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Research article 10 Dec 2018
Research article | 10 Dec 2018
Correspondence: Fangqun Yu (fyu@albany.edu)
HideCorrespondence: Fangqun Yu (fyu@albany.edu)
New particle formation (NPF) is known to be an important source of atmospheric particles that impacts air quality, hydrological cycle, and climate. Although laboratory measurements indicate that ammonia enhances NPF, the physicochemical processes underlying the observed effect of ammonia on NPF are yet to be understood. Here we present a comprehensive kinetically based H_{2}SO_{4}–H_{2}O–NH_{3} ternary ion-mediated nucleation (TIMN) model that is based on the thermodynamic data derived from both quantum-chemical calculations and laboratory measurements. NH_{3} was found to reduce nucleation barriers for neutral, positively charged, and negatively charged clusters differently, due to large differences in the binding strength of NH_{3}, H_{2}O, and H_{2}SO_{4} to small clusters of different charging states. The model reveals the general favor of nucleation of negative ions, followed by nucleation on positive ions and neutral nucleation, for which higher NH_{3} concentrations are needed, in excellent agreement with Cosmics Leaving OUtdoor Droplets (CLOUD) measurements. The TIMN model explicitly resolves dependences of nucleation rates on all the key controlling parameters and captures the absolute values of nucleation rates as well as the dependence of TIMN rates on concentrations of NH_{3} and H_{2}SO_{4}, ionization rates, temperature, and relative humidity observed in the well-controlled CLOUD measurements well. The kinetic model offers physicochemical insights into the ternary nucleation process and provides a physics-based approach to calculate TIMN rates under a wide range of atmospheric conditions.
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). H_{2}SO_{4} and H_{2}O 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 H_{2}SO_{4}. The contribution of H_{2}O to the nucleation is related to the hydration of H_{2}SO_{4} clusters (or, in the other words, modification of the composition of nucleating clusters), which reduces the H_{2}SO_{4} vapor pressure and hence diminishes the evaporation of H_{2}SO_{4} from the pre-nucleation clusters. NH_{3}, 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 NH_{3} 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 H_{2}SO_{4}–H_{2}O–NH_{3} nucleation rates on concentrations of NH_{3} ([NH_{3}]) and H_{2}SO_{4} ([H_{2}SO_{4}]), 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 m^{3} 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 H_{2}SO_{4}–H_{2}O–NH_{3} vapor mixtures, Kirkby et al. (2011) reported that an increase in [NH_{3}] 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 H_{2}SO_{4}, H_{2}O, and NH_{3} 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 [NH_{3}], [H_{2}SO_{4}], 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 H_{2}SO_{4}–H_{2}O–NH_{3} 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.
Most nucleation models developed in the past for H_{2}SO_{4}–H_{2}O binary homogeneous nucleation (e.g., Vehkamäki et al., 2002), H_{2}SO_{4}–H_{2}O ion-induced nucleation (IIN; e.g., Hamill et al., 1982; Raes et al., 1986; Laakso et al., 2003), and H_{2}SO_{4}–H_{2}O–NH_{3} ternary homogeneous nucleation (Coffman and Hegg, 1995; Korhonen et al., 1999; Napari et al., 2002) have been based on the classical approach, which employs capillarity approximation (i.e., assuming that small clusters have the same properties as bulk) and calculates nucleation rates according to the free energy change associated with the formation of a “critical embryo”. Yu and Turco (1997, 2000, 2001) developed a neutral and charged binary H_{2}SO_{4}–H_{2}O nucleation model using a kinetic approach that explicitly treats the complex interactions among small air ions, neutral and charged clusters of various sizes, precursor vapor molecules, and preexisting aerosols. The formation and evolution of cluster size distributions for positively and negatively charged cluster ions and neutral clusters affected by ionization, recombination, neutralization, condensation, evaporation, coagulation, and scavenging have been named IMN (Yu and Turco, 2000). The IMN theory significantly differs from classical IIN theory (e.g., Hamill et al., 1982; Raes et al., 1986; Laakso et al., 2003), which is based on a simple modification of the free energy for the formation of a critical embryo by including the electrostatic potential energy induced by the embedded charge (i.e., Thomson effect; Thomson, 1888). The classical approach does not properly account for the kinetic limitation to embryo development, enhanced stability and growth of charged clusters associated with dipole–charge interaction (Nadykto and Yu, 2003; Yu, 2005), and the important contribution of neutral clusters resulting from ion–ion recombination to nucleation (Yu and Turco, 2011). In contrast, these important physical processes are explicitly considered in the kinetic-based IMN model (Yu, 2006b).
Since the beginning of the century, nucleation models based on the kinetic approach have also been developed in a number of research groups (Lovejoy et al., 2004; Sorokin et al., 2006; Chen et al., 2012; Dawson et al., 2012; McGrath et al., 2012). Lovejoy et al. (2004) developed a kinetic ion nucleation model, which explicitly treats the evaporation of small neutral and negatively charged H_{2}SO_{4}–H_{2}O clusters. The thermodynamic data used in their model were obtained from measurements of small ion clusters, ab initio calculations, the thermodynamic cycle, and some approximations (adjustment of Gibbs free energy for neutral clusters calculated based on liquid droplet model, interpolation, etc.). Lovejoy et al. (2004) did not consider the nucleation on positive ions. Sorokin et al. (2006) developed an ion cluster–aerosol kinetic (ICAK) model, which uses the thermodynamic data reported in Froyd and Lovejoy (2003a, b) and empirical correction terms proposed by Lovejoy et al. (2004). Sorokin et al. (2006) used the ICAK model to simulate dynamics of neutral and charged H_{2}SO_{4}–H_{2}O cluster formation and compared the modeling results with their laboratory measurements. Chen et al. (2012) developed an approach for modeling NPF based on a sequence of acid–base reactions, with sulfuric acid evaporation rates (from clusters) estimated empirically based on measurements of neutral molecular clusters taken in Mexico City and Atlanta. Dawson et al. (2012) presented a semiempirical kinetics model for nucleation of methanesulfonic acid (MSA), amines, and water that explicitly accounted for the sequence of reactions leading to formation of stable particles. The kinetic models of Chen et al. (2012) and Dawson et al. (2012) consider only neutral clusters.
McGrath et al. (2012) developed the Atmospheric Cluster Dynamics Code (ACDC) to model the cluster kinetics by solving the birth–death equations explicitly, with evaporation rate coefficients derived from formation free energies calculated by quantum chemical methods (Almeida et al., 2013; Olenius et al., 2013). The ACDC model applied to the H_{2}SO_{4}–dimethylamine (DMA) system considers zero to four base molecules and zero to four sulfuric acid molecules (Almeida et al., 2013). Olenius et al. (2013) applied the ACDC model to simulate the steady-state concentrations and kinetics of neutral and negatively and positively charged clusters containing up to five H_{2}SO_{4} and five NH_{3} molecules. In ACDC, the nucleation rate is calculated as the rate of clusters growing larger than the upper bounds of the simulated system (i.e., clusters containing four or five H_{2}SO_{4} molecules) (Kürten et al., 2016).
The kinetic IMN model developed by Yu and Turco (1997, 2001) explicitly simulates the dynamics of neutral, positively charged, and negatively charged clusters, based on a discrete-sectional bin structure that covers the clusters containing 0, 1, 2,…, 15,…H_{2}SO_{4} molecules to particles containing thousands of H_{2}SO_{4} (and H_{2}O) molecules. In the first version of the kinetic IMN model (Yu and Turco, 1997, 2001), due to the lack of thermodynamic data for the small clusters, the compositions of neutral and charged clusters were assumed to be the same and the evaporation of small clusters was accounted for using a simple adjustment to the condensation accommodation coefficients. Yu (2006b) developed a second-generation IMN model that incorporated newer thermodynamic data (Froyd, 2002; Wilhelm et al., 2004) and physical algorithms (Froyd, 2002; Wilhelm et al., 2004) and explicitly treated the evaporation of neutral and charged clusters. Yu (2007) further improved the IMN model by using two independent measurements (Marti et al., 1997; Hanson and Eisele, 2000) to constrain monomer hydration in the H_{2}SO_{4}–H_{2}O system and by incorporating experimentally determined energetics of small neutral H_{2}SO_{4}–H_{2}O clusters that became available then (Hanson and Lovejoy, 2006; Kazil et al., 2007). The first and second generations of the IMN model were developed for the H_{2}SO_{4}–H_{2}O binary system, although the possible effects of ternary species such as the impact of NH_{3} on the stability of both neutral and charged pre-nucleation clusters have been pointed out in these previous studies (Yu and Turco, 2001; Yu, 2006b). The present work extends the previous versions of the IMN model in the binary H_{2}SO_{4}–H_{2}O system to the ternary H_{2}SO_{4}–H_{2}O–NH_{3} system, as described below.
