Atmospheric OH production—The O(¹D) + H 2 O reaction rate

Successful modeling of the chemistry of the troposphere and stratosphere requires accurate knowledge of the production rate of the OH molecule, since it occupies a central position in a number of chemical cycles. The reaction between O(¹D) and H 2 O is believed to be the principal primary OH source, and an evaluation of the literature shows a discrepancy of about a factor of two between two of the most recent rate coefficients for this reaction. We have generated O(¹D) from pulsed 1600 Å photolysis of O 2 , and observed the 6300 Å O(¹D → ³P) transition. The rate coefficient for H 2 O quenching of O(¹D) has been directly determined, and found to be (2.6 ± 0.5) × 10 −10 cm³molec −1 s −1 at 300°K, in good agreement with the measurements of Streit, Howard, Schmeltekopf, Davidson, and Schiff .