4.4 HOW DO WE USE THE ATMOSPHERE'S RADIATIVE PROPERTIES TO MAKE MEASUREMENTS OF OZONE AND OTHER ATMOSPHERIC CONSTITUENTS?

4.4.1a Remote Sensing
4.4.1b Forward Modeling and the Inverse Problem
4.4.2a Instrument Calibration
4.4.2b Scattering and Attenuation
4.4.3 Beer-Bouger-Lambert Law
4.4.4 Ozone Measurements

4.4.1a Remote Sensing

Remote Sensing means the measurement of any physical, chemical, or biological quantities in the atmosphere, the land, and the oceans, where these measurements are made from the vantage point of a spaceborne instrument. All such measurements are based on the measurement of electromagnetic radiation. Remote sensing techniques are divided into two categories: active and passive. In the case of active remote sensing, the radiation measured originates in the spaceborne instrument. An example of this is microwave radar that is used for topographic measurements of the land and ocean surfaces: A microwave transmitter aboard the spacecraft bounces a pulse off the Earth's surface, and then measures the characteristics of the returned pulse. Passive remote sensing techniques measure the naturally occurring radiation coming up from the Earth. In most cases, this radiation is either the thermal blackbody radiation of the Earth's land, ocean, and atmosphere (infrared and radio regions), or is reflected or scattered sunlight (visible and ultraviolet regions), or a combination of the two. In some passive measurements, called occultation, the light source (usually either the Sun or a star) is directly viewed through the earth's atmosphere as it rises or sets over the horizon seen by the satellite.

Measurements made from ground-based instruments can provide very accurate information about the chemical, physical, and biological state of the immediate region around the location where the measurement is made. Ground-based measurements may directly sample the air, water, soil, and vegetation, and so may use a variety of chemical and physical techniques. A spaceborne instrument has the advantage of being able to sample large regions of the Earth's surface in a very short period of time. For example, a geostationary satellite, such as the GOES meterologicical instruments, located at a height of 35,900 km, sees almost a complete hemisphere of the Earth's surface instantaneously. A polar-orbiting satellite, at around 500 to 1,000 km altitude, views a much smaller region, but in the fourteen orbits it makes in a 24-hour period, can cover almost the complete surface of the Earth. A clear advantage of spaceborne measurement is the ability to make measurements in the most remote or inhospitable regions of the planet. Another advantage is that the measurements are all made with the same instrument. If a number of separate instruments are used in a ground-based study, they must be carefully cross-calibrated to one another.

	Advantages	Disadvantages
Remote Sensing	-Wide geographical coverage (up to full-earth). - Automatic operation ensures continuous, daily coverage. - All measurements are made with a single, well-calibrated instrument. - Wide range of quantities can be measured simultaneously. - Environmental effects on the measurements can be minimized	- Once in orbit, instrument cannot be recovered for repair or laboratory calibration. - Instrument longevity is subject to many kinds of failures. - Only radiation-based measurements are possible. Physical quantities must be "retrieved" from radiance measurements. - Atmospheric profile retrieval is difficult, and some retrievals have considerable uncertainty - Measurements may only be made at time of satellite overpass. - Development, launch, on-orbit maintenance, and data processing are very expensive.
In-situ Measurements	- Instruments can be calibrated before, during, and after field measurements. - Measurement techniques may be physical, spectroscopic, or chemical. - Individual instruments may be quite long-lived. - Measurements may be made any time of day or night. - Instruments are much less expensive to develop, build, and deploy than spaceborne instruments	-Measurement valid for very small geographical region - Limited by accessibility of location, availability of power. - Instruments may be affected by environmental factors.

[For more information about spaceborne instruments, click here-link to instrumentation chapter]

The next table lists a number of quantities that have been measured from space, and, where possible, links to relevant web sites.

Quantity	Instruments	World Wide Web Links
Ozone, total column	TOMS, HALOE, SBUV, SBUV/2, GOME
Ozone, profile	BUV, SBUV, SBUV/2, MLS, SAGE, TOVS, GOME
ClO	HALOE
NO2	HALOE, GOME
CO
H2O	GOES, THIR, AVHRR
Atmospheric temperature profile	GOES, TOVS,
Surface temperature	GOES, AVHRR,
Aerosols	TOMS, AVHRR
Cloud cover, type, height	GOES, AVHRR, THIR, TOMS, SeaWiFS
Sea surface height	TOPEX/POSEIDON, SAR(?)
Tropical rainfall	TRMM
Surface topography	SAR, SLR, SAREX? (Canadian instrument?)
Ocean phytoplankton	CZCS, OCTS, SeaWiFS
Land vegetation cover	AVHRR, SeaWiFS

For studies of the atmosphere, we would like to be able to characterize the chemical composition as a function of geographical coordinates (latitude and longitude), and, if possible, as a function of altitude. Even though the Earth's atmosphere appears from space as a very thin layer, it is vertically very inhomogeneous. That is, as you go up through the atmosphere, its physical state (temperature, pressure) and its chemical state (concentrations of different chemical species) change. The graph of a particular quantity as a function of altitude is called a profile. Some satellite-based measurements can be made of the profiles for quantities such as ozone concentration, water vapor content, and temperature. Other physical quantities are measured for their total column amount, that is, the total amount, in the entire atmosphere, between the satellite and the surface of the Earth over a given geographical site.

For an explanation of the model that is used to accomplish remote sensing measurements of the atmosphere, see the section on Forward Scattering and the Inverse Problem (4.4.1b)

4.4.2a Instrument Calibration

Another problem with measurements made in space is that a technique for calibrating the instruments must be developed. Optical instruments degrade over time, affecting the measurements. Once an instrument is in space there is no way to directly measure the degradation. (Unlike ground-based instruments, which can always be hauled into a laboratory for calibration.) Measurements of the changes in a spaceborne instrument must be done remotely.

