GLOBAL TRENDS

Coordinator

Principal Authors

Other Contributors

2.0 INTRODUCTION

Measuring trends in ozone, and most other geophysical variables, requires that a small systematic change with time be determined from signals that have large periodic and aperiodic variations. Their time scales range from the day-to-day changes due to atmospheric motions through seasonal and annual variations to 11-year cycles resulting from changes in the sun ultraviolet output. Aperiodic variations include the irregular quasi-biennial oscillation, with a period of roughly 26-28 months, approximately 4-year variations, and other sources of interannual differences.

Because the magnitude of all of these variations is not well known and highly variable, it is necessary to measure over more than one period of the variations to remove their effects. This means at least 2 or more times the 11-year sunspot cycle. Thus, the first requirement is for a long-term data record. The second related requirement is that the record be consistent; a small effect is being sought, and changes in instrumentation or data analysis method will obscure changes in the atmosphere. A third requirement is for reasonable global sampling, to ensure that the effects are representative of the entire Earth. Therefore, the various observational methods relevant to trend detection are reviewed in Section 2.1 to characterize their quality and time and space coverage. Available data are then examined for long-term trends or recent changes in ozone total content (Section 2.2) and vertical distribution (2.3), as well as in related parameters such as stratospheric temperature (2.4), source gases and tropospheric ozone (2.5), and aerosols (2.6). The relation between trends in total column ozone and variations observed in the solar ultraviolet radiation at the ground are discussed in Section 2.7, and outstanding issues in trends detection are emphasized in Section 2.8.

2.1 OBSERVATIONAL METHODS RELEVANT TO TREND DETECTION

2.1.1 Introduction

For the individual measurements, the most critical quantity is stability, or the absence of time-dependent systematic errors. A systematic error, i.e., an error that is consistently present, is the amount by which the mean of a large number of individual observations of the same ozone value could differ from the "true" value. This is also referred to as the accuracy. The relevance of the accuracy to trend determination is further discussed in Section 2.1.6.

In the following sections, the term precision is used to refer to the random variations or spread of values that an instrument would report when observing a constant ozone value. It is sometimes referred to as repeatability, and depends on the random errors of the measuring system. This is important for many studies of atmospheric processes, and could be important for trend studies when only a few observations of a particular kind are available. However, most quantities compared in trend studies involve averaging a large amount of data, reducing the spread, so precision is often not of primary importance.

The section is organized as follows: Section 2.1.2 describes the measurements that have resulted in the data that have been used to derive trends, or closely related observations. The techniques that are expected to add to the trend data for future analyses are presented in Section 2.1.3. Both of these sections are divided into ground-based observations (including balloons or rockets launched from the ground) and satellite observations. Intercomparisons of different types of data, and what they indicate about the capabilities of the different techniques, are described in 2.1.4, which is divided into sections on present and future trend data. This portion of the chapter concludes with brief discussions of the sampling requirements in time and space (2.1.5) and an overview of trend-measuring capabilities (2.1.6).

2.1.2 Measurements Used in Current Trend Analyses

2.1.2.1 Total Ozone Measurements

Absorption spectroscopy affords a sensitive means for monitoring the column abundance of ozone from the ground. The attenuation of monochromatic radiation is related to the number of absorbing molecules in the optical path as they undergo state transitions that absorb incoming radiation. Absorption features in the ultraviolet and visible regions of the spectrum mainly involve electronic transitions and hence are generally relatively insensitive to temperature and pressure, which is a feature that considerably simplifies the reduction of the data to slant-column abundances.

The source of incoming light may be a celestial body, usually the sun, but also the moon, or a star, or it may be the light scattered from the sunlit zenith sky. Direct-light absorption measurements can be carried out only during clear periods from the ground, since an unobstructed view of the light source is required. Scattered light measurements can be obtained in cloudy weather by some absorption systems.

Dobson Spectrophotometer

The standard instrument in the Global Ozone Observing System is the Dobson spectrophotometer (Dobson, 1957). The instrument is a quartz double monochromator which measures the relative intensities of pair wavelengths in the Huggins ozone absorption band (300-350 nm) from which total ozone in a vertical column of the atmosphere can be deduced. Most precise observations are made on direct sunlight and standard double-pair wavelengths designated AD (305.5/325.4 and 317.6/339.8 nm). Less precise observations are made on clear or cloudy zenith skylight or, infrequently, moonlight. (For a more detailed description of the Dobson spectrophotometer and its operation, see OTP.) Direct sun AD observations generally provide the most precise measurements if the secant of the zenith angle, , is less than 3.

Long-term ozone measurement precision for the Dobson spectrophotometer, for annual means, is estimated to be 1% (at the 2 level), based on the standard deviation o from analyses of mean data from individual stations (WMO, 1980, 1981, 1982). Attainment of this precision requires that observations at all times be made on correct wavelengths; that changes in spectral characteristics of the instrument be accounted for using data derived from periodic standard lamp tests; that recalibration of the wedge be performed, if evidence suggests it is needed; that periodic recalibrations of the instrument be performed by the Langley method (Dobson and Normand, 1962) or through intercalibration with a primary standard Dobson instrument (Komhyr et al., 1989); that observational errors be minimized; that the temperature dependence of the ozone absorption coefficients be taken into account; and that relations derived empirically between direct sun and clear or cloudy zenith sky observations be adequately quantified. For use of the total ozone data in global trend analyses, it is necessary, furthermore, that observations be made on a sufficient number of days each month in order to obtain representative monthly mean data (see Section 2.1.5), and that the observations not be unduly influenced by local interfering absorbing species, such as sulfur dioxide and nitrogen dioxide or ozone produced photochemically in locally polluted air.

Until 1968, Dobson instrument calibrations at different stations were generally conducted randomly and independently. From 1974 onwards, increasing numbers of instruments have been modernized, refurbished, and calibrated by direct intercomparison with the WMO designated World Primary Standard Dobson Spectrophotometer No. 83, maintained at the World Dobson Spectrophotometer Central Laboratory in Boulder, Colorado. In subsequent intercomparisons, these instruments have shown typical calibration changes of 0-2% for direct sun observations on AD wavelengths. The long-term (1962-1987) ozone measurement stability of Primary Standard Dobson Instrument No. 83 is reported to have been maintained at \'b10.5% (Komhyr et al., 1989). Since the mid-1970s, virtually all (90) Dobson instruments of the global Dobson instrument station network have been calibrated several times either directly with instrument No. 83 or indirectly through intercalibrations with Secondary Standard Dobson spectrophotometers calibrated in Boulder in 1977.

