Ozone dynamics over Greece as derived from satellite and! in situ measurements

A comprehensive analysis of the records of surface ozone available for Athens, Greece ( 38° N, 24° E) for the periods 1901–1940 and 1987–1990 is presented. Both records are analysed to explore the intraseasonal fluctuations and the harmonic components of surface ozone and also compared to other historical surface ozone records. The variation in surface ozone concentration during rainfall is also investigated, using the hourly measurements of the surface ozone concentration obtained by a network of four stations within the Greater Athens area. The results indicate that, during rainfall events which are associated with the passing of a cold front, an important decrease of the surface ozone concentration is observed. Daily measurements of surface ozone and NO_x, from five stations in the Greater Athens Basin overthe period 1986–1990 are also used in order to examine the main features of basin-wide 0₃-HC-NO_x relations. A simple regression model between the surface ozone concentration and the temperature at the 850 hPa level, which was first tested in Los Angeles, gave satisfactory results in reproducing the mean monthly ozone variation in Athens, when coefficients extracted from local data were used in the regression equation. A series of vertical ozone soundings over Athens has been also performed in order to explore the tropospheric ozone variations and to examine further the transport that occurs at the 700hPa level with advection from the north-western sector. The relevant results are discussed. The existing uncertainties concerning the stratosphere-troposphere exchange of ozone which mainly occurs during midlatitude tropopause folding as well as during cut-off low events are also discussed. The examination of the role of the atmospheric circulation in the lower stratosphere in relation to the laminated structure of ozone is also attempted. The data collected during the balloon ascents have been compared with those during the balloon descents. Both profiles are compared with the total ozone measurements derived from the TOMS on the Nimbus-7 satellite and the Dobson spectrophotometer. The data collected for the vertical distribution of ozone and temperature have been compared with the satellite-derived reference models which provide the monthly latitudinal variations of vertical structure of both ozone and temperature. We have also used total ozone measurements obtained with a Dobson spectrophotometer ( No. 118) which has been instituted in Athens from 1989 in order to examine the consistency of data from TOMS with the corresponding Dobson data on a daily basis. Furthermore monthly mean total ozone data were first estimated for the entire period and were then Fourier analysed to obtain the amplitude, phase and percentage contribution to the total variance of the first, second and third harmonics.     

On the accuracy of total ozone measurements made with a Dobson spectrophotometer in Athens

Since 1989, total ozone measurements with the Dobson spectrophotometer No. 118 have been made in Athens (38°N, 24°E), which has been suffering from severe air pollution problems during the last fifteen years. These measurements are subject to errors caused by interfering gases that absorb solar ultraviolet radiation in the region of the Dobson instrument's wavelengths (300-400 nm). In the urban area of Athens, the measured S0₂ and N0₂ mixing ratios have been found to lead to total ozone errors of up to 2-5 per cent during the year 1991. To examine the consistency of Dobson measurements, the integrated ozone profiles from a number of ozonesoundings performed at Athens have been used also.     

Comparison of vertical ozone profiles as deduced from remote and in situ sensing techniques

A number of vertical ozone profiles up to 35 km in height, have been measured using balloon-based sensors at Athens, Greece (38^° N, 24^° E). The measurements were made during the winter of 1991-1992, as part of the European Arctic Stratospheric Ozone Experiment (EASOE). The data collected during the balloon ascents have been compared with those during the balloon descents. Both profiles are compared with the total ozone measurements derived from the TOMS on the Nimbus-7 satellite and the Dobson spectrophotometer.     

Three years of total ozone measurements over Athens obtained using the remote sensing technique of a Dobson spectrophotometer

Total ozone measurements with a Dobson spectrophotometer (No. 118) have been instituted in Athens (37.9° N, 23.8° E) from 1989 and they now form part of the data published by the World Meteorological Organization, World Ozone Data Center. We have used this data set in order to examine the consistency of data from the Total Ozone Mapping Spectrometer (flown on the Nimbus-7 satellite) with the corresponding Dobson data on a daily basis.

The results show that the Athens station may be used to provide ground truth for satellite-based total ozone measurements and also for providing reasonably accurate total ozone values for south eastern Europe.     