Figure 1 schematically illustrates the evolution of charged and neutral clusters–droplets explicitly simulated in the kinetic H_{2}SO_{4}–H_{2}O–NH_{3} TIMN model. Here, H_{2}SO_{4} (S) is the key atmospheric nucleation precursor driving the TIMN process while ions, H_{2}O (W), and NH_{3} (A) stabilize the H_{2}SO_{4} clusters and enhance H_{2}SO_{4} nucleation rates in this way. Ions also enhance cluster formation rates due to the interaction with polar-nucleating species, leading to enhanced collision cross sections (Nadykto and Yu, 2003). The airborne ions are generated by galactic cosmic rays (GCRs) or produced by radioactive emanations, lightning, corona discharge, combustion, and other ionization sources. The initial negative ions, which are normally assumed to be ${\mathrm{NO}}_{\mathrm{3}}^{-}$, are converted into ${\mathrm{HSO}}_{\mathrm{4}}^{-}$ core ions (i.e., S^{−}) and then to larger H_{2}SO_{4} clusters in the presence of gaseous H_{2}SO_{4}. The initial positive ions H^{+}W_{w} are converted into ${{\mathrm{H}}^{+}{\mathrm{A}}_{\mathrm{1}-\mathrm{2}}\mathrm{W}}_{\mathrm{w}}$ in the presence of NH_{3}, H^{+}S_{s} W_{w} in the presence of H_{2}SO_{4}, or H^{+}A_{a} S_{s}W_{w} in the case that both NH_{3} and H_{2}SO_{4} are present in the nucleating vapors. Some of the binary H_{2}SO_{4}–H_{2}O clusters, both neutral and charged, transform into ternary ones by taking up NH_{3} vapors. The molar fraction of ternary clusters in nucleating vapors depends on [NH_{3}], the binding strength of NH_{3} to binary and ternary pre-nucleation clusters, cluster composition, and ambient conditions such as T and RH.
Similar to the kinetic binary IMN (BIMN) model (Yu, 2006b), the kinetic TIMN model employs a discrete-sectional bin structure to represent clusters–particles. The bin index i represents the amount of core component (i.e., H_{2}SO_{4}) and i_{d} is the number of discrete bins. For small clusters (i≤i_{d} = 30 in this study), i is the number of H_{2}SO_{4} molecules in the cluster (i.e., i=s) and the core volume of the ith bin ${v}_{i}=i\times {v}_{\mathrm{1}}$, where v_{1} is the volume of one H_{2}SO_{4} molecule. When i > i_{d}, v_{i} = VRAT_{i} × v_{i−1}, where VRAT_{i} is the volume ratio of the ith bin to the (i−1)th bin. The discrete-sectional bin structure enables the model to cover a wide range of sizes of nucleating clusters–particles with the highest possible size resolution for small clusters (Yu, 2006b). For clusters with a given bin i, the associated amounts of water and NH_{3} and thus the effective radius of each ternary cluster are calculated based on the equilibrium of clusters–particles with the water vapor and/or ammonia, as described in later sections.
The evolution of positive, negative, and neutral clusters due to the simultaneous condensation, evaporation, recombination, coagulation, and other loss processes is described by the following differential equations obtained by the modification of those describing the evolution of the binary H_{2}SO_{4}–H_{2}O system (Yu, 2006b):
In Eqs. (1)–(6), the superscripts “+”, “−”, and “0” refer to positive, negative, and neutral clusters, respectively, while subscripts i, j, and k represent the bin indexes. ${N}_{\mathrm{0}}^{+,-}$ and Q are the concentration of initial ions not containing H_{2}SO_{4} (i.e., H^{+}A_{a}W_{w} and ${\mathrm{NO}}_{\mathrm{3}}^{-}$) and the ionization rate, respectively. N_{i} is the total number concentration (cm^{−3}) of all cluster/particles (binary + ternary) in bin i. For small clusters (i≤i_{d}), N_{i} is the number concentration (cm^{−3}) of all clusters containing i H_{2}SO_{4} molecules. For example, ${N}_{\mathrm{1}}^{\mathrm{0}}$ is the total concentration of binary and ternary neutral clusters containing one H_{2}SO_{4} molecule. Index i in Eq. (4) refers to the sum of H_{2}SO_{4} and ${\mathrm{HSO}}_{\mathrm{4}}^{-}$. The second term of Eq. (2) describes the reaction of ${\mathrm{HSO}}_{\mathrm{4}}^{-}$ + HNO_{3} → ${\mathrm{NO}}_{\mathrm{3}}^{-}$ + H_{2}SO_{4}. Although the rate of this reaction is generally negligible, we keep the term there for completeness. ${P}_{{\mathrm{H}}_{\mathrm{2}}{\mathrm{SO}}_{\mathrm{4}}}$ is the gas-phase production rate of neutral H_{2}SO_{4} molecules. ${L}_{i}^{+,-,\mathrm{0}}$ is the loss rate due to scavenging by preexisting particles and wall and dilution losses in the laboratory chamber studies (Kirkby et al., 2011; Olenius et al., 2013; Kürten et al., 2016). ${f}_{j,k,i}$ is the volume fraction of intermediate particles (volume = v_{j} + v_{k}) partitioned into bin i with respect to the core component – H_{2}SO_{4}, as defined in Jacobson et al. (1994). ${g}_{i+\mathrm{1},i}={v}_{\mathrm{1}}/({v}_{i+\mathrm{1}}-{v}_{i})$ is the volume fraction of intermediate particles of volume (${v}_{i+\mathrm{1}}-{v}_{\mathrm{1}}$) partitioned into bin i. ${\mathit{\delta}}_{j,\mathrm{2}}=\mathrm{2}$ at j=2 and ${\mathit{\delta}}_{j,\mathrm{2}}=\mathrm{1}$ at j≠2. ${\mathit{\gamma}}_{i}^{+}$, ${\mathit{\gamma}}_{i}^{-}$ , and ${\mathit{\gamma}}_{i}^{\mathrm{0}}$ are the mean (or effective) cluster evaporation coefficients for positive, negative and neutral clusters in bin i, respectively. ${\mathit{\beta}}_{i,j}^{+}$, ${\mathit{\beta}}_{i,j}^{-}$, and ${\mathit{\beta}}_{i,j}^{\mathrm{0}}$ are the coagulation kernels for the neutral clusters–particles in bin j interacting with positive, negative, and neutral clusters–particles in bin i, respectively, which reduce to the condensation coefficients for H_{2}SO_{4} monomers at j=1. ${\mathit{\eta}}_{j,k}^{+}$ and ${\mathit{\eta}}_{j,k}^{-}$ are coagulation kernels for clusters–particles of like sign from bin j and clusters–particles from bin k. It should be noted that the electrostatic repulsion is too strong for small clusters to gain more than one charge. However, small charged clusters can be scavenged by large preexisting particles of the same polarity. Large preexisting particles serve as the sink for small clusters in the model and the effect of multiple charge is small and thus is not tracked. ${\mathit{\alpha}}_{i,j}^{+,-}$ is the recombination coefficient for positive clusters–particles in bin i interacting with negative clusters–particles in bin j, while ${\mathit{\alpha}}_{i,j}^{-,+}$ is the recombination coefficient of negative clusters–particles from bin i interacting with positively charged clusters–particles from bin j.
The methods for calculating β, γ, η, and α for binary H_{2}SO_{4}–H_{2}O clusters have been described in our previous publications (Yu and Turco, 2001; Nadykto and Yu, 2003; Yu, 2006b). Dipole–charge interaction (Nadykto and Yu, 2003), image capture, and three-body trapping effects (Hoppel and Frick, 1986) are considered in the calculation of these coefficients. Since β, η, and α depend on the cluster mass (or size) rather than on the cluster composition, schemes for calculating these properties in binary and ternary clusters are identical. In contrast, γ is quite sensitive to cluster composition. The evaporation rate coefficient of H_{2}SO_{4} molecules from clusters containing i H_{2}SO_{4} molecules (γ_{i}) is largely controlled by the stepwise Gibbs free energy change $\mathrm{\Delta}{G}_{i-\mathrm{1},i}$ of formation of an i-mer from an (i−1)-mer (Yu, 2007)
where R is the molar gas constant, N^{o} is the arbitrary number concentration of a hypothetical gas consisting solely of the species for which the calculation is performed (generally under the reference vapor pressure P of 1 atm). ΔH^{o} and ΔS^{o} are enthalpy and entropy changes under the standard conditions (T=298 K, P=1 atm), respectively. The temperature dependence of ΔH^{o} and ΔS^{o}, which is generally small and typically negligible over the temperature range of interest (Nadykto et al., 2009), was not considered.
