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.     

Comparison of total ozone measurements from a differential optical absorption filter instrument and a Dobson spectrophotometer

Measures of total ozone at Lisbon in July 1996 taken with a differential optical absorption filter instrument (UAM-DOAF) and a Dobson spectrophotometer are compared. Correlation of the UAM-DOAF measures and NOAATOVS data from April to October 1996 and NASA Earth Probe TOMS data from July to October 1996 for Madrid (40 N, 3 W) are given. Results show that the UAM-DOAF instrument can be used with approximately a 3 per cent error in estimating short-term total ozone variations.     

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.     

Global validation of FY-3A total ozone unit (TOU) total ozone columns using ground-based Brewer and Dobson measurements

Analysis of the accuracy and variability of total ozone columns (TOC) has been conducted by many studies, while the TOC observations derived from the total ozone unit (TOU) on board the Chinese FengYun-3A (FY-3A) satellite platform are notably less well documented. Therefore, in this present study, we mainly focus on the global-scale validation of TOU-derived total ozone column data by comparing them with spatially and temporally co-located ground-based measurements from the well-established Brewer and Dobson spectrophotometer for the period July 2009 through December 2011. The results show that TOU-derived total ozone column data yields high accuracy, with the root mean square error less than 5% in comparison with ground-based measurements. Meanwhile, TOU underestimates Brewer measurements by 1.1% in the Northern Hemisphere and overestimates Dobson total ozone 0.3% globally. In addition, TOU-derived total ozone shows no significant dependence on latitude in comparison with either Brewer or Dobson total ozone measurements. Nevertheless, a significant dependence of TOU-derived total ozone is observed on the solar zenith angle (SZA) in comparison with both Brewer and Dobson, demonstrating that TOU underestimates at large SZA and overestimates at small SZA. Finally, the dependence of satellite – ground-based relative difference for total ozone values shows fair agreement when total ozone values are in the range 250–450 Dobson units (DU). Overall, the Chinese FY-3A/TOU performs well on total ozone retrieval with high accuracy, and the total ozone data derived from the TOU can be used as a reliable data source for ozone monitoring and other atmospheric applications.     

Temporal and spatial variability of total ozone column using TOMS satellite observations and comparison with measurements from the Dobson network

This article presents a detailed analysis of seasonal and interannual variability of total ozone content (TOC) at 16 different stations in Africa using Nimbus-7 Total Ozone Mapping Spectrometer (TOMS) data for a period of 14 years (January 1979–December 1992). The analysis provides not only an estimate of the long-term annual and seasonal trends but also statistics of means and variability of ozone on temporal and spatial scales. For example, high negative deviations were observed for stations in Northern Africa in spring (March–May), with as much as –20 DU in Alexandria. A comparison of total ozone column data retrieved from the TOMS satellite with measurements obtained from the Dobson ground-based network is further presented for Cairo, Irene, Nairobi and Springbok. Estimates of the percentage seasonal difference between TOMS satellite and Dobson ground-based measurements reveal that the ground measurements were higher in magnitude at all stations with the exception of Nairobi. To verify the level of correlation between the ground-based and satellite observations, rank correlation coefficients were determined for all stations using daily and monthly observations. The results show that there is good correlation between the compared data sets, with daily coefficient of determination (r ²) values of 0.87, 0.76, 0.58 and 0.87 for Cairo, Irene, Nairobi and Springbok, respectively.     

Comparison of total ozone column observations from space-borne Ozone Monitoring Instrument with ground-based Dobson Ozone Spectrophotometer measurements at an urban location in Indo-Gangetic Basin

