Both global measurements of ozone made from satellite platforms and localized measurements of ozone made from balloon, aircraft- and ground based platforms use remote sensing techniques. Each of the four platforms has a passive and an active remote sensing capability. Passive techniques include backscatter ultraviolet (BUV), occultation, limb emission, and limb scattering. These four techniques are used on satellite platforms, and it is in this context that they are discussed here. However, the viewing geometry concepts behind them can also be used the other platforms, including ground based ones. An active technique is lidar, which may be used from any one of the four platforms.
In this section, we will discuss the techniques available for passive remote sensing of ozone. As listed above, there are four techniques based on viewing geometry concepts used for passive remote sensing. In Section 4.1, we will discuss these four techniques in the context of examples of actual satellite-platform instruments that utilize them. Nonsatellite platforms, by which we mean ground based, aircraft, and balloon platforms, incorporate some of the same viewing geometry concepts, though in this Chapter we limit the discussion to two types of ground based spectrophotometers (4.2).
In Section 5, we will discuss an important technique for active remote sensing of ozone, the lidar technique. A more detailed discussion of measurement platforms and missions appears in Section 6.
Four passive remote sensing techniques exist for satellite platforms based on different viewing geometry concepts. The four techniques for passive remote sensing are (1) the backscatter ultraviolet (BUV) technique, (2) the occultation technique, (3) the limb emission technique, and (4) the limb scattering technique. Each technique involves a different viewing geometry. The viewing geometry affects the measurements of atmospheric radiation. Figure 7.01 illustrates the four viewing geometries for remote sensors.
Each of the four techniques is explained in the following sections in more detail. We will refer back to Figure 7.01 throughout Section 4.1 for schematic representations of the viewing geometries that underlie each of the four techniques. Satellite instruments designed for remote sensing usually employ one of these techniques. The actual way in which an atmospheric parameter, such as ozone concentration, is derived from the observed radiation involves the use of retrieval algorithms. These algorithms convert the measurements of radiation to an atmospheric parameter (such as ozone concentration). They depend on which of the four techniques is used; hence, they depend on the viewing geometry.
4.1.1 Backscatter Ultraviolet (BUV) Technique -- In the backscatter ultraviolet (BUV) technique, measurements are made of solar ultraviolet (UV) light entering the atmosphere (referred to as the irradiance) at a particular wavelength and of the solar UV that is either reflected from the surface or scattered back from the atmosphere (referred to as the radiance) at the same wavelength. This technique is schematically illustrated in Figure 7.01 A.
(a) Measuring radiance and irradiance -- For determining total ozone, two pairs of measurements are made. One measurement of incoming UV light (irradiance) and backscattered UV light (radiance) is made at a wavelength that is strongly absorbed by ozone. The other measurement of incoming UV irradiance and backscattered UV radiance is made at a wavelength that is weakly absorbed by ozone. The measurements of incoming UV irradiance and backscattered UV radiance at the weakly absorbing wavelength are the control case. They tell us how much backscattered UV light we would expect to measure if there was no change due to ozone absorption. At the other wavelength, the UV light is continuously being absorbed as it passes through the atmosphere by the amount of ozone along the light path. The differences in the pair measurements at the two wavelengths are used to infer how much ozone is present in the atmosphere.
(b) Measuring total column ozone -- Total column ozone is the amount of ozone in a column of air of unit area from the surface to the top of the atmosphere. It is estimated by measuring backscattered radiances at wavelengths between 312 nm and 340 nm. Incoming solar radiation at these wavelengths penetrates into the lower atmosphere (i.e. the troposphere, see Chapter 2) where it undergoes multiple scattering and reflections off cloud and terrestrial surfaces. The ratios of radiance to irradiance measurements at these wavelengths provide estimates of the column ozone amount.
(c) Measuring vertical profiles -- Information on the vertical structure of ozone can be derived using the BUV profiling technique. To understand what this is, we first recognize the fact that the longer the wavelength of the incoming UV irradiance, the less absorption of this radiation by ozone occurs. Such UV light is able to penetrate far into the atmosphere. Conversely, the shorter the incoming UV irradiance wavelength, the more absorption of this radiation by ozone occurs. Such UV light does not penetrate as far into the atmosphere. The absorption increases with decreasing wavelength, such that radiation at progressively shorter wavelengths is significantly absorbed at progressively higher altitudes. So the backscattered radiation at specific UV wavelengths can only be scattered from above a particular height. Below this level, all the radiation is absorbed and there is no backscattered radiance. This allows us to make a vertical measurement of ozone. Measurements at certain UV wavelengths are sensitive to specific portions of the ozone vertical profile, and the full profile can be obtained by measuring radiation at a series of wavelengths and using a retrieval algorithm that converts each radiance measurement to and atmospheric quantity.
