Absorption cross sections for water vapor from 183 to 193 nm

Absorption cross sections for water vapor at 184.9 nm were measured using a standard low‐pressure mercury lamp light source, optically filtered to isolate the spectral region near the emission line. The light from the source was detected using a solar‐blind phototube. Experiments were performed over a wide range of water column amounts, using neat water vapor and water vapor/nitrogen mixtures, with four methods to determine the water vapor concentration in the cell. Absorption cross sections were measured between 0 and 80°C. The results from these experiments yield a cross section of 7.14 (± 0.2) × 10 −20 cm² molecule −1 at 25°C with a small positive temperature dependence (about 4% between 0 and 80°C). This result is about 30% greater than current recommendations. The absorption spectrum was also measured at discrete wavelengths between 183 and 193 nm using a scanning double monochromator system. The cross section of deuterated water vapor was determined at 184.9 nm. The implications of the changes in the H 2 O cross section for the calibration of atmospheric OH and HO 2 measurements using the 184.9 nm photolysis of water vapor are discussed.