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.     

Remote sensing ‘ground-based automatic UV / visible spectrometer’ for the study of atmospheric trace gases

An advance remote sensing instrument, the ‘ground-based automatic UV / visible spectrometer’, has been developed indigenously at Pune (18° 31′ N, 73° 55′ E) to cover the spectra (462–498 nm) of zenith sky scattered light. A spectrometry technique is used to find out the vertical column density (VCD) of many atmospheric trace gases, such as NO₂, O₃, H₂O and O₄. The VCDs of these gases are extracted from observed spectra by comparing the magnitude of the differential optical depth (DOD) of each species in the 462–498 nm spectral range. Slant column densities (SCDs) of each species are found to increase with solar zenith angle (SZA), due to the approaching higher path length of sunlight. The VCDs of O₃ and NO₂ derived by the UV / visible spectrometer are compared with the ozone monitoring instrument (OMI) Aura satellite and ground-based Brewer spectrometer data. The compared VCD values are found to be close to satellite and ground-based measurements.     

Zenith scattered light measurement: observations of BrO and OClO

Stratospheric BrO and OClO observations have been made for the first time over a tropical station, Pune (18° 31′ N, 73° 55′ E) using a Differential Optical Absorption Spectroscopy (DOAS) technique by measuring zenith sky scattered light spectra in the wavelength range of 346–358 nm by ultraviolet (UV)/visible spectrometer. The Differential Optical Density (DOD) fitting technique is applied for the right selection of a suitable spectral region for the analysis to minimize interference and poorly fitting absorption features, and also to minimize the residual of the fit. Observed DODs of O₃, NO_2, BrO, OClO, O₄, Rayleigh and Ring are well fitted with the calculated DODs and the percentage DODs are found to vary up to 0.5%, 0.8%, 0.15%, 0.13%, 1.5%, 1.2% and 1.3% respectively. Chlorine and bromine species play an important role in the ozone depletion, hence O₃, NO₂, BrO and OClO Slant Column Densities (SCDs) are derived between 76° and 94° Solar Zenith Angles (SZAs). The SCDs of O₃ are found to be decreased in the twilight period (i.e. between 90° and 94° SZA) in the presence of sufficient BrO and OClO. Total Column Densities (TCDs) of O₃, NO₂, BrO and OClO are derived by UV/visible spectrometry, Brewer spectrometry and satellite-based Scanning Imaging Absorption spectrometer for Atmospheric Cartography (SCIAMACHY) for Pune and the higher latitude station Kanpur (26° 28′ N, 80° 24′ E) during the period 1 April–31 June 2008. The day-to-day variations in O₃ and NO₂ TCDs over Pune are found to be more than over Kanpur. BrO TCDs vary between 1.9?×?10¹³ and 4?×?10¹³ molecules cm^?2 over Pune, which are derived by UV/visible spectrometry, while they vary for the high-altitude station Kanpur between 0.5?×?10¹³ and 3.5?×?10¹³ molecules cm^?2 derived by SCIAMACHY. The OClO TCDs are found to have an increasing trend with variations between 2?×?10¹³ and 4.5?×?10¹³ molecules cm^?2 during the above period.     

Analysis of sunlight absorption spectra related to atmospheric trace gases in the tropics

Zenith sky-scattered light intensity spectra of wavelength ranges of 325–500 nm have been recorded with UV-visible spectrometer over tropical station Pune (18° 31′ N, 73° 55′ E). Zenith scattered light spectra in the spectral range of 346–358 nm are analysed to find out differential optical depth (DOD) for the period 15–18 November 2010. In DOD spectra, depths are noticed at relevant wavelength due to the absorption by atmospheric gases such as NO₂ (nitrogen dioxide), O₃ (ozone), BrO (bromine monoxide), and OClO (chlorine dioxide). These DOD spectra are analysed by a matrix inversion technique to calculate individual DOD spectrum of the gases. The observed and calculated DODs are found to be in a good agreement. The coefficient of determination (R²) between observed and calculated DODs of NO₂, O₃, BrO, OClO, O₄ (oxygen dimer), and Ring effect are observed to be 0.55, 0.77, 0.73, 0.75, 0.82, and 0.91, respectively. Filling-in of solar Fraunhofer lines in the observed zenith scattered sunlight is known as ‘Ring effect’. The slant column densities of the above gases are found to be increased due to increasing absorption path length with solar zenith angles. The vertical column densities (VCDs) of O₃ and NO₂ derived using ground-based spectrometer are compared with the Ozone Monitoring Instrument (OMI) on board Aura satellite during the period 1 March–31 December 2010. The day-to-day variations are found to be similar; however, the percentage differences in VCDs of O₃ between ground-based spectrometer and satellite-based OMI are observed to be varying from 1% to 15%, while for NO₂, they vary from 1% to 10%. Also, the seasonal mean values of VCDs of O₃ and NO₂ are discussed. The O₃ mean values in the rainy season are found to be higher than that of in the summer and winter seasons from both ground- and satellite-based measurement. Whereas, the NO₂ mean values in the winter season are found to be higher than that of in the summer and rainy seasons from both the measurement techniques. The VCDs of O₃ are observed to be lowest in winter season due to the loss of ozone within NO₂ and O₃ reaction active during the winter season.     