A big advantage of the BUV technique is that by looking directly down at the atmosphere below in a viewing geometry called nadir viewing, the satellite is able to get a good horizontal resolution. The total area of the field of view seen by the instrument is referred to as the footprint. The point on Earth directly beneath the satellite is called the subsatellite point. The main disadvantage of the BUV technique is that the effects of increased multiple scattering and reduced sensitivity to the shape of the profile lead to poor vertical resolution in the region below the ozone peak (about 30 km).
(d) Examples of BUV instruments: SBUV, TOMS, and GOME
(1) SBUV and SBUV/2 --The Solar Backscatter Ultraviolet (SBUV) instruments on the Nimbus-7 satellite and the NOAA polar orbiter series are examples of BUV profiling instruments that employ nadir viewing geometry and sample in 12 wavelengths. The SBUV instrument onboard the Nimbus-7 satellite had a nadir angle field of view that was a square 200 km on a side. It was operational from November 1978 to July 1990. The NOAA-11 and later the NOAA-14 SBUV instruments are jointly referred to as SBUV/2. They are the same type of instrument as the Nimbus-7 SBUV. The NOAA-11 SBUV/2 provided hemispheric total ozone measurements from 1989 to 1994. The NOAA-14 SBUV/2 continues these measurements. It takes about 7 days to create a global map.
(2) TOMS -- The Total Ozone Mapping Spectrometer (TOMS) samples backscatter UV at six wavelengths and provides a contiguous mapping of total column ozone. The TOMS instrument, also on-board the Nimbus-7 satellite, provided almost complete global coverage of ozone outside the polar night region (see Chapters 2 and 11) from November 1978 to May 1993. The Nimbus-7 TOMS scanned back and forth through a set of angles about the nadir angle. Its nadir angle field of view was a square 50 km on a side. The METEOR-3 TOMS instrument measured ozone from August 1991 to December 1994. It had a nadir angle field of view of about 60 km on a side. Other TOMS instruments are continuing to provide global coverage, such as the Earth Probe (EP) TOMS instrument, launched in July 1996. Its nadir angle field of view is about 39 km on a side. Total ozone maps are created once a day.
(3) GOME -- The GOME (Global Ozone Monitoring Experiment) was launched onboard the European Space Agency's European Remote Sensing-2 satellite (ERS-2) in April 1995. It employs a nadir viewing BUV technique that measures radiances from 240 nm and 793 nm, corresponding to the ultraviolet and visible regions of the electromagnetic spectrum. It measures column densities of ozone, nitrogen dioxide, water vapor, sulfur dioxide, chlorine and bromine monoxide, and other trace constituents. The nadir angle field of view is 320 km by 40 km. It takes about 2 days to create a global map. Future missions carrying GOME instruments are planned (see section 6.1.8).
4.1.2 Occultation Technique -- Another method for measuring the ozone vertical profile from a satellite platform is the occultation technique. Occultation instruments measure solar, lunar, and even stellar radiation directly though the limb of the atmosphere during satellite Sun, Moon, and star rise and set events (depending on which celestial radiator is being used by the satellite instrument). By measuring the amount of absorption of radiation through the atmosphere at different wavelengths (e.g. UV, visible, infrared), occultation instruments can infer the vertical profiles of a number of trace constituents, including ozone. Figure 7.01 B shows this schematically.
(a) Advantage: improved vertical resolution -- The ratio of the atmospherically altered radiation to the unaltered radiation measured outside the atmosphere gives the atmospheric transmission at specified wavelengths as a function of height. In this way, we can infer the profiles of various trace gases in the atmosphere. The vertical resolution of measurements from a solar occultation instrument is typically about 1 to 2 km, which is better than the BUV profiling technique. Since the same instrument is used to measure the attenuated and unattenuated radiation, any long-term instrument changes disappear when we take the ratio. It is for this reason that these instruments are often called self-calibrating.
(b) Disadvantage: limited spatial coverage -- Until recently the occultation technique used only solar radiation for measuring radiation through the limb of the atmosphere. Measurements could only be made at instrument sunrise and sunset. This meant that the instrument provided only limited spatial coverage per orbit. Such instruments require many orbits to get the same global coverage provided by BUV instruments (such as TOMS) on a daily basis with fewer orbits. Improved designs of occultation instruments allow for use of the Moon and even the stars as the occulting light sources, which will expand the spatial coverage. Such instruments will fly aboard future satellite missions, as outlined below.
