Vertical gradients of HONO, NO x and O 3 in Santiago de Chile Guillermo Villena a, b , Jörg Kleffmann a, * , Ralf Kurtenbach a , Peter Wiesen a , Eduardo Lissi b , Maria A. Rubio b , Giovanna Croxatto c , Bernhard Rappenglück c a Bergische Universität Wuppertal, FB C, Physikalische Chemie, Gaußstraße 20, 42119 Wuppertal, Germany b Facultad de Química y Biología, Universidad de Santiago de Chile, Chile (USACH), Casilla 40, C. 33, Santiago, Chile c Department of Earth and Atmospheric Sciences, University of Houston, 4800 Calhoun Rd, Houston, TX 77204-5007, USA article info Article history: Received 15 November 2010 Received in revised form 24 January 2011 Accepted 28 January 2011 Keywords: Oxidation capacity OH radical Nitrous acid Nitrogen oxides Gradient study abstract Gradients of HONO, NO x (NO þ NO 2 ) and O 3 were measured during an early summer campaign on a high-rise building in downtown Santiago de Chile. Distinct gradients of decreasing concentrations with altitude were observed for HONO and NO x , while O 3 concentrations were found to increase with altitude. From the observed daytime maximum of the HONO/NO x ratio the existence of a strong daytime source of HONO is proposed, thus confirming recent results from another measurement site in downtown San- tiago, where a high contribution of HONO to the OH radical initiation sources of >50% was observed. Since the HONO/NO x ratio and its daytime maximum were found to be independent of the altitude, it is concluded that HONO is an important OH radical source in Santiago not only close to the ground surface, but also at higher altitudes of the boundary layer. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction The hydroxyl radical (OH) is the major oxidant responsible for the degradation of most pollutants in the atmosphere and is thus called the “detergent of the atmosphere”. Accordingly, the identi- fication and quantification of OH radical sources are of paramount importance to understand oxidative processes in the atmosphere. For more than three decades it has been known that the photolysis of nitrous acid (HONO) is an important source of the OH radical in the early morning when other radical sources, like the photolysis of ozone and formaldehyde, are still of minor impor- tance (Perner and Platt, 1979): HONO þ hn/OH þ NO; l < 400 nm (R1) This “OH-morning peak” results from HONO which has accu- mulated during the night by direct emissions (Kurtenbach et al., 2001) and heterogeneous ground surface sources (Kleffmann et al., 2003). While night-time concentrations can be more or less well explained by known sources, i.e. heterogeneous reactions of NO 2 on humid surfaces and direct emissions (Vogel et al., 2003), daytime levels are generally much higher than expected (Kleffmann et al., 2005; Acker et al., 2006). Recent field studies have demonstrated that HONO can also be a major source of OH radicals close to the ground surface during daytime (Kleffmann, 2007 , and references therein). These observations motivated laboratory studies in which new photochemical HONO sources were identified. Postulated heterogeneous daytime sources are the photolysis of adsorbed HNO 3 /nitrate (Zhou et al., 2003) and photosensitized conversion of NO 2 on organics, like humic acids (George et al., 2005; Stemmler et al., 2006), which are mainly ground surface sources (Stemmler et al., 2007). Based on these sources, strong HONO gradients are expected in the atmosphere during daytime due to the short photolytic lifetime of HONO of only w10 min (R1). Thus, the contribution of HONO as an important OH source could be limited only to the lowest part of the planetary boundary layer. However, most recent gradient measurements generally show only small gradients in the boundary layer during daytime (Kleffmann, 2007; Zhang et al., 2009; Häseler et al., 2009). This indicates that volume sources of HONO, for example due to gas-phase reactions (Bejan et al., 2006; Li et al., 2008) or reactions on particle surfaces (Ziemba et al., 2010), may also be of importance. Accordingly, the identification and quantification of new HONO sources in the atmosphere and the identification of their source regions is an ongoing and controversial topic in atmospheric science. In two recent studies in downtown Santiago de Chile (Elshorbany et al., 2009a, 2010) very high HONO levels between 1 and 2 ppbv were observed during daytime. Accompanying model * Corresponding author. Tel.: þ49 202 439 3534; fax: þ49 202 439 2505. E-mail address: kleffman@uni-wuppertal.de (J. Kleffmann). Contents lists available at ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv 1352-2310/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2011.01.073 Atmospheric Environment 45 (2011) 3867e3873