A historical overview is presented of Dobson spectrophotometer total ozone observations made by the U.S.A.'s National Oceanic and Atmospheric Administration (NOAA) since the early 1960s. NOAA's contributions in upgrading the quality of total ozone measurements in the global Dobson instrument network, and in providing `ground truth' to satellite ozone observations, are also briefly described. Because ozone trend estimates to within +/- 1% per decade require highest quality data, a program was initiated in 1992 to reprocess the NOAA data with application to the data of final corrections. Trend results for a number of stations are presented and compared with trends derived from satellite instrument ozone measurements. Ozone variations and the downward trend in global ozone of recent years, including the accelerated ozone decrease that began in 1992 and has continued into 1993, are discussed in light of photochemical and dynamical processes that are potentially capable of altering global ozone abundance. Although the Mount Pinatubo eruption of 1991 is suspected of contributing to the most recent decrease in ozone, the mechanisms responsible for the ozone changes observed more than a year after the eruption remain under investigation.

Total ozone measurements have been performed at Uccle (50 degree(s)48'N, 4 degree(s)21'E) with Dobson spectrophotometer 40 since August 1971 and with Brewer instrument 16 since July 1983. The reduced thickness of sulfur dioxide (SO₂) as calculated from measurements with the Brewer spectrophotometer shows a decreasing trend that is in good agreement with the trend of in situ measurements of SO₂ density near the ground in the urban area of Brussels. From the mean annual variation of the ratio of both variables the corresponding correction for SO₂ interference of the Dobson instrument data was calculated on a month-by-month basis. From a comparison of the total ozone trends before and after SO₂ correction, it appeared that over the whole time period (August 1971 to May 1993) the SO₂ decrease at Uccle has induced a fictitious Dobson total ozone trend that amounts to a mean value of -1.47% per decade. More than half of the stations which have been used in the past in ozone trend studies are located within an urban area with a population of over 100,000 and are in consequence possibly affected by air pollution.

Rnmd iajor volcanic eruptions have a dramatic influence on the global ozone layer. The effect has to be evaluated when trends are evaluated for limited periods, 10 - 20 years. In this study the influence of the eruptions of El Chichon is thoroughly studied, and the depleting effects of the eruptions of Mt. Agung and Mt. Pinatubo are identified.

The Oslo ozone station on 60 degree(s) N has been in continuous operation since 1978. The measured ozone values for this station seem to be rather representative for the Arctic region. All the observed data have recently been reanalyzed and the results are given. In the period from 1978 to 1993 the ozone decrease is approximately 4.5%. The winter values (average of December, January, February, and March) decreased by approximately 9% whereas the summer values (average of May, June, July and August) decreased 2.3% in the 15 year period. The volcano eruptions from El Chichon and Mt. Pinatubo seem to influence the ozone layer.

The Dobson station situated at Reykjavik (64 degree(s)08'N, 21 degree(s)54'W), Iceland is one of the few sites where a continuous long term monitoring of total ozone has been performed since the establishment of the Dobson network in 1957. A thorough analysis of the data has been performed with the aid of a statistical model which was developed to reproduce the data and quantify the role of different physical parameters in determining ozone variations. The data set for the entire period has been critically examined for possible inconsistencies. Trend analysis shows a statistically meaningful decline of about 6.5% over the last 14 years during the summer. While no significant wintertime ozone decline is detected since 1977 the annual amount declines at a rate of 0.03 +/- 0.11% per year which is about three times the rate over the earlier twenty years. Signatures of the 11 year solar cycle are exceedingly large during the early months of the year and in the westerly phase of the QBO. Comparison with the TOMS data shows an excellent agreement at times of satellite overpass. Strong responses to major solar proton events and atmospheric nuclear bomb testings appear in the data. On shorter time scales the variations are largely determined by tropospheric synoptic scale disturbances.

