Space Based Remote Sensing

The ozone profiling technique with the greatest heritage uses the BUV technique. By using several wavelengths shorter than those used for total column measurements, information on the ozone distribution in the middle and upper stratosphere can be obtained. This technique makes use of the fact that with decreasing wavelengths light is absorbed at higher altitudes in the atmosphere because of the corresponding increased absorption cross section (see Fig. 1). The vertical resolution of this technique is quite broad, however, some 7 km in the middle and upper stratosphere and close to 10 km below the peak in the ozone layer (typically 20 to 25 km depending on latitude). Through its use on the SBUV series of instruments, this technique has provided extensive information on the latitude and seasonal dependence of ozone's vertical structure in the middle and upper stratosphere. One strong conclusion to come from this is the clear demonstration of statistically significant ozone losses near 40 km, especially at high latitudes.

The other space-based technique with the longest heritage uses the absorption of radiation at occultation (the rising and setting of the sun with each orbit). The occultation technique (see Fig. 2b) has several notable advantages. First, because it involves an along-path length against a rising or setting sun, signals are strong. Second, the technique is inherently "self-calibrating" in that for each measurement an observation is made at the top of the atmosphere and in darkness, so any changes in the performance of the instrument can be determined, at least to first order. Third, the technique has the capability for relatively high (~1 km) vertical resolution to be implemented fairly easily, although in the lower stratosphere, where ozone mixing ratios vary rapidly with altitude and one must view through the peak in the ozone layer, vertical resolution may be degraded.

The main limitation of the solar occultation technique is in spatial coverage. Since the sun rises and sets only once per orbit, at most two latitudes of data per orbit are obtained. These latitudes will change fairly rapidly with time. Depending on the orbital inclination and the orientation of the spacecraft orbit, a complex pattern of observations versus time is obtained; this may complicate the determination of seasonal distributions. If the spacecraft is in a polar sun-synchronous orbit, sunrises and sunsets are only obtained at high latitudes. Another disadvantage of this technique is high sensitivity to aerosol loading. When stratospheric aerosol loading is high, such as following a major volcanic eruption (e.g., Mt. Pinatubo in 1991), the high aerosol abundance provides a great deal of extinction that can complicate the retrieval of ozone amounts. This technique cannot penetrate clouds, and therefore can provide little information on the tropics below the tropopause, as high-level clouds are typically present near the tropical tropopause. An additional limitation of this method is that it actually observes number density as a function of altitude; most atmospheric scientists work with mixing ratios as a function of pressure. Unless temperature is measured together with ozone amounts, the conversion from the observed to desired quantities requires externally supplied (and noncollo-cated) meteorological information.

The occultation technique has been implemented using both UV/visible/near-infrared and purely infrared wavelengths. The Stratospheric Aerosol and Gas Experiment (SAGE) series of instruments has been the longest term implementation of this technique. SAGE I, which flew aboard the AEM-2 satellite, obtained data from 1979 to 1981, while the SAGE II instrument, which flies aboard the Earth Radiation Budget Satellite (ERBS), has obtained data since its launch in late 1984. Both instruments flew in a 57° inclination orbit; the latitude and time dependence of the solar occultations for SAGE II is shown in Figure 7. It is seen that it typically takes weeks for the occultations to scan the full range of latitudes. The nonuniform nature of the coverage is quite obvious—in some months, some latitudes are not sampled at all. High latitudes are only sampled occasionally. The SAGE instruments, which make measurements at a total of 4 (SAGE I) and 7 (SAGE II) chan-

Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun

Figure 7 Spatial coverage of solar occultations From the SAGE 11 instrument as a function of season. Sunrise and sunset occultations are indicated with different symbols. There is excellent year-to-year repeatability of the times and locations of the occultations. See ftp site for color image.

Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun

Figure 7 Spatial coverage of solar occultations From the SAGE 11 instrument as a function of season. Sunrise and sunset occultations are indicated with different symbols. There is excellent year-to-year repeatability of the times and locations of the occultations. See ftp site for color image.

nels, also measured both nitrogen dioxide (N02) and stratospheric aerosols; the SAGE 11 instrument also measures water vapor (H20). The main ozone measurement channel uses the Chappuis bands at 600 nm, but the measurement must account for the presence of aerosols, which also contribute to the 600-nm extinction.