Total ozone column measurements using an ultraviolet multi-filter radiometer

We have developed and used a method to retrieve total ozone column (TOC), from Ultraviolet Multi-filter Rotating Shadowband Radiometer (UVMFR) measurements in combination with radiative transfer model calculations. Look-up tables of ratios of the direct solar irradiance at (DI) 305 and 325nm in terms of TOC, solar zenith angle, and aerosol optical depth (AOD) have been constructed and compared with TOC retrievals estimated directly from UVMFR irradiance measurements. Sensitivity analysis of the influence of AOD on the calculated TOC has been investigated and found to be 1 Dobson unit per 0.1 change in AOD. We also examined the impact of ozone effective temperature on the TOC retrieval and found that it leads to a 0.9% change in TOC per K. UVMFR direct irradiance measurements in Athens, Greece, during the period July 2009–May 2014 were used to create a time series of high-temporal-frequency measurements (1 min for cloudless conditions) of TOC, which facilitated an analysis of the diurnal variation of TOC. Comparison of the TOC retrievals from the UVMFR with co-located and synchronous daily TOC retrievals from a Brewer MKIV spectrophotometer showed very good agreement (correlation coefficient 0.98). Daily TOC retrievals from the UVMFR were within ±3% compared with the ones measured by the Ozone Monitoring Instrument overpasses on board the Aura satellite.     

Annual,semi-annual and terannual waves in total ozone as derived from TOMS data at the subtropics

The reprocessed daily total ozone measurements made by the Total Ozone Mapping Spe trometer (TOMS) on Nimbus-7 over Athens (37·9° N, 23·8°E) have been analysed from January 1979 to January 1992. Monthly total ozone data are first estimated for the entire time period and are then Fourier analysed to obtain the amplitude, phase and percentage contribution to the total variance of the first, second and third harmonics. Findings presented in this paper concern the recent results in the literature according to which the northern mid-latitude total ozone trend shows a maximum seasonal cycle reaching greater than 0–8 per cent per year of ozone depletion in late winter.     

Possible impact of polar stratospheric processes on mid-latitude vertical ozone distributions

Balloon-borne ozone soundings have also been performed as part of European Arctic Stratospheric Ozone Experiment (EASOE) at Lindenberg, Hohenpeissenberg and Athens. Differences between these stations will be discussed with regard to the vertical ozone distributions and the features of stratospheric circulation during winter. The results give some evidence on the impact of chemically-disturbed air from the Arctic stratosphere to mid-latitudes from January until the middle of February during EASOE-winter in 1992. Ozone depletion is significantly pronounced at greater altitudes in mid-latitudes as it develops in polar latitudes.     

Total ozone over Ankara and its forecasting using regression models

The present study is concerned with the total ozone variation in Ankara, Turkey (39°57′ N; 32°53′ E) and developing a total ozone prediction model by Total Ozone Mapping Spectrometer (TOMS) and single-site aerological data (1984–2006). The daily averages of the total column ozone over Ankara show a seasonal variation, with larger values in spring/winter and lower in autumn/summer and a large day-to-day variability. In this study, in order to forecast the total column ozone over Ankara, a multi-linear regression equation was performed. Predictors are selected by stepwise regression method. The ozone value on the previous day, temperature at the 100 hPa pressure level, previous day's temperature for the 700 hPa pressure level and temperature difference for 50 hPa and 500 hPa are the most important predictors. The success of monthly prognostic total column ozone models built up for Ankara is tested by using Brewer MK III Spectrophotometer data and satellite (OMI) data with the upper-air data in 2007. The Brewer MKIII has been in operation since November 2006 in Ankara. The ground-based one-year data show good agreement with the satellite OMI data.     

Cover Application of the NOAA-AVHRR images to the study of the large forest fires in Spain in the summer of 1994.

Ozone vertical profiles derived from Umkehr observations by the Dobson spectrophotometer at Belsk (52.50° N, 20.47° E) and from ozone soundings carried out at the nearest aerological station, Legionowo (52.24° N, 20.58° E), have been compared with those measured by the MLS instrument on board the Aura spacecraft during the sites' overpasses for the period 2004–2005. It is assumed that the satellite station distance should be less than 2° and less than 4° for the latitudinal and longitudinal difference, respectively. The bias, RMS error, and the correlation coefficients between the ozone content in the Umkehr layers have been calculated using Dobson/sonde/MLS data. The ozone mixing ratio at selected levels in the lower and mid‐stratosphere (from 215 hPa up to 6.8 hPa) have been compared using the sonde/MLS data. The number of analysed daily values was ～40 (Dobson/MLS), 60 (sonde/MLS), and 60 (Dobson/sonde) since August 2004. The comparisons show a good correspondence (bias ～±5%, RMS <10%, correlation coefficient >0.5) between the ozone content in Umkehr layers 4–8 and ozone mixing ratio at pressures <50 hPa. At lower stratosphere (Umkehr layer 3) and upper stratosphere (Umkehr layer 9), there is also statistically significant relationship between the data, but the biases and RMS are ～2 times larger, while the correlation coefficients are still high (>0.7).     