ΔH, ΔS, and ΔG values needed to calculate cluster evaporation rates (Eq. 7) for the TIMN model can be derived from laboratory measurements and computational quantum chemistry (QC) calculation. Thermochemical properties of neutral and charged binary and ternary clusters obtained using the computational chemical methods and comparisons of computed energies with available experimental data and semi-experimental estimates are given in Tables A1–A4 and discussed in the Appendix. As an example, Fig. 2 shows ΔG associated with the addition of water ($\mathrm{\Delta}{G}_{+\mathrm{W}}^{o}$), ammonia ($\mathrm{\Delta}{G}_{+\mathrm{A}}^{o}$), and sulfuric acid ($\mathrm{\Delta}{G}_{+\mathrm{S}}^{o}$) to binary and ternary clusters as a function of the cluster hydration number w. H_{2}O has high proton affinity and thus H_{2}O is strongly bonded to all positive ions with low w. $\mathrm{\Delta}{G}_{+\mathrm{W}}^{o}$ expectedly becomes less negative and binding of H_{2}O to binary and ternary clusters weakens due to the screening effect as the hydration number w grows (Fig. 2a). The presence of NH_{3} in the clusters weakens binding of H_{2}O to positive ions. For example, $\mathrm{\Delta}{G}_{+\mathrm{W}}^{o}$ for H^{+}A_{1}W_{w}S_{1} is ∼3–4 kcal mol^{−1} less negative than that for H^{+}W_{w}S_{1} at w=3–6. The addition of one more NH_{3} to the clusters to form H^{+}A_{2}W_{w} and H^{+}A_{2}W_{w}S_{1} further weakens H_{2}O binding by ∼1.5–6 kcal mol^{−1} at w=1–3, while exhibiting a much smaller impact on hydration free energies at w > 3. Both the absolute values and trends in $\mathrm{\Delta}{G}_{+\mathrm{W}}^{o}$ derived from calculations are in agreement with the laboratory measurements within the uncertainty range of ∼1–2 kcal mol^{−1} for both QC calculations and measurements. This confirms the efficiency and precision of QC methods in calculating thermodynamic data needed for the development of nucleation models.
The proton affinity of NH_{3} is 204.1 kcal mol^{−1}, which is 37.5 kcal mol^{−1} higher than that of H_{2}O (166.6 kcal mol^{−1}) (Jolly, 1991). The hydrated hydronium ions (H^{+}W_{w}) are easily converted to H^{+}A_{1}W_{w} in the presence of NH_{3}. The binding of NH_{3} and H_{2}O molecules to H^{+}W_{w} exhibits a similar pattern. In particular, binding of NH_{3} to H^{+}W_{w} decreases as w is growing, with $\mathrm{\Delta}{G}_{+\mathrm{A}}^{o}$ for H^{+}A_{1}W_{w} ranging from −52.08 kcal mol^{−1} at w=1 to −8.32 kcal mol^{−1} at w=9. The binding of NH_{3} to H^{+}W_{w}S_{1} ions is also quite strong, with $\mathrm{\Delta}{G}_{+\mathrm{A}}^{o}$ for H^{+}A_{1}W_{w}S_{1} ranging from −33.14 kcal mol^{−1} at w=1 and to −10.57 kcal mol^{−1} at w=6. The addition of the NH_{3} molecule to H^{+}A_{1}W_{w} (to form H^{+}A_{2}W_{w}) is much less favorable thermodynamically than that to H^{+}W_{w}, with the corresponding $\mathrm{\Delta}{G}_{+\mathrm{A}}^{o}$ being −22 and −6 kcal mol^{−1} at w=2 and w=6, respectively. The $\mathrm{\Delta}{G}_{+\mathrm{A}}^{o}$ values for H^{+}A_{2}W_{w} are 3–5 kcal mol^{−1} more negative than the experimental values at w=0–1; however, they are pretty close to experimental data at w=2–3 (Fig. 2b and Table A2). While it is possible that the QC method overestimates the charge effect on the formation free energies of smallest clusters, the possible overestimation at w=0–1 will not affect nucleation calculations because most H^{+}A_{2}W_{w} clusters in the atmosphere contain more than two water molecules (i.e., w > 2) due to the strong hydration (see Table A2 and Fig. 2a).
A comparison of QC and semi-experimental estimates of $\mathrm{\Delta}{G}_{+\mathrm{S}}^{o}$ values associated with the attachment of H_{2}SO_{4} to positive ions shown in Fig. 2c indicates that computed $\mathrm{\Delta}{G}_{+\mathrm{S}}^{o}$ values agree well with observations for H^{+}W_{w}S_{1} and H^{+}A_{1}W_{w}S_{1} but differ by ∼2–4 kcal mol^{−1} from semi-experimental values for H^{+}A_{2}W_{w}S_{1}. As seen from Fig. 2a and c, the attachment of NH_{3} to H^{+}W_{w}S_{1} weakens the binding of both H_{2}O and H_{2}SO_{4} to the clusters. This suggests that the attachment of NH_{3} leads to the evaporation of H_{2}SO_{4} and H_{2}O molecules from the clusters. In other words, H_{2}SO_{4} is less stable in H^{+}A_{1}W_{w}S_{1} than in H^{+}W_{w}S_{1} (Fig. 2c). While this may be taken for the indication that NH_{3} inhibits nucleation on positive ions at the first look, further calculations show that binding of NH_{3} to H^{+}A_{1}W_{w}S_{1} is quite strong (Fig. 2b) and that H_{2}SO_{4} in the H^{+}A_{2}W_{w}S_{1} cluster is much more stable than that in H^{+}A_{1}W_{w}S_{1}, with $\mathrm{\Delta}{G}_{+\mathrm{S}}^{o}$ being more negative by ∼7 kcal mol^{−1} at w > 2. The H^{+}A_{2}W_{w}S_{1} cluster can also be formed via the attachment of H_{2}SO_{4} to H^{+}A_{2}W_{w}. In the presence of sufficient concentrations of NH_{3}, a large fraction of positively charged H_{2}SO_{4} monomers exist in the form of H^{+}A_{2}W_{w}S_{1} and hence NH_{3} enhances nucleation of positive ions. Since positively charged H_{2}SO_{4} dimers are expected to contain a large number of water molecules, we have not yet computed and derived quantum chemical data for these clusters. The CLOUD measurements do indicate that once H^{+}A_{2}W_{w}S_{1} clusters are formed, they can continue to grow to larger H^{+}A_{a}W_{w}S_{s} clusters along $a=s+\mathrm{1}$ pathway (Schobesberger et al., 2015).
Figure 2 clearly shows that the calculated values in most cases agree with measurements within the uncertainty range that justifies the application of QC values in the case that no reliable experimental data are available.
Nucleation barriers and cluster evaporation rates are critically important for calculations of nucleation rates. This section describes the methods employed to calculate the evaporation rates of nucleating clusters of variable sizes and compositions (i.e., γ in Eqs. 1–6) in the TIMN model.
In the atmosphere, [H_{2}O] is much higher than [H_{2}SO_{4}] and thus H_{2}SO_{4} clusters–particles are always in equilibrium with water vapor (Yu, 2007). In the lower troposphere, where most of the nucleation events were observed, [H_{2}SO_{4}] is typically at a sub-parts per trillion to parts per trillion level, while [NH_{3}] is in the range of sub-parts per billion to parts per billion levels (Butler et al., 2016; Warner et al., 2016) (note that, in what follows, all references to vapor mixing ratios – parts per billion and parts per trillion – are by volume). This means that small ternary clusters can be considered to be in equilibrium with H_{2}O and NH_{3} vapors. Like the previous BIMN model derived assuming equilibrium of binary clusters with water vapor, the present TIMN model treats small clusters containing a given number of H_{2}SO_{4} molecules as being in equilibrium with both H_{2}O and NH_{3}. Their relative concentrations are calculated using the thermodynamic data shown in Tables A1–A4. It should be noted that the system may deviate from equilibrium and the model scheme is probably not suitable when [NH_{3}] is less than or close to [H_{2}SO_{4}]. Under such cases, the equilibrium assumption may overestimate nucleation rates.
Figure 3 shows the relative abundance (or molar fractions) of small positive, negative, and neutral clusters (${f}_{s,a,w}^{+,-,\mathrm{0}}$) containing a given number of H_{2}SO_{4} molecules at the ambient temperature of 292 K and three different combinations of RH and [NH_{3}] values. As a result of relative instability of H_{2}SO_{4} in H^{+}A_{1}W_{w}S_{1} compared to H^{+}W_{w}S_{1} or H^{+}A_{2}W_{w}S_{1} (Fig. 2c), most of positive ions with one H_{2}SO_{4} molecule exist in the form of either H^{+}W_{w}S_{1} or H^{+}A_{2}W_{w}S_{1} (i.e, containing either zero or two NH_{3} molecules; Fig. 3a). When [NH_{3}] = 0.3 ppb (with T=292 K), most of the positive ions containing one H_{2}SO_{4} molecule do not contain NH_{3} and their composition is dominated by H^{+}W_{w}S_{1} ($\stackrel{\mathrm{\u203e}}{w}=\sim \mathrm{7}$). At the given T and [NH_{3}] = 0.3 ppb, around 17 % of positive ions with one H_{2}SO_{4} molecule contain two NH_{3} molecules at RH = 38 %. The fraction of positive ions containing one H_{2}SO_{4} and two NH_{3} molecules decreases to 0.9 %, when RH=90 %. At T=292 K and RH=38 %, the increase in [NH_{3}] by a factor of 10 to 3 ppb leads to the domination of H^{+}A_{2}W_{w}S_{1} (∼95 %) in the composition of positively charged H_{2}SO_{4} monomers. As expected, the composition of positive ions and their contribution to nucleation depends on T, RH, and [NH_{3}]. The incorporation of the quantum chemical and experimental clustering thermodynamics in the framework of the kinetic nucleation model enables us to study all these dependencies.