Along with ozonesondes, the SAGE II instruments provide a critical data set for the long-term variation of stratospheric ozone. In Figure 8. a zonally and seasonally averaged representation of the long-term trend in stratospheric ozone as a function of altitude is shown. Clear evidence for both upper stratospheric loss in ozone amounts (largest at high latitudes) and lower stratospheric amounts is observed. An accurate characterization of this change as a function of altitude remains an important research area.

The occupation technique using very similar wavelengths was also implemented using the polar ozone aerosol monitor (POAM-2) instrument, which flew aboard the French SPOTS satellite and obtained data for 3 years from late 1993 to late 1996 (Bevilacqua ct al„ 1997). Since the SPOT-3 satellite was in a polar sun-synchro no us orbit, all occultations were at middle and high latitudes, and the ROAM data provided an important picture of how high-latitude ozone profiles vary over the course of the year. An example of this is in Figure 9 in which the ozone number density at 20 km is shown inside the Antarctic polar vortex from May through December for the 3 years of POAM-2 observation. The slow decline of ozone in the winter, the rapid falloff in September, and the slow recovery in October and November are all clcarly evident. Intcrannual variability in these 3 years is quite small.

The occultation technique has also been applied using infrared wavelengths. The atmospheric trace molecule spectroscopy (ATMOS) instrument, a Fourier transform spectrometer, flew four times aboard the Space Shuttle (1985, 1992, 1993, 1994). ATMOS is notable for its measurements of a very broad range of trace constituents based on its very high spectral resolution (~0.01 cm-1). Its measurement of ozone (and many other constituents) helped serve as an important validation tool for measurements made from NASA's upper atmosphere research satellite (UARS), launched in September 1991. The Halogen Occultation Experiment (HALOE), a combination broadband radiometer and gas cell correlation radiometer has measured ozone (and several other trace gases) during the more than 6 years of UARS operations. Since UARS is in a 57° inclination orbit, the spatial coverage of HALOE is very similar in character to that of SAGE. Most recently, the improved limb atmospheric spectrometer (ILAS) instrument flew aboard Japan's ADEOS satellite and obtained data for nearly a year from August 1996 to June, 1997. Since ADEOS was in a polar sun-synchronous orbit ILAS data were restricted to high latitudes.

Emission technology has also been used for measurement of atmospheric ozone. These involve looking at the limb of the atmospheres and measuring the thermal emission from ozone (or some other species). Such measurements do not require the presence of a light source and can thus be made over a complete orbit (both day and night). When made from a polar sun-synchronous orbit, they are made at roughly the same time every day (typically once in the daytime and once in the nighttime), which facilitates studies of diurnal variation. On an inclined orbiter, such as UARS, the measurement time will vary and therefore intermix diurnal and seasonal dependence. As typically implemented, limb observations have vertical resolution of ~3 km, although the exact amount can vary higher or lower depending on the measuring instrument. Since limb emission is a thermally driven process, simultaneous measurement of temperatures to high accuracy is required. Infrared emission observations of ozone involve the measurement of the emission from the relatively small population of vibrationally excited molecules that exist in thermal equilibrium with the large majority of ground-state molecules, while in microwave emission, it is emission from rotationally excited molecules that is measured. In some cases (especially daytime in the mesosphere), nonthermal process may populate the excited vibrational states of ozone, and these must be accounted for in determining ozone concentrations from infrared emission measurements. Such nonthermal distribution of population states typically does not occur in the microwave, where the smaller energy quantum allows for thermal equilibrium to be maintained.

Implementations of emission techniques for ozone include the limb infrared monitor of the stratosphere (LIMS) instrument, which flew aboard the Nimbus 7 satellite and obtained data from October 1978 to May 1979. Later implementations include two instruments aboard UARS—the improved stratospheric and meso-spheric sounder (ISAMS) and the cryogenic limb array etalon spectrometer (CLAES) instruments. The CLAES instrument, which used a solid cryogen cooler, worked for approximately 20 months until the depletion of the cryogen. The ISAMS instrument on UARS provided 7 months of data.