This article presents a comparison analysis of OMIT (Ozone Monitoring Instrument retrieved overpass total ozone column (TOC)), and DOST (Dobson Ozone Spectrophotometer observed TOC) over Delhi during a period from October 2004 to June 2011. Megacity Delhi, located in Indo-Gangetic Basin, is an important site for comparison of ground-based and satellite retrieved TOCs due to significant anthropogenic emissions of ozone precursors, large shift in seasons, and large-scale crop residue burning in the region. DOST and OMIT data show an overall bias of 3.07% and significant correlation with coefficient of determination R² = 0.73. Large seasonal fluctuations in the biases and correlations have been observed ranging from 2.46% (winter) to 3.82% (spring), and R² = 0.84 (winter) to R² = 0.09 (summer), respectively. The large biases are attributed to changes in temperature, cloud cover, pollutants emissions from urban area, and crop-residue burning events. We also find notable variations in correlations between the datasets due to the varying burden of absorbing aerosols from open field crop-residue burning. The R² has changed from 0.67 (for aerosol optical depth, AOD 1.5–3.5) to 0.77 (for AOD 0–0.99). The dependence of the bias on solar zenith angle, cloud fraction, and satellite distance is also discussed. A simple linear regression analysis is applied to check the linkage between DOST and OMIT. The influence of atmospheric air temperature and relative humidity on OMIT at different pressure levels between 1000 and 20 hPa has been discussed.     

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.     

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.     

Effect of the out-of-band stray light on the retrieval of the Umkehr Dobson ozone profiles

The technique of producing a vertical ozone profile using ground-based observations of the Umkehr effect is well established and is widely used for ozone trend analysis. Still, observations of the Umkehr effect by Dobson ozone spectrophotometers have a documented problem: simultaneous measurements from collocated instruments produce noticeably different results, with the difference shown to be dependent on solar zenith angle and total ozone. Ozone profiles are thus difficult to compare from station to station. Current investigations into the level and effects of stray light within the instrument suggest that much of the difference can be attributed to out-of-band stray light characteristics of individual Dobson instruments. Resulting changes in retrieved ozone profiles suggest that adjusting measurements for stray light components removes much of the existing bias between Umkehr profiles and other ozone measuring datasets. This new information has implications for both observational methods and completeness of the existing datasets.     

Atmospheric total ozone column evaluation with a smartphone image sensor

A new method for quantifying the total ozone column (TOC) using a smartphone image sensor has been developed and validated. The TOC has been evaluated for relatively cloud free days at high air masses for solar zenith angles between 49.7° and 76.7° at a sub-tropical site. The method is based on the evaluation of the direct solar irradiances at 305 and 312 nm using the red colour pixel values of the solar disc recorded at these wavelengths by a smartphone camera. Narrow bandpass filters of 2 nm full width at half maximum at each of the two wavelengths were used in turn placed over the camera sensor to directly image the solar disc. The calibration of the pixel values of the solar disc to provide the direct solar irradiances at each of these two wavelengths allowed evaluation of the TOC calibrated to a portable sun photometer. The root mean square error (RMSE) for the smartphone-derived ozone values calibrated to corresponding values from a portable sun photometer was 4.3 Dobson Units (DU). The validation measurements for the smartphone-derived ozone values provided an average residual of 3.5% (up to a maximum of 11%) compared to the corresponding portable sun photometer values, with an RMSE of 8.4 DU during days of intermittent inclement weather conditions. The evaluation of the TOC based on a widely available device such as a smartphone has the potential to increase current citizen science initiatives valued by the general public and school-aged learners by enhancing knowledge and awareness of ozone and the resulting influences on the solar ultraviolet environment.     

On the relationship between total ozone and temperature in the troposphere and the lower stratosphere

Total ozone and atmospheric temperature are two variables that are closely connected with two important environmental problems: ozone depletion and global warming. Vertical temperature profiles and column ozone data are analysed to explore the relationship between these two variables at various levels in the troposphere and lower stratosphere over Athens, Greece during the period 1992–2004. The results obtained show an out-of-phase relationship between total ozone and temperature in the troposphere and an in-phase relationship in the lower stratosphere. Such a phase change is characteristic of all seasons.     