The Airborne Arctic Stratospheric Expedition (AASE) carried out measurements from Jan. 3 to Feb. 15, 1989. Enhanced levels of chlorine compounds were found in the Arctic stratosphere, and on two single flights ozone decrease of 17% were measured, interpreted as essential features of the Arctic stratosphere, caused by a combined effect of enhanced amounts of chlorine compounds and the presence of polar stratospheric clouds. Related model calculations also indicate extended ozone depletion maximizing in late March 1989 and amount to 5 - 8% in column at 70 degree(s) N. Ground-based ozone measurements, however, show that the most characteristic features during this period are temporal variations and a strong enhancement of ozone, probably due to an extended stratospheric warming. From these measurements it is hard to see any effect of an eventual enhanced burden of stratospheric chlorine, which might show up as an extended and long-lasting decrease of stratospheric ozone, but its eventual existence is masked by the temporal variations.

UV-Vis spectrometry measurements at zenith were carried out at Keflavik (64 N, 23 W) during the EASOE Campaign in the winter of 1991/92. During the middle of January, a situation of low ozone was observed associated to a very low temperature field in the lower stratosphere. Ozonesounding on day 15 January 1992 exhibits two different patterns, up and down 70 hPa. From the potential vorticity fields and backward trajectories analysis, the different origin of the air masses at the two layers are shown. While at the lower levels the ozone deficiency is the result of low latitudes advection, at the upper levels a more structured profile of 30% less than 10 days before is found to be air from inside the vortex presumably processed by PSC. The partial chemical depletion is supported by the very low NO₂ content measured at the same period at the station.

In this work the influence of solar protons of ground level event (GLE) type on stratospheric ozone were investigated. The asymmetry of ozone `mini-holes' caused by GLEs between the Arctic and Antarctic polar caps (N/S asymmetry) in terms of their spatial variations and possible mechanisms which are responsible for this asymmetry are discussed. There were considered GLE events: 29.09.89; 19.10.89; 24.10.89 and 21 - 28 May 1990 on data of balloon measurements and ground based ozone data of some Arctic and Antarctic observatories.

In a number of experiments on ozone layer investigations the short-term ozone variations with hour or several hours periods at heights 20 - 50 km were observed. We suppose that this phenomenon may originate due to interaction of internal gravity waves with stratified shear flow in the vicinity of critical layer, where the phase velocity of the wave is equal to flow velocity. The amplitude and period of ozone density oscillations are found in the gravity wave of constant amplitude, when the flow pattern near the critical layer is determined by nonlinearity, and viscosity effects are small. We have found that the density variation of passive admixture can be comparable with unperturbed value.

The radiance of large, ice covered land masses has been used to monitor TOMS instrument sensitivities. Greenland and Antarctica provide uniform and stable ice surfaces whose average albedo appears to be constant within the desired accuracy for instrument monitoring. Instrument radiance response will depend upon view and illumination angles, the sun-earth distance, and atmospheric conditions. Restriction to nadir views eliminates view angle dependence, and corrections are made for sun-earth distance. The effect of atmospheric conditions, such as ozone and clouds, is minimized by monitoring at wavelengths above 340 nm and by the high surface radiance. Relative instrument response is determined by the ratio of signals measured at different times using a binning technique to account for differences in solar illumination angles. The only remaining limit to long term monitoring accuracy is the albedo stability of the ice surface itself. Changes in the Nimbus-7/TOMS instrument response at long wavelengths are monitored within 1% accuracy over the lifetime of the instrument.

The Total Ozone Mapping Spectrometer (TOMS) ozone measurement is derived by comparing measured backscattered ultraviolet (BUV) radiances with theoretical radiances which are pre- computed using standard climatological ozone profiles and stored in a look-up table. Profile shape errors occur in this algorithm at high latitudes (or more specifically, high optical path lengths) when the actual vertical ozone distribution differs significantly from the standard profile used in constructing the tables. These errors are estimated using sensitivities derived from radiative transfer calculations and measurements of the actual ozone profile from Solar Backscatter Ultraviolet (SBUV) and Balloonsonde. These estimates include a short term uncertainty with a standard deviation of 10% in total column ozone amount as well as a systematic error in the long-term trend at very high solar zenith angles. At the maximum retrieval solar zenith angle of 88 degrees, these calculations indicate that TOMS long-term ozone depletions are over-estimated by 5%/decade.