As a result of very weak binding of H_{2}O and NH_{3} to small negative ions (Table A4), nearly all negatively charged clusters with s = 0–1 do not contain water and ammonia (not shown). In the case that s grows to 2, all S^{−}S_{2}A_{a}W_{w} clusters still do not contain NH_{3} (i.e., a=0), while only 20 %–40 % of them contain one water molecule (w=1) (Fig. 3b). As s further increases to 3, NH_{3} begins to enter some of the negatively charged ions. The fraction of S^{−}S_{3}A_{a}W_{w} clusters containing one NH_{3} molecule is 9 % at RH = 38 % and [NH_{3}] = 0.3 ppb, 3 % at RH = 90 % and [NH_{3}] = 0.3 ppb, and 50 % at RH = 38 % and [NH_{3}] = 3 ppb. Most S^{−}S_{3}W_{w} clusters are hydrated while the fraction of S^{−}S_{3}A_{a}W_{w} clusters containing two NH_{3} molecules at these ambient conditions is negligible. The fraction of negative cluster ions containing two NH_{3} molecules becomes significant at s=4 (Fig. 3b) and increases from 28 % at [NH_{3}] = 0.3 ppb to 80 % at [NH_{3}] = 3 ppb at RH = 38 %. At [NH_{3}] = 0.3 ppb, the increase in RH from 38 % to 90 % reduces the fraction of NH_{3}-containing S^{−}S_{3}A_{a}W_{w} clusters (i.e, a > =1) from to 95 % to 70 %, demonstrating a significant impact of RH on cluster compositions and emphasizing the importance of accounting for the RH in calculations of ternary nucleation rates.
The equilibrium distributions of neutral clusters are presented in Fig. 3c (H_{2}SO_{4} monomers and dimers) and Fig. 3d (H_{2}SO_{4} trimers and tetramers). Hydration is accounted for in the case of monomers and dimers and not included, due to lack of thermodynamic data, in calculations for trimers and tetramers. Based on the thermodynamic data shown in Table A3, the dominant fraction of neutral monomers is hydrated (79 % at RH = 38 % and 94 % at RH = 90 %) while the fraction of monomers containing NH_{3} is negligible (0.02 % at [NH_{3}] = 0.3 ppb and 0.2 % at [NH_{3}] = 3 ppb, RH = 38 %). As a result of the growing binding strength of NH_{3} with the cluster size (Table A3), the fraction of neutral sulfuric acid dimers containing one NH_{3} molecule reaches 18 % at [NH_{3}] = 0.3 ppb and 69 % at [NH_{3}] = 3 ppb when T = 292 K and RH = 38 %. In the case of H_{2}SO_{4} trimers and tetramers, data shown in Fig. 3d are limited to the relative abundance of unhydrated clusters only. Under the given conditions, most trimers contain two NH_{3} molecules while most tetramers contain three NH_{3} molecules. At [NH_{3}] = 3 ppb, ∼2 % of trimers contain three NH_{3} molecules (i.e., $s=a=\mathrm{3}$) and 55 % of tetramers contain four NH_{3} molecules (i.e., $s=a=\mathrm{4}$). As a result of a significant drop of $\mathrm{\Delta}{G}_{+\mathrm{A}}^{o}$ in the case that the a∕s ratio exceeds one (Table A3), the fraction of neutral clusters with $a=s+\mathrm{1}$ is negligible. The cluster distributions clearly indicate that small sulfuric acid clusters are still not fully neutralized by NH_{3} even if [NH_{3}] is at a parts per billion level and that the degree of neutralization (i.e., a:s ratio) increases with the cluster size.
In the TIMN model, the equilibrium distributions are used to calculate number-concentration-weighted stepwise Gibbs free energy change for adding one H_{2}SO_{4} molecule to form a neutral, positively charged, and negatively charged cluster containing s H_{2}SO_{4} molecules (${\stackrel{\mathrm{\u203e}}{\mathrm{\Delta}G}}_{s-\mathrm{1},s}$):
where ${f}_{s,a,w}^{+,-,\mathrm{0}}$ is the equilibrium fraction of a particular cluster within a cluster type as shown in Fig. 3.
In the atmosphere, where substantial nucleation is observed, the sizes of critical clusters are generally small (s < ∼5–10) (e.g., Sipilä et al., 2010) and nucleation rates are largely controlled by the stability (or γ) of small clusters with s < ∼5–10. QC calculations and experimental data on clustering thermodynamics available for clusters of small sizes (Tables A2–A4) are critically important as the formation of these small clusters is generally the limiting step for nucleation. Nevertheless, thermodynamics data for larger clusters are also needed to develop a robust nucleation model that can calculate nucleation rates under various conditions. Both measurements and QC calculations (Tables A2–A4) show significant effects of charge and charge signs (i.e., positive or negative) on the stability and composition of small clusters. These charge effects decrease quickly as the clusters grow due to the short-ranged nature of dipole–charge interaction and the quick decrease in electrical field strength around charged clusters as cluster sizes increase (Yu, 2005). Based on experimental data (Kebarle et al., 1967; Davidson et al., 1977; Wlodek et al., 1980; Holland and Castleman, 1982; Froyd and Lovejoy, 2003b), the stepwise ΔG values for clusters decrease exponentially as the cluster sizes increase and approach to the bulk values when clusters contain more than ∼8–10 molecules (Yu, 2005). Cluster compositions measured with an atmospheric pressure interface time-of-flight (APi-TOF) mass spectrometer during CLOUD experiments also show that the difference in the composition of positively and negatively charged clusters quickly decreases as the number of H_{2}SO_{4} molecules increases from 1 to ∼10 and exhibits little further changes (Schobesberger et al., 2015).
In the present TIMN model, we assume that both neutral and charged clusters have the same composition when s≥10 and the following extrapolation scheme is used to calculate $\mathrm{\Delta}{G}_{s-\mathrm{1},s}$ for clusters up to s=10:
where $\mathrm{\Delta}{G}_{{s}_{\mathrm{1}}-\mathrm{1},{s}_{\mathrm{1}}}$ is the stepwise mean Gibbs free energy change for H_{2}SO_{4} addition for a specific type (neutral, positive, or negative) of clusters at s=s_{1} that can be derived from QC calculation and/or experimental measurements, and $\mathrm{\Delta}{G}_{{s}_{\mathrm{2}}-\mathrm{1},{s}_{\mathrm{2}}}$ is the corresponding value for clusters at s=s_{2} (= 10 in the present study) that is calculated in the capillarity approximation accounting for the Kelvin effect. c in Eq. (10) is the exponential coefficient that determines how fast $\mathrm{\Delta}{G}_{s-\mathrm{1},s}$ approaches to bulk values as s increases. In the present study, c is estimated by fitting $\mathrm{\Delta}{G}_{s-\mathrm{1},s}$ at s=2 and s=3 based on Eq. (10) to the corresponding $\mathrm{\Delta}{G}_{s-\mathrm{1},s}$ from experimental (Hanson and Lovejoy, 2006; Kazil et al., 2007) or QC data (Table A3). Apparently the interpolation approximation Eq. (10) is subject to uncertainty. Nevertheless, it is a reasonable approach to connect thermochemical properties of QC data for small binary and ternary clusters that cannot be adequately described by the capillarity approximation with those for large clusters that can be adequately described by the very same capillarity approximation, and it is the best approach we can come up with at this point in order to develop a model that can be applied to all conditions. Further QC and experimental studies of the thermodynamics of relatively larger clusters can help to reduce the uncertainty.
For clusters with s≥s_{2}, the capillarity approximation is used to calculate $\mathrm{\Delta}{G}_{s-\mathrm{1},s}$ as
where P is the H_{2}SO_{4} vapor pressure and P_{s} is the H_{2}SO_{4} saturation vapor pressure over a flat surface with the same composition as the cluster. σ is the surface tension and v_{1} is the volume of one H_{2}SO_{4} molecule. r_{s} is the radius of the cluster and N_{A} is the Avogadro's number.
The scheme to calculate bulk $\mathrm{\Delta}{G}_{s-\mathrm{1},s}$ (s≥10) for H_{2}SO_{4}–H_{2}O binary clusters has been described in Yu (2007). For ternary nucleation, both experiments (Schobesberger et al., 2015) and QC calculations (Table A4) indicate that the growth of relatively large clusters follows the s=a line (i.e, in the composition of ammonia bisulfate). In the present TIMN model, the bulk $\mathrm{\Delta}{G}_{s-\mathrm{1},s}$ values for ternary clusters are calculated based on parameterized H_{2}SO_{4} saturation vapor pressure over ammonia bisulfate as a function of temperature, derived by Marti et al. (1997) from vapor pressures measured at a temperature between 27 and 60 ^{∘}C and surface tension measured at 298 K from Hyvärinen et al. (2005). The uncertainty in saturation vapor pressures and surface tension used in the calculation of the bulk $\mathrm{\Delta}{G}_{s-\mathrm{1},s}$ values is another source of uncertainty in the TIMN model, although it is likely to be small compared to other uncertainties as the nucleation is generally limited by the formation of small clusters.