Prediction of mean monthly total ozone time series – application of radial basis function network

The primary objective of the present paper is to apply Artificial Neural Network in the form of Radial Basis Function network to predict the mean monthly total ozone concentration over Arosa, Switzerland (46.8° N/9.68° E). The satellite observations of the total ozone content are based on the total ozone observations performed by the ground‐based instrumentation. While analysing the dataset it was found that January, February and March are the months of maximum variability in the mean monthly total ozone over the stated region. Then, these three months were considered as the target months to frame the predictive model. After appropriate training and testing, it was found that Radial Basis Function network is a suitable neural net type for predicting the aforesaid time series. Moreover, this kind of neural net was found most adroit in predicting the mean monthly total ozone in the month of January.     

Comparison of atmospheric CO2 observed by GOSAT and two ground stations in China

The atmospheric carbon dioxide (CO₂) column concentrations observed by the Greenhouse Gases Observing Satellite (GOSAT) and ground stations at Mt Waliguan (36.29° N, 100.90° E) and Lulin (23.47° N, 120.87° E) in China are compared. The data covered time periods from June 2009 to November 2011 for GOSAT and from July 2009 to December 2010 for the ground stations. The GOSAT monthly mean data tend to be generally smaller than those of the ground measurements by 5–10 ppm. The spatial and temporal variations of the atmospheric XCO₂ (dry air, column averaged, molar fraction of CO₂) concentrations, especially in the regions of China, are analysed by using the GOSAT monthly mean data. The variations are more significant in the northern hemisphere than in the southern hemisphere and show relatively high values and obvious fluctuations in the 15° N–45° N latitudinal band. These are generally consistent with the measurements of the Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY) and Atmospheric Infrared Sounder (AIRS). The satellite data show significant seasonal variations, with maximum in April and May and minimum in September and October. This feature is in general agreement with that of the ground observations and previous reports. In the regions of China, the XCO₂ ranged from 355 ppm to 385 ppm with a mean of 374 ppm, which is in agreement with the global concentration.     

An analysis of the distribution of nitrogen dioxide in the south-eastern Mediterranean for the period 1985–1989

The abundance of ozone in the stratosphere depends upon a photochemical cycle in which nitrogen dioxide ( N0₂) plays a predominant role. Although the zonal mean cross-sections for N0₂ averaged for 34 months have been calculated–on the basis of L1MS data–lack of detailed information at local or regional scale could result in difficulties in terms of modelling the distribution of ozone in the stratosphere. In this paper we use SAGE II measurements to calculate the altitude profiles of N0₂ mixing ratios for the geographical region 15 to 30° E and 32·5 to 42·5° N for the period 1985 to 1989.Analysis of the findings shows that N0₂ experiences higher values in summer months, that the N0₂ maximum mixing ratios decrease from 1985 to 1989, that maximum mixing ratios are found at an altitude ofapproximately 33 km, with variations depending on season and year and that N0₂ mixing ratios for the period 1985–1989 are higher compared to the relevant mixing ratios for 1979–1981.     

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.     

Total ozone amount trend at St Petersburg as deduced from Nimbus-7 TOMS observations

The observations of the Total Ozone Mapping Spectrometer (TOMS) flown on the Nimbus-7 satellite have been used in order to detect the monthly trend in total ozone concentration over St Petersburg (60°N, 30°E) during the period from November 1978 to January 1992. The trend analysis suggests that the total ozone depletion over the 13-year period shows strong variations from month to month reaching —13 per cent for December but with a slightly positive trend for September.     

