Abstract. Polar Ozone and Aerosol Measurement (POAM) III, a follow-on to the successful POAM II, is a spaceborne experiment designed to measure the vertical profiles of ozone, water vapor, nitrogen dioxide, and aerosol extinction in the polar stratosphere and upper troposphere with a vertical resolution of 1-2 km. Measurements are made by the solar occultation technique. POAM III, now in polar orbit aboard the SPOT 4 satellite, is providing data on north-and southpolar ozone phenomena, including the south-polar ozone hole, and on the spatial and temporal variability of stratospheric aerosols, polar stratospheric clouds, and polar mesospheric clouds.

The second Polar Ozone and Aerosol Measurement instrument (POAM II) is a spaceborne experiment designed to measure the vertical profiles of ozone, water vapor, nitrogen dioxide, aerosol extinction, and temperature in the polar stratosphere and upper troposphere with a vertical resolution of about i km. Measurements are made by the solar occultation technique. The instrument package, which has a mass of less than 25 kg, is carried on the Satellite Pour l'Observation de la Terre (SPOT) 3 spacecraft and has a design lifetime of 3-5 years. POAM II has provided data on the south polar ozone hole, north and south polar ozone phenomena, the spatial and temporal variability of stratospheric aerosols and polar stratospheric clouds, and has detected polar mesospheric clouds. IntroductionThe second Polar Ozone and Aerosol Measurement (POAM II) instrument is a nine-channel photometer designed to measure constituents and properties of the atmosphere that are critical for understanding ozone chemistry in the polar regions. The measured constituents and approximate altitude ranges are shown in Table 1. Measurements are made by the solar occultation technique [Russerr et at., 1993]: the Sun is observed through the Earth's atmosphere as it rises and sets, as seen from the satellite. These measurements are performed with an altitude accuracy of about 1 km and a vertical resolution of 0.8 km; the vertical resolution of the geophysical profiles inferred from the measurements is about 1 km. Horizontal resolution, as determined by the geometry of the line of sight (LOS) through the atmosphere and the angular size of the Sun, is about 200 km along the LOS and 30 km across it. Additionally, the LOS moves horizontally by about 100 km during the approximately 15 s it takes for the Sun to traverse the atmosphere. POAM II, funded by the Ballistic Missile Defense Organization, was launched on September 26, 1993, on the Satellite Pour l'Observation de la Terre (SPOT) 3 spacecraft and is performing successfully. The Naval Research Laboratory's POAM program is currently constructing two more instruments of this type, one for SPOT 4 and one for the fourth Space Test Experiment Platform (STEP 4; this satellite will not be in polar orbit). The basic POAM concept originated at the Department of Physics and Astronomy of the University of Wyoming, which built a three-channel, ozone-only version (POAM I). Unfortunately, the experiment failed immediately after launch in 1985. The SPOT 3 satellite is in a Sun-synchronous orbit at an altitude of 833 km, an inclination of 98.7 ø, and a period of 101.5 min; the descending node is crossed at 1030 LT. As seen from the satellite, the Sun rises in the north polar region and sets in the south polar region 14.2 times per day. This orbit makes SPOT 3 an excellent platform for polar ozone observations. From launch to late 1995, data have been gathered from approximately 14,000 solar occultation events, and preliminary scientific results have been presented on polar ozone phenomena in general and the formation and ...

Measurements of polar mesospheric clouds (PMCs) from three different satellite instruments are compared. These instruments are the Solar Mesospheric Explorer (SME), the Wind Imaging Interferometer (WINDII), and the Polar Ozone and Aerosol Measurement (POAM II). These measurements have been put on a common basis, correcting for differences in the wavelengths and measurement techniques used. This common basis is the probability distribution of the excess extinction ratio (EER) at a standard wavelength of 265 nm, where the EER is the ratio of the PMC extinction coefficient to the background molecular Rayleigh scattering coefficient. The results indicate that the POAM and WINDII measurements in the Southern Hemisphere had a higher probability of observing bright PMCs during the 1993–1996 time period than SME did a decade earlier in 1983–1986. Local time variations identified in WINDII data are interpreted in terms of a diurnal and semidiurnal component of average EER. These results are qualitatively similar to those found from lidar soundings of noctilucent cloud at sites in Norway and at the South Pole. Differences in interannual variability, local time of the measurements, assumed particle size distributions, and solar cycle effects are ruled out as possible explanations of the differences.

Abstract. The second Polar Ozone and Aerosol Measurement (POAM II) instrument is a space-borne visible/near IR photometer which uses the solar occultation technique to measure vertical profiles of ozone, nitrogen dioxide, and water vapor as well as aerosol extinction and atmospheric temperature in the stratosphere and upper troposphere. Here we report on the detection of polar mesospheric clouds (PMCs) in the high-latitude southern hemisphere by POAM II during the 1993 and 1994 summer seasons. These measurements are noteworthy because they are the first measurements of PMCs in atmospheric extinction. The POAM II PMC data set has been analyzed using a simple geometric cloud model. We find that mean cloud altitudes deduced from these data are 82-83 km, consistent with previous ground-based and satellite measurements. In addition, the 0.7 km vertical resolution of POAM II allows for accurate determination of cloud thickness. For the PMCs detected by POAM II we find a mean thickness of 2.4 km. The mean peak slant optical depth was determined to be 1.2 x 10 -3 for the 1993 season and 1.8 x 10 '3 for the 1994 season, corresponding to a cloud extinction coefficient of 3.9 x 10 -6 and 6.1 x 10 -6 km -• respectively.The multichannel capability ofPOAM II also makes it possible to study the wavelength dependence of the measured slant optical depth for the clouds with largest extinction. The results of this analysis suggest an upper limit to the modal particle radii for these clouds of approximately 70 nm.

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