Error estimations of dry deposition velocities of air pollutants using bulk sea surface temperature under common assumptions Yung-Yao Lan a , Ben-Jei Tsuang a, * , Noel Keenlyside b , Shu-Lun Wang c , Chen-Tung Arthur Chen d , Bin-Jye Wang d , Tsun-Hsien Liu f a Department of Environmental Engineering, National Chung Hsing University, Taichung 40227, Taiwan b Leibniz-Institut fuer Meereswissenschaften Duesternbrooker Weg 20, D-24105 Kiel, Germany c Department of Marine Environmental Engineering, National Kaohsiung Marine University, Kaohsiung 81157, Taiwan d Institute of Marine Geology and Chemistry, National Sun Yat-Sen University, Kaohsiung 80446, Taiwan f Department of Atmospheric Sciences and Graduate Institute of Atmospheric Physics, National Central University, Tao-yuan 32054, Taiwan article info Article history: Received 12 March 2009 Received in revised form 11 April 2010 Accepted 14 April 2010 Keywords: South China sea Deposition resistance Henry constant Chemical enhancement factor Dry deposition abstract It is well known that skin sea surface temperature (SSST) is different from bulk sea surface temperature (BSST) by a few tenths of a degree Celsius. However, the extent of the error associated with dry depo- sition (or uptake) estimation by using BSST is not well known. This study tries to conduct such an evaluation using the on-board observation data over the South China Sea in the summers of 2004 and 2006. It was found that when a warm layer occurred, the deposition velocities using BSST were underestimated within the range of 0.8e4.3%, and the absorbed sea surface heat flux was overestimated by 21 W m 2 . In contrast, under cool skin only conditions, the deposition velocities using BSST were overestimated within the range of 0.5e2.0%, varying with pollutants and the absorbed sea surface heat flux was underestimated also by 21 W m 2 . Scale analysis shows that for a slightly soluble gas (e.g., NO 2 , NO and CO), the error in the solubility estimation using BSST is the major source of the error in dry deposition estimation. For a highly soluble gas (e.g., SO 2 ), the error in the estimation of turbulent heat fluxes and, consequently, aerodynamic resistance and gas-phase film resistance using BSST is the major source of the total error. In contrast, for a medium soluble gas (e.g., O 3 and CO 2 ) both the errors from the estimations of the solubility and aerodynamic resistance are important. In addition, deposition esti- mations using various assumptions are discussed. The largest uncertainty is from the parameterizations for chemical enhancement factors. Other important areas of uncertainty include: (1) various parame- terizations for gas-transfer velocity; (2) neutral-atmosphere assumption; (3) using BSST as SST, and (4) constant pH value assumption. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Due to its large surface area and considerable biological activity, the open ocean is both a significant source of as well as a sink for air pollutants (such as SO 2 , CO 2 ,O 3 , NO 2 , NO and CO). The sea surface temperature (SST) is a key parameter for estimating the exchange rates of mass and energy between air and sea, such as turbulent heat fluxes and dry deposition rates of air pollutants. Nonetheless, it is well known that skin sea surface temperature (SSST, or SST at 0-m depth) is different from bulk sea surface temperature (BSST, or SST at 4-m depth). The skin sea surface temperatures are typically a few tenths of a degree Celsius cooler than the temperatures some tens of centimeters below (Saunders, 1967; Paulson and Simpson, 1981; Wu, 1985; Fairall et al., 1996; Wick et al., 1996; Donlon et al., 2002). This uppermost layer is referred to as the molecular sub- layer, the skin layer, or the cool skin of the ocean. As a result of new satellite remote sensing methodologies that are emerging for the estimates of airesea fluxes (e.g., Chou et al., 2003), the cool skin is now recognized as an important feature of the ocean viscous layer. Generally speaking, the structure of the viscous layer is known to be related to the molecular viscosity, surface winds, and airesea flux exchanges. It has been found that when using the BSST for esti- mating turbulent heat fluxes, the error can be as high as 10 W m 2 (e.g., Fairall et al., 1996). However, in many global and regional climate models, it is commonly assumed that the deposition rate can be determined using BSST under neutral conditions (e.g., Wanninkhof, 1992; Bye, 1996; Frankignoulle et al., 1996; Brostrom, 1996, 2000; Keir et al., 2001; Rannik, 2001; Vesovic et al., 2001; * Corresponding author. Tel.: þ49 886 4 22851206; fax: þ49 886 4 22862587. E-mail address: tsuang@nchu.edu.tw (B.-J. Tsuang). Contents lists available at ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv 1352-2310/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2010.04.021 Atmospheric Environment 44 (2010) 2532e2542