A probe into the chaotic nature of total ozone time series by correlation dimension method

The endeavor of the present paper is to investigate the existence of chaotic behavior in the underlying dynamics of the total ozone concentration over Arosa, Switzerland (9.68°E, 46.78°N). For this purpose, the correlation dimension method is employed to the mean monthly total ozone concentration data collected over a period of 40 years (1932–1971) at the above location. Based on the observation of a low correlation dimension value of 1 for this data set, the study reports the existence of low-dimensional chaotic behavior in the ozone concentration dynamics.     

Unusual diurnal variation in surface ozone observed after the 26 December 2004 tsunami over the rural site of Bay of Bengal,India

Measurements of surface ozone (O₃) and nitrogen dioxide (NO₂) were studied from December 2004 to February 2005, covering the giant tsunami event on 26 December 2004 at Tranquebar (11° N, 79.9° E, 9 m) over the west coast of the Bay of Bengal, India. An unusual maximum O₃ concentration of 28 parts per billion by volume (ppbv) was observed in the morning and a minimum (16 ppbv) in the evening, indicating that pronounced chemical loss of O₃ occurred in the daytime after the tsunami over this coastal region. An increase in NO₂ concentration from 5.6 ppbv before the tsunami to 10.5 ppbv after the tsunami was observed in the daytime. The observed unusual diurnal changes in O₃ were not due to mass transport processes as the five-day back trajectories of air parcels transport before and after the tsunami remained unchanged. Similarly, meteorological and micrometeorological parameters were found to be normal before and after the tsunami. The unusual low O₃ level during the daytime was possibly due to prolonged excess emission of iodocarbons from the sea surface after the tsunami, which resulted in enhanced inorganic iodine (I _x ) concentration, leading to massive destruction of O₃. Similarly, unusually high O₃ levels during the night-time were possibly due to the intrusion of ozone-rich air after the tsunami from the free troposphere into the surface layer when the boundary layer height shrinks after midnight. The present work can be extended on a regional scale by incorporating modelling studies using recent remote sensing tools.     

Roles of ozone depleting substances and solar activity in observed long-term trends in total ozone column over Indian region

An additive time-series decomposition analysis was performed on the Multi Sensor Reanalysis-2 (MSR-2) monthly mean total ozone column (TOC) time-series dataset spanning over 34 years (January 1979–December 2012) for Indian region (0.0–40.0 °N; 67.5–97.5° E). Statistically significant (p-value <0.05) long-term trends in TOC were estimated in the deseasonalized TOC time series. The role of multiple natural and anthropogenic factors: quasi biennial oscillations (QBO), El-Nino Southern Oscillations (ENSO), cyclic variation in solar activity (SA), and ozone depleting substances (ODS) was investigated to explain the long-term trends in TOC over Indian region. Over sub-tropical Indian region (25.0° N– 40.0° N), declining long-term linear trends were estimated, which varied from ?0.30% to ?1.10% per decade. Interestingly a positive long-term linear-trend (0.10–0.30% per decade) was observed over equatorial-tropical part of Indian region. No statistically significant long-term trend was observed for 30mb Equatorial Zonal Winds and Nino 3.4 index – indicators for QBO and ENSO; however, a positive long-term linear trend of magnitude 17.00 ± 1.18% per decade was observed in effective equivalent stratospheric chlorine – a proxy for ODS, and a negative long-term linear trend of magnitude 12.72 ± 2.86% per decade was observed in 10.7 cm Solar Radio Flux – a representative for SF. It is inferred that over the Indian region above tropic of cancer, about 85.00% of the estimated negative long-term linear trend in TOC can be explained by the increase in the stratospheric concentration of ODS; whereas, decrease in the solar activity accounted for 15.00% of the estimated negative long-term linear trend in TOC over sub-tropical Indian region.     

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).     

Intercomparison of ozone models derived by remote and in situ sensing techniques with recent local measurements at middle latitudes

In the framework of the European Arctic Stratospheric Ozone Experiment (1991–1992) a scries of balloon ascents for ozone and temperature in situ measurements up to 35 km height have been performed at Athens, Greece (38° N, 24° E). This is the first time that such an intensive sounding campaign has been performed in Athens. 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. The comparison shows that at the middle stratosphere there is very good agreement between the Satellite Reference Model and the in situ ozone measurements. There is also very good agreement between the Satellite Reference Model and the in situ temperature measurements, thus confirming the recently published findings by Varotsos and Helmis.     

