Infra-red and microwave instruments also measure atmospheric composition. The intensity of the emitted thermal flux depends on the temperature and the abundance of the emitter (i.e. on the probability per molecule of emission of a photon of given wavelength, and on the number of emitting molecules present). So, once the temperature has been determined from the emission by a molecule of known abundance, such as CO2, measurements of the same atmospheric path at a different wavelength in an emission band of a different species can yield information on the concentration of the second species.
Among the first targets for global measurement campaigns were the oxides of nitrogen, N2O, NO, NO2, and N2O5, following the discovery that they are involved in the catalytic destruction of ozone, posing a threat to the Earth's shield against solar ultraviolet radiation. Most members of the NOy family are very reactive and the amounts available to interact with ozone vary dramatically with space and time and cannot be followed effectively by traditional measurements from aircraft or balloons. Global measurements of nitrogen dioxide reveal large gradients vertically, diurnally, and with latitude, and nitric oxide in the upper stratosphere shows a strong correlation with solar activity. NO2 is removed by conversion to 'reservoir' species such as dinitrogen pentoxide and nitric acid in the long polar night. Reactions involving chlorine and other halogen-bearing species also have a major role and measurements of such reactive molecules as chlorine oxide (ClO) have also become a priority. An understanding of the chemistry going on in all regions of the atmosphere is crucial to predicting the fate of the ozone layer at all latitudes.
The lower atmosphere is experiencing a gradual change due to the accumulation of minor constituents such as carbon dioxide, carbon monoxide, chlorofluorocar-bons, nitrous oxide, and methane. These are radiatively active gases that affect the energy balance at the surface, in part by radiating back absorbed radiative energy (the greenhouse effect), leading to a general warming. Changes in reactive gases such as ozone have a profound effect on the chemical mix that affects not only the 'greenhouse' but also leads to phenomena such as acid rain. The first remote-sensing experiments to measure tropospheric carbon monoxide globally for the first time in 1976 found high concentrations, not over the urban industrial centres of N America and Europe, as expected, but downwind of the forests of S America and Africa. Since then, the importance of biomass burning has been in the headlines. Today, remote-sounding instruments on satellites are monitoring a wide range of greenhouse gases, ozone-destruction precursors, and other pollutants in a global check on the health of the planet.
The Infrared Interferometer Spectrometer (IRIS) was a very early example of an instrument designed to conduct spectral 'surveys' of the Earth, that is, highresolution measurements spanning a wide spectral range, which could be used for a variety of investigations, not all of which need to be planned in advance. A number of versions have flown, not only in Earth orbit but also to Mars, Jupiter, Uranus and Neptune, where they made excellent progress in understanding atmospheric structure and composition.
IRIS was launched on Nimbus 3 in 1969, becoming the first Michelson interferometer in space. A similar version with higher spectral resolution (2.8 versus 5 cm-1) and a narrower field-of-view (5° versus 8°) flew a year later on Nimbus 4. Both had potassium bromide beamsplitters, giving a spectral range of 400 to 1600 cm-1 (6.3 to 25 pm); the relatively modest spectral resolution corresponds to a total mirror travel of only a few mm. This travel occurs at constant speed, with sampling of the signal from the detector at regular small intervals. The mirror position at the sample points is required to very high accuracy, and is achieved by using a second interferometer with a monochromatic source at a frequency much higher than the greatest infra-red frequency being measured. In IRIS, this was a neon discharge lamp, or a He-Ne laser in some versions. The sampling point was then tracked by counting the zero crossings of the signal from the reference source when it passed through the same interference path as the infra-red beam.
Background radiation from the instrument components was reduced by thermally isolating the baseplate from the warm spacecraft, and allowing the housing to cool radiatively to space, down to an electrically thermostatted value of 250 K. The thermistor bolometer detector was not cooled below this temperature, and had a sensitivity that dictated an integration time of 10.9 s to scan the spectrum. In this time an orbiting spacecraft moves nearly 100 km, tending to change the scene below during the course of an integration, and leading to aliasing of any spatial variations into the spectrum. This was prevented by image-motion compensation, using a scan mirror in front of the interferometer. The same mirror scanned to cold space and to a warm target for two-point radiometric calibration.
