Sulfur dioxide, a short-lived atmospheric constituent, is oxidized to sulfate aerosols, a climate agent. Main
sources are volcanoes, smelters, and fossil fuel combustion. Satellite monitoring of SO2 began with TOMS data in
1978 that detected volcanic eruption clouds. Hyperspectral instruments, like OMI and GOME, have a twenty-fold
improvement in sensitivity. Degassing volcanoes, smelters, and large power plants are now monitored for a database
of SO2 emission to the atmosphere. SO2 is a distinctive marker for volcanic ash clouds, a hazard to aircraft.
All but one of the backscatter UV (BUV) instruments have used solar reflective diffusers made of ground aluminum to maintain instrument calibration after launch. These diffusers have been sued throughout mission life-times, which range from less than 1 years to over 14 years. Means for monitoring diffuser reflectance include mechanisms on the instruments as well as methods to infer reflectance using earth radiance data. We compare changes in diffuser reflectance for the various instruments and find some common behavior as well as significant differences. Changes which appear to occur at different rates are actually quite similar when corrections are made for the amount and direction of incident solar irradiation. However, a class of instruments, the SBUV/2, has significantly lower degradation rates. We find, as have previous authors, that spacecraft self-contamination is the most likely cause of diffuser changes and observed differences. Observed changes suggest that contaminant layer thickness is the main reflectance degradation mechanism in the first few years of operation.
The geostationary tropospheric pollution satellite (GEO TROPSAT) mission is a new approach to measuring the critical constituents of tropospheric ozone chemistry: ozone, carbon monoxide, nitrogen dioxide, and aerosols. The GEO TROPSAT mission comprises a constellation of three instruments flying as secondary payloads on geostationary communications satellites around the world. This proposed approach can significantly reduce the cost of getting a science payload to geostationary orbit and also generates revenue for the satellite owners. The geostationary vantage point enables simultaneous high temporal and spatial resolution measurement of tropospheric trace gases, leading to greatly improved atmospheric ozone chemistry knowledge. The science data processing, conducted as a research (not operational) activity, will provide atmospheric trace gas data many times per day over the same region at better than 25 km ground footprint. The high temporal resolution identifies short time scale processes, diurnal variations, seasonal trends, and interannual variation.
The improved TOMS instruments, flight models 3, 4, and 5, are to be flown aboard Earth probe (EP), Japanese ADEOS, and Russian Meteor-3M satellites, respectively. TOMS obtains the total column amount of the atmospheric ozone from measurements of the extra-terrestrial solar spectral irradiance and the backscattered earth spectral radiance at six ultraviolet wavelengths between 308.6 nm and 360 nm. The added scientific goal of new generation instruments is to monitor the trend of the global burden of the atmospheric ozone, which requires an accuracy of 1% in the calibration for the ratio of the radiance to the irradiance measurements. The emphasis of the prelaunch-calibration approaches was to maximize the accuracy in the ratio of the calibration for the two measurement modes and to minimize possibility of the systematic errors. The source geometry was maintained as close as possible in the calibration setup for the two measurement modes so that the uncertainty associated with the source could be canceled out in the ratio of the two calibrations. Also, multiple calibration techniques and radiometric sources have been used to check consistency of the calibration. The FM-3 calibration results show a three sigma standard errors of the mean for the ratio calibration that range from 0.28% to 0.63% in descending order of the wavelength.
Three TOMS (Total Ozone Mapping Spectrometers) of a new design series are scheduled to be launched successively over the next several years. Changes have been made in the area of instrument calibration which should significantly improve the precision of TOMS ozone measurements over their predecessors. In the BUV method for determining ozone overburden, the precision of retrieved ozone amounts is directly related to knowledge of changes in diffuser reflectance. A three solar diffuser system employed on a previous TOMS proved capable of detecting a 0.25% (2 (sigma) ) ozone error over the three year mission. In addition to multiple diffusers, the new TOMS have on board a system for monitoring diffuser reflectance which alone should maintain instrument calibration with a precision at least double that of earlier TOMS. Improvements in prelaunch calibration techniques should result in closer inter- instrument agreement, an important consideration when measuring trends with multiple instruments. Unfortunately, the agreement between instruments is not likely to be better than about 1% ozone.
A new TOMS requirement is to measure total ozone trends of 1% per decade. The ozone calibration depends on knowledge of diffuser plate reflectance and spectrometer wavelength changes. Absolute diffuser reflectance changes are now measured with a reflectance calibration assembly, containing a UV Hg-phosphor lamp. Three diffusers are flown to permit frequent solar calibrations and infer relative diffuser changes when exposed at different rates. Finally, an ambiguity in the wavelength monitor was corrected. These three changes to the original Nimbus-7 TOMS design are expected to produce 0.5% ozone trend data, based on pre launch test results.
The radiance of large, ice covered land masses has been used to monitor TOMS instrument sensitivities. Greenland and Antarctica provide uniform and stable ice surfaces whose average albedo appears to be constant within the desired accuracy for instrument monitoring. Instrument radiance response will depend upon view and illumination angles, the sun-earth distance, and atmospheric conditions. Restriction to nadir views eliminates view angle dependence, and corrections are made for sun-earth distance. The effect of atmospheric conditions, such as ozone and clouds, is minimized by monitoring at wavelengths above 340 nm and by the high surface radiance. Relative instrument response is determined by the ratio of signals measured at different times using a binning technique to account for differences in solar illumination angles. The only remaining limit to long term monitoring accuracy is the albedo stability of the ice surface itself. Changes in the Nimbus-7/TOMS instrument response at long wavelengths are monitored within 1% accuracy over the lifetime of the instrument.