These instruments have provided significant information on the behavior of ozone at a given pressure level, especially the relationship between ozone amounts and the meteorology of the stratosphere. An example is shown in Figure 10, in which the variation of ozone in the lower stratosphere (~30mbar) during a major stratospheric warming in the Northern Hemisphere in the winter of 1979 is shown using LIMS data. In this figure, the polar vortex region of high ozone usually found over the pole (e.g., February 6, 1979) is split into two, as shown in the later analyses for February 16 and February 23. The March 1 panel shows that the ozone has filled in the low region subsequent to a stratospheric warming that had occurred during this time.

Feb. 6,1979 Feb. 16,1979

90 90

Feb. 6,1979 Feb. 16,1979

90 90

Figure 10 Contour maps showing the evolution of lower stratospheric ozone obtained from LIMS during the vicinity of a major stratospheric warming.

As typically implemented, limb emission observations are made only at longitude as the satellite moves along its orbital track; no cross-track scanning (as is done in TOMS) or imaging is carried out to "fill in" the interorbit gaps. One instrument that is an exception to this is the cryogenic infrared spectrometers and telescopes for the atmosphere (CRISTA) instrument, which was deployed from the Space Shuttle in 1994 and 1997. The CRISTA instrument has three telescopes and infrared detectors viewing 18° apart from each other, thus obtaining higher horizontal resolution than any other atmospheric chemistry profiling instrument.

Several other space-based techniques have been used for measurement of atmospheric ozone. In one, a variant of the infrared emission technique, emission is measured not from vibrationally excited ozone (as was done on LIMS, CLAES, and ISAMS), but from electronically excited molecular oxygen [02('A)] produced following ultraviolet photolysis of ozone. This technique, which was used on the Solar Mesosphere Explorer (SME) satellite in the early 1980s, is applicable mainly in the thermosphere and mesosphere; at lower attitudes, the 02(1A) is quenched so rapidly that there is insufficient signal for detection. UV limb scattering was also implemented on SME but was limited to observation in the mesosphere and topmost part of the stratosphere. Since this is a scattering technique, it provides the possibility for good spatial coverage and also can potentially allow for good vertical resolution. More recently, this technique was tested with a pair of instruments (the Shuttle Ozone Limb Sounder Experiment and the Limb Ozone Retrieval Experiment) that flew aboard the Space Shuttle in 1997. Finally, the technique of stellar occultation (in which stars are the source of the radiation) has been tested using the ultraviolet imaging and spectral imagers (UVISI) instrument that flies aboard the U.S. Department of Defense's Midcourse Space Experiment (MSX) spacecraft. The stellar occultation technique has the potential to overcome the spatial limitation of the solar occultation technique because of the existence of many stars that can serve as a source in a given orbit. The technique also has potential for high vertical resolution; the main complication is the need to overcome the much reduced photon flux for a star as opposed to that of the sun.

5 FUTURE MEASUREMENTS

There will be significant activity in the next few years in ozone measurements, especially in the implementation of several space-based measurement systems. These include additional copies of existing instruments, next-generation instruments based on current techniques and significantly new instruments. The first decade of the twenty-first century will also see to at least two space platforms devoted to studying the composition of the atmosphere and providing new insight into both the distribution of ozone and the chemical processes responsible for its measured abundance.

TOMS/SBUV

An updated version of the TOMS instrument, which will probably have a much wider spectral range than TOMS, as well as a large number of channels, is an instrument to be provided by the Dutch government aboard the Aura spacecraft of NASA's Earth Observing System, scheduled for launch in 2004. This instrument, currently known as OMI (ozone monitoring instrument), will also make use of spatial imaging through the use of a detector array, eliminating the need for cross-track scanning and the use of a photomultiplier tube for detection. In the longer-term, a total ozone mapping instrument is scheduled for inclusion in the National Polar Orbiting Environmental Satellite System (NPOESS) being developed by the United States. The exact nature of this instrument has not yet been determined, although it will likely have many of the observing goals of TOMS. The first NPOESS spacecraft will fly no earlier than 2010. Additional SBUV instruments are planned for several NOAA operational meteorological spacecraft through 2004.

