Hygroscopicity and evaporation of ammonium chloride and ammonium nitrate: Relative humidity and size effects on the growth factor Dawei Hu a , Jianmin Chen a, b, * , Xingnan Ye a , Ling Li a , Xin Yang a, b, * a Center for Atmospheric Chemistry Study, Department of Environmental Science & Engineering, Fudan University, Shanghai 200433, China b Institute of Global Environment Change Research, Fudan University, Shanghai 200433, China article info Article history: Received 25 November 2010 Received in revised form 28 January 2011 Accepted 10 February 2011 Keywords: Hygroscopicity Evaporation Ammonium chloride Ammonium nitrate H-TDMA abstract The hygroscopicity and evaporation of ammonium chloride and ammonium nitrate in the size range of 40e200 nm are investigated from 20% to 86% RH using a self-assembled hygroscopic tandem differential mobility analyzer (H-TDMA) system. The hygroscopicity of 100 nm (NH 4 ) 2 SO 4 is also measured for comparison. The measured hygroscopic growth factors (GFs) of (NH 4 ) 2 SO 4 agree well with the theoretical Köhler curve. Great discrepancies between the measured GFs and the theoretical values are observed for NH 4 Cl and NH 4 NO 3 due to their volatile properties. The evaporation of NH 4 Cl below the deliquescence RH (DRH) is significantly promoted while RH increases. Similar trend is also observed for NH 4 NO 3 particle less than 50 nm. The proposed mechanism suggests that the increase of RH alters the chemical equilibrium among NH 4 X (s) (X represents Cl  or NO 3  ), NH 3(g) and HX (g) , i.e., NH 4 X ðsÞ $NH 3 þ HX ðgÞ , by converting NH 3(g) and HX (g) into NH 3 $nH 2 O and HX$nH 2 O, which accelerates the evaporation of NH 4 X (s) . When RH is higher than the DRH, the GFs of NH 4 X increase with initial particle size (D 0 ) throughout the investigated size range. In this study, the iso-GF curves are also drawn to illustrate the effects of D 0 and RH on the GFs. Different from (NH 4 ) 2 SO 4 , NaCl, Na 2 SO 4 and NaNO 3 , the GFs of NH 4 X are more sensitive to D 0 than RH due to the unique volatility of NH 4 Cl and NH 4 NO 3 particles. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Atmospheric particles play an important role in affecting radia- tive forcing and climate (Charlson et al.,1992; IPCC, 2007). Ammonia (NH 3 ) from combustion, fertilizers and biological decay is the dominant volatile base in the atmosphere (Bouwman et al., 1997). By reacting with sulfuric acid (H 2 SO 4 ), nitric acid (HNO 3 ) and hydrogen chloride (HCl), particulate ammonium salts such as (NH 4 ) 2 SO 4 , NH 4 NO 3 and NH 4 Cl are formed and account for the predominant inorganic components of atmospheric aerosols (Renard et al., 2004; Seinfeld and Pandis, 2006). Unlike (NH 4 ) 2 SO 4 , NH 4 NO 3 and NH 4 Cl are salts with higher vapor pressure. Aerosol particles containing NH 4 NO 3 and NH 4 Cl are generally in equilibrium with gaseous NH 3 , HNO 3 and HCl species, and their partition between particles and gases is commonly a strong function of ambient temperature and relative humidity (RH) (Stelson and Seinfeld, 2007; Finlayson-Pitts and Pitts Jr, 2000; Dassios and Pandis, 1999). Previous works have revealed that NH 4 NO 3 and NH 4 Cl particles (especially 100 nm) deliquesced at 62% and 78% RH at 298 K, from which the solid particle spontaneously absorbed water and produced a saturated aqueous solution (Seinfeld and Pandis, 2006; Lightstone et al., 2000; Martin, 2000; Topping et al., 2005). Studies also demonstrated the hygroscopic properties of inorganic salts to be initial particle size (D 0 ) dependent (Russell and Ming, 2002; Biskos et al., 2006a,b; Park et al., 2009; Hu et al., 2010). The hygro- scopic growth factors (GFs) of sulfate ((NH 4 ) 2 SO 4 , Na 2 SO 4 ), nitrate (NaNO 3 ) and chloride (NaCl) particles were reported to increase with D 0 when the RH is above the deliquescence RH (DRH) (Russell and Ming, 2002; Biskos et al., 2006a; Park et al., 2009; Hu et al., 2010). Until now, the hygroscopicity of NH 4 NO 3 and NH 4 Cl in wide-size range still not be studied and the corresponding size- effect is also out of discussion. The evaporation of monodisperse NH 4 NO 3 and NH 4 Cl under different RH has been measured extensively by continuously and rapidly removing gaseous NH 3 and HNO 3 (or HCl) from aerosols (Allen et al., 1989; Tang and Munkelwitz, 1989; Harrison et al., 1990; Larson and Taylor, 1983). Larson and Taylor (1983) studied the size change of NH 4 NO 3 aqueous droplet upon passing through * Corresponding authors. Center for Atmospheric Chemistry Study, Department of Environmental Science & Engineering, Fudan University, Shanghai 200433, China. Tel.: þ86 21 65642298; fax: þ86 21 65642080. E-mail addresses: jmchen@fudan.edu.cn (J. Chen), yangxin@fudan.edu.cn (X. Yang). 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.2011.02.024 Atmospheric Environment 45 (2011) 2349e2355