The uneven geographical distribution of the existing Dobson spectrophotometer network (Figure 2.1-2) gives rise to a spatial sampling error when attempts are made to determine global ozone content and trends. At most, the Dobson instruments can provide trend information for specific regions of the globe. An important utility of the Dobson instruments is their ability to provide correlative data for satellite instruments that measure ozone on a global scale (Fleig et al., 1986; 1989a; 1989b; Bojkov et al., 1988; McPeters and Komhyr, 1989), but that are subject to calibration drifts and are not highly sensitive to tropospheric ozone.

Filter Ozonometers M-83 and M-124

Since 1957, routine ground-based total column ozone measurements have been made at more than 40 stations in the USSR using a filter ozonometer instrument designated as type M-83 (not to be confused with the Standard Dobson ozone spectrophotometer, which was the 83rd instrument manufactured of the Dobson type). The filter-type instrument is based upon the same principle as the Dobson spectrophotometer in using differential absorption of ultraviolet radiation in the 300-350 nm Huggins band of ozone. The M-83 instrument, however, uses two broadband filters and measures the relative attenuation of the solar ultraviolet radiances either directly from the sun or indirectly from the zenith sky (Gustin, 1963).

Direct intercomparisons between M-83 filter instruments and Dobson spectrophotometers prior to 1971 (Bojkov, 1969) revealed that the M-83 records show 65% less ozone when the observations are restricted to an air mass < 1.5, and 20% to 30% more ozone when data are taken for > 2.0. A strong dependence on turbidity was also detected, with 9% to 14% higher ozone readings when the surface visibility was less than 5 km. These strong deviations for > 2.0 make many of the high-latitude measurements in the USSR very uncertain, especially for measurements prior to 1972, as described below.

Improved filters were introduced into the M-83 instrument starting in 1972-1973 (Gustin, 1978). The new filters have maximum transmittance at 301 nm and 326 nm, and their band passes are less than those in the earlier version: 22 nm (291-312 nm) and 15 nm (319-334 nm). Comparison of Nimbus-4 BUV satellite overpasses over M-83 stations in the USSR demonstrated a standard deviation of about 50 Dobson Units before 1973, and about 25 DU afterward (WMO, 1980, 1983). The Nimbus-4 BUV overpasses of Dobson stations maintained a standard deviation of about 17 DU during the 1970-1977 lifetime of the satellite.

A much newer, reportedly improved instrument designated as M-124 has been installed in many stations since 1986 (Gustin and Sokolenko, 1985), but no ozone data have been reported yet for this instrument. No trend data with the M-124 can be expected for about a decade unless the data can be satisfactorily cross-calibrated with the M-83 data from the same location.

Satellite observations

Total Ozone Monitoring Spectrometer (TOMS)

The TOMS, launched on the Nimbus-7 spacecraft in 1978, is an instrument (Heath et al., 1975) whose primary measurement goal is to obtain contiguous mapping of the total column ozone amount over the globe (Bowman and Krueger, 1985; Schoeberl et al., 1986). To achieve this, TOMS step scans across the sub-orbital track, sampling radiation backscattered from the underlying surface and atmosphere. Ozone column amounts are inferred by utilizing the wavelength dependence of the Earth's ultraviolet albedo in the Huggins band of the ozone absorption spectrum. The TOMS raw data are measurements of the direct and backscattered solar UV radiation at six fixed wavelength channels (312.5, 317.5, 331.2, 339.8, 360, and 380 nm). Data from the first four channels are used in pairs to provide three estimates of the total column ozone amount by the differential absorption method. The remaining two channels, which are free of ozone absorption, are used to determine the effective background albedo.

It was recognized from the outset that this technique was intrinsically capable of very high accuracy and stability, since the requirement was for a relative measurement of the ratio of Earth's backscattered UV radiance to the solar UV irradiance at the same wavelength. Because both measurements could in principle be made with the same instrument, the determination of albedo as a function of wavelength should not depend on either the absolute calibration of the instrument nor on long-term variations in the sensitivity of the instrument. However, a serious uncertainty is introduced by the use of the diffuser plate, which is not common to both measurements but only used to transform the solar irradiance into a radiance that is comparable in magnitude to the backscattered Earth radiance, and can be measured in the same manner. Observations of the solar irradiance indicated that the diffuser plate was degrading and becoming less reflective with time. Cebula et al. (1988) developed a model of the diffuser and instrument degradation, which was adopted by the Ozone Processing Team (OPT). Application of this model in the reduction of the TOMS data led to a downward drift of the TOMS results compared to those of the ground-based Dobson network (see above) reported by Fleig et al. (1986, 1989a), Bojkov et al. (1988).

Extensive analysis reported by the Trends Panel indicated that the OPT model had large uncertainties and had almost certainly underestimated the true diffuser plate degradation. The errors were large enough to allow agreement with the Dobson data. Subsequently, some results for trends in total ozone in the Trends Report were based on normalizing the TOMS data to the Dobson values. This allows extension to global coverage, but is at best only an approximation and provides no independent information from TOMS on the trends.

Subsequently, a major advance in the analysis of the TOMS data has been made. Bhartia et al. (1989) pointed out that, for the related Solar Backscatter Ultraviolet (SBUV) experiment, there was a wavelength at 305.8 nm whose data could be used with those from the SBUV 312.5 nm channel to form a new "D pair." Results from this pair are less sensitive to diffuser degradation for two reasons. First, the degradation varies with wavelength, but the D pair wavelengths are only 6.7 nm apart, compared to the 18.7 for the A pair. In addition, because the difference in ozone absorption coefficients is larger for the D pair than for the other pairs, results are estimated to be only 0.22 times as sensitive to diffuser drift than the archived values, which are based on a latitude-dependent weighted sum of the A, B, and C pairs. Because of the large absorption coefficients, D pair values can only be obtained in the tropics, where the total column amount is small, but a comparison showed that they follow the Dobson values much more closely than the archived values do. This provides confirmation that the archived data are in error, as well as strong corroboration that the error mechanism has been identified (Bhartia et al., 1989; Hudson et al., 1989, private communication).

Generalizing from these insights, McPeters et al. (1989, private communication) have developed the "pair justification" method. The basis is that different pairs have different sensitivity to diffuser degradation errors, resulting in a drift between pairs than can be measured accurately. By requiring that all pairs obtain the same total ozone result, and assuming that the uncorrected diffuser degradation is approximately linearly dependent on wavelength, a unique determination of the total ozone is obtained that is independent of diffuser degradation. The implementation makes use of pairs denoted A, B, and C (312.5/331.2; 317.5/339.8; 331.2/339.8 nm). There appears to be a fundamental limit of 0.5-1.0% per decade to the stability that can be obtained. Test results are described in Section 2.1.4.