Variation of total column ozone along the monsoon trough region over north India

This study examined the total column ozone (TCO) variations over New Delhi (28.65° N, 77.217° E) and Varanasi (25.32° N, 83.03° E), which lie along the monsoon trough region, and over the tropical station Kodaikanal (10.23° N, 77.46° E), which lies outside the monsoon trough. Monthly, seasonal and annual TCO variations were determined using data from ground-based Dobson spectrophotometers during 2000–2008, Brewer spectrophotometers during 2000–2005 and the satellite-based Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY) during 2002–2008. We found that Dobson, Brewer and SCIAMACHY TCO variations showed negative trends, indicating a decreasing tendency during the period studied at all three stations. Over Varanasi, the trend decreased further by about 3 DU year^?1. Quasi-Biennial Oscillation (QBO) influences were seen in the time series of TCO over New Delhi and Varanasi, and weaker QBO signals over Kodaikanal. Comparisons were made between ground-based Dobson and Brewer spectrophotometer and SCIAMACHY satellite monthly mean TCO values. The differences between SCIAMACHY and Dobson TCO were 0.4–4.2% for New Delhi and 2.3–6.2% for Varanasi. The differences between SCIAMACHY and Brewer TCO values were 2.0–6.4% for Kodaikanal. In the peak monsoon months (July and August), decreases in TCO values over New Delhi and Varanasi (the monsoon trough region) may be due to the deep convection present during the monsoon season. During the monsoon season, several intense cyclonic systems appear over the monsoon trough region and may cause lowering of the TCO. Kodaikanal shows opposite features, with high values being observed during the peak monsoon months. TCO values over New Delhi were found to be higher than those over Varanasi and Kodaikanal, and TCO values over Varanasi were higher than over Kodaikanal. It was concluded that TCO values increase with increasing latitude.     

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.     

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.     

Ozone depletion over Scotland as derived from Nimbus-7 TOMS measurements

The TOMS (Total Ozone Mapping Spectrometer) flown on the Nimbus-7 satellite has been measuring the total ozone concentration over the globe since November 1978. Recent investigations based on TOMS data have shown that in the latitude belt 40–70° N the spring ozone depletion rate reaches the value of —0·8 per cent per year. This paper reports trends derived from the TOMS reprocessed total ozone data for the case of Dundee (56·5°N, 3°W) from January 1979 to January 1992. The depletion rate of the mean monthly total ozone concentration over this 13-year period shows a strong variation from more than —15 per cent in December and January to about +0·5 per cent in February and June, while the overall mean is about —7 per cent.     

Aerosol optical properties from sun photometric measurements in Hangzhou district,China

Along with fast economic development, the Yangtze Delta region is experiencing rapid environmental changes. A high concentration of aerosols with diverse properties is emitted in this region, providing a unique opportunity for understanding the impact of environmental change on climate systems, especially in urban regions. Aerosol optical properties including aerosol optical depth (AOD), the Ångström exponent (α), and water vapour and their relationship with aerosol particle concentrations were examined using ground-based measurements at three sites in the Hangzhou district: Hangzhou (120.01° E, 30.26° N), Lin'an (119.73° E, 30.26° N) and Qiandaohu (119.05° E, 29.55° N). The results from these observations show that there exists strong temporal and spatial variation in aerosol optical properties and particle matter (PM) concentrations. Monthly averaged AODs at the Hangzhou and Lin'an sites show a similar trend. AODs in Qiandaohu have the highest values in winter and spring and the lowest values in summer and autumn. In 2008, the minimum monthly mean α for every site was lowest in December due to the weather conditions. From spring to summer, these three locations experienced a drop in AOD and an increase in α due to the humidity-swelling effect. There is a positive correlation between AOD and aerosol particle concentrations; the correlation coefficients were obtained as 0.40, 0.55 and 0.69 in Hangzhou, Lin'an and Qiandaohu, respectively. The association between daily PM₁₀ concentration and AOD is marginally statistically significant at the three sites, which suggests that AOD is a useful tool for mapping PM distribution over large spatial domains.