(c) Measurement capabilities on current and future satellite missions
(1) SAGE I, II, and III -- The Stratospheric Aerosol and Gas Experiment II (SAGE II) instrument aboard the Earth Radiation Budget Satellite (ERBS) has been measuring ozone since October 1984. Like its SAGE I predecessor, it employs a solar occultation technique to measure trace gases such as ozone, nitrogen dioxide, and water vapor, as well as stratospheric aerosols (like sulfuric acid droplets) in the UV and visible regions. The next generation SAGE III, scheduled for initial launch in mid-1999 aboard the Russian Meteor-3M spacecraft will have expanded capabilities to measure in the near infrared region (see Section 6.1.5). It will also be able to use the Moon as a light source. This lunar occultation mode will increase the spatial coverage of the occultation technique. Measurements in the infrared will allow for characterization of more atmospheric constituents.
(2) HALOE -- The Halogen Occultation Experiment (HALOE) instrument aboard the Upper Atmospheric Research Satellite (UARS), launched in 1991, also measures solar occultation. However, HALOE measures in the infrared region at specific, preselected wavelengths. As noted above, observations in the infrared region allow more atmospheric constituents to be measured than with the current SAGE II. In addition to ozone and water vapor it also can measure methane, hydrochloric acid, and reactive nitrogen species.
(3) POAM II and III -- The Polar Ozone and Aerosol Measurement II (POAM II) instrument was launched aboard the French Space Agency's Satellite Pour l'Observation de la Terre-3 (SPOT-3) satellite in September 1993. It only remained operational until November 1996, but it has since been replaced by the POAM III instrument, which was launched aboard the SPOT-4 satellite in March 1998. The POAM II and its successor POAM III are solar occultation devices that are designed to measure the vertical distribution and overall stratospheric abundances of ozone, water vapor, nitrogen dioxide, and various aerosols.
(4) ATMOS -- The Atmospheric Trace Molecule Spectroscopy (ATMOS) Experiment was an instrument flown three times the Space Shuttle over a period of about 10 years. This instrument also used the Sun as a source of light, but the instrument used an interferometer over a wide range of wavelengths. The power of an interferometer is that it can observe the absorption features of various trace constituents at very high wavelength resolution. This allowed for measurements of trace gases that could not be measured by any other technique. SAGE and HALOE used only limited wavelength channels to make their measurements.
(5) GOMOS -- The Global Ozone Monitoring by Occultation of Stars (GOMOS) instrument is scheduled to be flown on the ENVISAT-1 mission (see Sections 4.1.4 and 6.1.7). As its name suggests, it will use the stars as sources for its occultation measurement. They will significantly increase the spatial coverage of this technique, since the satellite carrying GOMOS will frequently encounter star rises and star sets. The only disadvantage of the use of the stars is that measurements may not go as low in the atmosphere as solar occultation.
4.1.3 Limb Emission -- A third technique for remote sensing measurements of ozone from a satellite platform is the limb emission technique. Instruments based upon the limb emission technique infer ozone amounts from measurements of longwave radiation (infrared or microwave) thermally emitted in the atmosphere along the line of sight of the instrument. The viewing geometry for this technique is shown in Figure 7.01 C. The altitude to which the instrument can see is called the tangent altitude. In theory, the instrument could look all the way to the surface, but below a certain altitude (under 10 km), clouds interfere with the emitted longwave radiation. This radiation emission occurs along the geometric path between the tangent altitude and the satellite instrument. This horizontal path is quite long compared to the tangent altitude.
Though the limb emission viewing geometry looks similar to the occultation viewing geometry, the techniques are very different. Instruments using the occultation technique measure absorption of solar, lunar, or stellar radiation through the atmosphere. Instruments using the limb emission technique measure longwave radiation emitted by trace gases at different altitudes.
(a) Vertical resolution by limb viewing -- The limb emission viewing technique is also referred to as limb viewing. In limb viewing, the vertical fields of view of the instruments are narrow. The measured radiances are an accumulation of radiation emitted along a long horizontal path with little vertical range. Because of the rapid decrease in atmospheric density with height, the primary contribution to the measured radiation at a specific altitude originates close to that altitude. This is because the number of, and hence the longwave radiation emission from, molecules higher up is much less. As the satellite revolves around Earth, the tangent altitude above a given location changes. By taking a number of measurements above a given location, the limb emission sensors are able to create a vertical profile of trace gas concentrations. The resulting vertical resolution is quite good, usually on the order of 3 kilometers.
