1.What is the basic difference between in-situ and remote sensing measurements?1b.What platforms are used for ozone measurements?
2.How is passive remote sensing different from active sensing? Give an example of each.
2b.How can remote sensing be used to differentiate atmospheric components?
2c.Are there platforms unsuitable for remote sensing? Explain.
3a. Explain how the backscatter ultraviolet technique is used to measure ozone concentration.
3b. How can this technique be modified to arrive at a vertical ozone profile?
3c. What is a limitation of the BUV technique?
4a. What is the occultation measurement technique?
4b. Why are occultation instrument referred to as "self calibrating'?
4c. What radiation sources are used in occultation?
4d. What is the disadvantage of a solar occultation instrument?
5a. Explain the limb emission technique.
5b. Why can you say that radiation measured by limb emission at a particular wavelength was emitted at that altitude?
5c. What advantage does the limb emission technique have over solar occultation?
6a. What is the limb scattering technique?
6b. What is the altitude range of this technique and why is it important to ozone research?
7. Explain how the DIAL lidar technique is used to generate vertical ozone profiles.
8a. What is an ozonesonde?
8b. Explain briefly how an ozonesonde is used to determine ozone concentration.
Consider the various platforms for atmospheric measurements (space based, balloon, aircraft, ground). Which platform(s) would you select to accomplish the following and why:
9a. study transport dynamics for a chemical species in the stratosphere with high spatial resolution
9b. obtain global information on an column amount of an atmospheric constituent
9c. test an instrument under development that will fly in space
9d. gather data for assessing long-term changes in an atmospheric trace constituent variability over a specific location.
The appendix provides a list of atmospheric constituents important to the study of stratospheric ozone along with measurement capabilities currently available or planned through NASA programs.
ANSWERS
1a. In-situ measurements involve a direct sampling of the atmosphere. A sample of the atmosphere is brought into the instrument and can be analyzed for its properties or its relative amount of various chemical components. In remote sensing measurements the observation platform does not directly sample the atmospheric parameter of interest (such as amount of ozone), but instead it measures the change in electromagnetic radiation, either thermal or shortwave, that is due to the presence of the parameter.
1b. Ozone measurements can be made from the ground, aircraft, balloons, and satellites.
2a. Passive remote sensing measurements involve observing the atmosphere in various regions of the electromagnetic spectrum while active remote sensing involves interacting with the atmosphere and measuring the response. An example of active remote sensing is the lidar (light detection and ranging) techniques which employs a laser as a light source to probe the atmosphere. Direct measurement of ultraviolet light from the Sun scattered by the atmosphere is passive.
2b. Remote sensing of the atmosphere relies on the fact that each atmospheric molecule has its own spectroscopic behavior (or fingerprint) when it is observed by an instrument which is sensitive to radiation at specific wavelengths.
2c. All platforms (ground, aircraft, balloons, and satellites) can be used for remote sensing.
3a. In the backscatter ultraviolet (BUV) technique measurements are made of solar ultraviolet (UV) light entering the atmosphere (irradiance) at a particular wavelength and of the solar UV that is either reflected from the surface or scattered back from the atmosphere (radiance) at the same wavelength. For total ozone, 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 are the control case. They tell us how much backscattered light we would expect to measure if there was no change due to ozone absorption. At the other wavelength, the 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.
3b. In the BUV profiling technique, measurements are made at certain wavelengths that are sensitive to specific portions of the ozone vertical profile. 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 an atmospheric quantity.
3c. The limitation 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).
4a. Occultation instruments measure solar, lunar, and even stellar radiation directly through 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). 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.
4b. The same instrument is used to measure the attenuated and unattenuated radiation, so any long-term instrument changes disappear when we take the ratio. It is for this reason that these types of instruments are called "self-calibrating."
4c. Solar, lunar, and even stellar radiation can be used in the occultation technique.
4d. The disadvantage of the occultation method has been that measurements could only be made at instrument sunrise and sunset since the instruments depended on solar radiation. This resulted in poor spatial coverage on a daily basis. The spatial coverage is improving with the new generation of occultation instruments that also can use lunar and even stellar radiation.
5a. 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.
5b. 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, hence the longwave radiation from, molecules higher up is much less.
5c. 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.
6a. Limb scattering measures light that is scattered along the line of sight of the limb of the atmosphere. Because of the fact that this technique observes scattered light rather than a direct light source such as the Sun or the Moon, data can be taken nearly continuously.
6b. Until recently 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.
7. Differential Absorption Lidar (DIAL) technique consists of transmitting an intense beam of light (almost always a laser) into the atmosphere, where it is scattered by interactions with aerosol particles and air molecules. Some fraction of the light is scattered in the backward direction and can be collected by a telescope located near the transmitting laser. The signal is then collected by a detector. It is stored as a function of time of the transmitted laser pulse. The time duration between transmission of the laser pulse and its detection can be converted into a geometric altitude, provided that the beam is transmitted vertically.
8a. An ozonesonde is an in-situ measurement instrument that is carried onboard a weather balloon. An ozonesonde is used to measure ozone vertical profiles. Ozonesondes are composed of an ozone sensor, battery, a small gas pump, and some electronic circuit boards. The ozone sensor is connected to a meteorological radiosonde. This radiosonde transmits values of air temperature, air pressure, relative humidity, detector current, detector temperature, and pump speed to a ground receiving station.
8b. The ozone sensor consists of two small chambers containing solutions of potassium iodide (KI). Each chamber has a platinum electrode at its base and the electrodes are connected by an ion bridge (a wire). Air is pumped into one chamber and the ozone reacts with the potassium iodide which produces iodine (I2). When the iodide changes to iodine the two cells are no longer in electrical equilibrium. Electrons flow from one cell to another in order to reestablish equilibrium. As the amount of ozone in the air increases, the faster the iodide is changed to iodine and the more electrons flow between the cells (current). The amount of current that flows between the chambers is measured and sent to the ground receiving station. The ozone amount can be derived by the simple equation: P=C*i*Tp*t, where P = ozone partial pressure (nanobars); C= constant; i = current; Tp = pump temperature; t = amount of time to force 100 milliliters of air through the system.
9a. Aircraft observations provide snap-shots of stratospheric conditions. The aircraft data are obtained from these highly calibrated instruments at a very high precision, accuracy, and spatial resolution. In principle, such observations are superior for understanding the stratosphere.
9b. Global information on a chemical constituent is best derived from satellite missions where coverage is more complete.
9c. Balloon-based platforms provide platforms for testing instruments under development since they are relatively inexpensive to launch and can obtain profiles of a constituent or property at varying altitudes.
9d. Ground-based measurements use high-quality research instruments for the determination of long-term changes in atmospheric trace constituent variability.
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