Solar Backscatter Ultraviolet Spectrometer

The Solar Backscatter Ultraviolet (SBUV) spectrometer (described below in Section 2.1.2.2) also measures the solar ultraviolet radiation that is backscattered by the Earth and atmosphere. Data from four of its wavelength channels, which are the same as four of the TOMS channels, are used in the same way to determine total ozone amounts, but only in the nadir directly below the spacecraft. SBUV ozone values are systematically slightly lower than those from TOMS, but they vary with time in the same way. Because SBUV also relies on the same diffuser plate, its total ozone values have also decreased with time due to diffuser degradation.

2.1.2.2 Profile Measurements

Standard Dobson Umkehr

The Dobson instrument can also be used to obtain information on the vertical distribution of ozone. Measurements of the downward scattered radiation at two wavelengths are made for solar zenith angles Z from 60-86.5 degrees. From the ratio of these radiances as a function of Z, which reverses for Z near 90 degrees, the vertical profile can be inferred. (The method takes its name from the German word for a reversal.) The solution is usually given in terms of the layer mean ozone partial pressure (nb) of ozone in layers about 5-km thick, but with layer 1, the troposphere, about 10-km thick.

Although the retrieval provides data for Umkehr layers 1 through 9 (0-48 km), the averaging kernels for the present standard algorithm indicate that only data for layers 4 to 8 (19-43 km) should be used for trend analysis, while other layers should be used with caution. The vertical resolution of the retrieval for these layers is from 11 to 14 km. The systematic uncertainty or bias is estimated to be 15-20% in the OTP, and 5-12% by DeLuisi et al. (1989a). Random errors, due to the dependence of the ozone absorption coefficients on the varying temperature, tropospheric and stratospheric aerosols, thin clouds invisible from the surface, and instrumental effects, are of the order of 5-10%, some of which could be reduced by monitoring the clarity of the zenith sky. Neither the systematic nor random errors should seriously affect estimation of long-term trends. Trend estimation will be affected by instrument drifts, by changes in calibration, by stratospheric aerosols, and by real atmospheric temperature trends. Calculations indicate that realistic temperature trends (<2degC/decade) will induce errors < 0.2%/decade in ozone trends for layers 4 through 8 (Figure 3-12, Chapter 3, OTP). Serious errors from stratospheric aerosols are easily recognized in the data and such contaminated data may be edited out before trend estimation. Moderate and smaller errors may be adequately corrected by the methods of DeLuisi et al. (1989a) or by statistical methods using proxy data (Reinsel et al., 1989). Finally, instrumental factors may be handled by statistical methods (e.g., sudden calibration changes as by Reinsel et al., 1989) or by re-evaluation of the observations using new calibration data.

To reduce the time required for an Umkehr measurement, the Short method was developed and tested by Mateer and DeLuisi (1984). This method uses the A-C-D Dobson wavelength pairs in place of the C pair that is used for the Standard Umkehr method. By using the triple pair, ozone profile information is obtained when measurements are made during a solar zenith angle change of 80 to 89 degrees. In contrast, the C pair Standard Umkehr method that has provided most of the past data requires a solar zenith angle change of 60 to 90 degrees. An exploratory ozone profile retrieval algorithm for the Short Umkehr has been developed for testing. An operational algorithm will be developed after the present Standard Umkehr algorithm (Mateer and Duetsch, 1964) has been updated.

Although Umkehr observations date back to the 1930s, the current Umkehr profile archive at the World Ozone Data Centre (viz., retrievals using the present standard C-Umkehr algorithm) begins about the time of the IGY (ca. July 1957). The archive comprises over 35,000 ozone profile retrievals for some 68 stations, uniformly processed using the standard retrieval algorithm (Mateer and Duetsch, 1964). For 26 of these stations, the entire station record consists of fewer than 100 profiles. Reinsel et al. (1984), found only 13 stations with a sufficient number of observations and length of record for use in their trend analyses, while Reinsel et al. (1989) only used 10. One of the stations, Mont Louis, no longer makes observations. The record of Aspendale (the only Southern Hemisphere station in the trend set) ends in 1982. From 1983 onward the record is being continued at Melbourne, 25 km away. The remaining trend stations cover the latitude range from 24 to 53deg North.

It appears that additional stations, especially those in the Automated Dobson Network (Komhyr et al., 1985), will have a sufficient number of observations and length of record to be used for trend estimation within the next 5 years. There are presently seven automated Dobson instruments that routinely obtain Standard and Short Umkehr measurements. These instruments were developed by NOAA/GMCC (Komhyr et al., 1985) and are located at Perth (Australia); Lauder (New Zealand); Huancayo (Peru); Mauna Loa (Hawaii); Boulder, Colorado (United States); Haute Provence (France); and Fairbanks, Alaska (United States). The frequency of Umkehr measurements is significantly increased by the automated Dobson because observers are not required, and a shorter observing time (for the Short Umkehr) decreases the chance of cloud interference. Ancillary measurements of sky conditions are also made at the automated Dobson sites. These measurements consist of zenith-sky clouds and turbidity. In addition, lidar measurements of stratospheric aerosols for correcting errors to the Umkehr profiles (DeLuisi et al., 1989a) are routinely made at Mauna Loa, Boulder, and Haute Provence.

Balloon Ozonesondes

Balloon ozonesondes are compact, lightweight, balloon-borne instruments flown with standard meteorological radiosondes for measurement of ozone, air pressure, and air temperature up to altitudes of about 30 km. Regener (1960, 1964) developed a very fast response chemiluminescent ozonesonde, but these instruments often exhibited considerable variations in sensitivity to ozone (which was later corrected). Thus, while an early measurement program provided a great deal of useful information on the ozone climatology over the North American continent, the variable response characteristics associated with the measurements and the relative shortness of the record render the data unsuitable for ozone trend studies in the troposphere and stratosphere.