Figure 4 presents stepwise (${\stackrel{\mathrm{\u203e}}{\mathrm{\Delta}G}}_{s-\mathrm{1},s}$) and cumulative (total) ${\stackrel{\mathrm{\u203e}}{\mathrm{\Delta}G}}_{\mathrm{s}}$ Gibbs free energy changes associated with the formation of neutral, positively charged, and negatively charged binary and ternary clusters containing s H_{2}SO_{4} molecules under the conditions specified in the figure caption. The clusters are assumed to be in equilibrium with water (Yu, 2007) and ammonia (Fig. 3). As seen from Fig. 4, the presence of NH_{3} reduces the mean ${\stackrel{\mathrm{\u203e}}{\mathrm{\Delta}G}}_{s-\mathrm{1},s}$ for larger clusters, which can be treated as the bulk binary H_{2}SO_{4}–H_{2}O solution (Schobesberger et al., 2015), by ∼3 kcal mol^{−1}, indicating a substantial reduction in the H_{2}SO_{4} vapor pressure over ternary solutions (Marti et al., 1997). The comparison also shows that the influence of NH_{3} on ${\stackrel{\mathrm{\u203e}}{\mathrm{\Delta}G}}_{s-\mathrm{1},s}$ of small clusters ($s\le \sim \mathrm{4}$) is much lower than that on larger ones and bulk solutions. For example, at [NH_{3}] = 0.3 ppb, the differences in ${\stackrel{\mathrm{\u203e}}{\mathrm{\Delta}G}}_{s-\mathrm{1},s}$ between binary and ternary positive ions with s=1 and neutral clusters with s=2 are only 0.45 and ∼1 kcal mol^{−1}, respectively. In the case of negative ions, zero and 0.27–0.45 kcal mol^{−1} differences at s≤2 and s=3–4, respectively, were observed. The reduced effect of ammonia on smaller clusters is explained (Tables A2–A4) by ammonia's weaker bonding to smaller clusters than to larger ones, which in turn yields lower average NH_{3}-to-H_{2}SO_{4} ratios (Fig. 3). It should be noted that QC data for positively charged clusters are very limited and the interpolation approximation is subject to large uncertainty. In order for the nucleation on positive ions to occur, the first step is for H_{2}SO_{4} to attach to a positive ion that does not contain H_{2}SO_{4}. Unlike negative ions, the effect of charge on the bonding of H_{2}SO_{4} with positive ions is much weaker and thus the stepwise Gibbs free energy change for the addition of one H_{2}SO_{4} molecule to form a positively charged cluster is likely to be similar to that of neutral clusters, i.e., decreasing with cluster size. Therefore, the QC data for positively charged clusters containing one H_{2}SO_{4} molecule provide a critical constraint. The success of the model in predicting the [NH_{3}] needed for nucleation on positive ions to occur (see Sect. 3) shows the usefulness of the first-step data and approximation.
As seen from Fig. 4, bonding of H_{2}SO_{4} to small negatively charged clusters (s < 3) is much stronger than that to neutrals and positive ions. As a result, at s < 3 the formation of negatively charged clusters is barrierless (${\stackrel{\mathrm{\u203e}}{\mathrm{\Delta}G}}_{s-\mathrm{1},s}$ < 0). These small clusters cannot be considered nucleated particles because ${\stackrel{\mathrm{\u203e}}{\mathrm{\Delta}G}}_{s-\mathrm{1},s}$ (Fig. 4a) first increases and then decreases with growing s, reaching the maximum barrier values at $s=\sim \mathrm{3}$–6. ${\stackrel{\mathrm{\u203e}}{\mathrm{\Delta}G}}_{s-\mathrm{1},s}$ can become positive for larger clusters due to the charge effect decreasing quickly as the clusters are growing. The effect of NH_{3} on negative ions becomes important at $s\ge \sim \mathrm{4}$, when bonding between the clusters and NH_{3} becomes strong enough to contaminate a large fraction of binary clusters with ammonia (Fig. 3). In contrast, the impact of NH_{3} on neutral dimers and positively charged monomers of H_{2}SO_{4}, as well as on ${\stackrel{\mathrm{\u203e}}{\mathrm{\Delta}G}}_{s-\mathrm{1},s}$ for both positively charged and neutral clusters, monotonically decreases for all s values, including s≤5.
${\stackrel{\mathrm{\u203e}}{\mathrm{\Delta}G}}_{s-\mathrm{1},s}$ for charged and neutral clusters converges into the bulk values at $s=\sim \mathrm{10}$, when impact of the chemical identity of the core ion on the cluster composition becomes diffuse (Schobesberger et al., 2015) and when the contribution of the electrostatic effect to ${\stackrel{\mathrm{\u203e}}{\mathrm{\Delta}G}}_{s-\mathrm{1},s}$ becomes less than ∼0.5 kcal mol^{−1}. The comparison of cumulative (total) ${\stackrel{\mathrm{\u203e}}{\mathrm{\Delta}G}}_{\mathrm{s}}$ (Fig. 4b) indicates the lowest nucleation barrier for the case of negative ions, followed by positive ions and neutrals. The barrierless formation of clusters with s ranging from 1 to 3 substantially reduces the nucleation barrier for negatively charged ions and facilitates their nucleation. The presence of 0.3 ppb of NH_{3} lowers the nucleation barrier for negative, positive, and neutral clusters from ∼17, 24, and 38 kcal mol^{−1} to 2, 7, and 16 kcal mol^{−1}, respectively. A relatively low nucleation barrier for charged ternary clusters is explained by the simultaneous effect of ionization and NH_{3}, which also reduces the size of the critical cluster (s^{*}).
It is important to note that the size of the critical cluster, commonly used to “measure” the activity of nucleation agents in the classical nucleation theory (Coffman and Hegg, 1995; Korhonen et al., 1999; Vehkamäki et al., 2002; Napari et al., 2002; Hamill et al., 1982) is no longer a valid indicator, when charged molecular clusters and small nanoparticles are considered. As seen from Fig. 4, positively charged ternary critical clusters (${s}^{*}=\mathrm{3}$–4) are smaller than the corresponding negatively charged ones (${s}^{*}=\mathrm{4}$–5); however, the nucleation barrier for ternary positive clusters under the condition specified in the figure caption is more than 3 times higher than that for ternary negatives ones.
As we mentioned earlier, H_{2}SO_{4} is the key atmospheric nucleation precursor driving the formation and growth of clusters in the ternary H_{2}SO_{4}–H_{2}O–NH_{3} system while ions, H_{2}O, and NH_{3} act to stabilize the H_{2}SO_{4} clusters. The clustering thermodynamic data derived from QC calculations and measurements (Sect. 2.3) are used to constrain size- and composition- dependent Gibbs free energy changes and evaporation rates of H_{2}SO_{4}, which are critically important. Average or effective rates of H_{2}SO_{4} molecule evaporation from positively charged, negatively charged, and neutral clusters containing s H_{2}SO_{4} molecules (${\stackrel{\mathrm{\u203e}}{\mathit{\gamma}}}_{\mathrm{s}}^{+,-,\mathrm{0}}$) are calculated from ${\stackrel{\mathrm{\u203e}}{\mathrm{\Delta}G}}_{s-\mathrm{1},s}$ as
where N^{o} is as defined in Eq. (6). The present model assumes only a single H_{2}SO_{4} molecule evaporates, i.e., no water ligands, for instance, are attached to it. This is likely the dominant evaporation pathway as hydrated H_{2}SO_{4} molecules are generally more stable.
Figure 5 gives the mean evaporation rate ($\stackrel{\mathrm{\u203e}}{\mathit{\gamma}}$) of a H_{2}SO_{4} molecule from these clusters under the conditions corresponding to Fig. 4. The shapes of $\stackrel{\mathrm{\u203e}}{\mathit{\gamma}}$ curves are similar to those of ${\stackrel{\mathrm{\u203e}}{\mathrm{\Delta}G}}_{s-\mathrm{1},s}$ (Fig. 4a) as $\stackrel{\mathrm{\u203e}}{\mathit{\gamma}}$ values are largely controlled by ${\stackrel{\mathrm{\u203e}}{\mathrm{\Delta}G}}_{s-\mathrm{1},s}$ (Eq. 12). The presence of ammonia, as expected, significantly reduces the vapor pressure of H_{2}SO_{4} over bulk aerosol (Marti et al., 1997) and hence the H_{2}SO_{4} evaporation rate. The evaporation rates of both neutral and positive clusters decrease as s increases, and the positive clusters are uniformly more stable than corresponding neutral clusters. $\stackrel{\mathrm{\u203e}}{\mathit{\gamma}}$ for negative ions first increases and then decreases as s increases, peaking at around $s=\sim \mathrm{3}$–6. The presence of NH_{3} reduces the evaporation rates of larger clusters by more than 2 orders of magnitude and the effect decreases for smaller clusters, as the binding of NH_{3} to small neutral and charged clusters is weaker compared to that for larger clusters (Fig. 4). [NH_{3}] influences the average NH_{3} : H_{2}SO_{4} ratio (Fig. 3) and the evaporation rates of these small clusters. The nucleation rates, limited by formation of small clusters (s < ∼5), depend strongly on the stability or evaporation rate of these small clusters. While the binding of NH_{3} to small neutral and charged clusters is weaker compared to that to larger clusters, small clusters containing NH_{3} are much more stable than those without (Fig. 4) and thus ammonia is important for nucleation.
