Research Staff: Lawrence P. Giver, Peter Pilewskie, Warren J. Gore and Charles Chackerian, Jr.

Several groups have recently been working to improve the near-infrared spectrum of water vapor in HITRAN. The unit-conversion errors found previously have now been corrected in the recently released HITRAN-2000. But new measurements by other workers show the total intensity of the water vapor band at 1130 nm about 38% stronger than the sum of the line intensities on the revised HITRAN list in this region.

It was important to quickly determine if the HITRAN intensity values are in error by as much as claimed by others. Only intensity errors for the strong lines could result in the total band intensity being in error by such a large amount. To quickly get a number of spectra of the near-infrared region from 650 to 1650 nm, we used the Solar Spectral Flux Radiometer (SSFR) in our laboratory with the 25-meter base path White absorption cell. This moderate resolution spectrometer is a flight instrument that has flown on the Sandia Twin Otter for the ARESE II experiment. The measured band profiles were then compared to calculated spectra using the latest HITRAN line intensities, convolved with the SSFR instrumental resolution.

Our spectra of the 1130 nm band in fact have somewhat more absorption than the HITRAN simulations, but not as much as the 38% intensity increase for this band suggested in the literature. Although not uniform over the entire band, an intensity increase of about 20% on average would be more compatible with our data. The HITRAN line intensities in the 1130 nm band were adopted from values in the literature. We expect that these measurements of the strong lines have significant errors because the experimental conditions were not optimized to make good intensity measurements of these lines. These lines were typically saturated at the long path lengths used to determine good positions, assignments, and intensities of the many weaker lines.

Collaborators: Prasad Varanasi, SUNY at Stony Brook; Richard S. Freedman, SPRI

Point of Contact: Lawrence P. Giver, 650/604-5231, lgiver@mail.arc.nasa.gov