Brewer and Milford (1960) described an electrochemical ozone detector employing the well-known oxidation of potassium iodide (KI) by ozone as the basic reaction. Subsequently, Griggs (1961) investigated the physical and chemical aspects of a similar balloon-borne instrument commonly referred to as the Brewer "bubbler" ozonesonde. Versions of the instrument are manufactured in the United States, G.D.R., and India. Because the air pumps of the Brewer ozonesondes are lubricated with a thin film of oil that may destroy ozone, they must be conditioned with ozone prior to flight time to minimize the ozone destruction. With proper conditioning, ozone losses can be kept to a few percent, as has been the case at Hohenpeissenberg Observatory (F.R.G.). A few soundings from routine ozone measurement programs conducted in past years, however, have exhibited ozone losses, in extreme cases of up to 50%. Improvement in data quality is achieved through normalization of all soundings to Dobson spectrophotometer total ozone. The normalization factor for any sounding is a constant by which ozone values at all altitudes are multiplied. Use of a constant multiplication factor for normalization may, however, not be justified for soundings that exhibit large ozone losses within the instrument (Hilsenrath et al., 1986). Brewer ozonesonde data having normalization factors that range from 0.95-1.25 are generally acceptable; in an analysis later in this chapter the range 0.9-1.2 is used. The average correction factor for more than 1,000 ozonesondes flown over 20 years at Hohenpeissenberg is 1.07. Other uncertainties associated with ozone measurements with Brewer ozonesondes stem from variations in pump air flow rates above 10 mb, particularly for lightly lubricated pumps that become "dry" after several hours of operation. Build-up of AgI on the platinum anode during operation may also affect sensor performance. The significance of errors due to these effects has not been adequately assessed.

A considerable amount of useful atmospheric ozone vertical distribution data has nevertheless been obtained with the Brewer ozonesondes. The data are suitable for ozone trend analyses, particularly from stations that have maintained unchanged instrument preflight conditioning and test procedures throughout the entire measurement program.

Komhyr (1969) developed an electrochemical concentration cell (ECC) ozonesonde utilizing the reaction of ozone with KI, but with platinum cathode and anode electrodes contained in separate sensor chambers connected by an ion bridge. This sensor evolved from a carbon-iodine (CI) ozonesonde (Komhyr, 1964) that is no longer in use in the United States, but continues to be used in Japan. Because the chemical composition of the electrodes of the ECC sensor remain unchanged during operation, the sensor can be used indefinitely without deterioration of performance. The ECC sonde incorporates a miniature air pump fabricated from Teflon reinforced with glass fibers. The pump is not lubricated, so that minimal conditioning with ozone is required to prevent ozone loss within it. Early versions (type 3A) of the ECC ozonesonde employed a pump of rectangular cross section. Because of non-uniformity of manufacture, some of the pumps exhibited excessive leakage at pressure altitudes above 15 mb, which in extreme cases have been underestimating the ozone by more than 15%. A newer instrument version (type 4A) employs a Teflon pump of circular cross section with external O-ring seals that is claimed to render the pumps and instruments suitable for use at higher altitudes.

ECC ozonesondes using a 1% KI cathode electrolyte yield measured total ozone amounts that agree closely with Dobson spectrophotometer total ozone. For example, for 525 soundings made by NOAA since 1984 over a wide range of operating conditions, from the tropics to the polar regions, the mean ECC sonde-Dobson spectrophotometer total ozone normalization factor was 1.01 \'b1 0.05 (1o) (Komhyr et al., 1989). ECC ozonesondes are, therefore, suitable for use at stations where independent measurements of total ozone are not available, e.g., in polar regions during the polar night.

Ozone measurement uncertainty for ECC sondes is estimated to be \'b110% in the troposphere, \'b15% in the stratosphere up to 10 mb, with the uncertainty increasing to 20 \'b1 5% at 3 mb (Hilsenrath et al., 1986). Two ECC ozonesondes flown with an ozone UV-photometer during the MAP-GLOBUS campaign of 1983 (Aimedieu et al. 1987) gave ozone values that differed from the photometer values by 2.1 + 1.1% at 8.1 + 1.1 mb, and by -0.6 \'b1 3.0% at 3.9 \'b1 0.4 mb. The low ozone values measured above 10 mb, if real, may stem from application during data processing of inadequate pump efficiency corrections. Variability in the data at these altitudes is attributable at least in part to variations in manufacture of the pump components. It must be pointed out that small, continuing improvements over the years have led to more accurate measurements, but they may also result in spurious indications of trends.

Rocket ozonesondes

Over the last 2 to 3 decades, several groups in various countries have developed and used rocketsondes. In the USSR, rocket optical ozonesondes (Brezgin et al. 1977) and chemiluminescent sondes (Konkov and Perov, 1976; Perov and Khrgian, 1980; Perov and Tishin, 1985) have been introduced. A solar photometer (Subbaraya and Lal, 1981) and another optical sonde (Somayajulu et al., 1981) have been developed in India. However, the longest data record has been collected in the United States by the ROCOZ and ROCOZ-A systems, which are described here.

The ROCOZ-A ozonesonde is a four-filter, sequential-sampling, ultraviolet radiometer. The instrument was originally developed for stratospheric soundings aboard an ARCAS rocket (Kreuger and McBride, 1968a,b). This instrument was subsequently modified for launch aboard a smaller diameter Super-Loki launch vehicle. In 1982, an instrument improvement program was initiated at NASA's Goddard Space Flight Center/Wallops Flight Facility (WFF). Changes were made to the center wavelengths and spectral shapes of the ultraviolet filters. An integrated calibration facility was established (Holland et al., 1985), and new data reduction procedures were designed (Barnes et al., 1986). A description of the present design of the radiometer has been published by Barnes and Simeth (1986). Because of the short data record of the present instrument, these data are not directly applicable to trend studies at this time.

The ROCOZ-A ozonesonde is propelled aloft by a Super-Loki booster rocket. At rocket burnout, the instrument and its carrier coast to a nominal apogee of 70 km, where the payload is ejected for deployment on a parachute. The radiometer measures the solar ultraviolet irradiance over its filter wavelengths as it descends through the atmosphere. The amount of ozone in the path between the radiometer and the sun is then calculated from the attenuation of solar flux as the instrument falls. In addition, radar from the launch site measures the height of the payload throughout its descent. Finally, knowledge of the solar zenith angle allows calculation-of the fundamental ozone value measured by the radiometer; that is, ozone column amount as a function of geometric altitude. Ozone number density is the derivative of ozone column amount versus altitude. Combined with auxiliary atmospheric soundings for pressure and temperature, ROCOZ-A can duplicate the fundamental ozone values of backscatter ultraviolet (Barnes, 1988) and solar occultation measurements (Cunnold et al., 1989) from satellites. ROCOZ-A comparisons have also been made with the ultraviolet spectrometer on the SME satellite (Barnes et al., 1987b).

Details of the measurements of the precision of ROCOZ-A ozone column amounts and ozone number densities have been published (Holland et al., 1985; Barnes et al., 1986). In addition, there are published results from an equatorial ozone measurement campaign (Barnes et al., 1987a) that found very low variability in stratospheric ozone, temperature, and pressure. From the results of this campaign, it is possible to estimate the precision of the remaining measurements in the ROCOZ-A data set. The full set of precision estimates for ROCOZ-A is given in Barnes et al. (1989). For ozone number densities and mixing ratios, the profile-to-profile repeatability of ROCOZ-A measurements is estimated at 3 to 4%.