ATM OS (atmospheric trace molecular spectroscopy) differed from IRIS principally by using the Sun as a source, and by employing cooled detectors, so that the signal-to-noise ratio obtained was very high. It took advantage of this by having an enormous physical mirror displacement, which provided the very high spectral resolution needed to detected pollution molecules present in the atmosphere in quantities of less than one part per billion by volume, and the short scan time required to obtain spectra before the tangent path through the atmosphere changed significantly due to the orbital motion of the space shuttle to which it was mounted. By moving both mirrors through a maximum physical displacement of 25 cm in 2.2 s, spectra covering from 600 to 4800 cm-1 with a resolution of 0.013 cm-1 were achieved, while the line of sight between the instrument and the Sun moved only about 2 km relative to the surface of the Earth at the tangent point. Figure 10.12 shows a schematic of the optical layout. Most of the complexity results from the need to fold and focus the beams, and to drive both mirrors. Note also the inclusion of a camera that images the Sun continuously in the visible, in order to verify post facto the positioning of the field-of-view in the centre of the disc, and to check for sunspots, or refractive effects in the Earth's atmosphere, both of which affect the spectrum and its baseline. The detector is cadmium mercury telluride, cooled by a mechanical refrigerator.
The Tropospheric Emission Spectrometer (TES) is an infra-red imaging Fourier transform spectrometer with mechanically cooled optics and detectors, operating in the spectral range 650-3050 cm-1(3.3-15.4 ¡m). TES is designed to measure thermal emission from tropospheric minor constituents with high spectral and high spatial resolution from the Earth Observing System satellite AURA, launched in June 2004.
The main focus is on tropospheric ozone, a key constituent in pollution studies for which no good global data exist, but the wide spectral range means dozens of species (including NOy, CO, SO2 and other sulphur compounds) are observed to address the atmospheric chemistry part of the greenhouse puzzle. An advanced feature of TES is that it can scan continuously between the nadir and the limb. The interesting tropopause region is observed with high vertical resolution, high spectral resolution and continuous spectral coverage, allowing problems of stratosphere-troposphere exchange to be addressed. Convective transport across the tropopause is inhibited by the temperature structure, and the ways in which
flG. 10.12. A schematic diagram of the Tropospheric Emission Spectrometer, showing the main subsystems. TES is basically a Michelson interferometer with two pairs of moving mirrors to give a long path difference in a compact instrument. The purpose of the filters, which, along with the detectors, are cooled to around 85 K, is to limit the total amount of radiation falling on the detectors and hence reduce the photon-induced noise. Several filters are mounted on wheels, as shown, which can be rotated to define different spectral regions. A high-resolution spectrum is then obtained within that region by interferometry. Four spectra, in different spectral regions, are obtained simultaneously by the use of dichroics (wavelength-sensitive beam splitters) and multiple detector/filter assemblies, as shown.
Moving mirrors flG. 10.12. A schematic diagram of the Tropospheric Emission Spectrometer, showing the main subsystems. TES is basically a Michelson interferometer with two pairs of moving mirrors to give a long path difference in a compact instrument. The purpose of the filters, which, along with the detectors, are cooled to around 85 K, is to limit the total amount of radiation falling on the detectors and hence reduce the photon-induced noise. Several filters are mounted on wheels, as shown, which can be rotated to define different spectral regions. A high-resolution spectrum is then obtained within that region by interferometry. Four spectra, in different spectral regions, are obtained simultaneously by the use of dichroics (wavelength-sensitive beam splitters) and multiple detector/filter assemblies, as shown.
pollutants from the surface reach the stratosphere, and stratospheric ozone enters the troposphere, are still very poorly understood.
In many ways, TES is the ultimate remote-sensing instrument, having nadir and limb viewing, cryogenic detectors and optics, wide spectral coverage and high spectral resolution capable of resolving individual spectral lines with good signal-to-noise ratio. In principle, TES measurements should be capable of making accessible to the experimenter virtually all of the information the Earth's passively emitted thermal radiation field contains.
The principal components and general layout of TES are shown in Fig. 10.12. These gather, focus, modulate, filter, detect, amplify and calibrate the signal from a well-defined field of view in the atmosphere, either downwards or sideways to the limb. The detectors feature 41x16 element optically conjugated focal plane arrays each optimized for a different spectral region and operating at a temperature of 63 K using mechanical refrigerators. In addition, each focal plane is equipped with interchangeable cooled filters that limit the instantaneous spectral bandwidths to about 250 cm-1. This provides essential control over the instrument thermal background and reduces the data rate. Except for two external mirrors (part of the pointing system), the entire optical path is radiatively-cooled to about 165 K, to reduce further the instrument background.
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