(b) Limb viewing vs nadir viewing -- By measuring limb emissions, the limb viewing technique provides good vertical resolution, but as noted above, this comes at the expense of horizontal resolution. The long horizontal path between the satellite and the tangent altitude provides for a poor horizontal resolution. This is the opposite of nadir viewing techniques, such as the BUV technique, which provide good horizontal resolution, but poor vertical resolution. This is the fundamental tradeoff in passive remote-sensing from satellite platforms.
(c) Infrared limb sounding instruments on current and future satellite missions -- Limb emission measurements are made in the infrared wavelength range. But limb sounders provide a better coverage than solar occultation instruments, since longwave emissions from the limb of Earth's atmosphere can be measured continuously through the day and night. The solar occultation technique can only be used each time the satellite experiences sunrise or sunset.
(1) LIMS and CLAES -- Limb sounders such as the Limb Infrared Monitor of the Stratosphere (LIMS) aboard the Nimbus-7 satellite, and the Cryogenic Limb Array Etalon Spectrometer (CLAES) aboard the Upper Atmospheric Research Satellite (UARS), were designed to make measurements of many atmospheric constituents. These include HNO3 (nitric acid), NO2 (nitrogen dioxide), and HCl (hydrochloric acid), all of which are important to ozone chemistry. The disadvantage to these instruments in the past has been their limited lifetimes because their detectors require expendable coolants.
(2) HiRDLS and TES -- The solar occultation sunrise-sunset limitation is overcome for a new generation of infrared limb sounding instruments will fly on the Earth Observing System (EOS) Chemistry mission satellite, known as EOS-Chem, scheduled for launch in December 2002. The EOS-Chem platform is discussed in more detail in Section 6.1.4. Here we note in particular two instruments, the High Resolution Dynamics Limb Sounder (HiRDLS) and the Tropospheric Emission Spectrometer (TES). These instruments will employ low power consuming mechanical coolers for their detectors and measure even more atmospheric constituents because of advanced technology.
HiRDLS is a limb scanning instrument designed to measure temperature in the upper atmosphere and to determine concentrations of ozone, water vapor, methane, nitrous oxide, nitrogen dioxide, chlorine nitrate, and various chlorofluorocarbon (CFC) compounds, as well as the location of polar stratospheric clouds. It has a vertical resolution of about 1 km. The TES instrument is capable of both nadir and limb viewing. It will measure all radiatively active gases in the atmosphere by measuring thermal longwave emissions from the surface and the atmosphere. TES has a vertical resolution of about 2 km.
(d) Microwave sounding instruments on current and future satellite missions -- Microwave emission instruments have the unique ability to see through clouds because they observe at very long wavelengths and therefore can see lower into the atmosphere (as they are not obstructed by clouds). They can detect important atmospheric species not observable by any of the techniques mentioned earlier. The most well know of these types of instruments is the Microwave Limb Sounder (MLS) flying on board the previously cited UARS. This instrument has simultaneously detected ozone and key active species involved in the catalytic destruction of ozone (see Chapters 5 and 11). An advanced version MLS will fly on EOS-Chem. The main disadvantage of microwave instruments is their large size, weight (nearly 300 kg) and demands for power. The application of limb emission measurements is discussed in Section 6.1.
4.1.4 Limb Scattering -- The final technique for passive remote sensing is called limb scattering. This technique is shown schematically in Figure 7.01 D. It employs aspects of the other three techniques. The viewing geometry is similar to that of both limb emission and occultation, which provides good vertical resolution. It also measures scattered solar radiation in a manner similar to the BUV measurement, but the light source is in Earth's limb. This allows for coverage through the entire atmosphere, which provides for good column measurements similar to those of BUV instruments like TOMS.
Limb scattering was employed on earlier satellites, but the altitude range was limited to the upper stratosphere and mesosphere. With recent technological advances, the technique is now being tested for collecting data in the altitude region of the upper troposphere and lower stratosphere (UT/LS) which is important to understanding the mixing between these two layers. Because this technique observes scattered light rather than a direct light source such as the Sun or the Moon, data can be taken nearly continuously. This technique works best with ozone, however other trace gases like water vapor, nitrogen dioxide, and sulfur dioxide are also measurable. The measurements are made in the UV, visible, and near infrared regions. Another application of the technique is the detection of aerosols in the UT/LS region.