The estimates of the accuracy of the ROCOZ-A profiles are also given in Barnes et al. (1989). For ROCOZ-A ozone amount, the accuracy estimates come from an internal, unpublished error analysis. The analysis is based on errors of the effective ozone absorption coefficients used to convert the radiometer readings into ozone profiles, plus the differences between the ozone values at altitudes where two ROCOZ-A channels give simultaneous readings (Barnes et al., 1986). A laboratory flight simulator, based on long pathlength photometry (DeMore and Patapoff, 1976; Torres and Bandy, 1978), has been constructed to measure the accuracy of ROCOZ-A ozone readings. Publication of a detailed error analysis will follow the conclusion of experiments with the simulator. It will complete the primary characterization of the ROCOZ-A ozonesonde. ROCOZ-A ozone number densities and ozone mixing ratios are estimated to be accurate to 5 to 7% and 6 to 8%, respectively.

The vertical resolution of ROCOZ-A ozone profiles is 4 km (Barnes et al., 1986). This resolution comes from the data reduction algorithm for the profiles, since measurements from the instrument are less than 100 meters apart during flight. ROCOZ-A ozone column amounts are smoothed before differentiation for ozone density. The vertical resolution of the smoothed profiles has been adjusted to roughly match the 8-km resolution quoted for the two limb scanning instruments on the Solar Mesospheric Explorer (Rusch et al., 1984), and the 1- to 5-km vertical resolution for SAGE II (Mauldin et al., 1985b).

Satellite Observations

The Stratospheric Aerosol and Gas Experiment (SAGE I and II)

SAGE I and SAGE II are both satellite-borne multi-wavelength radiometers employing solar extinction (occultation) techniques to measure stratospheric aerosols and gases. Ozone profiles are determined from measurements of attenuation of solar radiation by ozone in the most intensely absorbing portion of the Chappuis band, at 600 nm. SAGE I was launched aboard the dedicated Applications Explorer Mission-2 in February 1979 and operated for 34 months until November 1981, when the spacecraft electrical system failed. SAGE II was launched on the Earth Radiation Budget Satellite in October 1984, and has operated continuously since then. Both SAGE I and SAGE II are in approximately 600 km circular orbits with inclination angles of 56 and 57 degrees, respectively, such that the latitudinal coverage is almost identical. Detailed descriptions of the instruments are given by McCormick et al. (1979), and Mauldin et al. (1985a,b). The capabilities of the two instruments are not identical because of small differences in the instrument configurations and data processing algorithms.

In the solar occultation technique, measurements are made of the solar radiation transmitted through the atmosphere as the sun sets (or rises) behind it relative to the spacecraft. The transmission for a given ray path is the ratio of the signal strength for that ray path to the signal when the sun is above the atmosphere. The vertical distribution of ozone is determined from the changes in the transmission. Other wavelengths allow the concentrations of other gases and aerosols to be determined; the aerosol effects on the ozone transmittance may then be corrected. The retrieval algorithms are described in the OTP, Ch. 3, or in Chu and McCormick (1979), Mauldin and Chu (1982), and by Chu (1986). Because of the high signal-to-noise ratio, a vertical resolution of 1 km is achieved in the stratosphere.

It is important to note that the measurements performed by the SAGE instruments are self-calibrating, in that only relative radiance measurements are required to determine the transmission and, therefore, the distribution of atmospheric species such as ozone. Consequently, no absolute radiance calibration is necessary. The only requirement is that the instrument retain constant responsivity for the duration of each spacecraft sunrise or sunset, usually about 100 seconds. However, the position of the line of sight must be accurately known, which requires very accurate data on the position of the spacecraft during the observing events and a reasonably stable spacecraft. The OTP analysis indicates that absolute accuracy of the ozone values determined by SAGE I and SAGE II is about 6 to 9%, depending on altitude. However, the stability, or ability to detect changes, was 2 to 7% for SAGE I, and 1.3 to 4% for SAGE II. The uncertainty in the difference between SAGE I and SAGE II ozone values allow changes of 2% over the several years between their observing periods to be detected from 25 to 45 km.

A limitation of the occultation technique is the relatively small amount of data obtained, two vertical profiles per orbit, and the changing location of the observed latitudes, rendering comparisons between occultation instruments difficult. The ozone trends detection uncertainty estimated above for the SAGE I and II ozone data do not include errors caused by the undersampling of the natural variability of the ozone distribution in both the spatial and temporal domains.

Ozone data from SAGE II between November 1984 to November 1988 have been archived at the National Space Science Data Center (NSSDC). The SAGE I ozone data have also been archived at NSSDC. Reprocessing of the SAGE I ozone data using the updated temperature correction information from NOAA-NMC has been performed for the ozone trends study, and the data are currently being rearchived at NSSDC. The new temperature data introduce only small differences at high altitudes in the low-latitude regions.

Solar Backscatter Ultraviolet Spectrometer (SBUV)

The SBUV is a downward viewing double monochromator that was launched on the Nimbus-7 spacecraft in 1978 to measure the UV albedo of the atmosphere and surface for the purpose of determining the vertical profile of ozone, in addition to total ozone. Singer and Wentworth (1957) suggested that observations from above the atmosphere, in which the fraction of sunlight reflected back to space (the planetary albedo) is measured as a function of wavelength, could be used to deduce the concentration of ozone as a function of pressure (i.e., altitude). Other experiments utilizing the same principle have flown on Kosmos-65, OGO-4, Nimbus-4, Atmospheric Explorer D, and most recently, NOAA-9 and the Japanese OHZORA satellite.

The SBUV made measurements in 12 wavelength channels, located at (in nm) 255.5, 273.5, 283.0, 287.6, 292.2, 297.5, 301.9, 305.8, 312.5, 317.5, 331.2, and 339.8. Measurements in the channel at 255.5 nm were not used because they were contaminated by NO fluorescence; the next seven were used for extracting profile information, while the last four (which are common with TOMS) are used to determine the total ozone. The solutions are given in terms of ozone amounts within the Umkehr layers. The analysis presented in the OTP shows that only data in layers 6 to 9 (approximately 28 to 48 km) are independent and reliable for trend studies. The vertical resolution of the solutions is 8 to 10 km.