On the total ozone depletion over Greece derived from satellite-flown and ground-based instruments

There is abundant evidence of the depletion of atmospheric ozone concentration, both from ground-based measurements and from satellite data. However, one set of data published in 1989 by Bais et al. relating to Thessaloniki, in northern Greece, reported no depletion but rather said ‘the period March 1982 to March 1988 shows an insignificant change of total ozone, of about +0-02 per cent per year’. The data of Thessaloniki, obtained from the same instrument (Brewer spectrophotometer no. 5) as that used by Bais et al. has now also been published in the Red Book. In this paper we examine TOMS data for Thessaloniki, Athens and Crete for the six-year period from March 1982 to March 1988 and find a common ozone depletion of the order of 30 per cent during this period, for each of these three sites, including Thessaloniki; this is in contradiction to the results of Bais et al. for Thessaloniki. We have analysed the Red Book data for Thessaloniki and found that it shows total ozone depletion consistent with the results obtained from TOMS data, which are themselves consistent with results from other ground-based measurements in mid-latitudes. It is concluded that there are errors both in the data presented by Bais et al. and in their analysis and, in particular, that their statement quoted above is incorrect and should be disregarded.     

Validation of the MetOp-A total ozone data from GOME-2 and IASI using reference ground-based measurements at the Iberian Peninsula

One of the most important atmospheric composition products derived from the first EUMETSAT Meteorological Operational satellite (MetOp-A) is the total ozone column (TOC). For this purpose, MetOp-A has two instruments on board: the Global Ozone Monitoring Experiment 2 (GOME-2) that retrieves the TOC data from the backscattered solar ultraviolet-visible (UV-Vis) radiance, and the Infrared Atmospheric Sounding Interferometer (IASI) that uses the thermal infrared radiance to derive TOC data. This paper focuses on the simultaneous validation of the TOC data provided by these two MetOp-A instruments using the measurements recorded by five well-calibrated Brewer UV spectrophotometers located at the Iberian Peninsula during the complete 2009. The results show an excellent correlation between the ground-based data and the GOME-2 and IASI satellite observations (R² higher than 0.91). Differences between the ground-based and satellite TOC data show that the IASI instrument significantly overestimates the Brewer measurements (about 4.4% when all five ground-based stations are jointly used). In contrast, the GOME-2 instrument shows a slight underestimation (~ 1.6%). In addition, the absolute relative differences between the Brewer and GOME-2 data are quite smaller (about a factor higher than 2) than the Brewer-IASI absolute differences. The satellite viewing geometry (solar zenith angle and the view zenith angle) has no significant influence on the Brewer-satellite relative differences. Moreover, the analysis of these relative differences with respect to the ground-based TOC data indicates that GOME-2 instrument presents a slight underestimation for high TOC values. Finally, the IASI-GOME-2 correlation is high (R² ~ 0.92), but with a mean relative difference of about ± 6% which could be associated with the bias between UV-Vis and infrared spectroscopy used in the retrieval processes.     

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.     

Surface and total ozone investigations in the region of Sofia,Bulgaria

Atmospheric ozone behaviour over Sofia has been investigated with remote-sensing and in situ techniques. Surface ozone and boundary layer observations performed in recent years at three city sites have been analysed. It was found that, in the autumn period, at close meteorological conditions, diurnal ozone variations show stable behaviour from year to year during the analysed period. It may be assumed that the boundary layer and ozone precursor concentrations, which are involved in photochemical ozone formation, keep up their state from year to year at the mentioned conditions. These findings may be interesting when surface ozone trends and climate change influence on ozone are investigated. The analysis of the long-term total ozone content (TOC) variations did not find a total ozone trend in the 1997–2008 period.     

On Improving the accuracy with Auto-Encoder on Conjunctivitis

Applying the classification approach in machine learning to medical field is a promising direction as it could potentially save a large amount of medical resources and reduce the impact of error-prone subjective diagnosis. However, low accuracy is currently the biggest challenge for classification. So far many approaches have been developed to improve the classification performance and most of them are focusing on how to extend the layers or the nodes in the Neural Network (NN), or combining a classifier with the domain knowledge of the medical field. These extensions may improve the classification performance. However, these classifiers trained on one datasets may not be able to adapt to another dataset. Meanwhile, the layers and the nodes of the neural network cannot be extended infinitely in practice. To overcome these problems, in this paper, we propose an innovative approach which is to employ the Auto-Encoder (AE) model to improve the classification performance. Specifically, we make the best of the compression capability of the Encoder to generate the latent compressed vector which can be used to represent the original samples. Then, we use a regular classifier to perform classification on those compressed vectors instead of the original data. In addition, we explore the classification performance on different extracted features by enumerating the number of hidden nodes which are used to save the extracted features. Comprehensive experiments are conducted to validate our proposed approach with the medical dataset of conjunctivitis and the STL-10 dataset. The results show that our proposed AE-based model can not only improve the classification accuracy but also be beneficial to solve the problem of False Positive Rate.     