4.4.2b Scattering and Attenuation

As they travel through the atmosphere, visible and ultraviolet photons change their direction of travel through Rayleigh and Mie scattering processes. At the same time, a photon of a certain wavelength has a certain probability of encountering an ozone molecule (or some others) having a certain absorption cross section at that wavelength, and therefore has a certain probability of being absorbed. The quantitative description of the amount of light that makes it to a certain point in space, having traveled through an absorbing medium is called the Beer- Bouger-Lambert law (4.4.3)

4.4.4 Ozone Measurements

All techniques for measuring ozone make use of the known attenuation due to ozone absorption described above. Global measurements of ozone from satellite platforms make use of passive remote sensing techniques.

The most accurate technique for remote sensing of ozone from space is the backscatter ultraviolet (BUV) technique [click to section in Measurements chapter], in which the solar ultraviolet irradiance entering the atmosphere and the radiance scattered back into space are measured at certain wavelengths. Since nothing in the Earth-atmosphere system emits UV radiation, the radiation escaping the atmosphere(and seen by the satellite) has either been scattered by atmospheric constituents, or reflected from the Earth's surface. Two pairs of measurements are made: one at a wavelength that is strongly absorbed by ozone, and one that is weakly absorbed. The measurements of the incoming and backscattered light at the weakly absorbed wavelength tell us how much backscattered light we would expect to measure if there were no attenuation due to ozone absorption. At the other wavelength, ozone absorbs some of the light as it passes through the atmosphere, and the radiation backscattered to space is highly attenuated. The more ozone in the atmosphere, the greater the attenuation. Thus, the differences between the pair measurements at the two wavelengths are used to infer how much ozone is present in the atmosphere. Total column ozone is estimated by measuring backscattered radiances at wavelengths between 312 nm and 380 nm. Incoming solar radiation at these wavelengths penetrates into the lower troposphere where it undergoes multiple scattering and reflection off cloud and terrestrial surfaces. The ratios of radiance to irradiance measurements at these wavelengths provide estimates of the column ozone amount, but provide no information on the vertical structure of the ozone. At shorter UV wavelengths, however, the incoming radiation is absorbed more strongly by ozone, and thus 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.

Measurements at these wavelengths are sensitive to specific portions of the ozone profile, and the full profile can be obtained by measuring radiation at a series of wavelengths. This is called the BUV profiling technique. The disadvantage of the BUV technique is that the effects of increased multiple scattering low in the atmosphere leads to a reduced sensitivity to the shape of the profile and poor vertical resolution in the region below the ozone peak (about 30 km).

The Solar Backscatter Ultraviolet instrument on the Nimbus-7 satellite is an example of a BUV profiling instrument. The contribution functions for each wavelength measurement are shown in Figure?? [figure in measurements chapter??] which demonstrates how each wavelength measures backscattered radiances from a different altitude range in the atmosphere. The vertical resolution is about 5 km in the middle and upper stratosphere, increasing to 8 km or more in the lower stratosphere.

Another method for measuring the ozone profile from a satellite platform is the solar occultation technique. Solar occultation instruments measure solar radiation directly though the limb of the atmosphere during satellite sunrise and sunset events [click to schematic in measurements chapter???]. The ratio of the atmospherically-attenuated solar radiation to the unattenuated solar radiation measured outside the atmosphere gives the atmospheric transmission (the fraction of transmitted light is 1 - A, where A is the fraction of absorbed light) at specified wavelengths as a function of height. From this from the profiles of a number of constituents, including ozone can be inferred. The vertical resolution of measurements from a solar occultation instrument are typically on the order of 1 km, which is much better than a BUV profiling instrument. Identical optics are used to measure the attenuated and unattenuated solar radiation so that any long-term instrument change cancels in the ratio. Because of this, these instruments are often called "self-calibrating". The disadvantage of the occultation method is that measurements can only be made at instrument sunrise and sunset so the instrument has poor spatial coverage. Unfortunately, the advantage of good precision and vertical resolution, even below the ozone peak, is offset by a low coverage rate.

A third technique for measuring ozone from a satellite is the limb sounding technique. Instruments based upon the limb sounding technique infer ozone amounts from measurements of longwave radiation (infrared or microwave) emitted in the atmosphere along the line of sight of the instrument [click to schematic in measurements section]. The vertical fields of view of the instruments are narrow, such that the measured radiances are a cumulation 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 radiation measured at a specific altitude originates very near that altitude, because the number, and thus the contribution from atmospheric particles at higher altitudes is relatively lower. As a result, limb sounders can make measurements at high vertical resolution. Since the measured radiation only comes from above the instrument line of sight, scattering off aerosols, clouds, and terrestrial surfaces does not interfere with the measurements. Limb sounders also provide a better horizontal resolution than solar occultation instruments, since emission from the limb can be measured continuously through the day.

There are also ground-based techniques that use backscattered radiation to remotely measure properties of the atmosphere. Lidars (Light Detection and Ranging) are active remote sensing instruments which infer temperature, density, and trace constituent concentration profiles from measurements of backscattered laser light [click to schematic in measurements section]. Lidars operate in a variety of modes. Measurements of the ozone profile use the differential absorption (DIAL) technique. In this technique the lidar emits radiation at two wavelengths: one which is strongly absorbed by ozone, and one which is very weakly absorbed. The ratio of the backscattered signals is used to derive ozone amounts. By dividing the backscattered radiation into small time increments (called binning), the altitude range in which the scattering took place can also be determined and the ozone profile can be derived.. Lidars are capable of making measurements at 1 km resolution from roughly 15 to 50 km.

A final technique that makes use of radiative properties of the atmosphere is a ground-based microwave sounder. ...missing text...

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