Again, in principle, this technique requires only the measurement of the ratio of the backscattered Earth radiance to the solar irradiance, with the attendant insensitivity to calibration accuracy and stability, but the use of a diffuser plate involves another optical element in the measurement of the solar irradiance which eliminates this advantage. On Nimbus-7, SBUV and TOMS shared the same diffuser plate. SBUV observations of the solar irradiance also showed that the diffuser plate was degrading with time, most rapidly at the shortest wavelengths. The diffuser model of Cebula et al. (1988) was developed from the SBUV observations and used by the OPT for the SBUV data reduction also. The archived results showed very large (25%) downward ozone changes in the upper stratosphere over 8 years, which was the reason for the original appointment of the Ozone Trends Panel. In this case also, it was found that the uncertainties in the diffuser model were much larger than expected, and that the changes in the stratosphere were probably smaller than indicated by the archived data and could even be slightly positive. In this case, the D pair again supports the assertion that the degradation is larger than predicted by the model, but there has been no method developed to date for finding an independent determination of the diffuser degradation at the shorter wavelengths. Thus, at the moment the SBUV data can provide no independent information on long-term trends in the ozone profile (see OTP).

Limb Infrared Monitor of the Stratosphere

The Limb Infrared Monitor of the Stratosphere (LIMS) is a six-channel limb scanning infrared radiometer that also was launched on Nimbus-7 in 1978 (Gille and Russell, 1984; OTP). This type of instrument measures the infrared radiation emitted by atmospheric molecules as the instrument scans across the limb. Because of the geometry, this technique has an inherently high vertical resolution and the ability to sound to high altitudes. Because it measures infrared emission, it can obtain measurements at all local times, resulting in very dense coverage. LIMS obtained ozone data from 15 to 64 km altitude, with a vertical resolution of 2.5 km. The absolute accuracy was about 10%, and the precision a few percent.

Because of the small signals involved, it is necessary to cool the detectors. LIMS life was limited by the technology of that period, which dictated the use of a solid cryogen cooler. This resulted in a 7-month lifetime. Otherwise, it should be useful for trend measurements, especially since it provides a 1978/79 determination. LIMS data were used in the OTP to help evaluate other data.

2.1.3 Measurements for Future Trend Analyses

2.1.3.1 Total Ozone Measurements

Ground-based Measurements

The Brewer Spectrophotometer

The Atmospheric Environment Service of Canada developed the fully automated Brewer ozone spectrophotometer in the early 1980s. Development of the Brewer instrument was based, in part, on earlier work by Wardle et al. (1963) and Brewer (1973) and was carried out with the goal to replace or supplement the Dobson instrument in the world network. There are presently about 30 instruments operating in 15 countries, including the seven Canadian stations (Kerr et al., 1988). At present, five stations are reporting data to the WODC. The record of routine measurements of total ozone made with the Brewer spectrophotometer started in 1982. These records are presently not of sufficient length to carry out a proper independent trend analysis.

The spectrophotometer is a modified Ebert type with a 1,800-lines/mm holographic grating used in the second order. It simultaneously measures the intensity of light at 5 wavelengths in the ultraviolet absorption spectrum of ozone with a resolution of 0.6 nm. The operational wavelengths are 306.3 nm, 310.0 nm, 313.5 nm, 316.8 nm, and 320.0 nm. Measurements at these wavelengths allow correction for the effects of sulfur dioxide, a potential interferent for Dobson ozone measurements in polluted air. A more complete description of the automated instrument is given by Kerr et al. (1985). The Brewer instrument has been calibrated on an absolute scale using the ozone absorption coefficients of Bass and Paur (1985). Intercomparison with the Dobson instrument has shown that the Dobson AD direct sun total ozone measurement is about 3% larger than the Brewer measurement. When this bias is removed, the long-term agreement between field Brewer and Dobson instruments at Toronto and Edmonton has been within 1% (Kerr et al., 1988; 1989a).

Although the method to measure total ozone with the Brewer instrument is similar to that for the Dobson instrument, there are differences in sampling and reporting daily data. These differences arise primarily because the Brewer instrument is fully automatic and thereby capable of sampling continuously throughout the day. Measurements are screened and only those of good quality are used to determine the daily mean total ozone value that is reported. In general, good quality direct sun measurements can be made on about 75% of the days at a typical mid-latitude station.

High Resolution Visible/Ultraviolet Absorption Spectroscopy

Absorption spectroscopy affords a sensitive means for monitoring the column abundance of several stratospheric species (e.g., Noxon et al, 1979; McKenzie and Johnston, 1982; Pommereau et al., 1988a, 1988b). A useful approach is to measure the absorption of light scattered by the sunlit zenith sky. An important feature of such measurements is that they can be carried out on cloudy as well as clear days. The optical paths (or air mass factors) relevant to a species in a stratospheric layer become very large at solar zenith angles greater than about 88 degrees, maximizing the slant-column abundance of absorber and hence the observed percent absorption.

Modern optics, electronics, computers, and detectors allow atmospheric absorption spectroscopy to be done with very high precision, typically 0.05% absorption. Multichannel detectors eliminate the uncertainties that scanning optics introduce. Least-squares fitting of the observed spectra to those recorded in the laboratory with the same instrument for several atmospheric gases allows the abundance of more than one atmospheric species to be determined simultaneously. Under favorable spectral conditions, the sensitivity stated above can be enhanced a factor of 50 by such fitting. The stratospheric species that are most amenable to observing with ultraviolet/visible absorption spectroscopy have proven to be O3, NO2, NO3, and OClO. The characteristics of one instrument that has been applied to these species is used here as an example of the type that is achievable today.

The example (Mount et al, 1987) is a crossed Czerny-Turner spectrometer, with a grating chosen so that the appropriate color filters provide second-order light in the red region of the spectrum, approximately 605-685 nm (for NO3 observations) and third-order light in the blue region, approximately 400 to 450 nm (for O3, NO2, and OClO observations). The instrument is used at approximately 0.5-nm spectral resolution. The detection system is a Reticon diode array cooled by a refrigerator to about -70degC; the array contains 1,024 independent silicon diodes that simultaneously measure the spectrum over the indicated wavelengths. Eleven diodes cover the full width at half maximum of the instrumental spectral profile. Thus, spectral lines are highly oversampled.

An important advantage of simultaneous measurement of the desired spectral interval (as opposed to scanning) is the elimination of time-dependent changes during the course of a measurement (particularly atmospheric scintillation effects). These can be quite important when the measured species absorb only a few tenths of a percent of the incoming light, as is the case for ozone and OClO in general and for NO2 at small zenith angles. After least squares fitting the observed sky data to the reference absorption spectra measured in the laboratory, the one-sigma standard errors of the fitted column abundances are typically 2-5% for large solar zenith angles for NO2. Ozone columns can be determined to 10-15% accuracy using the Chappuis bands. Future development is expected to improve this considerably. While most efforts with this technique have been on the measurement of other gases, the technology presumably could be applied to ozone measurements as well.