The impact of chlorofluorocarbon (CFC) release on the earth's ozone shield is becoming more evident with every passing year. It is imperative that the long term continuous monitoring of this environmental global experiment proceeds by every means possible. The National Oceanic and Atmospheric Administration (NOAA) has 14 years of TIROS Operational Vertical Sounder (TOVS) earth radiation data that includes ozone sensitive 9.7 micrometer information. The current 14 year sequence of daily TOVS ozone maps from February 1979 to May 1993 covers a period of unprecedented decline of the earth's ozone shield. Recent Antarctic ozone holes are developing earlier, growing larger and lasting longer than ever before. While there is no evidence as yet of persistent Arctic `sub-250 Dobson Unit' ozone values, the robust Arctic spring ozone peaks of earlier decades have shrunk by a third. Worst of all, 1993 Northern mid-latitude TOVS ozone values dropped to levels 11% below normal. TOVS ozone data is essential for understanding the recent polar ozone declines and their relationship to ozone declines in populous temperate zone regions.

Accurate, detailed maps of total ozone were not available until the launch of the Total Ozone Mapping Spectrometer (TOMS) in late 1978. However, the Scanning Radiometer (SR) on NOAA satellites during the 1970s included a visible channel that overlapped closely with the Chappuis absorption band of ozone around 600 nm. We use a simple calculation of two-way transmittance to obtain total ozone from data taken by the Advanced Very High Resolution Radiometer (AVHRR). THe AVHRR was the successor to the SR on NOAA satellites, and because it flew during the 1980s, we are able to compare our derived ozone amounts with those derived from TOMS. Our method works only over scenes whose albedos are large and unvarying.

We discuss the results of lidar observations about aerosol and ozone in the stratosphere obtained over the course of a year (June 1991 - April 1992) and being of current interest owing to a great deal of aerosol appearing in the stratosphere due to the eruption of Pinatubo volcano in June 1991. These studies have aroused considerable interest because of possible interaction of volcanic aerosol with ozone which results in ozone reduction.

The aerosol correction of the results oflaser sounding of stratospheric ozone concentration obtained by the method of differential absorption in the presence of powerful aerosol formations (as a rule of volcanic origin) is one of the "hottest" are as in current research.

We present some results of complex gas analysis of a tropospheric ozone cycle using a gas analyzer `TRAL' along a specially constructed slightly tilted path between the two buildings of Tomsk Akademgorodok (the distance between these buildings was approximately 0.5 km; the height of the path was 12 to 18 m at its terminal points). Included are temporal profiles of ozone (O₃) and carbon dioxide (CO₂) concentrations. To increase the gas-analysis sensitivity, especially in determining background concentrations of ozone, four runs of a laser beam along the 2 km path were used.

The data obtained in 1989 during the launchings to the stratosphere of a two-cascade impactor from the test ground in Apatity have been discussed. The aerosol samples have been analyzed using an electronic microscope to have information on the structure and size distribution of aerosol particles. The chemical and elemental analyses have been made using the methods of mass-spectrometry, IR spectroscopy, neutron activation, and x-ray fluorescence.

The ground-based ozone monitoring network in Canada began operating in the late 1950s and since then has expanded as a result of the increasing concerns regarding depletion of the ozone layer. Since the late 1980s routine measurements of UV-B radiation have been made in addition to stratospheric ozone. Ozone data from the Canadian network have contributed to the determination of global trends in stratospheric ozone. Real-time measurements of ozone and UV-B are used operationally in the Ozone Watch and UV Index programs which were established in 1992 to advise the Canadian public on the current state of the ozone layer and UV-B radiation. Some results from analysis of ozone and UV-B measurements are given and the Ozone Watch and UV Index programs are described.

Radiative perturbation theory is a technique for calculating the influence of selected atmospheric variations on certain radiative effects, such as fluxes, heating rates, etc. This technique has already demonstrated its utility by its ability to handle the wide variability of aerosol properties. In the present study, we have applied the technique to an examination of changes in deleterious UV radiation which reaches the ground as a function of changing levels of both ozone and stratospheric aerosol. Perturbation theory permits us to write our results in a particularly simple form, with an accuracy at least as good as the available biological response data. Currently, we are turning our attention to the photochemistry involved in the production and destruction of stratospheric ozone. In collaboration with colleagues at the C.S.I.R.O., we are examining the usefulness of perturbation theory to the calculation of actinic fluxes in the Schuman-Runge bands of oxygen. Progress in this area is reported.

