{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,5,20]],"date-time":"2026-05-20T15:56:21Z","timestamp":1779292581853,"version":"3.51.4"},"reference-count":18,"publisher":"American Geophysical Union (AGU)","issue":"10","license":[{"start":{"date-parts":[[1999,5,15]],"date-time":"1999-05-15T00:00:00Z","timestamp":926726400000},"content-version":"vor","delay-in-days":0,"URL":"http:\/\/onlinelibrary.wiley.com\/termsAndConditions#vor"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Geophysical Research Letters"],"published-print":{"date-parts":[[1999,5,15]]},"abstract":"<jats:p>Measurements of the concentrations of OH and HO<jats:sub>2<\/jats:sub> (HO<jats:sub>x<\/jats:sub>) in the high\u2010latitude lower stratosphere imply the existence of unknown photolytic sources of HO<jats:sub>x<\/jats:sub>. The strength of the additional HO<jats:sub>x<\/jats:sub> source required to match the observations depends only weakly on solar zenith angle (SZA) for 80\u00b0 &lt; SZA &lt; 93\u00b0. The wavelengths responsible for producing this HO<jats:sub>x<\/jats:sub> 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 &lt; 45 kcal mole<jats:sup>\u22121<\/jats:sup>. We speculate that peroxynitric acid (HNO<jats:sub>4<\/jats:sub>) dissociates after excitation to an unknown excited state with an integrated band cross section of 2\u20103 \u00d7 10<jats:sup>\u221220<\/jats:sup> cm\u00b2 molecule<jats:sup>\u22121<\/jats:sup> nm (650 &lt; \u03bb &lt; 1250 nm).<\/jats:p>","DOI":"10.1029\/1999gl900255","type":"journal-article","created":{"date-parts":[[2002,9,17]],"date-time":"2002-09-17T22:30:28Z","timestamp":1032301828000},"page":"1373-1376","source":"Crossref","is-referenced-by-count":86,"title":["Twilight observations suggest unknown sources of HO<sub>x<\/sub>"],"prefix":"10.1029","volume":"26","author":[{"given":"P. O.","family":"Wennberg","sequence":"first","affiliation":[],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"R. J.","family":"Salawitch","sequence":"additional","affiliation":[],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"D. 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