Satellite Stratospheric Aerosol Measurement Validation

I The validity of the stratospheric aerosol measurements made by the satellite sensors SAM II and SAGE has been tested by comparing their results with each other and with results obtained by other techniques (lidar. dustsonde. filter. impactor). The latter type of comparison has required the development of special techniques that (1) convert the quantity measured by the correlative sensor (e.g. particle backscatter. number. or mass) to that measured by the satellite sensor (extinction). and (2) quantitatively estimate the uncertainty in the conversion process. The results of both types of comparisons show agreement within the measurement and conversion uncertainties. Moreover. the satellite uncertainty is small compared to aerosol natural variability (caused by seasonal changes, volcanoes. sudden warmings, vortex structure. etc.). Hence. we conclude that the satellite measurements are valid. INTRODUCTION The satellite sensors Stratospheric Aerosol Measurement (SAM II) and Stratospheric Aerosol and Gas Experiment (SAGE) have been developing a global data base on stratospheric aerosol behavior since they were launched in October 1978 and February 1979, respectively /1-6/. To test the validity of this data base. an extensive program of comparisons has been conducted, both between the two satellite sensors and between other sensors and the satellite sensors. COMPARISONS BETWEEN SAM II AND SAGE As described in more detail by Yue et al. /7/. the SAM II measurements are confined to the two latitude bands: 64°_80 0 N and 64°-80·S. whereas the SAGE measurements range from 72°S_ to 72°N. Thus. there are occasions when the measurements nearly overlap, permitting comparisons. Although SAM II and SAGE use the same measurement principle (sun photometry through the Earth's atmospheric limb, yielding vertical profiles of aerosol extinction), there are significant differences between the instruments used and data-reduction procedures. For example. the sensors fly on separate satellites in different orbits. leading to considerably different viewing geometries and sampling durations even when the sampled air volumes are adjacent. Also, the SAGE instrument uses a holographic grating to disperse the sunlight. while the SAM II instrument uses an interference filter for spectral discrimination. SAGE measurements are quantized to 12 bits accuracy (1 part in 4096), while SAM II measurements are quantized to 10 bits accuracy (1 part in 1024). The higher quantization error for SAM II measurements is apparent when the measured aerosol extinction levels are low. The handling and processing of the data from each instrument are also different. The telemetry and raw data processing for the two satellite systems are separately operated under different organizations at NASA's Goddard Space Flight Center. In addition. the data reduction schemes for the two experiments are considerably different. For example. the geometric height determination for the SAM II-inferred vertical profiles is based primarily on the refracted sunshape fitting scheme /8/. For the SAGE data. it is determined by matching the measured slant-path optical depth profiles at the short wavelength channel to a computed profile from the given vertical temperature and pressure profiles. The inversion algorithms for the two instruments are also different. SAGE inversion uses a nonlinear iterative inversion algorithm where all the four wavelength data are cyclically updated. while the SAM II inversion algorithm is basically a linear inverse matrix-type inversion applied to the Single wavelength measurement. The comparisons between SAM II and SAGE 1.0-um wavelength inverted data. therefore. can provide a critical assessment of