Surface ozone measurements are running at The Auroral Observatory, Tromso. In this report measurements throughout the period Nov. 17, 1992 to Jan. 11, 1993 are given. No diurnal variations can be seen, probably due to the lack of sunlight during the polar night. However, peculiar ozone decreases occur normally during the day time when substantial traffic exists in the nearby town, and it is considered that the ozone is depleted through reactions with pollutants such as nitric oxides and hydro carbons. The American ozonometer has proved its reliability through world-wide use, and the Russian ozonometer gives almost identical results. In addition the Russian ozonometer is most userfriendly. The Russian ozonometer is still under development, and our device is a prototype made for testing.

Variations of ozone concentration (O₃) and intensity of solar radiation (ISR) near the earth surface with periods 3.1 (mu) at 6.2 min were observed. The ozone measurements were carried out with the help of special chemiluminescent ozonometer.

Troposphere ozone concentration has been monitored since March 1991 at the Mt. Cimone Observatory (44 degree(s)12' N, 10 degree(s)42' E, 2165 m asl), a mountain site far from sources of anthropogenic pollution. The yearly trends of surface ozone concentration clearly evince a variation marked by peaks in spring and summer. That no ozone diurnal variations are usually found in late autumn, winter and early spring means that Mt. Cimone can be considered a baseline station over this period. In the warmer months, the daily ozone variation is affected by local mixing with the upwelling of ozone-poor low tropospheric air over the mid-daytime hours. Screening out the latter values, as well as those induced by horizontal transport phenomena from the pollutant-rich Po valley lowlands, yields free tropospheric ozone concentrations.

Continuous measurements of surface ozone have been taken since August 1991 at Belsk, Poland. Distinct temporal variability of ozone concentration is observed: seasonal variation with maximum in summer months and minimum in autumn months and diurnal variation especially pronounced from March to October. Daily maximum occurs in the afternoon, while daily minimum occurs before sunrise. A certain number of nocturnal maximums occur especially in autumn and winter. Extremely high values (up to 96 ppb), probably caused by intense local photochemical production of ozone, was observed in August 1992 simultaneously with extremely high values of temperature. The results of UVB, temperature, and ozone measurements are positively correlated in the summer months. Correlation coefficient between ozone and temperature reached .86, between ozone and UVB radiation reached .74 in June 1992. For a relatively warm and sunny period in November 1991 correlation coefficient between ozone concentration and UVB radiation reached .93.

Surface ozone measurements within the city limits are presented. Measurements have been performed by a chemiluminescence ozonometer at various sites of industrial and residential areas and in the suburbs as well. The background ozone level is 15 - 25 ppb. Weak diurnal variation is observed. Decremental O₃ concentrations in the city are in some tens of percentage in the winter period. The urban plume is extended at least up to 10 km from the city limits.

The interaction of ozone with oxides occurring in aerosols (Al₂O₃, ZnO, MgO, TiO₂) at temperatures 22 divided by -63.5 degree(s)C was provided. Laboratory experiments have shown that activity of investigated oxides with respect to ozone grows under irradiation of their surface. To calculate correctly atmospheric ozone aerosol sink one must take account of both heterogeneous processes proceeding without action of light and photocatalytic ones.

The kinetics of stratospheric sulfuric acid aerosols accumulation is modeled. The chemical scheme includes 10 main reactions of the oxygen cycle of ozone and SO₂ photo-oxidation. Accounting of SO₂ photo-oxidation by the ozone is the peculiarity of this scheme. This reaction proceeds with participation of triplet excited SO₂ molecules, its rate constant being equal to approximately 10^-12cm³s^-1. Satisfactory agreement between calculated and experimental temporary variations of sulfuric acid aerosols concentration was obtained.

The lidar measurements at Verhnetulomski Observatory ((phi) equals 68.6 degree(s)N, (lambda) equals 31.8 degree(s)E) in the Murmansk region detected the considerable increase of stratospheric aerosol concentration after a solar proton event of GLE (ground level event) type at 16.02.1984. This increase at 17 km altitude reached 40% at 20.02.1984. Some details of trigger influence of high energetic incident solar protons on stratospheric ozone layer, including aerosol formations, are discussed.

