G. Tsaknakis

A study on the atmospheric concentrations of primary and secondary air pollutants in the Athens basin performed by DOAS and DIAL measuring techniques

In this work an analysis of continuous Differential Optical Absorption Spectroscopy (DOAS) measurements of primary and secondary air pollutants (SO(2), NO(2) and O(3)) in the Athens basin is performed combined with Differential Absorption Lidar (DIAL) vertical ozone measurements obtained inside the Planetary Boundary Layer (PBL) and the lower free troposphere. The measurements took place during the period May 2005-February 2007, at the National Technical University of Athens Campus (200 m above sea level (asl.), 37.96 °N, 23.78 °E). The SO(2) and NO(2) DOAS measurements showed maximum 1-hour mean values (around 20 μg/m(3) and 74 μg/m(3), respectively) in winter and did not exceed the current European Union (EU) air quality standards (European Council Directive 2008/50/EC), in contrast to ozone, which shows its maximum (around 128 μg/m(3)) in summer and frequently exceeds the EU standard for human health protection (120 μg/m(3)). If the measurements are classified according to the two most frequent flow-patterns of the air masses in the Athens basin (northern-southern circulation), it is observed that in general the atmospheric concentrations of all measured pollutants including ozone are higher when the southern circulation occurs, in comparison to the corresponding values under the northern circulation. The vertical ozone profiles obtained by DIAL were also higher under the southern circulation. During the summer months a mean difference (between the southern-northern circulations) of the order of 15-20 μg/m(3), maximized at the 0.9-1.1 km and 1.7-1.8 km height, was observed within the PBL. It was also observed that the summer surface ozone levels remained relatively high (around 80-110 μg/m(3)) even during strong northerly winds, verifying the high levels of rural surface ozone in the surrounding area reported by previous studies.

Optical, size and mass properties of mixed type aerosols in Greece and Romania as observed by synergy of lidar and sunphotometers in combination with model simulations: A case study

A coordinated experimental campaign aiming to study the aerosol optical, size and mass properties was organized in September 2012, in selected sites in Greece and Romania. It was based on the synergy of lidar and sunphotometers. In this paper we focus on a specific campaign period (23-24 September), where mixed type aerosols (Saharan dust, biomass burning and continental) were confined from the Planetary Boundary Layer (PBL) up to 4-4.5 km height. Hourly mean linear depolarization and lidar ratio values were measured inside the dust layers, ranging from 13 to 29 and from 44 to 65sr, respectively, depending on their mixing status and the corresponding air mass pathways over Greece and Romania. During this event the columnar Aerosol Optical Depth (AOD) values ranged from 0.13 to 0.26 at 532 nm. The Lidar/Radiometer Inversion Code (LIRIC) and the Polarization Lidar Photometer Networking (POLIPHON) codes were used and inter-compared with regards to the retrieved aerosol (fine and coarse spherical/spheroid) mass concentrations, showing that LIRIC generally overestimates the aerosol mass concentrations, in the case of spherical particles. For non-spherical particles the difference in the retrieved mass concentration profiles from these two codes remained smaller than ±20%. POLIPHON retrievals showed that the non-spherical particles reached concentrations of the order of 100-140 μg/m(3) over Romania compared to 50-75 μg/m(3) over Greece. Finally, the Dust Regional Atmospheric Model (DREAM) model was used to simulate the dust concentrations over the South-Eastern Europe.

EARLINET observations of the Eyjafjallajökull ash plume over Europe

EARLINET, the European Aerosol Research Lidar NETwork, established in 2000, is the first coordinated lidar network for tropospheric aerosol study on the continental scale. The network activity is based on scheduled measurements, a rigorous quality assurance program addressing both instruments and evaluation algorithms, and a standardised data exchange format. At present, the network includes 27 lidar stations distributed over Europe. EARLINET performed almost continuous measurements since 15 April 2010 in order to follow the evolution of the volcanic plume generated from the eruption of the Eyjafjallajökull volcano, providing the 4-dimensional distribution of the volcanic ash plume over Europe. During the 15-30 April period, volcanic particles were detected over Central Europe over a wide range of altitudes, from 10 km down to the local planetary boundary layer (PBL). Until 19 April, the volcanic plume transport toward South Europe was nearly completely blocked by the Alps. After 19 April volcanic particles were transported to the south and the southeast of Europe. Descending aerosol layers were typically observed all over Europe and intrusion of particles into the PBL was observed at almost each lidar site that was affected by the volcanic plume. A second event was observed over Portugal and Spain (6 May) and then over Italy on 9 May 2010. The volcanic plume was then observed again over Southern Germany on 11 May 2010.

One-year observations of the vertical structure of Saharan dust over Athens, Greece monitored by NTUA's lidar system in the frame of EARLINET

Suspended aerosol particles play a significant role in the Global Change issue. Every year huge quantities of Saharan dust are transported to the European continent and North Atlantic Sea. Since May 1, 2000, an elastic backscatter lidar station is in operation in Athens, Greece, in the frame of the European LIDAR Network (EARLINET), aiming at the monitoring of extreme aerosol events over the Eastern Mediterranean Sea. During the first year of operation 20 cases of Saharan dust transport to the East Mediterranean Sea (EMS), were successfully observed. The lidar data are correlated with meteorological and satellite observations, including the aerosol index (AI data from the Total Ozone Mapping Spectrometer (TOMS) and the SeaWiFS satellite images. The analysis of the data collected so far, made possible a first statistical approach of the vertical and horizontal extent and of the seasonal variation of free tropospheric Saharan dust layers over the EMS area. The main results of this work are: (1) multiple dust layers of variable thickness (0.5 - 4 km) are systematically observed in the altitude region 2 - 6 km, (2) Saharan dust outbreaks take place all year round, with more pronounced frequencies during the summer period and can persist for a few days (1- 5), (3) 24 - 48 hours are usually needed for the Saharan air masses to reach the EMS area, and finally (4) the TOMS AI values can reach 1.5 - 3.5 over Greece. These lidar data gave the aerosol vertical structure over the Eastern Mediterranean Sea during Saharan dust outbreaks and can be a direct input for global radiative transfer models.