The evolution of cluster/particle size distributions can be obtained by solving the dynamic Eqs. (1)–(6). Since the concentrations of clusters of all sizes are predicted, the nucleation rates in the kinetic model can be calculated for any cluster size larger than the critical size of neutral clusters (i > i^{*}) (Yu, 2006b),
where ${J}_{i}^{+}$, ${J}_{i}^{-}$, and ${J}_{i}^{\mathrm{0}}$ are nucleation rates associated with positive, negative, and neutral clusters containing i H_{2}SO_{4} molecules. As a result of scavenging by preexisting particles or wall loss, the steady-state J_{i} decreases as i increases. To compare with CLOUD measurements, we calculate nucleation at a cluster mobility diameter of 1.7 nm (J_{1.7}).
Many practical applications require information on the steady state nucleation rates. For each nucleation case presented in this paper, constant values of [H_{2}SO_{4}] (i.e., ${N}_{\mathrm{1}}^{\mathrm{0}}$), [NH_{3}], T, RH, Q, and ${L}_{i}^{+,-,\mathrm{0}}$ are assumed. The preexisting particles with fixed surface area or wall loss serve as a sink for all clusters. Under a given condition, cluster distribution and nucleation rate reach steady state after a certain amount of time. We calculate size-dependent coefficients for a given case and then solve Eqs. (1)–(6) to obtain the steady-state cluster distribution and nucleation rate, with the approach described in Yu (2006b).
Figure 6 shows a comparison of the model TIMN rates J_{1.7} with CLOUD measurements, as a function of [NH_{3}] under two ionization rates. It should be noted that Dunne et al. (2016) developed a simple empirical parameterization (denoted hereafter as “CLOUDpara”) of binary, ternary, and IIN rates in CLOUD measurements as a function of [NH_{3}], [H_{2}SO_{4}], T, and negative ion concentration. The predictions of CLOUDpara (Dunne et al., 2016) and ACDC based on nucleation thermochemistry obtained using the RI-CC2//B3LYP method (McGrath et al., 2012; Kürten et al., 2016) are also presented in Fig. 6 for comparisons.
Like the CLOUD measurements, the TIMN predictions reveal a complex dependence of J_{1.7} on [NH_{3}], and an analysis of the TIMN results shows this behavior can be explained by the differing responses of negative, positive, and neutral clusters to the presence of ammonia (Fig. 4). Under the conditions specified in Fig. 6, nucleation is dominated by negative ions for [NH_{3}] < ∼0.5 ppb, by both negative and positive ions for [NH_{3}] from ∼0.5 ppb to ∼10 ppb (with background ionization) or ∼20 ppb (with pion-enhanced ionization), and by neutrals at higher [NH_{3}]. According to TIMN, [NH_{3}] values of at least 0.6–1 ppb are needed before positive ions contribute significantly to nucleation rates – in good agreement with the threshold found in the CLOUD experiments (Kirkby et al., 2011; Schobesberger et al., 2015). TIMN simulations also extend CLOUD data at [NH_{3}] of ∼1 ppb to include a “zero-sensitivity zone” in the region of 1–10 ppb, followed by a region of strong sensitivity of J_{1.7} to [NH_{3}] commencing at [NH_{3}] > ∼10–20 ppb. The latter zone may have important implications for NPF in heavily polluted regions, including much of India and China, where [NH_{3}] may exceed 10–20 ppb (Behera and Sharma, 2010; Meng et al., 2018). It is noteworthy in Fig. 6 that the dependence of J_{1.7} on [NH_{3}] and Q predicted by the ACDC model (McGrath et al., 2012) and the CLOUD data parameterization (Dunne et al., 2016) deviate substantially from the experimental data as well as the TIMN simulations. CLOUDpara does not consider impacts of positive ions and key controlling parameters such as RH and surface area of preexisting particles. Dunne et al. (2016) reported that CLOUDpara is also very sensitive to the approach to parameterize T dependence, showing that the contribution of ternary IIN to NPF below 15 km in altitude has grown from 9.6 % to 37.5 %, after the initial empirical temperature function was replaced with a simpler one.
Figure 7 presents a more detailed comparison of TIMN simulations with CLOUD measurements of J_{1.7} as a function of [H_{2}SO_{4}], T, and RH. The TIMN model reproduces both the absolute values of J_{1.7} and its dependencies on [H_{2}SO_{4}], T, and RH in a wide range of temperatures (T = 208–292 K) and [H_{2}SO_{4}] (5 × 10^{5}–5 × 10^{8} cm^{−3}). As expected, nucleation rates are very sensitive to [H_{2}SO_{4}] and T. For example, J_{1.7} increases by 3 to 5 orders of magnitude with an increase in [H_{2}SO_{4}] of a factor of 10 and by roughly 1 order of magnitude for a temperature decrease of 10 ^{∘}C, except in cases in which the nucleation rate is limited by Q (for example, [H_{2}SO_{4}] = ∼10^{8}–10^{9} cm^{−3} at T = 278 and 292 K, shown in Fig. 7a). The key difference between CLOUDpara and TIMN predictions is that the dlnJ_{1.7} ∕ dln[H_{2}SO_{4}] ratio predicted by CLOUDpara is nearly constant while TIMN shows that this ratio depends on both [H_{2}SO_{4}] and T. The CLOUD measurements taken at T = 278 K clearly show (in agreement with the TIMN) that dlnJ_{1.7} ∕ dln[H_{2}SO_{4}] is not constant. CLOUDpara overestimates J_{1.7} compared to both measurements and TIMN simulations, except for the case in which T = 278 K and [H_{2}SO_{4}] range from ∼7 × 10^{6} to 5 × 10^{7} cm^{−3}, with a deviation of CLOUDpara from experimental data and TIMN growing with the lower temperature.
Both CLOUD measurements and TIMN simulations (Fig. 7b) show an important influence of RH on nucleation rates. In particular, CLOUD measurements indicate a 1–5 order of magnitude rise in J_{1.7} after RH increases from 10 % to 70 %–80 % and a stronger effect of RH on nucleation rates at higher temperatures under the conditions shown in Fig. 7b. The RH dependence of J_{1.7} predicted by the TIMN model is consistent with measurements, being slightly weaker than that measured at high RH.
Figure 8 compares TIMN model predictions with all 377 data points of CLOUD measurements reported in data Table S1 of Dunne et al. (2016). The vertical error bars show the range of J_{model} associated with the uncertainty in the [H_{2}SO_{4}] measured (−50 %, +100 %). The effect of uncertainty in measured [NH_{3}] (−50 %, +100 %) is not included. In the presence of ionization (Fig. 8a), J_{model} agrees with CLOUD measurements within the uncertainties under mainly all conditions, although J_{model} tends to be slightly lower than J_{obs} when T = 292–300 K and J_{obs} is relatively small (< ∼1 cm^{−3} s^{−1}). For the neutral nucleation (Fig. 8b), the model agrees well with observations at low T (T = 205–223 K) but deviates from observations as T increases. The underprediction of the model for neutral nucleation at T = 278–300 K cannot be explained by the uncertainties in measured [H_{2}SO_{4}] and [NH_{3}]. Apparently for neutral nucleation the model predicts much stronger temperature dependence than the CLOUD measurements. The possible reasons for the difference include the uncertainties in both the model (especially the thermodynamics data and approximation) and measurements. It should be noted that under the conditions of high T and absence of ions, the role of cluster evaporation (i.e., thermodynamics) becomes more important (i.e., higher evaporation and/or generally less tightly bound clusters) and the effect of the possible biases of the used thermochemistry can be more clearly revealed. The contamination (by amines) in the CLOUD measurements (Kirkby et al., 2011) can be another possible reason. The level of contamination in the cloud chamber appears to increase with temperature (Kürten et al., 2016), which may explain the good agreement at low T and increased deviation at higher T. Further research is needed to identify the source of the difference for neutral ternary nucleation at high T.
A comprehensive kinetically based H_{2}SO_{4}–H_{2}O–NH_{3} TIMN model, constrained with thermodynamic data from QC calculations and laboratory measurements, has been developed and used to shed new light on physicochemical processes underlying the effect of ammonia on NPF. We show that the stabilizing effect of NH_{3} grows with the cluster size and that the reduced effect of ammonia on smaller clusters is caused by weaker bonding that in turn yields lower average NH_{3}-to-H_{2}SO_{4} ratios. NH_{3} was found to impact nucleation barriers for neutral, positively charged, and negatively charged clusters differently due to the large difference in the binding energies of NH_{3}, H_{2}O, and H_{2}SO_{4} to small clusters of different charging states. The lowest and highest nucleation barriers are observed in the case of negative ions and neutrals, respectively. Therefore, nucleation of negative ions is favorable, followed by nucleation of positive ions and neutrals. Different responses of negative, positive, and neutral clusters to ammonia result in a complex dependence of ternary nucleation rates on [NH_{3}]. The TIMN model reproduces both the absolute values of nucleation rates and their dependencies on the key controlling parameters and agrees with the CLOUD measurements for all the cases at the presence of ionization. For the neutral ternary nucleation, the model agrees well with observations at low temperature but deviates from observations as temperature increases.
