D. Balis

Record low total ozone during northern winters of 1992 and 1993

The last two winter-spring seasons (DJFM) distinguished themselves by being with the lowest ever total ozone over all three continental size regions between 45°N and 65°N of North America, Europe and Siberia. The total ozone deficiencies for the entire season over all of the above mentioned regions were about 11% and 13% below the long-term normal during the two consecutive years (1991/92 and 1992/93 respectively ). This helped to pull down the cumulative ozone decline since the winter-spring of 1969/70 to be about 14% in the latitude belt of the 45°N–65°N. Frequencies of days with ozone values deviating below the long-term mean by more than 2σ have been ten times higher than their 35-year average. There are evidences deduced from trajectories on potential temperature surfaces that transport of poor in ozone air masses forced in addition by vertical motions, could account for a number of the extreme cases. There is also evidence that cold air, known to have excess ClO content, has moved over the sun lighted latitudes on many occasions, when chemical ozone destruction could have been favored. These ozone deficiencies do not have similar rates of decline and did not reach even close to the extreme low values regularly observed during the Antarctic-spring ozone hole phenomena.

Comparison of Models Used for UV Index Calculations

Eighteen radiative transfer models in use for calculation of UV index are compared with respect to their results for more than 100 cloud‐free atmospheres, which describe present, possible future and extreme conditions. The comparison includes six multiple‐scattering spectral models, eight fast spectral models and four empirical models. Averages of the results of the six participating multiple‐scattering spectral models are taken as a basis for assessment. The agreement among the multiple‐scattering models is within ±0.5 UV index values for more than 80% of chosen atmospheric parameters. The fast spectral models have very different agreement, between ±1 and up to 12 UV index values. The results of the empirical models agree reasonably well with the reference models but only for the atmospheres for which they have been developed. The data to describe the atmospheric conditions, which are used for the comparison, together with the individual results of all participating models and model descriptions are available on the Internet: http://www.meteo.physik.uni‐muenchen.de/strahlung/cost/.

Correcting global solar ultraviolet spectra recorded by a brewer spectroradiometer for its angular response error.

We present a methodology for correcting the global UV spectral measurements of a Brewer MKIII spectroradiometer for the error introduced by the deviation of the angular response of the instrument from the ideal response. This methodology is applicable also to other Brewer spectroradiometers that are currently in operation. The various stages of the methodology are described in detail, together with the uncertainties involved in each stage. Finally global spectral UV measurements with and without the application of the correction are compared with collocated measurements of another spectroradiometer and with model calculations, demonstrating the efficiency of the method. Depending on wavelength and on the aerosol loading, the cosine correction factors range from 2% to 7%. The uncertainties involved in the calculation of these correction factors were found to be relatively small, ranging from ~0.2% to ~2%.

Atmospheric aerosol and gaseous species in Athens, Greece

Abstract Measurements of aerosol species including Cl - , NO - 3 , SO 2- 4 , NH + 4 , Na + , K + , Mg 2+ and Ca 2+ and gaseous pollutants like HCl, HNO 3 , NO 2 , SO 2 and O 3 were conducted at the centre of Athens. Results show moderate levels of pollution with aerosol species typical of dust emissions and secondary aerosol of anthropogenic origin. Most gaseous pollutants correlate well with aerosol species. There is a pronounced elevation in the concentration of acidic trace gases HCl and HNO 3 arriving at the city from the direction of the Saronikos gulf, indicating poor neutralisation of such species over the sea. On the contrary, air masses coming from inland are characterised by considerably higher ammonium aerosol. It was concluded that in addition to heavier aerosol load pollution episodes in Athens during the summer period are associated with higher concentrations of gaseous acids.

