Direct observation of OH radicals after 565 nm multi-photon excitation of NO 2 in the presence of H 2 O Damien Amedro, Alexander E. Parker, Coralie Schoemaecker, Christa Fittschen ⇑ Université Lille Nord de France, Laboratoire PC2A, UMR CNRS 8522, Cité Scientifique, Bâtiment C11, 59655 Villeneuve d’Ascq, France article info Article history: Available online 23 July 2011 abstract The study of a controversial reaction, NO  2 þ H 2 O as a potential new source of OH radicals in the atmo- sphere, has been performed using the coupling of a FAGE instrument to a laser photolysis cell. No OH rad- icals were observed using an unfocused excitation laser beam at 565 nm, but OH formation was observed by focusing the beam even though this reduces the instrument sensitivity. The dependence of the OH- fluorescence signal with the laser energy shows that the OH radicals originate from a complex mecha- nism including multiphoton absorption of NO 2 . Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction OH is the principle atmospheric oxidant and is at the origin of the majority of chemical transformations in the atmosphere. The chemical mechanisms leading to its formation and consumption are very complex and understanding of the different pathways is a key to the ability to accurately model atmospheric chemistry and to predict future evolution of the atmosphere. As different fields campaigns have shown an underestimation of OH concentra- tion by models [1], identification of possible new OH source reac- tions are highly desired. One such reaction was proposed by Li et al. [2] through excitation of NO 2 with visible light NO 2 þ hmðk > 420 nmÞ! NO  2 ð1Þ and subsequent reaction with H 2 O: NO  2 þ H 2 O ! OH þ HONO ð2Þ They studied the excitation of NO 2 in the range 560–640 nm in the presence of water, using a focused OPO laser, and followed OH formation through in-situ laser induced fluorescence. They found that the concentration of generated OH radicals has a linear depen- dence on the laser energy. Even though a graph of OH-LIF signal as a function of laser energy showed a negative offset, the linearity was taken as proof that OH radicals are formed in the reaction se- quence (1) and (2), following the absorption of one photon. They determined a rate constant of 1.7  10 13 cm 3 s 1 for reaction (2) and a yield of 0.001 was calculated. Through supplementary exper- iments, the formation of OH radicals by dissociation of NO 2 leading to O( 1 D) or O( 3 P) was ruled out. From these results, Wenneberg and Dabdud [3] estimated the impact of such additional OH- generation on O 3 production and found that this could lead to an increase of up to 40% in polluted areas. The same reaction sequence had been studied previously by Crowley and Carl [4] using a similar technique. They observed OH radicals at wavelengths below 500 nm following O( 1 D) forma- tion from 2-photon absorption, but did not observe any OH-radical formation at 532 nm, where the 2-photon process is not energetic enough to form O( 1 D) atoms. From their experiments, they esti- mated an upper limit of 7  10 5 for the reactive quenching of NO  2 by water vapour relative to collisional quenching, i.e. more than an order of magnitude lower than Li et al. The large disagreement between both studies, together with the high impact of this reaction for atmospheric chemistry, has trig- gered new studies. Carr et al. [5] tried to detect OH radicals from the reaction of excited NO 2 with H 2 O, but with using an unfocused excitation beam in He buffer gas. They concluded that, if the rate constant reported by Li et al. [2] is correct, they should have ob- served OH radicals in their experiments and estimated an upper limit of the OH-yield from reaction (2) at least 17 times lower than reported by Li et al., in good agreement with Crowley and Carl. They concluded that the OH observed in Li’s experiments were due to multiphoton effects or a mechanism implying two NO  2 . In the response of Li et al. [6] to the comment of Carr et al. [5] they pointed out that ‘the reasons for the differences in the results of the two studies are not obvious’ and mentioned the difficulty of the study due to ‘rapid quenching of NO  2 by water and low product yield’. In order to provide a further perspective on this reaction by using a different experimental technique and to try to conclude whether this reaction is of importance to OH production under atmospheric conditions, we have studied this reaction using a laser photolysis cell coupled to the detection of OH radicals by FAGE (Fluorescence Assay be Gas Expansion): the high sensitivity of 0009-2614/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2011.07.062 ⇑ Corresponding author. Fax: +33 3 20 43 69 77. E-mail address: christa.fittschen@univ-lille1.fr (C. Fittschen). Chemical Physics Letters 513 (2011) 12–16 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett