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GEOPHYSICAL RESEARCH LETTERS, VOL. 26, NO. 10, PAGES 1373–1376, 1999

Twilight Observations Suggest Unknown Sources of HO_x

P. O. Wennberg

California Institute of Technology, Pasadena

R. J. Salawitch

Jet Propulsion Laboratory, California Inst. of Tech., Pasadena

D. J. Donaldson

Department of Chemistry, University of Toronto, Canada

T. F. Hanisco

Department of Chemistry, Harvard University, Cambridge, MA

E. J. Lanzendorf

Department of Chemistry, Harvard University, Cambridge, MA

K. K. Perkins

Department of Chemistry, Harvard University, Cambridge, MA

S. A. Lloyd

Applied Physics Lab., Johns Hopkins Univ., Laurel, MD

V. Vaida

Department of Chemistry, University of Colorado, Boulder

R. S. Gao

NOAA Aeronomy Laboratory, Boulder, CO

E. J. Hintsa

Department of Chemistry, Harvard University, Cambridge, MA

R. C. Cohen

Department of Chemistry, University of California, Berkeley

W. H. Swartz

Applied Physics Lab., Johns Hopkins Univ., Laurel, MD

T. L. Kusterer

Applied Physics Lab., Johns Hopkins Univ., Laurel, MD

D. E. Anderson

Applied Physics Lab., Johns Hopkins Univ., Laurel, MD

Abstract

Measurements of the concentrations of OH and HO₂ (HO_x) in the high-latitude lower stratosphere imply the existence of unknown photolytic sources of HO_x. The strength of the additional HO_x source required to match the observations depends only weakly on solar zenith angle (SZA) for 80° < SZA < 93°. The wavelengths responsible for producing this HO_x must be longer than 650 nm because the flux at shorter wavelengths is significantly attenuated at high SZA by scattering and absorption. Provided that the sources involve only a single photon, the strength of the bonds being broken must be < 45 kcal mole⁻¹. We speculate that peroxynitric acid (HNO₄) dissociates after excitation to an unknown excited state with an integrated band cross section of 2-3 × 10⁻²⁰ cm² molecule⁻¹ nm (650 < λ < 1250 nm).

Received 21 December 1998; accepted 24 March 1999.

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Citation: Wennberg, P. O., et al. (1999), Twilight Observations Suggest Unknown Sources of HO_x, Geophys. Res. Lett., 26(10), 1373–1376.

Copyright 1999 by the American Geophysical Union.