Measurements of OH are a direct probe of the chemistry of the upper stratosphere and lower mesosphere. OH column abundance has been measured over various sites since 1977, providing a long record of the natural variability in this atmospheric region. Such OH column measurements will be needed for validating OH concentration profiles obtained by the Microwave Limb Sounder (MLS) that will be launched in 2003 onboard NASA's Aura satellite. The observed variability in the OH column, however, has been difficult to interpret, and the absolute column abundance over different sites has shown larger than expected disagreement.

The column abundance of OH over the Jet Propulsion Laboratory's Table Mountain Facility (TMF) has been measured regularly since July 1997 using the Fourier-Transform Ultraviolet Spectrometer (FTUVS). Empirical linear fits for OH column as a function of solar zenith angle for each year and for each month have been derived. The annual average OH column over TMF for 1998 - 2000 is 10 - 15% larger than that reported over Tokyo for 1992 - 1995 by another group and 25 - 50% smaller than that reported over Colorado for 1980 - 1990 and for 1996 by a third group. The measured OH column over TMF is comparable to or smaller than that predicted by standard photochemical models. The afternoon OH column typically is larger than the morning OH column at the same solar zenith angle for all months, with the steepest slopes (for OH column as a function of solar zenith angle) found in winter. Statistically significant variability in the normalized OH column abundance is evident on time scales with periods ranging from 1 - 2 weeks to annual. (Periodic variability on longer time scales cannot be assessed quantitatively yet.) The calculated amplitude for OH column variations, based on variations in H₂O and O₃ as observed at 10 - 80 km altitude by HALOE, is sensitive to the fraction of the OH column in the mesosphere. Predicted differences are as large as a factor of two, based on published model calculations and measurements. The measured amplitude for OH column variations over TMF, however, is larger than the largest of the calculated amplitudes. The results from our analysis of the TMF OH column measurements, model sensitivity analyses, and comparisons between measured and predicted OH column abundances will be presented.

This work was performed at the Jet Propulsion Laboratory, California Institute of Technology. Support was provided by the NASA Upper Atmosphere Research Program, Caltech, and the Naval Research Laboratory.