Homogenized total ozone data records from the European sensors GOME/ERS-2, SCIAMACHY/Envisat, and GOME-2/MetOp-A

Within the European Space Agencys Climate Change Initiative, total ozone column records from GOME (Global Ozone Monitoring Experiment), SCIAMACHY (SCanning Imaging Absorption SpectroMeter for Atmospheric CartograpHY), and GOME-2 have been reprocessed with GODFIT version 3 (GOME-type Direct FITting). This algorithm is based on the direct fitting of reflectances simulated in the Huggins bands to the observations. We report on new developments in the algorithm from the version implemented in the operational GOME Data Processor v5. The a priori ozone profile database TOMSv8 is now combined with a recently compiled OMI/MLS tropospheric ozone climatology to improve the representativeness of a priori information. The Ring procedure that corrects simulated radiances for the rotational Raman inelastic scattering signature has been improved using a revised semi-empirical expression. Correction factors are also applied to the simulated spectra to account for atmospheric polarization. In addition, the computational performance has been significantly enhanced through the implementation of new radiative transfer tools based on principal component analysis of the optical properties. Furthermore, a soft-calibration scheme for measured reflectances and based on selected Brewer measurements has been developed in order to reduce the impact of level-1 errors. This soft-calibration corrects not only for possible biases in backscattered reflectances, but also for artificial spectral features interfering with the ozone signature. Intersensor comparisons and ground-based validation indicate that these ozone data sets are of unprecedented quality, with stability better than 1% per decade, a precision of 1.7%, and systematic uncertainties less than 3.6% over a wide range of atmospheric states.

Further ozone decline during the northern hemisphere winter-spring of 1994-1995 and the new record low ozone over Siberia

Ground-based total ozone observations show that during the second half of January 1995 through February, most of March, and the first week of April very low ozone values, again were observed in northern middle and high latitudes. Over Eastern and Southern Europe and North America the deficiency was 10–12% with a few periods down to 15–20%. Since mid January the ozone values fall 25% below the long-term mean over Siberia and Yakutia. This anomaly lasted on the average for twelve consecutive weeks. For a number of days in early February and in the second half of March, the deficiency exceeded 35–40%. The monthly mean lower stratosphere temperatures over Siberia, where the polar vortex was displaced were 10°C below the normal for January and February falling down below −75°C to −80°C during the time of lowest ozone.

Changes in surface UV solar irradiance and ozone over the balkans during the eclipse of August 11, 1999

Abstract Intensive measurements of UV solar irradiance, total ozone and surface ozone were carried out during the solar eclipse of 11 August 1999 at Thessaloniki, Greece and Stara Zagora, Bulgaria, located very close to the footprint of the moons shadow during the solar eclipse with the maximum coverage of the solar disk reaching about 90% and 96% respectively. It is shown that during the eclipse the diffuse component is reduced less compared to the decline of the direct solar irradiance at the shorter wavelengths. A 20-minute oscillation of erythemal UV-B solar irradiance was observed before and after the time of the eclipse maximum under clear skies, indicating a possible 20-minute fluctuation in total ozone presumably caused by the eclipse induced gravity waves. The surface ozone measurements at Thessaloniki display a decrease of around 10–15 ppbv during the solar eclipse. Similarly, ozone profile measurements with a lidar system indicate a decrease of ozone up to 2 km during the solar eclipse. The eclipse offered the opportunity to test our understanding of tropospheric ozone chemistry. The use of a chemical box model suggested that photochemistry can account for a significant portion of the observed surface ozone decrease.

Evaluating a new homogeneous total ozone climate data record from GOME/ERS-2, SCIAMACHY/Envisat and GOME-2/MetOp-A†

The European Space Agencys Ozone Climate Change Initiative (O3-CCI) project aims at producing and validating a number of high-quality ozone data products generated from different satellite sensors. For total ozone, the O3-CCI approach consists of minimizing sources of bias and systematic uncertainties by applying a common retrieval algorithm to all level 1 data sets, in order to enhance the consistency between the level 2 data sets from individual sensors. Here we present the evaluation of the total ozone products from the European sensors Global Ozone Monitoring Experiment (GOME)/ERS-2, SCIAMACHY/Envisat, and GOME-2/MetOp-A produced with the GOME-type Direct FITting (GODFIT) algorithm v3. Measurements from the three sensors span more than 16 years, from 1996 to 2012. In this work, we present the latest O3-CCI total ozone validation results using as reference ground-based measurements from Brewer and Dobson spectrophotometers archived at the World Ozone and UV Data Centre of the World Meteorological Organization as well as from UV-visible differential optical absorption spectroscopy (DOAS)/Systeme D′Analyse par Observations Zenithales (SAOZ) instruments from the Network for the Detection of Atmospheric Composition Change. In particular, we investigate possible dependencies in these new GODFIT v3 total ozone data sets with respect to latitude, season, solar zenith angle, and different cloud parameters, using the most adequate type of ground-based instrument. We show that these three O3-CCI total ozone data products behave very similarly and are less sensitive to instrumental degradation, mainly as a result of the new reflectance soft-calibration scheme. The mean bias to the ground-based observations is found to be within the 1 ± 1% level for all three sensors while the near-zero decadal stability of the total ozone columns (TOCs) provided by the three European instruments falls well within the 1–3% requirement of the European Space Agencys Ozone Climate Change Initiative project.