Ozone vertical profile measurements on Kola peninsula are presented. The data have been obtained using tethered balloons up to altitudes of about 0.5 km, tethered sondes up to 120 m, and mountainous relief up to 1 km. The observations have been performed at various meteorological conditions and seasons. The possibilities of each method are considered.

An ordinary photochemical model of (O) and (O₃) distribution is modified to include additional sources of atomic oxygen formation due to the dissociation by electron and proton impact as well as to the dissociative recombination of ions O₂⁺ and NO⁺. It is shown that the atomic oxygen density depends linearly on ion production rate in D-region during AA and PCA disturbances. The paper shows that the additional sources of atomic oxygen substantially affect the distribution of ozone on the heights of ionospheric D-region. The calculation of (O) and (O₃) density height distribution for the specific AA and PCA disturbances is performed. Results are compared with experimental rocket (O) and (O₃) profiles.

There are several types of instruments that enable us to measure the ozone content in different conditions (in the outdoors, in moving platforms, in industrial buildings, etc.). They operate on the basis of different physical and chemical processes and have different values of parameters such as sensitivity, accuracy, response time, etc. The model of photometer for measuring the ozone concentration is described below. The instrument acts due to selective absorption of UV-radiation by ozone molecule. The distinguishing peculiarity of this device is the use of radioluminescent source (RLS) with very high long-term temporal emission stability instead of the generally used unstable mercury lamp. The RLS emission power is 3 (DOT) 10¹¹ quantum/s. Three subsystems (optical, gas-flow, and electronic) making up this photometer are described in detail. The approximate estimates of photometer threshold sensitivity and accuracy are made and the possible means for improving these parameters are outlined.

Even though observational data on atmospheric ozone has been steadily increasing during the last few decades, ozone in the upper stratosphere is still poorly resolved. A feasible method for retrieving this information is ground-based spectrophotometer observation of the Umkehr effect. At Sodankyla Observatory in northern Finland, Umkehr-observations have been performed for some years now. Here we present the first results from this work and show some of the potential of the technique.

At the German Arctic station (Koldewey station, 78.9 degree(s)N, 1.9 degree(s)E) in Ny- Alesund we are operating a combined Ozone & Aerosol LIDAR which is capable to measure stratospheric profiles in the UV at 308 nm and 353 nm and in the visible at 532 nm at night as well as during daytime. In order to extend our measurements further down into the troposphere we developed a new detector unit. In our experimental set-up used up to now the backscattered light from lower altitudes was completely blocked by means of a mechanical chopper to avoid saturation of the photomultipliers. A new design of the chopper will block the signal differentially which permits the receiving of a well-defined part of the signal. In this way we can obtain ozone profiles down to altitudes of about 5 km. In comparison to other tropospheric LIDARS, which require two acquisition systems for different altitude ranges, our design has the advantage that the better resolution of photon counting can be used for the whole range from 5 to 40 km altitude. The new detector unit has a completely new optical set- up. The alteration between night- and daytime configurations have been simplified and respective Raman-shifted signals by atmospheric N₂: 332, 385, and 607 nm have been added. These latter signals allow an aerosol correction to the ozone signal which is presently necessary due to the strong input of the volcanic aerosol in the stratosphere originating from the Mount Pinatubo eruption in 1991 and the generally higher aerosol content of the troposphere.

The Brewer Instrument in Søndre Strømfjord in Greenland (66.0 °N, 54.0 °W) operated by the Danish Met. Office, has been measuring ozone and UV-radiation since spring 1991 —before the Mt. Pinatubo eruption. Six wavelengths are involved in the ozone measurements: (302.2 nm), 306.3 nm, 310.0 nm, 313.5 nm, 316.8 nm and 320.0 nm 302.2 nm is used for calibration purposes. It occurred to us that the redundant information might be used to obtain information about the aerosol optical depth defined through the relations: I = Ioexp(—trn) t = tqg + tsg + tp I and Io denote solar irradiances at Earth' surface and outside the atmosphere respectively. m denotes total airmass (1/ cos 0 or Chapman Function). tag is optical depth due to absorption in gasses — mainly ozone, but sometimes also SO2. t is the Rayleigh scattering term and t, is the aerosol optical depth, which we aim at determining