The limb scattering technique has been tested on the Space Shuttle and has given good ozone profiles with high vertical resolution down to the tropopause. The technique will be fully exploited by the Scanning Imaging Absorption SpectroMeter for Atmospheric CHartographY (SCIAMACHY) flying on the European Space Agency's ENVISAT-1 satellite, scheduled for launch in May 2000 (see Section 6.1.7). The SCIAMACHY instrument will provide global measurements of various trace gases in the troposphere and stratosphere by measuring transmitted, backscattered, and reflected radiation from the atmosphere between 240 nm (ultraviolet) and 2400 nm (shortwave infrared). The large-wavelength range provides for the determination of aerosols and clouds. SCIAMACHY has both a nadir and a limb scanning geometry. The nadir viewing allows for measurements of total column amounts of various trace gases, while the limb scanning determines profiles of trace gases and aerosols in the stratosphere. Together, these measurements will provide global distribution maps of the various constituents in the UT/LS region of the atmosphere.
We now turn our attention to passive remote sensing techniques on ground based platforms. The instrumentation used incorporates some of the same viewing geometry concepts we saw in the satellite platforms. Here we will discuss two types of spectrophotometers. The spectrophotometer is an instrument that infers column ozone by measuring the amount of ultraviolet sunlight reaching it, and from this deducing how much UV absorption took place because of ozone.
We consider two types of spectrophotometers here. They are the Dobson Spectrophotometer (4.2.1) and the Brewer Spectrophotometer (4.2.2). Recall that as a passive remote sensing instrument, the spectrophotometer instrument only measures the amount of radiation that it intercepts. It does not actually measure the amount of ozone itself like an ozonesonde in-situ measurement would. Nor does it send out any electromagnetic radiation, such as the laser beam of a lidar, to make active remote sensing measurements.
4.2.1 Dobson Spectrophotometer -- The Dobson Spectrophotometer measures total column ozone from the ground to the top of the atmosphere in a column by measuring the amount of sunlight reaching Earth's surface in the region of the electromagnetic spectrum where ozone absorption occurs. This absorption by ozone occurs in the 290 to 320 nanometer wavelength region. It is the ultraviolet-B (UV-B) region (see Chapter 1). Since the presence of clouds, pollution, and aerosols (such as smoke) also affect the amount of light (shortwave radiation) reaching the ground, a region of the spectrum where ozone does not absorb is also simultaneously measured. This is similar to the two-pair measurements of the BUV technique. The assumption is made that clouds and other atmospheric contaminants reduce the amount of light equally across wavelengths (i.e. the reduction is the same at 300 nm as it is at 340 nm). By taking the ratio of the two measured values, the effects of the clouds and aerosols are canceled out, leaving the ozone absorption signal. The less ozone overhead, the more UV-B radiation at 305 nm reaches the detector. Thus the ratio of the 305 nm detector channel to the 325 nm detector channel is smaller than when there is more ozone overhead.
This method was pioneered by Gordon Dobson in the 1930's and the instrument he designed, the Dobson spectrophotometer, is still in use today. The light entering the spectrophotometer is split into two beams that fall alternately on a photomultiplier tube. The more intense beam is reduced in intensity by passing it through a piece of glass of varying thickness. (In our case, the more intense beam is 325 nm, since the solar output curve is lower at 305 nm than at 325 nm, as explained in the blackbody radiation section of Chapter 4. In addition, ozone absorbs much more strongly at 305 nm than at 325 nm.) The instrument operator adjusts the glass wedge until both beams are the same brightness. When the photomultiplier tube detects the beams are the same intensity, the position of the glass wedge is noted and the amount of ozone is then derived from lookup tables. The look-up tables and the equations used to derive total ozone from the intensities of the measured irradiances are not presented here. They can be found in reference books on the subject.
The Dobson spectrophotometer can be used to determine ozone by measuring light from either direct sunlight, diffuse light from clear or cloudy skies, and even from reflected sunlight from the Moon. The direct Sun measurements are preferred because the uncertainties of ozone measurements get larger as the amount of light entering the instrument decreases.
4.2.2 Brewer Spectrophotometer -- The Brewer spectrophotometer measures ozone based on the same technique as the Dobson instrument. Unlike the Dobson instrument, however, the Brewer spectrophotometer is completely automated and can be programmed by a laptop computer to make measurements at any given time during the day. Most Brewer instruments are programmed to take measurements at regular observation times. (This takes into account the angle of the Sun.) The instrument measures ultraviolet light at five wavelengths (306, 310, 313, 317, 320 nm). The total column ozone amount is calculated by using a more complicated form of equation used for the Dobson instrument that includes terms for sulfur dioxide. The absolute accuracy for a total ozone measurement made by a well calibrated Brewer instrument is estimated to be +/- 2.0%.