Possible particulate nitrite formation and its atmospheric implications inferred from the observations in Seoul, Korea Chul H. Song a, * , Mi E. Park a , En J. Lee a, b , Jae H. Lee a , Bo K. Lee c , Dong S. Lee c , Jhoon Kim d , Jin S. Han e , Kwang J. Moon e , Yutaka Kondo f a Department of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, Republic of Korea b Department of Environmental Engineering, Ajou University, Suwon 442-721, Republic of Korea c Department of Chemistry, Yonsei University, Seoul 120-749, Republic of Korea d Department of Atmospheric Science, Yonsei University, Seoul 120-749, Republic of Korea e Air Quality Division, National Institute of Environmental Research, Incheon 404-170, Republic of Korea f RCAST, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan article info Article history: Received 1 October 2008 Received in revised form 22 December 2008 Accepted 12 January 2009 Keywords: Particulate nitrite formation HONO particle alkalinity HONO deactivation abstract Simultaneous measurements of gaseous species and fine-mode, particulate inorganic components were performed at the University of Seoul, Seoul in Korea. In the simultaneous measurements, a certain level of nitrous acid (HONO) was observed in the gas-phase, indicating possible heterogeneous HONO production on the surface of the ambient aerosols. On the other hand, high particulate nitrite (NO 2 ) concentrations of 1.41(2.26) mg/m 3 were also measured, which sometimes reached 18.54 mg/m 3 . In contrast, low HONO-to-NO 2 ratios of 0.007(0.006) were observed in Seoul. This indicates that a significant fraction of HONO is dissolved in atmospheric aerosols. Around the Seoul site, sufficient alkalinity may have been provided to the atmospheric aerosols from the excessive presence of NH 3 in the gas-phase. Due to the alkaline particulate conditions (defined in this study as a particle pH >w3.29), the HONO molecules produced at the surface of the atmospheric aerosols appeared to have been converted into particulate nitrite, thereby preventing their further participation in the atmospheric O 3 /NO y /HO x photochemical cycles. It was estimated that a minimum average of 65% of HONO was captured by alkaline, anthropogenic, urban particles in the Seoul measurements. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Although the mechanism for atmospheric nitrous acid (HONO) formation is not well understood, HONO is considered to be an important source of hydroxyl radical (OH) in the atmosphere. HONO effectively absorbs the solar radiation at wavelengths between 330 nm and 380 nm, and efficiently produces OH radical by the photo-dissociation of HONO (i.e., HONO þ hn / OH þ NO). Regarding the formation mechanism of HONO, the following heterogeneous HONO productions at the dry/wet surfaces of ambient aerosols (and also at terrestrial surfaces and the bio-films of trees and vegetables) have been proposed and discussed from several laboratory studies and field observations (Harris et al., 1982; Lammel and Cape, 1996; Goodman et al., 1999; Kalberer et al., 1999; Alicke et al., 2003; Acker et al., 2006a,b): NO þ NO 2 þ H 2 O/2HONO (R1) 2NO 2 þ H 2 O/HONO þ HNO 3 (R2) Subsequently, HONO produced at the various aerosol surfaces is released into the atmosphere, in which it participates in the atmospheric photochemical cycles after its photo-dissociation into OH and NO. The HONO formation and the subsequent OH production are, therefore, of primary importance in atmospheric chemistry, particularly over rural and urban areas where HONO levels are typically high (Platt et al., 1980; Harris et al., 1982; Andre ´ s-Herna ´ndez et al., 1996; Alicke et al., 2003; Acker et al., 2006a). The contributions of the OH production via the HONO photo-dissociation to the atmospheric OH budget over the urban areas were estimated to be w80% (during early morning hours) and 20–30% (during 24 h period) (e.g., Alicke et al., 2003; Acker et al., 2006a). Acker et al. (2006a) estimated high OH production rates of 1–4  10 7 cm 3 s 1 during the first hour after sunrise with a maximum HONO level of w2 ppb in the urban area of Rome. * Corresponding author. Tel.: þ82 62 970 3276; fax: þ82 62 970 3404. E-mail address: chsong@gist.ac.kr (C.H. Song). Contents lists available at ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv 1352-2310/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2009.01.018 Atmospheric Environment 43 (2009) 2168–2173