The TIMN model developed in the present study may be subject to uncertainties associated with the uncertainties in thermodynamic data and interpolation approximation for pre-nucleation clusters. Further measurements and quantum calculations, especially for relatively larger clusters, are needed to reduce the uncertainties. While the TIMN model predicts nucleation rates in a good overall agreement with the CLOUD measurements, its ability to explain the NPF events observed in the real atmosphere is yet to be quantified and will be investigated in further studies.
All relevant data are available in the article, or from the corresponding authors upon request.
Thermochemical data for small neutral and charged binary H_{2}SO_{4}–H_{2}O and ternary H_{2}SO_{4}–H_{2}O–NH_{3} clusters have been reported in a number of earlier publications (Bandy and Ianni, 1998; Ianni and Bandy, 1999; Torpo et al., 2007; Nadykto et al., 2008; Herb et al., 2011, 2013; Temelso et al., 2012a, b; DePalma et al., 2012; Ortega et al., 2012; Chon et al., 2014; Husar et al., 2014; Henschel et al., 2014, 2016; Kürten et al., 2015). The PW91PW91/6-311$++$G(3df,3pd) method, which is a combination of the Perdue–Wang PW91PW91 density functional with the largest Pople 6-311$++$G(3df,3pd) basis set, has thoroughly been validated and agrees well with existing experimental data. In earlier studies, this method has been applied to a large variety of atmospherically relevant clusters (Nadykto et al., 2006, 2007, 2008, 2014, 2015; Nadykto and Yu, 2007; Torpo et al., 2007; Zhang et al., 2009; Elm et al., 2012, 2013; Leverentz et al., 2013; Xu and Zhang, 2012, 2013; Zhu et al., 2014; Bork et al., 2014; Elm and Mikkelsen, 2014; Peng et al., 2015; Miao et al 2015; Ma et al., 2016) and has been shown to be well suited to study the H_{2}SO_{4}–H_{2}O and H_{2}SO_{4}–H_{2}O–NH_{3} clusters, as evidenced by a very good agreement of the computed values with measured cluster geometries, vibrational fundamentals, dipole properties, formation Gibbs free energies (Nadykto et al., 2007, 2008, 2014, 2015; Nadykto and Yu, 2007; Herb et al., 2013; Elm et al., 2012, 2013; Leverentz et al., 2013; Bork et al., 2014), and high-level ab initio results (Temelso et al., 2012a, b; Husar et al., 2012; Bustos et al., 2014).
^{a} Froyd and Lovejoy (2003a). ^{b} Meot-Ner
(Mautner) et al. (1984). ^{c} Payzant et al. (1973).
^{d} Froyd (2002). ^{e} Froyd and Lovejoy (2012).
^{f} The $\mathrm{\Delta}{G}_{+\mathrm{S}}^{o}$ values given
here were calculated based on experimental $\mathrm{\Delta}{G}_{+\mathrm{S}}^{o}$
values at T = 270 K from Froyd and Lovejoy (2003a) and ΔS
values from quantum calculation.
^{a} Nadykto and Yu (2007). ^{b} Torpo et al. (2007). ^{c} Ortega et al. (2012). ^{d} Chon et al. (2007). ^{e} Kürten et al. (2015).
We have extended the earlier QC studies of binary and ternary clusters to larger sizes. The computations have been carried out using a Gaussian 09 suite of programs (Frish et al., 2009). In order to ensure the quality of the conformational search we have carried out a thorough sampling of conformers. We have used both a basin-hopping algorithm, as implemented in Biovia Materials Studio 8.0, and locally developed sampling code. The sampling code is based on the following principle: mesh, with molecules to be added to the cluster placed in the mesh nodes, is created around the cluster, and a blind search algorithm is used to generate the guess geometries. The mesh density and orientation of molecules, as well as the minimum distance between molecules and cluster, are variable. Typically, for each cluster of a given chemical composition a thousand to several thousands of isomers have been sampled. We used a three-step optimization procedure, which includes
pre-optimization of initial and guess geometries using the semiempirical PM6 method, separation of the most stable isomers located within 15 kcal mol^{−1} of the intermediate global minimum and duplicate removal, followed by
optimization of the selected isomers meeting the aforementioned stability criterion by the PW91PW91/CBSB7 method and
the final optimization of the most stable isomers at the PW91PW91/CBSB7 level within 5 kcal mol^{−1} of the current global minimum using the PW91PW91/6-311$++$G(3df,3pd) method.
Typically, only ∼4 %–30 % of initially sampled isomers reach the second (PW91PW91/CBSB7) level, at which ∼10 %–40 % of isomers optimized with PW91PW91/CBSB7 are selected for the final run. Typically, the number of equilibrium isomers of hydrated clusters is larger than that of unhydrated ones of similar chemical composition. Table A1 shows the numbers of isomers converged at the final PW91PW91/6-311$++$G(3df,3pd) optimization step for selected clusters and HSG (enthalpy, entropy, and Gibbs free energy) values of the most stable isomers used in the present study. The number of isomers optimized at the PW91PW91/6-311$++$G(3df,3pd) level of theory varies from case to case, typically being in the range of ∼10–200.
The computed stepwise enthalpy, entropy, and Gibbs free energies of cluster formation have been thoroughly evaluated and used to calculate the evaporation rates of H_{2}SO_{4} from neutral, positively, and negatively charged clusters.
Table A2 presents the computed stepwise Gibbs free energy changes under standard conditions (ΔG^{o}) for positive binary and ternary clusters, along with the corresponding experimental data or semi-experimental estimates. Figure 2 in the main text shows ΔG associated with the addition of water ($\mathrm{\Delta}{G}_{+\mathrm{W}}^{o}$), ammonia ($\mathrm{\Delta}{G}_{+\mathrm{A}}^{o}$), and sulfuric acid ($\mathrm{\Delta}{G}_{+\mathrm{S}}^{o}$) to binary and ternary clusters as a function of the cluster hydration number w. Both the absolute values and trends in $\mathrm{\Delta}{G}_{+\mathrm{W}}^{o}$ derived from calculations are in agreement with the laboratory measurements within the uncertainty range of ∼1–2 kcal mol^{−1} for both QC calculations and measurements. This confirms the efficiency and precision of QC methods in calculating thermodynamic data needed for the development of nucleation models. Nevertheless, it should be noted that the uncertainties in computed free energies of 1–2 kcal mol^{−1} may lead to large uncertainty in predicted particle formation rates. By increasing or decreasing all Gibbs free energies by 1 kcal mol^{−1}, Kürten at al. (2016) showed that, depending on the conditions, the modeled particle formation rate can change from less than an order of magnitude to several orders of magnitude. Uncertainties estimated by Kürten at al. (2016) represent the upper limit because computed free energies may be overestimated for some clusters and underpredicted for others, which leads to partial or, in some case, full error cancelation.
Table A3 presents the computed stepwise Gibbs free energy changes for the formation of ternary S_{s}A_{a}W_{w} clusters under standard conditions. The corresponding binary electrically neutral clusters can be found in previous publications (e.g., Nadykto et al., 2008; Herb et al., 2011). The thermodynamic properties of the S_{1}A_{1} have been reported in a number of computational studies (e.g., Herb et al., 2011; Kurtén et al., 2007; Nadykto and Yu, 2007). However, most of these studies, except for Nadykto and Yu (2007) and Henschel et al. (2014, 2016), did not consider the impact of H_{2}O on cluster thermodynamics. We have extended the earlier studies of Nadykto and Yu (2007) and Herb et al. (2011) to larger clusters up to S_{4}A_{5} (no hydration) and up to S_{2}A_{2} (hydration included). The free energy of binding of NH_{3} to H_{2}SO_{4} (or H_{2}SO_{4} to NH_{3}) obtained using our method is −7.77 kcal mol^{−1}, which is slightly more negative than values reported by other groups (−6.6–7.61 kcal mol^{−1}) and within less than 0.5 kcal mol^{−1} of the experimental value of −8.2 kcal mol^{−1} derived from CLOUD measurements (Kürten et al., 2015).
As it may be seen from Table A3, the NH_{3} binding to S_{1−2}W_{w} weakens as w increases. The average $\mathrm{\Delta}{G}_{+\mathrm{W}}^{o}$ for S_{1}W_{w} formation derived from a combination of laboratory measurements and quantum chemical studies are −3.02, −2.37, and −1.40 kcal mol^{−1} for the first, second, and third hydrations, respectively (Yu, 2007). This indicates that a large fraction of H_{2}SO_{4} monomers in the Earth's atmosphere are likely hydrated. Therefore, the decreasing NH_{3} binding strength to hydrated H_{2}SO_{4} monomers implies that RH (and T) will affect the relative abundance of H_{2}SO_{4} monomers containing NH_{3}. Currently, no experimental data or observations are available to evaluate the impact of hydration (or RH) on $\mathrm{\Delta}{G}_{+\mathrm{A}}^{o}$. Table A3 shows that the presence of NH_{3} in H_{2}SO_{4} clusters suppresses hydration and that $\mathrm{\Delta}{G}_{+\mathrm{W}}^{o}$ for S_{2}A_{2} falls below −2.0 kcal mol^{−1}. This is consistent with earlier studies by our group (Herb et al., 2011) and others (Henschel et al., 2014, 2016) showing that large S_{n}A_{n} clusters (n > 2) are not hydrated under typical atmospheric conditions. In the present study, the hydration of neutral S_{n}A_{n} clusters at n > 2 is neglected, due to the lack of thermodynamic data.
