Dimitris Balis

EARLINET correlative measurements for CALIPSO: First intercomparison results

A strategy for European Aerosol Research Lidar Network (EARLINET) correlative measurements for Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) has been developed. These EARLINET correlative measurements started in June 2006 and are still in progress. Up to now, more than 4500 correlative files are available in the EARLINET database. Independent extinction and backscatter measurements carried out at high-performance EARLINET stations have been used for a quantitative comparison with CALIPSO level 1 data. Results demonstrate the good performance of CALIPSO and the absence of evident biases in the CALIPSO raw signals. The agreement is also good for the distribution of the differences for the attenuated backscatter at 532 nm ((CALIPSO-EARLINET)/EARLINET (%)), calculated in the 1–10 km altitude range, with a mean relative difference of 4.6%, a standard deviation of 50%, and a median value of 0.6%. A major Saharan dust outbreak lasting from 26 to 31 May 2008 has been used as a case study for showing first results in terms of comparison with CALIPSO level 2 data. A statistical analysis of dust properties, in terms of intensive optical properties (lidar ratios, Angstrom exponents, and color ratios), has been performed for this observational period. We obtained typical lidar ratios of the dust event of 49 ± 10 sr and 56 ± 7 sr at 355 and 532 nm, respectively. The extinction-related and backscatter-related Angstrom exponents were on the order of 0.15–0.17, which corresponds to respective color ratios of 0.91–0.95. This dust event has been used to show the methodology used for the investigation of spatial and temporal representativeness of measurements with polar-orbiting satellites.

Characterization of the vertical structure of Saharan dust export to the Mediterranean basin

We present the results of our investigations into the vertical structure of several North African dust plumes exported to the Mediterranean in 1997. Two backscatter lidar systems were operated in the western and eastern parts of the Mediterranean basin during dust events identified using Meteosat visible images. Dust transport soundings have shown that dust particles are trapped and transported inside well-defined layers in the free troposphere. In general, the dust transport appeared to be multilayered, with several distinct layers at different altitudes between 1.5 and 5 km. The analysis of Meteosat IR images, the Total Ozone Mapping Spectrometer aerosol index, and back-trajectories clearly shows that these layers have different origins in Africa. Finally, in addition to the free troposphere transport, the presence of dust particles inside the planetary boundary layer has been assessed and quantified for two particular events with aerosol optical thickness of 0.3–0.4. using simultaneous lidar and Sun photometer measurements. In one case only, significant dust load (dust optical thickness of ∼0.1) occurred in the boundary layer.

Aerosol lidar intercomparison in the framework of the EARLINET project. 1.Instruments

In the framework of the European Aerosol Research Lidar Network to Establish an Aerosol Climatology (EARLINET), 19 aerosol lidar systems from 11 European countries were compared. Aerosol extinction or backscatter coefficient profiles were measured by at least two systems for each comparison. Aerosol extinction coefficients were derived from Raman lidar measurements in the UV (351 or 355 nm), and aerosol backscatter profiles were calculated from pure elastic backscatter measurements at 351 or 355, 532, or 1064 nm. The results were compared for height ranges with high and low aerosol content. Some systems were additionally compared with sunphotometers and starphotometers. Predefined maximum deviations were used for quality control of the results. Lidar systems with results outside those limits could not meet the quality assurance criterion. The algorithms for deriving aerosol backscatter profiles from elastic lidar measurements were tested separately, and the results are described in Part 2 of this series of papers [Appl. Opt.43, 977–989 (2004)]. In the end, all systems were quality assured, although some had to be modified to improve their performance. Typical deviations between aerosol backscatter profiles were 10% in the planetary boundary layer and 0.1 × 10-6 m-1 sr-1 in the free troposphere.

Ten years of GOME/ERS-2 total ozone data- : The new GOME data processor (GDP) version 4: 1. Algorithm description