The largest source of systematic error in the measurements is the evaluation of the air mass factors for scattered light that are used to convert measured slant column abundances to vertical column abundances. These errors largely affect the accuracy but not the precision of the data. Estimated error limits in air mass factors are no more than 10% for solar zenith angles near 90 degrees, and they are substantially less for smaller angles.

Satellite Observations

Future Flights of TOMS Instruments

TOMS data have proven of great value in observing and studying the polar ozone depletions, as well as in providing information on the global decrease of total ozone. With the development of independent techniques for determining the diffuser reflectivity, TOMS data should be able to provide a determination of total ozone decrease over all areas of the globe that is independent of the ground-based Dobson network, although it should agree with the latter in areas where both obtain measurements. For these reasons, it is extremely important to continue TOMS-type measurements. The TOMS instrument on Nimbus-7 is now over 10 years old, and efforts have been made for future flights of TOMS-type instruments on several spacecraft.

First flight of a new, technically upgraded instrument is scheduled for the fall of 1991 on board the Soviet Union's Meteor-3 spacecraft. Unlike the sun-synchronous orbit of Nimbus-7, the Meteor-3 orbit will precess with a period of 225 days. Thus, the coverage will not be as uniform as that obtained with previous data, and adjustments will have to be made for diurnal variations. TOMS has also been selected to fly on a small U.S. Explorer-class satellite in 1993, and on the Japanese ADEOS satellite, scheduled for launch in February 1995. In these cases the orbit will again be sun-synchronous.

An upgraded TOMS will also be part of the Global Ozone Monitoring Radiometer (GOMR), an operational instrument to make long-term ozone measurements. The GOMR, discussed further below, will be included in the complement of operational instruments on the NOAA afternoon free-flier, beginning in about 1997.

2.1.3.2 Profile Measurements

Ground-based Measurements

Brewer Umkehr

Although Umkehr observations have been made with the Brewer Ozone Spectrophotometer for several years (Mateer et al., 1985, Kerr et al., 1989b), this has been a developmental period during which the observational technique and the algorithm have changed. The present body of data is insufficient for trend analysis.

A Brewer Umkehr observation consists of zenith sky intensity ratio measurements for three wavelength pairs over the solar zenith angle range from 60 to 90 degrees. The profile retrieval data cover the altitude range of layers 1 to 10 (0-53 km). The averaging kernels for the present algorithm show that the retrievals for layers 4 to 8 (19-43 km) and marginally layer 9 (43-48 km) are suitable for trend estimation. Until a complete error analysis has been carried out for the Brewer Umkehr system, those for the standard Dobson Umkehr may be considered as only approximately applicable. Results of an intercomparison of Brewer Umkehr, Dobson Umkehr, and ozonesonde data have indicated that the quality of the Brewer Umkehr technique is comparable to that of the Dobson "short" Umkehr method (McElroy et al., 1989).

Lidar

The lidar measurements of the ozone vertical distribution are based on the Differential Absorption Laser technique (DIAL), which requires the simultaneous emission of two laser wavelengths characterized by a different ozone absorption cross section. Part of the laser radiation is scattered back to the surface, where it is collected by a telescope, detected by a photomultiplier and time-sampled to retrieve the altitude resolution of the measurement. The derivative of the logarithm of each signal is computed and the ozone number density is obtained from the difference of the derivatives, divided by the differential ozone cross section. The lidar technique has been described by Megie et al. (1977), Uchino et al. (1978), Pelon and Megie (1982), Werner et al. (1983), Pelon et al. (1986), Ancellet et al. (1988), Godin et al. (1989), and McDermid and Godin (1989).

In principle the technique is self-calibrating, and thus particularly suited for the detection of trends. Two types of error affect the ozone lidar measurement. The random or statistical error is related to the signal-to-noise ratio. The systematic error is related on one hand to errors in the value of the ozone absorption cross sections used and their dependence on temperature, and on the other to uncertainties in the different physical processes, such as the Rayleigh and Mie scattering or the absorption by other species (NO2, SO2). The systematic error can be reduced by additional measurements of temperature and aerosol vertical profiles or use of empirical models. The random error which results from this correction is less than 3% in the whole altitude range. The choice of the two wavelengths used in the experiment depends directly on the altitude range monitored. In the troposphere, the limiting factor is the systematic error, which becomes smaller as the difference in wavelength is reduced. A wavelength separation of less than 10 nm is used, and the time necessary to obtain a tropospheric ozone profile is about 10 min for an altitude resolution superior to 1 km and an overall uncertainty less than 5%. In the stratosphere, the separation of the wavelengths needs to be larger (~50 nm) in order to limit the statistical error. Furthermore, the range solution has to be degraded in the upper range of the measurement (above 30 km) to account for the rapid decrease of the signal-to-noise ratio with altitude. With the present systems, obtaining an ozone profile in the 15-50 km altitude range requires an integration time of 2 to 3 hours for an altitude resolution of 0.5-8 km and a corresponding uncertainty of 2% to 10%.

Microwave Radiometry

Microwave radiometers (MR) are being used increasingly to investigate the middle atmosphere. An MR measures the thermal emission from rotational transitions of the molecule under investigation. The frequency employed is typically below 300 GHz (1 mm wavelength) to reduce the effects of tropospheric opacity (e.g., clouds or aerosols) and local thermodynamic equilibrium (LTE) conditions apply to ~80 km altitude. The sensor operates on the superheterodyne principle that provides a very high spectral resolution, allowing the determination of the exact shape of the emission line. Since the shape of the emission lines are dominated by pressure broadening to 80 km altitude. profiles can be retrieved from an exact measurement of the emission line shape. At low altitudes (~15 km) the emission line widths become very large, and this, together with instrumental problems (base line and non-linearities) makes low-altitude retrievals very difficult. The altitude range over which high-quality ozone profiles can be determined from MR measurements is typically 15 to 70 km (Zommerfeld and Kunzi, 1989) with an altitude resolution of ~10 km. The accuracy claimed for this sensor is 1 part per million by volume, and the statistical error is of the order of 15%. The measuring time required for a complete profile is approximately 1 hour. Another instrument, developed by De la Noe et al. (1988) has obtained measurements of the ozone distribution with reported random errors from 1 to 5% at 40 km. Connor et al. (1987) present an error analysis for data obtained in Antarctica during 1986: they show measurement accuracies of 15-19% (depending on altitude), supported by intercomparisons with other observations. A detailed study performed recently by Bevilacqua and Olivero (1988) showed that the best vertical resolution attainable is 6 or 7 km.

