provide a conclusive demonstration of the principle of the enhancement of nonlinear optical response through the supramolecular engineering of polymers. It is expected that considerably larger enhancement can be observed in structures with better alignment of chromophores, such as certain biopolymers and derivatives of helical poly(tripheny1methylmethacrylate)~ (1 6).

Global distributions of CH 4 in the mesosphere and stratosphere have been measured continuously since October 11, 1991, by the Halogen Occultation Experiment (HALOE) onboard the UARS. CH 4 mixing ratio is obtained using the gas filter correlation technique operating in the 3.3-grn region. Since measurements are made during solar occultation in the 57 ø inclination orbit, data are collected 15 times daily for both sunrises and sunsets. This provides coverage of one hemisphere in a month period. One complete hemispheric sweep (from equator to -80 ø latitude) is made during the spring and summer seasons of two hemispheres, and a partial sweep (from equator to around 50 ø latitude) is made during the fall and winter seasons of two hemispheres. HALOE CH 4 measurements are validated using direct comparisons with correlative data and internal consistency checks using other HALOE-measured tracers, HF, and aerosols. It is estimated for the 0.3-to 50-mbar region that the total error, including systematic and random components, is less than 15 % and that the precision is better than 7%. The CH4 gas filter channel does not depend significantly on the Pinatubo aerosol extinction. An experimentally accurate measurement of CH 4 is very important because CH 4 is a primary interfering gas in the HALOE HC1 channel and, subsequently, can cause HC1 measurement error. Simultaneous measurements of CH4 and other HALOE species (03, H20, NO, NO2, HC1, HF, and aerosol extinction coefficients) provide important information on atmospheric dynamic and chemical processes, since CH4 can be used as a tracer and an indicator of atmospheric transport processes. Several new pieces of infolsnation on previously unreported HALOE-observed features are also presented. Introduction Atmospheric methane, CH 4, naturally produced and released from the surface, has an important role in the greenhouse effect, causing a global warming, and in atmospheric chemistry, by removing hydroxyl radicals in the troposphere and in the stratosphere by converting reactive chlorine atoms to HC1 and producing hydrogen species to form water vapor by oxidation (see summary [World Meteorological Organization (WMO), 1982]). Methane can be used as a tracer to study atmospheric transport processes when enough measurements are made on a regional or global scale. For example, the features of CH 4 and other species observed by the Halogen Occultation Experiment (HALOE) were reported by Russell et al. [1993a] for the springtime Antarctic and by Park and Russell [1994] for the summertime polar region. Both papers used CH 4 as a tracer, and in the latter paper it was used to separate dynamical effects and to assess the importance of chemical processes for ozone deficiency in the polar regions. Methane measurements in the stratosphere before the Upper Atmosphere Research Satellite (UARS) [Reber, 1993] include the 1979 global measurements by stratospheric and mesospheric sounder (SAMS) on Nimbus 7 [Rodgers et al., 1984] and otherwise sparse individual profiles obtained by in situ sampli...

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Revised cross‐section values for the excitation of the O I(3s ³S°‐2p ³P; λ1304 Å), O I(3d ³D°‐2p ³P; λ1027 Å), and O I(3s′ ³D°‐2p ³P; λ989 Å) resonance transitions by electron impact on atomic oxygen are presented from threshold to 300 eV. These results are smaller than the excitation cross sections used in some airglow models by a factor of ∼2.8. The revised values are in good agreement with recent quantum‐scattering calculations. The downward revision is required by new laboratory studies in which the direct and dissociative cross sections for λ1304 Å excitation were normalized with small probable error to the O and O2 ionization cross sections. The results also reflect new advances in VUV optical calibration techniques. A number of outstanding airglow problems are simplified by these revisions.

CH4 and N2O are useful as dynamical tracers of stratospheric air transport because of their long photochemical lifetimes over a wide range of altitudes. The cryogenic limb array etalon spectrometer (CLAES) instrument on the NASA UARS provided simultaneous global measurements of the altitude profiles of CH4 and N2O mixing ratios in the stratosphere between October 1, 1991, and May 5, 1993. Data between January 9, 1992, and May 5, 1993 (388 days), have been processed using version 7 data processing software, and this paper is concerned with the assessment of the quality of this data set. CLAES is a limb‐viewing emission instrument, and approximately 1200 profiles were obtained each 24‐hour period for each constituent over a nominal altitude range of 100 to 0.1 mbar (16 to 64 km). Each latitude was sampled 30 times per day between latitudes 34°S and 80°N, or 34°N and 80°S depending on the yaw direction of the UARS, and nearly all local times were sampled in about 36 days. This data set extends the altitude, latitude, and seasonal coverage of previous experiments, particularly in relation to measurements at high winter latitudes. To arrive at estimates of experiment error, we compared CLAES profiles for both gases with a wide variety of correlative data from ground‐based, rocket, aircraft, balloon, and space‐borne sensors, looked at the repeatability of multiple profiles in the same location, and carried out empirical estimates of experiment error based on knowledge of instrument characteristics. These analyses indicate an average single‐profile CH4 systematic error of about 15% between 46 and 0.46 mbar, with CLAES biased high. The CH4 random error over this range is 0.08 to 0.05 parts per million, which translates to about 7% in the midstratosphere. For N2O the indicated systematic error is less than 15% at all altitudes between 68 and 2 mbar, with CLAES tending to be high below 6.8 mbar and low above. The N2O random error is 20 to 5 ppb between 46 and 2 mbar, which also translates to 7% in the low to midstratosphere. Both tracers have useful profile information to as low as 68 mbar, excluding the tropics, and as high as 0.2 mbar (CH4) and 1 mbar (N2O). The global fields show generally good spatial correlation and exhibit the major morphological and seasonal features seen in previous global field data. Several morphological features are pointed out for regions and conditions for which there have been essentially no previous data. These include the differential behavior of the tracer isopleths near and inside the Antarctic winter vortex, and local maxima in the tropics in 1992, probably associated with the Mount Pinatubo sulfate aerosol layer. Overall, the results of this validation exercise indicate that the version 7 CH4 and N2O data sets can be used with good confidence for quantitative and qualitative studies of stratospheric and lower‐mesospheric atmospheric structure and dynamics.