The number of NH_{3} molecules in the cluster (or H_{2}SO_{4} to NH_{3} ratio) significantly affects $\mathrm{\Delta}{G}_{+\mathrm{S}}^{o}$ and $\mathrm{\Delta}{G}_{+\mathrm{A}}^{o}$ values. For example, $\mathrm{\Delta}{G}_{+\mathrm{S}}^{o}$ for S_{3}A_{a} clusters increases from −7.08 to −16.92 kcal mol^{−1} and $\mathrm{\Delta}{G}_{+\mathrm{A}}^{o}$ decreases from −16.14 to −8.93 kcal mol^{−1} as a grows from 1 to 3. For S_{4}A_{a} clusters, $\mathrm{\Delta}{G}_{+\mathrm{S}}^{o}$ increases from −7.48 to −16.26 kcal mol^{−1} and $\mathrm{\Delta}{G}_{+\mathrm{A}}^{o}$ decreases from −17.16 to −11.34 kcal mol^{−1} as a increases from 2 to 4. $\mathrm{\Delta}{G}_{+\mathrm{A}}^{o}$ for the S_{4}A_{1} cluster is less negative by 1.38 kcal mol^{−1} than that for S_{4}A_{2}. $\mathrm{\Delta}{G}_{+\mathrm{S}}^{o}$ for the S_{4}A_{1} cluster is also quite low (−4.16 kcal mol^{−1}), which might indicate the possible existence of a more stable S_{4}A_{1} isomer, which is yet to be identified. In the presence of NH_{3}, the uncertainty in the thermochemistry data for S_{4}A_{1} will not significantly affect ternary nucleation rates because most S_{4} clusters contain three or four NH_{3} molecules.
For the S_{s}A_{a} clusters with s=a, $\mathrm{\Delta}{G}_{+\mathrm{A}}^{o}$ increases as the cluster grows while $\mathrm{\Delta}{G}_{+\mathrm{S}}^{o}$ first increases significantly as S_{1}A_{1} is converted into S_{2}A_{2} and then levels off as S_{2}A_{2} is converted into S_{4}A_{4}. We also observe a significant drop in $\mathrm{\Delta}{G}_{+\mathrm{A}}^{o}$ when the NH_{3} ∕ H_{2}SO_{4} ratio exceeds 1. This finding is consistent with the ACDC model calculation showing that growth of neutral S_{s}A_{a} clusters follows the s=a pathway (Schobesberger et al., 2015).
Table A4 shows ΔG_{+W}, ΔG_{+A}, and ΔG_{+S} needed to form negatively charged clusters under standard conditions, along with available semi-experimental values (Froyd and Lovejoy, 2003b). H_{2}O binding to negatively charged S^{−}S_{s} clusters significantly strengths with increasing s, from $\mathrm{\Delta}{G}_{+\mathrm{W}}^{o}$ = −0.61 – −1.83 kcal mol^{−1} at s=1–2 to $\mathrm{\Delta}{G}_{+\mathrm{W}}^{o}$ = −3.5 kcal mol^{−1} at w=1 and −2.25 kcal mol^{−1} at w=4 at s=4. $\mathrm{\Delta}{G}_{+\mathrm{W}}^{o}$ values at s=3 and 4 are slightly more negative (by ∼0.1–0.9 kcal mol^{−1}) than those reported by Froyd and Lovejoy (2003b). Just like H_{2}O binding, NH_{3} binding to S^{−}S_{s} at s < 3 is very weak, with $\mathrm{\Delta}{G}_{+\mathrm{A}}^{o}$ ranging from +2.81 kcal mol^{−1} at s=0 to −4.85 kcal mol^{−1} at s=2. However, it significantly increases as s grows. In particular, at $s\ge \mathrm{3}\mathrm{\Delta}{G}_{+\mathrm{A}}^{o}$ ranges from −11.89 kcal mol^{−1} for S^{−}S_{3}A_{1} to −15.37 kcal mol^{−1} for S^{−} S_{4}A_{1}. NH_{3} clearly cannot enter small negative ions. However, it can easily attach to larger negative ions with s≥3, which is consistent with CLOUD measurements (Schobesberger et al., 2015). Since hydration weakens NH_{3} binding in S^{−}S_{3}A_{1}W_{w} and S^{−}S_{4}A_{1}W_{w} clusters, its impacts on the cluster formation and nucleation rates may potentially be important.
In contrast to H_{2}O and NH_{3}, binding of H_{2}SO_{4} to small negative ions (s < 3) is very strong. These ions are very stable even when they contain no NH_{3} or H_{2}O molecules. High electron affinity of H_{2}SO_{4} molecules results in the high stability of S^{−}S_{s} at s=1–2. However, the charge effect reduces as s grows. In particular, $\mathrm{\Delta}{G}_{+\mathrm{S}}^{o}$ of S^{−}S_{s} drops from −32.74 kcal mol^{−1} at s=1 to −10.58 kcal mol^{−1} and −8.28 kcal mol^{−1} at s=3 and 4, respectively. At the same time, $\mathrm{\Delta}{G}_{+\mathrm{A}}^{o}$ increases from 0.08 kcal mol^{−1} (s=1) to −11.89 kcal mol^{−1} (s=3) and −15.37 kcal mol^{−1} (s=4). The hydration of S^{−}S_{s} at s=3 and 4 enhances the strength of H_{2}SO_{4} binding, especially at s=4. $\mathrm{\Delta}{G}_{+\mathrm{S}}^{o}$ values for S^{−}S_{3−4}W_{w} are consistently ∼1.5–3 kcal mol^{−1} less negative than the corresponding semi-experimental estimates (Table A4). The possible reasons behind the observed systematic difference are yet to be identified and include the use of the low-level ab initio Hartree–Fock method to compute reaction enthalpies and uncertainties in experimental enthalpies in studies by Froyd and Lovejoy (2003b).
NH_{3} binding to S^{−}S_{3} significantly enhances the stability of H_{2}SO_{4} in the cluster by ∼7 kcal mol^{−1} compared to $\mathrm{\Delta}{G}_{+\mathrm{S}}^{o}$ for the corresponding binary counterpart. The binding of the second NH_{3} to S^{−}S_{3}A to form S^{−}S_{3}A_{2} is much weaker ($\mathrm{\Delta}{\mathrm{G}}_{+\mathrm{A}}^{o}$ = −7.27 kcal mol^{−1}) than that of the first NH_{3} molecule ($\mathrm{\Delta}{\mathrm{G}}_{+\mathrm{A}}^{o}=-\mathrm{11.89}$ kcal mol^{−1}). This indicates that most of S^{−}S_{3}A_{a} can only contain one NH_{3} molecule, in a perfect agreement with the laboratory study of Schobesberger et al. (2015). In the case of S^{−}S_{4}, binding of the first ($\mathrm{\Delta}{\mathrm{G}}_{+\mathrm{A}}^{\mathrm{o}}$ = −15.37 kcal mol^{−1}) and second (and −12.23 kcal mol^{−1}) NH_{3} molecules to the cluster is quite strong, while the attachment of NH_{3} leads to substantial stabilization of H_{2}SO_{4} in the cluster, as evidenced by $\mathrm{\Delta}{G}_{+\mathrm{S}}^{o}$ growing from −8.28 kcal mol^{−1} at a=0 to −11.76 kcal mol^{−1} and −16.71 kcal mol^{−1} at a=1 and a=2, respectively. The NH_{3} binding free energy to S^{−}S_{4}A_{2} (to form S^{−}S_{4}A_{3}) drops to −7.59 kcal mol^{−1}, indicating, in agreement with the CLOUD measurements (Schobesberger et al., 2015), that most S^{−}S_{4} clusters contain one or two NH_{3} molecules.
FY designed the study, led the development of the TIMN model, analyzed results, and wrote a major part of the paper. ABN, JH, KMN, and LAU designed, carried out, and evaluated quantum calculations. ABN also contributed to the writing of the paper. GL contributed to the result analysis and discussion. All contributed to the reviewing and editing of the paper.
The authors declare that they have no conflict of interest.
The authors thank Richard Turco (Distinguished Professor Emeritus, UCLA) for
comments that helped to improve the paper. This study was supported by the
NSF under grant 1550816, NASA under grant NNX13AK20G, and NYSERDA under
contract 100416. ABN, KMN and LAU would like to thank the Russian Science Foundation
(under grant 18-11-00247) and
the Ministry of Science and Education of Russia (under grants 1.6198.2017/6.7
and 1.7706.2017/8.9) for support and the Center of Collective Use of MSTU
“STANKIN” for providing resources. Edited by: Veli-Matti
Kerminen
Reviewed by: three anonymous referees
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