Assessment of total columnar ozone climatological trends over the Indian sub-continent

In the present study, long term satellite and Dobson spectrophotometer Total Column Ozone (TCO) data have been used to study the interannual variability and also to assess climatological trends in TCO over different geographical locations of Indian sub-continent. TCO data were analyzed for the period 1957 to 2015 over New Delhi (28.63° N, 77.18° E), Varanasi (25.30° N, 83.02° E), Pune (18.53° N, 73.84° E) and Kodaikanal (10.0° N, 77.47° E). An extensive validation was performed for Total Ozone Mapping Spectrometer (TOMS) and Ozone Monitoring Instrument (OMI) retrieved TCO data independently with Dobson Spectrophotometer TCO measurements over New Delhi, Varanasi, Pune and Kodaikanal. The results of this exercise showed good correlation coefficient (r) of 0.87 (0.88), 0.84 (0.82), 0.91 (0.80) and 0.84 (Data not available) respectively. Climatological mean TCO over New Delhi, Varanasi, Pune and Kodaikanal are 275.02 ± 6.44 DU, 269.03 ± 7.34 DU, 260.78 ± 5.07 DU and 258.71 ± 6.36 DU respectively for the period 1957 to 2015. An increasing trend over New Delhi (0.20 DU year^–1), Pune (0.18 DU year^–1), Kodaikanal (0.14 DU year^–1) and decreasing trend over Varanasi (0.01 DU year^–1) were observed. High significance of TCO trend was found at New Delhi (p-value < 0.0001), Pune (p-value = 0.002) and Kodaikanal (p-value = 0.003) with negligible trend over Varanasi with p-value of 0.84. The TCO variations at different geographical locations associated with upper atmospheric meteorological parameters such as lower Stratospheric Temperature (ST) at 65 hPa and Tropopause Height (TH) were also addressed. Annual lower stratospheric temperature shows positive relationship with TCO and Stratospheric ozone over the study sites. Further, decadal variability in TCO with respect to solar activity at New Delhi was also analyzed.     

Reply to the paper: On the total ozone depletion over Greece derived from satellite flown and ground-based instruments

This note answers questions raised by Cracknell et al. (preceding paper) on total ozone records over Thessaloniki, Greece, It also explains the physical theory and experimental methodology which was used in the objective re-evaluation of the Thessaloniki ozone record, which was also deposited to the WMO/World Ozone Data Centre in Toronto.     

Long-term trends of total ozone content over mid-latitudes of the Northern Hemisphere

Dynamic climatic normals and long-term trends of total ozone in the mid-latitudes of the Northern hemisphere (30°N–60°N) have been determined using data from satellite observations for the period of 1978–2017. The annual course of total ozone is shown as changing over the various regions during the period of observations. The specific features of alteration in the state of the ozone layer are discussed depending on latitude and longitude. Thus, a general increase in total ozone in winter, an increase in spring (with the exception of the northern latitudes of Europe, Asia, and Pacific), and a continuing decrease in summer (with the exception of the northern latitudes of America) during the last 17 years is revealed. The long-term trends of total ozone for different regions and latitude zones (30°N–40°N, 40°N–50°N, and 50°N–60°N) are given depending on season.     

On functioning accuracy of extremal control systems with a nonstationary nonlinear characteristic

The guaranteed estimates of the maximum possible deviation of the output signal of the extremal control system from the current maximum of the nonlinear nonstationary characteristic of the controlled plant, when the behavior information of the disturbing actions and the nonlinear characteristic itself is incomplete, are given.