ARTICLE IN PRESS Comparison of wet-only and bulk deposition at Chiang Mai (Thailand) based on rainwater chemical composition Somporn Chantara à , Nawarut Chunsuk Environmental Chemistry Research Laboratory, Chemistry Department, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand article info Article history: Received 11 October 2007 Received in revised form 18 March 2008 Accepted 19 March 2008 Keywords: Acid precipitation Wet deposition Bulk deposition Chemical composition abstract The chemical composition of 122 rainwater samples collected daily from bulk and wet- only collectors in a sub-urban area of Chiang Mai (Thailand) during August 2005–July 2006 has been analyzed and compared to assess usability of a cheaper and less complex bulk collector over a sophisticated wet-only collector. Statistical analysis was performed on log-transformed daily rain amount and depositions of major ions for each collector type. The analysis of variance (ANOVA) test revealed that the amount of rainfall collected from a rain gauge, bulk collector and wet-only collector showed no significant difference (p ¼ 0.05). The volume weight mean electro-conductivity (EC) values of bulk and wet- only samples were 0.69 and 0.65 mS/m, respectively. The average pH of the samples from both types of collectors was 5.5. Scatter plots between log-transformed depositions of specific ions obtained from bulk and wet-only samples showed high correlation (r40.91). Means of log-transformed bulk deposition were 14% (Na + and K + ), 13% (Mg 2+ ), 7% (Ca 2+ ), 4% (NO 3  ), 3% (SO 4 2 and Cl  ) and 2% (NH 4 + ) higher than that of wet-only deposition. However, multivariate analysis of variance (MANOVA) revealed that ion depositions obtained from bulk and wet-only collectors were not significantly different (p ¼ 0.05). Therefore, it was concluded that a bulk collector can be used instead of a wet-only collector in a sub-urban area. & 2008 Elsevier Ltd. All rights reserved. 1. Introduction Presently, emission of air pollutants is rapidly increasing mainly from anthropogenic activities due to large popula- tion, a rapidly growing economy, high-energy consumption, agricultural production and industrialization. The two major groups of air pollutants are SO x and NO x . Their oxidations result in particulate sulfate and particulate nitrate which ultimately determine the lifetime of those pollutants in the atmosphere (Bayraktar and Turalioglu, 2005). Acid deposition, the major removal pathway for secondary pollutants, can take place in two forms wet and dry deposition, which together is referred to as bulk deposition. Wet deposition is defined as the process by which secondary pollutants are incorporated into cloud and deposited back to the earth’s surface in the form of rain, snow and mist. In theory, wet deposition is best measured by using a wet-only collector which has a removable lid that covers a collecting bucket to exclude dry deposition during a dry period and opens whenever precipitation is detected by a precipitation sensor (Da ¨mm- gen et al., 2005; Staelens et al., 2005; Plaisance et al., 1998). However, it has the drawback of being expensive, requiring a power supply and sensors. Therefore, the use of bulk collectors to collect wet precipitation allows for the cost-effective operation of large networks (Da ¨ mmgen et al., 2005). The difference in chemical composition between bulk and wet-only deposition depends on deposition of dry gases and particles which could have been resuspended Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/atmosenv Atmospheric Environment 1352-2310/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2008.03.022 à Corresponding author. Tel.: +6653 943470; fax: +66 53 892277. E-mail address: sp_chan@chiangmai.ac.th (S. Chantara). Atmospheric Environment 42 (2008) 5511–5518