It should be noted that the accuracy can be improved by using observing sites at high altitudes, thus reducing the attenuation by tropospheric water vapor. For a mid-latitude site at 500 m above sea level (Bern, Switzerland), the observing statistics for favorable conditions are >80% of the time during fall, winter, and spring, and ~50% of the time for summer. Model calculations (Kunzi and Rubin, 1988) indicate that for >90% of available observing time the sensor has to be at 3,000 m altitude for the tropics, whereas in polar regions the site can be at any altitude.

MR instruments seem particularly well suited to perform long-term trend measurements of total stratospheric and mesospheric ozone and its vertical distribution, since an MR is easily calibrated with black body radiators and thereby has a response that can be made stable over long periods. Furthermore, the operation and data retrieval of an MR can be fully automated and require only little maintenance by skilled operators. However, at present, only little experience is available with MRs in ozone research and more intercomparisons and experience with sites in different climatic regions is needed.

Satellite Observations

Solar Backscatter Ultraviolet Radiometer--Version 2 (SBUV/2)

NOAA is currently flying the Solar Backscatter Ultraviolet Radiometer--Version 2 (SBUV/2) as an operational instrument for measurement of both total ozone and vertical profiles. The first SBUV/2 was launched on NOAA-9 in 1984 and continues to operate in overlap with the instrument launched on NOAA-11 in 1988. This series of instruments is planned to be in orbit through the lifetime of the present afternoon series of NOAA polar orbiting satellites (ca 1996). These satellites sound the atmosphere. Unfortunately, an onboard check of the diffuser plate incorporated in the NOAA-9 SBUV/2 failed to function properly. At the same time, the processed ozone data from this instrument indicate a time-dependent bias with respect to the ground-based information as well as the Nimbus-7 SBUV data that is opposite in sign to our general understanding of diffuser plate degradation. No data from either instrument are now available. This time-dependent bias is still under examination; if the cause for this disparity is discovered, the data will be reprocessed accordingly. An attempt has been made to correct the onboard diffuser plate check on the SBUV/2 follow-on units.

The failure to obtain data from the NOAA-9 SBUV/2 is especially damaging, since the first 2 years overlapped with the last 2 years of the SBUV. The SBUV/2 data could have been used to correct the SBUV diffuser model, allowing a reliable profile trend to be obtained over the 8-year life of SBUV. Alternatively, the differences between the first 2 years of SBUV and SBUV/2 could have been used to get a direct difference, as was done for data from SAGE I and SAGE II.

Upper Atmosphere Research Satellite

The Upper Atmosphere Research Satellite (UARS) is scheduled for launch by the U.S. National Aeronautics and Space Administration (NASA) in late 1991. The instruments have been designed for research purposes and the study of dynamical, chemical, and physical processes in the stratosphere, mesosphere, and lower thermosphere. Although it is not designed to measure ozone trends, a few of the instruments may provide data useful for trend studies.

Ideally, each of the ozone measurements will be sufficiently accurate that it can be compared with the existing body of previous data. However, because of the small trends and the often larger systematic biases between different types of instruments, the clearest indications are likely to occur when similar instruments are used. Some possibilities are: comparisons of ozone measurements among infrared limbscanners such as UARS IR limb scanners (CLAES and ISAMS) as compared to LIMS and to future EOS instruments (see following); comparisons between measurements made by the UARS Microwave Limb Sounder (MLS) and the future EOS MLS; comparisons between measurements made by the infrared occultation instruments (UARS HALOE, and the later ILAS [see following]); and perhaps also comparisons among the visible occultation observations performed by the successive SAGE instruments.

Infrared Limb Atmospheric Spectrometer (ILAS)

The ILAS is under definition study by the Japanese Environmental Agency for possible launch on the ADEOS satellite, now scheduled for February 1995. It is a solar occultation spectrometer, baselined to use three monochromators in the middle infrared. In addition to the measurement of ozone (9-10 micrometer bands), several other trace species and aerosols will be measured over the 10-60 km altitude range.

Earth Observing System (EOS)

The U.S. National Aeronautics and Space Administration (NASA), the European Space Agency (ESA), and the Japanese Space Agency (NASDA) are developing a co-ordinated international system of polar platforms for remote sensing of the atmosphere, oceans, land surface, biosphere, and solid Earth. The goals are to begin or continue a set of baseline observations of key variables that indicate the state of the entire Earth system, and continue them for at least 15 years. At this time, instrument selection is still underway.

NASA has selected instruments for the definition phase for NASA's polar orbiting platforms (NPOP-1 and -2). It includes, for NPOP-l, an infrared limb sounder and SAGE III occultation instruments that should provide observations of ozone with high vertical resolution, high accuracy, and precision from cloud tops to 80 km. The limb sounder will also be capable of high horizontal resolution. Launch is scheduled for late 1996. In addition, a SAGE III instrument is planned as an attached payload for the Space Station to provide profiles at low to mid-latitudes to complement the NPOP-l high latitude coverage. The Space Station is scheduled for launch in 1995. For NPOP-2, definition studies are being carried out for limb sounders operating in the infrared, sub-millimeter, and microwave portions of the spectrum, which will make additional measurements of ozone profiles.

The Global Ozone Monitoring Radiometer (GOMR) is planned to be the next in the series of NOAA operational satellite ozone monitoring systems. It is to fly on the satellite series subsequent to the current Advanced TIROS-N with launch about the mid-199Os. The GOMR will supplant the SBUV/2. The GOMR is currently envisioned as two subsystems: an ultraviolet nadir and side-scan sounder based on Nimbus-7 TOMS technology, as noted above, and an infrared limb sounder based on EOS limb sounding instruments. The nadir sounder will provide total ozone in the sunlit portion of the Earth while the limb sounder will provide stratospheric profiles of ozone and other minor constituents important to ozone photochemistry and climate. The final configuration of the satellite ozone monitoring system is undergoing evaluation.

Three instruments providing ozone measurements in the stratosphere have been selected by the European Space Agency for flight on the first ESA Polar Platform (EPOP-1). GOMOS (Global Ozone Monitoring by Occultation of Stars) is an instrument intended to monitor ozone by stellar occultation, observing the full UV/visible/near-infrared spectrum of stars using a double grating spectrometer. SCIAMACHY (Scanning Imaging Absorption Spectrometer) is a combination of two spectrometers operating also in the UV/visible/near-infrared part of the spectrum to observe transmitted, reflected, and scattered light. It is designed to be used for both nadir and limb sounding plus solar occultation and will provide measurements of tropospheric ozone. MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) is a limb sounder operating in the mid-infrared region of the spectrum. All these instruments are presently in Phase A studies.

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