoulos, R. E. Mamouri, A. Papayannis, P. Kokkalis, E. Giannakaki, S. Basart, I. Daglis,C. Zerefos
Annales Geophysicae (ANGEO) , 2009,
Abstract: A long-lasting Saharan dust event affected Europe on 18–23 May 2008. Dust was present in the free troposphere over Greece, in height ranges between the surface and approximately 4–5 km above sea level. The event was monitored by ground-based CIMEL sunphotometric and multi-wavelength combined backscatter/Raman lidar measurements over Athens, Greece. The dust event had the maximum of its intensity on 20 May. Three-dimensional dust spatial distribution over Greece on that day is presented through satellite synergy of passive and active remote sensing using MODIS and CALIPSO data, respectively. For the period under study, the ground-based measurements are used to characterize the dust event and evaluate the latest version of the BSC Dust Regional Atmospheric Modeling (BSC-DREAM) system. Comparisons of modeled and measured aerosol optical depths over Athens show that the Saharan dust outbreak is fairly well captured by BSC-DREAM simulations. Evaluation of BSC-DREAM using Raman lidar measurements on 20 May shows that the model consistently reproduces the dust vertical distribution over Athens.
Impact of solar activity on climate changes in Athens region, Greece  [PDF]
Nectaria A. B. Gizani,Kimon Papathanasopoylos,Leonidas Vatikiotis,Efthimios Zervas
Physics , 2011,
Abstract: The scope of this work is to study the role that the solar weather plays in terrestrial weather. For this reason we study the effect of the solar activity on the climate changes in Greece. In the current work we look for possible correlation between the solar activity data spanning the years from 1975 to 2000 and the meteorological data from two weather stations based inside the city of Athens, Greece (New Philadelphia) and in greater Athens in the north of Attica (Tatoi area). We examine the annual variations of the average values of six meteorological parameters: temperature, atmospheric pressure, direction and intensity of wind, rainfall and relative air humidity. The solar data include decade variations, within the above period, of the solar irradiance, mean sunspot number between two solar cycles, magnetic cycle influence, and solar UV driving of climate (radio flux).
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