On the longitude dependence of total ozone trends over middle-latitudes

It has recently been observed that the total ozone trends derived from certain geographical regions such as the Mediterranean and Athens (Greece) show similar values to those derived from the 40°N zonal averaged column ozone data. In this Letter, the total ozone concentration, collected by the Total Ozone Mapping Spectrometer (TOMS) flown on Nimbus-7 and Meteor-3 during the time-period January 1979-December 1993, as well as by Earth Probe during the time-period January 1997-May 2001, for the Mediterranean, Athens (Greece) and Srinagar (India), is analysed. Further, the harmonic analysis performed on total ozone time-series provides a proper tool to interpret the observed similarity in total ozone seasonal trends, which may probably be attributed to the effect of planetary waves on the ozone distribution.     

On the correction of the total ozone content over Athens,Greece as deduced from satellite observations

The observations with the Total Ozone Mapping Spectrometer ( TOMS) mounted aboard the Nimbus-7 satellite have previously been used to determine the trends of the total ozone amount over Athens, Greece ( 38° N, 24° E), since 1979, for various months ( Varotsos, C. A., and Cracknell, A. P., 1993, International Journal of Remote Sensing, 14, 2053–2059). The total ozone depletion over the 13-year time period showed a strong seasonal variation of the trend from more than 7 per cent in winter to about 2·5 per cent in summer. However, the TOMS instrument measures the back-scattered ultraviolet radiation in order to determine ozone content and is limited to observations above the cloud level. ln the presence of thick cloud the column ozone content is generally underestimated. This underestimation of the total ozone amount is quantitatively examined, especially in the synoptic cases where ozone-rich air has been transported into the lower troposphere. The influence of this underestimation on the total ozone depletion over Athens, Greece, deduced from TOMS observations, is finally attempted.     

Technical Report: Standardization of the Athens Dobson spectrophotometer versus Reference Dobson spectrophotometer 064

The comparison between Dobson spectrophotometer number 118 operating in Athens, Greece, and the Reference Instrument Dobson number 064 (Hohenpeissenberg, Germany) for column ozone observations, is discussed. The results obtained showed that the highest difference in total ozone observations derived from those instruments is almost 0.6% when the μ value ranges from 1.2 to 2.5, and hence no data reprocessing of historical data from Athens is necessary.     

A complex study of Etna's volcanic plume from ground‐based,in situ and space‐borne observations

Two periods of transboundary transport of volcanic aerosols and debris following recent eruptions of Mount Etna, Italy, were examined using ground‐based and satellite spectrophotometric measurements together with Light Detection And Ranging (LiDAR) and aerosol filter observations in Athens and Thessaloniki, Greece. Independent columnar SO₂ measurements from ground and space identified peaks at Greek sites after the volcanic eruptions. LiDAR measurements of the aerosol extinction at Thessaloniki and Athens performed in July 2001 have shown the height of the volcanic plume to be about 3.5 km asl and the optical thickness of the dust layer to be of the order of 3×10^?3 at 532 nm. Strong ozone depletion observed at the volcano plume level by using ozonesonde ascents may be attributed to the in‐plume processes that generate reactive halogens, which in turn destroy ozone. The chemical and elemental composition of aerosol samples, taken at the Earth's surface, was analysed and confirmed the volcanic origin of the dust.     