Spectroscopic study of Ce- and Cr-doped LiSrAlF6 crystals

Undoped, as well as Ce3+ and Cr3+ doped LiSrAlF6 (LiSAF) crystals have been examined by Raman spectroscopy in order to identify the system’s molecular vibrations and the role of doping. Furthermore the electron spin resonance (ESR) spectroscopic technique has been used to study the distortions of the local crystalline environment of Ce. The analysis of the main ESR resonance shows that the Ce3+ ions substitute for Sr2+ ones in the structure and permit the description of the electronic eigenfunctions and energy splitting of its 2F5/2 ground state. We have tackled the distortion of the trigonal crystalline environment, induced by Na+ codopants acting as charge compensators to Ce3+, and we have assigned the ESR resonances to Na occupying sites at various distances from the Ce atoms. Our results indicate growth of high quality crystals.

An ESR study of a Ce3+:Na+:LiSrAlF6 single crystal

The crystal field environment of the Ce+3 dopant in LiSrAlF6 (LiSAF) crystals has been studied by the Electron Spin Resonance (ESR) spectroscopic technique. Ce was found in a highly anisotropic trigonal environment which suggests that it substitutes for the Sr site. Na+ ions operate as charge compensators to Ce+3 possessing nearby to Ce, Sr sites. The ground state electronic wavefunction and the energy diagram of Ce are discussed in view of our ESR results.

Compact mobile lidar system based on the LabVIEW code: applications in urban air pollution monitoring in Athens, Greece

The LIDAR technique is an efficient tool for continuous monitoring of air pollution over urban areas, with high temporal and range resolution. The urban areas of Athens, Greece, exhibit high air pollution levels, especially those regarding suspended particulates, mainly linked with car traffic and industrial emissions. In this paper, we present the first mobile Greek LIDAR system, based on the LabVIEW code, now located at the Athens Technical University Campus, nearby the urban area of the city. The LIDAR dataset acquired, under various air pollution and meteorological conditions, gives specific indications of the diurnal variation of the backscattering coefficient and relative backscatter of the suspended particulates in the first 2500 - 3000 m ASL over the city of Athens. The LIDAR dataset acquired is analyzed in conjunction with meteorological data (temperature, humidity) and air pollution data (O3 CO, NOx), acquired at the same site, and conclusions are drawn.

Forest Fire Aerosols: Vertically Resolved Optical and Microphysical Properties and Mass Concentration from Lidar Observations

The influence of smoke on the aerosol loading in the free troposphere from EARLINET observations are examined in this paper. Several cases during 2001–2011 were identified over Thessaloniki and Athens, Greece, 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. The biomass burning regions were found to extend across Russia in the latitudinal belt between 45°N and 55°N, as well as in Eastern Europe. The highest frequency of agricultural fires occurred during the summer season (mainly in August). Emphasis is also given on the 2007 wild fires surrounding Athens and earlier studies performed in the frame of EARLINET. The data collected allowed the optical and microphysical characterization of the smoke aerosols that arrived over Greece, where limited information has so far been available and in synergy with AERΟNET and CALIPSO observation a first attempt is made for the vertically resolved mass concentration of the smoke plumes.

Air-pollution monitoring and atmospheric parameter profiling over Athens, Greece, using a compact mobile laser remote sensing system

In this paper we present the basic features of the laser remote sensing (lidar) system located at the National Technical University of Athens (NTUA), Greece. This compact and mobile lidar is in full operation since late 1999. This paper includes basic features of the lidar technique (technical description, temporal and spatial resolution, range etc.), a brief presentation of the instrumentation used and an outline of the algorithms applied to retrieve useful information from the backscattered lidar signals. We also present some examples of aerosol profiling measurements over Athens, which include monitoring of clouds and suspended aerosols (i.e. during Saharan dust and photochemical air pollution events), as well as the evolution of the Planetary Boundary Layer (PBL) over the Greater Athens Area (GAA).

Preliminary measurements of cloud properties over Athens, Greece, using a compact mobile LIDAR system

The Lidar technique is an efficient tool for continuous monitoring of clouds and aerosols, with high temporal and spatial resolution. Lidar systems can provide long-term accurate information on cloud top and base heights and their optical depth. The spatial distribution of clouds and the diurnal variation of cloud properties are important parameters in many operational and research applications (i.e. radiative transfer modeling, energy balance, meteorology, etc.). In this paper, we present the application of a compact mobile lidar system in monitoring of cloud properties, over Athens, Greece. Preliminary lidar measurements were performed during summer- autumn 1994, focusing on the study of short-time variability of spectral cloud properties at two wavelengths (355 nm and 532 nm), using a compact Nd:YAG laser. The Lidar dataset acquired is analyzed during selected cases and conclusions are drawn.