SAGE III

An improved version of the SAGE instrument was launched aboard a Russian Meteor-3M satellite in late 2001. The SAGE III instrument uses the occultation technique pioneered with the earlier SAGE instruments but has significant advances, including a wider spectral range (from 290 to 1500nm), and the use of relatively high resolution spectral information within several channels to help provide much improved detection of ozone and other trace species, as well as separation of ozone and aerosol extinction. SAGE III also makes collocated measurements of temperature and pressure to facilitate conversion of profiles from number density versus altitude to mixing ratio versus pressure. SAGE III also has a lunar occultation capability that will remove some of the spatial limitations associated with solar occultations. This is especially important for the Meteor-3M SAGE, which will be in a polar sun-synchronous orbit in which solar occultations are restricted to high latitude. The lunar occultations cover a much broader range of latitudes.

Platforms Dedicated to Atmospheric Chemistry

ENVISAT. ENVISAT, a program of the European Space Agency, was launched in 2002 and has three significant instruments for measurements of atmospheric ozone and related trace constituents. These include the scanning imaging absorption spectrometer for atmospheric chemistry (SCIAMACHY), the Michelsen interferometer for passive atmospheric sounding (MIPAS), and the global ozone monitoring through stellar occultations (GOMOS) instruments. SCIAMACHY is an enhanced version of the GOME instrument flying aboard ERS-2; it will utilize the DOAS technique as did GOME but will also have limb and occultation modes and will have infrared wavelengths that GOME did not have. MIPAS is a high-resolution infrared emission instrument, and GOMOS will use the stellar occupation technique to determine ozone profiles (Burrows, 1999).

EOS Aura. The EOS Aura spacecraft planned for launch in 2004 will have three ozone-measuring instruments in addition to the OMI. These are the high-resolution dynamics limb sounder (HIRDLS), the microwave limb sounder (MLS), and the troposphere emission spectrometer (TES). HIRDLS will use the technique of infrared emission to determine vertical profiles of ozone, temperature, aerosols, and a host of trace constituents. Unlike most previous infrared emission measurements, however, HIRDLS will have high vertical (~1 km) and horizontal (~5°) resolution; the latter comes from its making several scans away from the nadir as the spacecraft moves in its orbit. MLS is a significant improvement over the UARS MLS, with special attention being given to improving measurements in the upper troposphere and lower stratosphere, as well as the measurement of many trace constituents, such as OH, BrO, and N20 not measured with the UARS MLS. TES is designed to measure ozone and its precursors in the troposphere using highresolution infrared spectral measurements. It will use both nadir and limb viewing geometries to do this.

REFERENCES

Albritton, D. L., and R. T. Watson (Eds.), Scientific Assessment of Ozone Depletion: 1991, Global Ozone Research and Monitoring Project, Report No. 25, World Meteorological Organization, Geneva, 1991. Albritton, D. L., R. T. Watson, and J. J. Aucamp (Eds.), Scientific Assessment of Ozone Depletion: 1994, Global Ozone Research and Monitoring Project, Report No. 37, World Meteorological Organization, Geneva, 1994. Albritton, D. L., J. J. Aucamp, G. Megie and R. T. Watson (Eds.), Scientific Assessment of Ozone Depletion: 1998, Global Ozone Research and Monitoring Project, Report No. 44, World Meteorological Organization, Geneva, 1998. Bevilacqua, R. M., et al., Use of POAMII data in the investigation of the Antarctic ozone hole,

J. Geophys. Res., 102, 23643-23657, 1997. Burrows, J. P., Current and future passive remote sensing techniques used to determine atmospheric constituents, in A. F. Bouwman (Ed.), Approaches to Scaling of Trace Gas Fluxes in Ecosystems, Elsevier, Amsterdam, 1999, pp. 317-347

0 0

Post a comment