The Effect of Three Different Absorption Cross-Sections and their Temperature Dependence on Total Ozone Measured by a Mid-Latitude Brewer Spectrophotometer

Abstract The influence of variations in atmospheric temperature and ozone profiles on the total ozone column (TOC) derived from a Brewer MKII spectrophotometer operating in Thessaloniki, Greece, is investigated using three different sets of ozone absorption cross-sections. The standard Brewer total ozone retrieval algorithm uses the Bass and Paur (1985) cross-sections without accounting for the temperature dependence of the ozone cross-sections which produces a seasonally dependent bias in the measured TOC. The magnitude of this temperature effect depends on the altitude where the bulk of the ozone absorption occurs. Radiosonde measurements for the period 2000 to 2010 combined with climatological ozone profiles were used to calculate the effective temperature of ozone absorption and investigate its effect on the retrieved ozone column. Three different ozone absorption cross-section spectra convolved with the instruments slit function were used: those of Bass and Paur (hereafter BP), currently used in the standard Brewer retrieval algorithm; those of Brion, Daumont, and Malicet (Malicet et al., 1985; hereafter BDM); and the recently published set by Serdyuchenko et al. (2013 hereafter S13). The temperature dependence of the differential ozone absorption coefficient ranges between 0.09 and 0.13% per degree Celsius for BP, between −0.11 and −0.06% per degree Celsius for BDM, and between 0.018 to 0.022% per degree Celsius for S13, resulting in a seasonal bias in the derived TOC of up to 2%, 1.8%, and 0.4%, respectively. The temperature sensitivity of the differential ozone absorption coefficient for the Brewer spectrophotometer at Thessaloniki for the BP and BDM cross-sections is found to be within the range reported for other Brewer instruments in earlier studies, whereas the seasonal bias in TOC is minimized when using the new S13 cross-sections because of their small temperature dependence.

Estimation of the microphysical aerosol properties over Thessaloniki, Greece, during the SCOUT‐O3 campaign with the synergy of Raman lidar and Sun photometer data

[1] An experimental campaign was held at Thessaloniki, Greece (40.6°N, 22.9°E), in July 2006, in the framework of the integrated project Stratosphere‐Climate Links with Emphasis on the Upper Troposphere and Lower Stratosphere (SCOUT‐O3). One of the main objectives of the campaign was to determine the local aerosol properties and their impact on the UV irradiance at the Earth’s surface. In this article, we present vertically resolved microphysical aerosol properties retrieved from the inversion of optical data that were obtained from a combined one‐wavelength Raman/two‐wavelength backscatter lidar system and a CIMEL Sun photometer. A number of assumptions were undertaken to overcome the limitations of the existing optical input data needed for the retrieval of microphysical properties. We found acceptable agreement with Aerosol Robotic Network retrievals for the fine‐mode particle effective radius, which ranged between 0.11 and 0.19 for the campaign period. It is shown that under complex layering of the aerosols, general assumptions may result in unrealistic retrievals, especially in the presence of aged smoke aerosols. Furthermore, with this instrument setup, the inversion algorithm can also be applied successfully for the complex refractive index in cases of vertically homogeneous layers of continental polluted aerosols. For these inversion cases, the vertically resolved retrievals for the single‐scattering albedo resulted in values around 0.9 at 532 nm, which were in very good agreement with estimates from airborne in situ observations obtained in the vicinity of the lidar site.