Richard P. Cageao

High-resolution Fourier-transform ultraviolet–visible spectrometer for the measurement of atmospheric trace species: application to OH

A compact, high-resolution Fourier-transform spectrometer for atmospheric near-ultraviolet spectroscopy has been installed at the Jet Propulsion Laboratorys Table Mountain Facility (34.4 degrees N, 117.7 degrees W, elevation 2290 m). This instrument is designed with an unapodized resolving power near 500,000 at 300 nm to provide high-resolution spectra from 290 to 675 nm for the quantification of column abundances of trace atmospheric species. The measurement technique used is spectral analysis of molecular absorptions of solar radiation. The instrument, accompanying systems designs, and results of the atmospheric hydroxyl column observations are described.

Calculated hydroxyl A2 sigma --> X2 pi (0, 0) band emission rate factors applicable to atmospheric spectroscopy

A calculation of the A2 sigma --> X2 pi (0, 0) band emission rate factors and line center absorption cross sections of OH applicable to its measurement using solar resonant fluorescence in the terrestrial atmosphere is presented in this paper. The most accurate available line parameters have been used. Special consideration has been given to the solar input flux because of its highly structured Fraunhofer spectrum. The calculation for the OH atmospheric emission rate factor in the solar resonant fluorescent case is described in detail with examples and intermediate results. Results of this calculation of OH emission rate factors for individual rotational lines are on average 30% lower than the values obtained in an earlier work.

OH column abundance over Table Mountain Facility, California: AM-PM diurnal asymmetry

Observations of the OH column abundance have been made by the Fourier Transform Ultraviolet Spectrometer at the JPL Table Mountain Facility (TMF) near Los Angeles since July 1997. In the January 1998–December 2003 data set we used five OH lines to derive the OH column abundance in the atmosphere. This data set was used to quantify the OH morning/afternoon asymmetry (AMPMDA). An analysis of summer and winter data showed that the daily OH maximum occurred 26–36 minutes after solar transit. This phase lag appears to be the primary reason why OH in the afternoon is larger than at corresponding solar zenith angles in the morning throughout the year. A simple heuristic model suggests that the asymmetry is a direct consequence of the finite lifetime of OH. Comparison of the TMF data with earlier results from Fritz Peak Observatory, Colorado, by Burnett et al. reveals significant differences in the behavior of the AMPMDA between the two sites.

OH column abundance over Table Mountain Facility, California: Intra‐annual variations and comparisons to model predictions for 1997–2001

Measurements of the OH column abundance over the Jet Propulsion Laboratorys Table Mountain Facility (TMF) have been made since July 1997 at 10°–80° solar zenith angle using a Fourier transform ultraviolet spectrometer. The measured OH column at any solar zenith angle is typically larger in the afternoon than in the morning. The variations observed in the OH column abundance appear to result from changes in atmospheric conditions on a daily or longer timescale. The larger observed variations are statistically significant. Sensitivity coefficients describing how the OH column abundance is expected to change in response to changes in the concentrations of H_2O, O_3, NO, CO, and CH_4 have been calculated on the basis of an analytic model. On the basis of these sensitivity coefficients and Halogen Occultation Experiment observations of O_3, the net sensitivity of the OH column abundance to variations in O_3 should be close to zero. The observed OH column abundance over TMF increased by about 25% from July 1997 to December 2001. This interannual trend in OH column abundance is not consistent with calculations that incorporate observed trends in H_2O and O_3 and is at least a factor of 2 larger than the calculated difference between solar minimum and maximum. Comparisons between measured and calculated normalized OH column abundances suggest that the sensitivity of OH to variations in H_2O may be a factor of 2 larger than predicted in present models and that there is some other major driver for the observed variability in the OH column abundance that was not included in the present analysis.