The Global Ozone Monitoring Instrument (GOME) was launched on European Space Agencys ERS-2 platform in April 1995. The GOME data processor (GDP) operational retrieval algorithm has generated total ozone columns since July 1995. In 2004 the GDP system was given a major upgrade to version 4.0, a new validation was performed, and the 10-year GOME level 1 data record was reprocessed. In two papers, we describe the GDP 4.0 retrieval algorithm and present an error budget and sensitivity analysis (paper 1) and validation of the GDP total ozone product and the overall accuracy of the entire GOME ozone record (paper 2). GDP 4.0 uses an optimized differential optical absorption spectroscopy (DOAS) algorithm, with air mass factor (AMF) conversions calculated using the radiative transfer code linearized discrete ordinate radiative transfer (LIDORT). AMF computation is based on the TOMS version 8 ozone profile climatology, classified by total column, and AMFs are adjusted iteratively to reflect the DOAS slant column result. GDP 4.0 has improved wavelength calibration and reference spectra and includes a new molecular Ring correction to deal with distortion of ozone absorption features due to inelastic rotational Raman scattering effects. Preprocessing for cloud parameter estimation in GDP 4.0 is done using two new cloud correction algorithms: OCRA and ROCINN. For clear and cloudy scenes the precision of the ozone column product is better than 2.4 and 3.3%, respectively, for solar zenith angles up to 80°. Comparisons with ground-based data are generally at the 1-1.5% level or better for all regions outside the poles.

Chemical Ozone Loss in the Arctic Winter 1994/95 as Determined by the Match Technique

The chemically induced ozone loss inside the Arctic vortex during the winter 1994/95 has been quantified by coordinated launches of over 1000 ozonesondes from 35 stations within the Match 94/95 campaign. Trajectory calculations, which allow diabatic heating or cooling, were used to trigger the balloon launches so that the ozone concentrations in a large number of air parcels are each measured twice a few days apart. The difference in ozone concentration is calculated for each pair and is interpreted as a change caused by chemistry. The data analysis has been carried out for January to March between 370 K and 600 K potential temperature. Ozone loss along these trajectories occurred exclusively during sunlit periods, and the periods of ozone loss coincided with, but slightly lagged, periods where stratospheric temperatures were low enough for polar stratospheric clouds to exist. Two clearly separated periods of ozone loss show up. Ozone loss rates first peaked in late January with a maximum value of 53 ppbv per day (1.6 % per day) at 475 K and faster losses higher up. Then, in mid-March ozone loss rates at 475 K reached 34 ppbv per day (1.3 % per day), faster losses were observed lower down and no ozone loss was found above 480 K during that period. The ozone loss in hypothetical air parcels with average diabetic descent rates has been integrated to give an accumulated loss through the winter. The most severe depletion of 2.0 ppmv (60 %) took place in air that was at 515 K on 1 January and at 450 K on 20 March. Vertical integration over the levels from 370 K to 600 K gives a column loss rate, which reached a maximum value of 2.7 Dobson Units per day in mid-March. The accumulated column loss between 1 January and 31 March was found to be 127 DU (∼36 %).

Variability of UV‐B at four stations in Europe

The variability of solar irradiance in the UV-B spectral region is studied at four stations operating well-calibrated spectroradiometers during the period 1991–96. It is confirmed that total ozone is the controlling factor in long-term changes of solar irradianee below 320 nm received at the ground. This result is supported by the similar long-term variability under clear sky and all sky conditions during the past five years. The study includes estimates of the amplitude of the annual cycle and estimates of long-term changes. An important result of this note is, that calculated changes in solar irradiance depend on the length of the observational period and that any speculations on its future changes should be treated always with caution. It is estimated that a 2.7% change per year in solar irradiance at 305 nm at Thessaloniki is a rate that could possibly continue in view of the expected continuation of the ozone decline resulting to an increase of 27% per decade, which is comparable to the amplitude of the annual cycle at that station.

Regional levels of ozone in the troposphere over eastern Mediterranean

During the last 5 years, information on the regional tropospheric ozone levels over the eastern Mediterranean has become available but was confined to measurements at ground level. Here available information is expanded with measurements at two more rural ground level sites spanning 8° latitude, ozonesonde ascents, lidar observations, ship cruises, and aircraft flights. During wintertime the ozone monthly values are 10-20 ppbv higher than values at other European sites, while during summertime the values are comparable. This results in regional ozone background levels in the area that are above the European Union (EU) 32 ppb/24 hours phytotoxicity limit during the entire year. Late spring lidar observations show that south and southwestern synoptic flows which are associated with Saharan dust events result in lower ozone above the planetary boundary layer (PBL) by 20-35 ppbv as compared to these during northerly flows, which transport air from continental Europe. These lidar observations along with ship measurements during July show that ozone is enhanced 1.5-2.2 times in the continental outflow, when compared to aged maritime air or air from the African continent. These results along with ozonesonde observations suggest that ozone abatement in the area is largely beyond the control of regional emissions and can be controlled only with emission reductions on a European scale. During September, measured background NO and NO 2 levels between 3 and 4.5 km above sea level ranged from 1 to 221 pptv and from 102 to 580 pptv, respectively. The troposphere contains around 40-70 Dobson units of ozone during summer and around 20-30 Dobson units during winter.