Total ozone observations at Athens,Greece by satellite-borne and ground-based instrumentation

A comparison is made of total ozone (TOZ) content observations conducted by the Dobson spectrophotometer No. 118, the SCanning Imaging Absorption SpectroMeter for Atmospheric CHartographY (SCIAMACHY), the Total Ozone Mapping Spectrometer (TOMS) and the Ozone Monitoring Instrument (OMI) over Athens, Greece, during 1991–2008. Spearman's and Wilcoxon's tests were used to determine the measure of the agreement between the ground-based and satellite column ozone data. The correlation coefficient between Dobson and Nimbus-7, ADEOS, Earth Probe, OMI and SCIAMACHY observations was found to be 0.95, 0.96, 0.94, 0.93 and 0.87, respectively, while the correlation coefficient between total ozone observations of SCIAMACHY and Earth Probe-TOMS and OMI is 0.85 and 0.93, respectively. SCIAMACHY overestimates the column ozone with respect to Dobson, Earth Probe-TOMS and OMI by 10, 15 and 3 DU, respectively, while Dobson underestimates the column ozone with respect to Nimbus-7, ADEOS and OMI by 5, 10 and 8 DU. The results obtained confirm that the Athens Dobson station may continue to be considered as a ground-truth total ozone station for the validation of the satellite column ozone observations. In addition, linear regression analysis of the deseasonalized monthly mean column ozone, as derived from Dobson measurements, gives an increase of +0.33 ± 0.07% per year during 1991–2000 and a decrease of –0.33 ± 0.07% per year for the period 2001–2008.     

Improvement of the Umkehr ozone profile by the neural network method: analysis of the Belsk (51.80°N, 20.80°E) Umkehr data

Vertical profiles of atmospheric ozone by the neural network (NN) method are compared with those obtained by the standard Umkehr inversion algorithm – UMK92. Both methods used the same input, the so-called N values, derived from Umkehr measurements at Belsk (51.80°N, 20.80°E), Poland, by the Dobson spectrophotometer No 84. The vertical profiles of ozone from satellite observations, Microwave Limb Sounder (MLS) overpasses for the period 2004–2009, and from ozonesoundings performed at the nearby aerological station, Legionowo (52.4° N, 21.0° E), for the period 2000–2009 provide a reference data set for the NN model building. The NN methodology appears to be a promising tool for extracting information about the vertical ozone profile from ground-based Umkehr measurements, despite some limitations of the NN method itself, such as the results being limited to the analysed station, sensitivity to errors in the reference data sets, and lack of possibility to determine the actual retrieval errors. Accuracy of the NN ozone profiles is better for all Umkehr layers than that by the standard Umkehr inversion algorithm when NN and UMK92 profiles are compared with the reference profiles. It is especially pronounced for comparisons with the ozonesonde profiles for layers 4 and 1, where the absolute error changes from 10.6 Dobson units (DU) (UMK92) to 4.4 DU (NN) and from 6.6 DU (UMK92) to 3.5 DU (NN), respectively (1 Dobson unit is equal to 2.69 × 10²⁰ molecules/m²). The mean (over all Umkehr layers) correlation coefficient between NN-MLS, and NN-ozonesonde profiles is 0.75 and 0.85, respectively. The corresponding correlation coefficients for the comparison with UMK92 profiles are lower, i.e. 0.61 and 0.64, respectively.     

SCIAMACHY/Envisat,OMI/Aura,and ground-based total ozone measurements over Kyiv-Goloseyev station

Validation of satellite ozone measurements is important for data improvement due to instrumental long-term drifts and retrieval algorithm limitations. For satellite data quality estimation, we compare the total ozone content (TOC) derived from the satellite Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY)/Envisat and Ozone Monitoring Instrument (OMI)/Aura spectrometer overpass data and the ground-based measurements made with the Dobson spectrophotometer 040 at the GAW station No. 498 Kyiv-Goloseyev. The station was opened for Dobson ozone measurements in 2010. The results for Direct Sun, Zenith Blue, and Zenith Cloud observations are presented separately, in order to assess the influence of weather conditions (clear or cloudy sky) on the difference between satellite and ground-based measurements. Results from the SCIAMACHY–Dobson and OMI–Dobson difference analyses show small relative overestimation of TOC for satellite data. The ground-based Dobson 040 data are of high quality for Direct Sun and Zenith Blue from AD ((305.5 and 325.0 nm) and (317.5 and 339.9 nm)) pair measurements. Seasonal variations of the difference are seen with maximal satellite–Dobson data discrepancy near the winter solstice. Satellite TOC values are systematically higher than Dobson ones at solar zenith angles larger than 70°. This difference could be explained by seasonal non-uniformity in the satellite data.     

