Organization(s):Network For the Detection of Atmospheric Composition Change (NDACC)
University of Massachusetts at Amherst
What does this program measure?
The instrument consists of a sensitive heterodyne down-convertor feeding a 120 channel filter-bank spectrometer. It measures the spectral line at 110.836 GHz produced by a rotational transition of ozone. The altitude distribution is retrieved from the details of the pressure-broadened line shape. The retrieved distribution covers a range of 20-70 km, with vertical resolution < 10 km from 20-45 km. Precision is 4-5%, accuracy 5-7%. There are two levels of calibration: hourly, under computer control, and manually, three times per week. Black bodies at known ambient and cryogenic temperatures are used in the calibration procedures. Values are reported as a mixing ratio (ppmv)
How does this program work?
Excited molecules of ozone produce characteristic spectra, which are analogous to, for example, the characteristic spectrum of excited sodium in a streetlight that is perceived by the eye as yellow light. This light would appear as a series of sharp spectral lines when passed through a prism and displayed on a screen, rather than a rainbow-like continuous transition from red to violet. The lines occur because the light is emitted only at specific wavelengths. The ozone emission occurs at wavelengths that are longer than those of light by a factor of the order of 10,000, and the appropriate detector uses radio rather than photochemical techniques. The frequency of the line we observe is 110.8 GHz, which corresponds to a wavelength of 2.7 mm. The emission is excited by collisions between ozone and air molecules. The intensity is proportional to the ozone density, and the width of the line is proportional to the atmospheric pressure at the level at which it is emitted. Emission from all levels in the atmosphere reaches the ground, resulting in a complex observed line shape whose details depend on the ozone altitude distribution. Mathematical techniques are used to retrieve the altitude distribution from the measured line shape, with a vertical resolution varying between about 8 and 15 km. The UMass Stratospheric Ozone Instrument is a Millitech Corp, 110.8 GH Microwave Spectrometer that is operated with a frequency of 3 profiles per hour at MLO.
Why is this research important?
Long term trends in ozone are small, less than one percent per year. (Nonetheless, they are important, when periods of decades are considered.) Because the trends are small they are difficult to measure accurately with any individual technique. Careful joint consideration of measurements made with several different instruments and measuring techniques tends to average out drifts that may exist in records from individual instruments, and may point out problems in individual measurements. Thus, the combined records from several instruments at an individual site leads to the best estimate of the long term trend at that site. The microwave measurements contribute to the capability to determine long term trends at MLO with improved accuracy resulting from the availability of data from several different instruments. Because the microwave measurements are nearly fully automated, measurements are made nearly continuously and very high time resolution is obtained. In addition, the altitude range covered by the microwave technique extends above those of other instruments used at MLO and provides information on the upper stratosphere and mesosphere.
Are there any trends in the data?
A minimum record length is required for trends analysis, and the microwave record is only now becoming long enough for this purpose.
How does this program fit into the big picture?
What is it's role in global climate change?
Stratospheric ozone is measured and studied primarily because ozone filters out most of the biologically damaging short wavelength solar ultraviolet radiation. These studies form the scientific basis for international agreements, such as the Montreal Protocol, restricting or banning the release of substances, such as chlorofluorocarbons, that destroy ozone. Changes in ozone levels are not the primary cause of climate change. However, trends in the absorption of incoming solar ultraviolet radiation and outgoing infrared radiation from the Earth by ozone must be included in climate change studies for maximum accuracy in future climate predictions.
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