The GOME‐2 total column ozone product: Retrieval algorithm and ground‐based validation

The Global Ozone Monitoring Instrument (GOME-2) was launched on EUMESATs MetOp-A satellite in October 2006. This paper is concerned with the retrieval algorithm GOME Data Processor (GDP) version 4.4 used by the EUMETSAT Satellite Application Facility on Ozone and Atmospheric Chemistry Monitoring (O3M-SAF) for the operational generation of GOME-2 total ozone products. GDP 4.4 is the latest version of the GDP 4.0 algorithm, which is employed for the generation of official Level 2 total ozone and other trace gas products from GOME and SCIAMACHY. Here we focus on enhancements introduced in GDP 4.4: improved cloud retrieval algorithms including detection of Sun glint effects, a correction for intracloud ozone, better treatment of snow and ice conditions, accurate radiative transfer modeling for large viewing angles, and elimination of scan angle dependencies inherited from Level 1 radiances. Furthermore, the first global validation results for 3 years (2007–2009) of GOME-2/MetOp-A total ozone measurements using Brewer and Dobson measurements as references are presented. The GOME-2/MetOp-A total ozone data obtained with GDP 4.4 slightly underestimates ground-based ozone by about 0.5% to 1% over the middle latitudes of the Northern Hemisphere and slightly overestimates by around 0.5% over the middle latitudes in the Southern Hemisphere. Over high latitudes in the Northern Hemisphere, GOME-2 total ozone has almost no offset relative to Dobson readings, while over high latitudes in the Southern Hemisphere GOME-2 exhibits a small negative bias below 1%. For tropical latitudes, GOME-2 measures on average lower ozone by 0% to 2% compared to Dobson measurements.

Solar activity-total column ozone relationships : Observations and model studies with heterogeneous chemistry

In the present paper we examine the effect of long-term solar variability on total column ozone using the longest available ground-based (1964-1994) and satellite (1979-1992) zonally averaged total column ozone records. Numerical simulations with a two-dimensional (2-D) model, which incorporates heterogeneous chemistry, transport, and the representation of the long-term solar variability, are compared to the observations. Our analysis of the total column ozone records shows that the solar activity signal in total column ozone is better seen in the tropics and during periods with no volcanoes and no synergistic effects from El Nino-Southern Oscillation (ENSO) and quasibiennial oscillation (QBO). On decadal timescales the solar activity component in total column ozone is confirmed to have a relatively small amplitude (1-2% of the 1964-1994 mean). The slowly varying decadal solar activity component in total column ozone has an amplitude 3 to 5 times larger than that of the 27-day solar rotation oscillation as seen in total ozone. The observations confirm model calculations of a larger amplitude in total column ozone at solar maximum as compared to solar activity minimum conditions for the 27-day period. The heterogeneous 2-D chemical transport model results are in good agreement with the long-term ground-based observations. It is noted that in the tropics both the interannual variability of total column ozone and the long-term impact from chlorofluorocarbons (CFCs) through the action of polar stratospheric clouds (PSCs) are small and therefore the 1-2% solar activity signal in the data can be more clearly seen above the noise level at these lower latitudes.

Role of urban and suburban aerosols on solar UV radiation over Athens, Greece

The scope of this paper is to provide an experimental and modelling contribution on the role of suspended lower tropospheric particles to the UV spectral irradiance reaching the ground in urban environments. The UV spectral irradiance was measured with a UV spectrophotometer, while the aerosol data were obtained simultaneously by a lidar system operating in the 300 nm spectral region. Both UV and lidar instruments were operated inside the Athens basin during various air pollution and meteorological conditions, in the frame of the European MEDCAPHOT-TRACE Campaign in summer 1994. Aerosol vertical profiles were also obtained by a second lidar system operated outside the Athens basin (in a sub-urban area) during the same time period and were intercompared to the urban profiles. Using the lidar data as input to a radiative transfer model (MODTRAN), we provide comparisons of measured and modelled spectral UV irradiances under distinct environmental conditions.