The present status of the total ozone depletion over Greece and Scotland: a comparison between Mediterranean and more northerly latitudes

Results showing ozone concentrations over Athens and over Dundee, determined from satellite remote sensing measurements, are reported along with the results of ground-based ozone measurements in Athens and ground-based solar ultraviolet measurements in Dundee. The spatial variation of ozone concentrations within Greece is also discussed.     

The EOLE Project: A multiwavelength laser remote sensing (lidar) system for ozone and aerosol measurements in the troposphere and the lower stratosphere. Part II: Aerosol measurements over Athens,Greece

Ground-based measurements of suspended aerosols in the lower troposphere (0.6-3.5 km height) in Athens, Greece were performed for a period of 10 days in November 1994. The measurements were performed in a suburban area in the northern part of the Athenian basin, using a two-wavelength backscattering lidar system operating at 355 nm and 532 nm, simultaneously. The lidar system was pointing vertically and was operated under different meteorological conditions. A lidar inversion algorithm was applied to determine the vertical profile of the aerosol backscattering coefficient in the 0.6-3.5 km altitude range. The analysis of the bi-dimensional cross-section of the aerosol backscattering coefficient and slope rate, for two selected cases, in conjunction with radiosonde, solar UV-B irradiance and in situ air pollution data, gave the possibility to study air pollution characteristics inside the Athens basin.     

应用MIPAS/ENVISAT卫星遥感资料研究平流层爆发性增温过程中大气化学成分变化特征

2002年12月~2003年1月北半球高纬地区平流层发生了一次爆发性增温过程（SSW）,使该地区的动力、热力场和大气化学成分的分布发生剧烈变化。选取了搭载在欧洲环境探测卫星ENVISAT上的对地遥感观测仪器麦克耳逊干涉被动大气探测仪（MIPAS）的观测数据,首先利用臭氧探空资料和再分析气象资料对MIPAS观测的臭氧和温度廓线资料进行了对比验证;其次使用该资料对此次SSW过程进行了研究。对比结果显示,MIPAS资料与NCEP和臭氧探空资料都有较好的一致性。在这次平流层爆发性增温事件中,增温过程随时间向下传播影响到平流层下层。N₂O和CH₄的水平和垂直浓度分布与位涡反映的极涡始终都有很好的一致性。O₃和H₂O的浓度分布也受到极涡的限制,O₃体积混合比随增温而增大,有极涡外的O₃进入极涡。NO₂并未受到极涡分布的严格限制,在极区维持低浓度区。HNO₃浓度场在极区显示为极大值,但并不与极涡对应。     

Retrieval of land and sea surface temperature using NOAA-11 AVHRR· data in north-eastern Brazil

The split-window method is investigated and a simple airborne atmospheric correction model for thermal data is proposed. Analysis shows that the atmospheric transmittance has a quadratic behaviour with the water vapour content in the first few kilometres of a wet atmosphere. The effect of the emissivity is evaluated for the retrieval of surface temperature using data from NOAA-11 AVHRR channels 4 and 5 for two extreme atmospheres. The results indicate that a spectral emissivity variation (Δε) in channels 4 and 5 of ±0.01 is not as important for a wet atmosphere as for a dry atmosphere. The split-window algorithm developed in this work has its parameters dependent on the atmospheric state and the values of these parameters are determined by using radiosonde profiles. Data from eighty-five radiosondes have been used to determine and check the local seasonal equatorial split-window parameters. The results of surface temperature retrieval show that the local seasonal equatorial and daily split-window parameters (given by daily radiosonde profiles) for NOAA-11 AVHRR data exhibit good agreement between their surface temperature results and the results of in situ measurements for two days. Comparisons with in situ measurements show that the maximum difference from retrieving a vegetated surface temperature using AVHRR data is less than 1.0°C in a wet atmosphere. Although the seasonal parameters have demonstrated a good performance when applied for two particular days, it does not indicate that they can be used successfully for other times when the atmospheric state differs from the average seasonal profile determined in this work. Evidence of this fact is shown through the variation of water vapour amount in the eighty-five radiosonde atmospheric profiles that have been analysed. This variation presents a wide range in water vapour from 2.8g cm^-2, to 4.92g cm^-2, which can significantly modify the retrieval of surface temperature using remote infrared sensors. Discussion of this problem is given in this paper.