New particle formation (NPF), an important source of particles in the atmosphere, is a dynamic process involving interactions among precursor gas molecules, small clusters, and preexisting particles (Yu and Turco, 2001; Zhang et al., 2012). H2SO4 and H2O are known to play an important role in atmospheric particle formation (e.g., Doyle, 1961). In typical atmospheric conditions, the species dominating the formation and growth of small clusters is H2SO4. The contribution of H2O to the nucleation is related to the hydration of H2SO4 clusters (or, in the other words, modification of the composition of nucleating clusters), which reduces the H2SO4 vapor pressure and hence diminishes the evaporation of H2SO4 from the pre-nucleation clusters. NH3, the most abundant gas-phase base molecule in the atmosphere and a very efficient neutralizer of sulfuric acid solutions, has long been proposed to enhance nucleation in the lower troposphere (Coffman and Hegg, 1995), although it has been well recognized that earlier versions of the classical ternary nucleation model (Coffman and Hegg, 1995; Korhonen et al., 1999; Napari et al., 2002) significantly overpredict the effect of ammonia (Yu, 2006a; Merikanto et al., 2007; Zhang et al., 2010).
The impacts of NH3 on NPF have been investigated in a number of laboratory studies (Kim et al., 1998; Ball et al., 1999; Hanson and Eisele, 2002; Benson et al., 2009; Kirkby et al., 2011; Zollner et al., 2012; Froyd and Lovejoy, 2012; Glasoe et al., 2015; Schobesberger et al., 2015; Kürten et al., 2016) including those recently conducted at the European Organization for Nuclear Research (CERN) in the framework of the CLOUD (Cosmics Leaving OUtdoor Droplets) experiment that has provided a unique dataset for quantitatively examining the dependences of ternary H2SO4-H2O-NH3 nucleation rates on concentrations of NH3 ([NH3]) and H2SO4 ([H2SO4]), ionization rate (Q), temperature (T), and relative humidity (RH) (Kirkby et al., 2011; Kürten et al., 2016). The experimental conditions in the CLOUD chamber, a 26.1 m3 stainless steel cylinder, were well controlled, while impacts of potential contaminants were minimized (Schnitzhofer et al., 2014; Duplissy et al., 2016). Based on CLOUD measurements in H2SO4-H2O-NH3 vapor mixtures, Kirkby et al. (2011) reported that an increase in [NH3] from ∼0.03 ppb (parts per billion, by volume) to ∼0.2 ppb can enhance ion-mediated (or induced) nucleation (IMN) rate by 2-3 orders of magnitude and that the IMN rate is a factor of 2 to > 10 higher than that of neutral nucleation under a typical level of contamination by amines. In the presence of ionization, common highly polar atmospheric nucleation precursors such as H2SO4, H2O, and NH3 molecules tend to cluster around ions, and charged clusters are generally much more stable than their neutral counterparts with enhanced growth rates as a result of dipole-charge interactions (Yu and Turco, 2001).
Despite various laboratory measurements indicating that ammonia enhances NPF, the physicochemical processes underlying the observed different effects of ammonia on the formation of neutral, positively charged, and negatively charged clusters (Schobesberger et al., 2015) are yet to be understood. To achieve such an understanding, a nucleation model based on the first principles is needed. Such a model is also necessary to extrapolate data obtained in a limited number of experimental conditions to a wide range of atmospheric conditions, in which [NH3], [H2SO4], ionization rates, T, RH, and surface areas of preexisting particles vary widely depending on the region, pollution level, and season. The present work aims to address these issues by developing a kinetically based H2SO4-H2O-NH3 ternary IMN (TIMN) model that is based on the molecular clustering thermodynamic data. The model predictions are compared with relevant CLOUD measurements and previous studies.
