Activation and growth of cloud condensation nuclei are investigated. A one‐dimensional cloud model including detailed microphysics is developed. The system studied consists of NaNO3 particles and condensing water and HNO3 vapors. According to numerical simulations, enhanced concentrations of atmospheric nitric acid vapor affect cloud formation by increasing the number of cloud droplets and decreasing their mean size compared to circumstances where no condensable vapors other than water are present. It is argued that the cloud albedo increases because of these effects.

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Density functional theory is used to calculate the surface tension of planar and slightly curved surfaces, which can be written as gamma(R)=gamma(infinity)(1-2delta(infinity)R), where R is the radius of curvature of the surface. Calculations are performed for a Lennard-Jones fluid, split into a hard-sphere repulsive potential and an attractive part. The repulsive part is treated using the local density approximation. The attractive part is treated using a high temperature approximation (HTA) in which the pair correlation function is approximated by the Percus-Yevick pair correlation function of a uniform hard-sphere fluid evaluated at a position-dependent average density. An expression relating the Tolman length delta(infinity) to the density profile of the planar surface is derived. Numerical results are presented for the planar surface tension gamma(infinity) and for delta(infinity) and are compared with those using mean field theory (MFT) and with those using the square-gradient approximation. Values for gamma(infinity) using the HTA are 30%-40% higher than those using MFT. Values for delta(infinity) using the HTA are around -0.1 (in units of the Lennard-Jones parameter sigma) and only weakly dependent on temperature. These values are less negative than the values from MFT. The square-gradient approximation gives reasonable estimates of the more accurate nonlocal results for both the MFT and the HTA.

It is shown that the classical expression for the change in grand potential of a system on formation of a cluster of radius R is modified by a factor [1−(2w+6δT)/R], to first order in 1/R, where w is a correction due to the nonzero compressibilities of liquid and vapor (near the triple point, w is approximately equal to the product of liquid compressibility and surface tension), and δT is the coefficient in the expression relating the surface tension of the droplet, γ(R), to the planar surface tension, γ∞, i.e., γ(R)=γ∞(1−2δT/R). An expression for δT is derived involving the pair and triplet correlation functions and the density profile of the planar surface. This complements the expression for δT involving the pair distribution function derived by Blokhuis and Bedeaux; the equivalence of the two expressions in the low density limit is demonstrated. Calculations of δT and w are performed using mean-field density functional theory for the Yukawa potential and an r−6 potential, as well as using the square-gradient approximation. δT is found to be negative for all conditions investigated; its magnitude depends on the potential used, and tends to increase with increasing temperature. However, the ratio δT/w is found to be relatively insensitive to potential and to temperature, being between about −1.2 and −1.5 for the conditions investigated. The effect of using a weighted density approximation in place of the local density approximation for the hard-sphere part of the potential is estimated in a square-gradient approximation and found to be small.

The results of stochastic simulations of growth and evaporation of small clusters in vapor are reported. Energy dependent growth rates are determined from the monomer-cluster collision rate and decay rates are found from a detailed balance, with the equilibrium size and energy distribution of clusters calculated using the capillarity approximation and the equilibrium vapor pressure. These rates are used in simulations of two-dimensional random walks in size and energy space to determine the fraction of clusters in supersaturated vapor of size (i(min)+1) that reach a size i(max). By assuming that clusters of size i(min) are in equilibrium, this fraction can be related to the nonisothermal nucleation rate. The simulated rates show good agreement with the previously published analytical results. In the absence of an inert carrier gas, the nonisothermal nucleation rates are typically between 1% and 5% of the isothermal rates.