Size dependence of volume and surface nucleation rates for homogeneous freezing of supercooled water droplets

Abstract

Abstract. The relative roles of volume and surface nucleation were investigated for the homogeneous freezing of pure water droplets. Experiments were carried out in a cryogenic laminar aerosol flow tube using supercooled water aerosols with maximum volume densities at radii between 1 and 3 µm. Temperature-and size-dependent values of volumeand surface-based homogeneous nucleation rates between 234.8 and 236.2 K were derived using a microphysical model and aerosol phase compositions and size distributions determined from infrared extinction measurements in the flow tube. The results show that the contribution from nucleation at the droplet surface increases with decreasing droplet radius and dominates over nucleation in the bulk droplet volume for droplets with radii smaller than approximately 5 µm. This is interpreted in terms of a lowered free energy of ice germ formation in the surface-based process. The implications of surface nucleation for the parameterization of homogeneous ice nucleation in numerical models are considered.

Rate of Homogenous Nucleation of Ice in Supercooled Water

“…6). For comparison the parameterization of Kuhn et al 15 and limiting data from There are considerable differences between the estimates of deq(T) from the four experiments in the present study, which results from the differences in fitted volume and surface nucleation rates coefficients. This highlights the uncertainty in quantifying the rate of surface nucleation.…”

“…The volume and surface nucleation rate coefficients derived from each run are compared with one another in Figure 7 (a and b) and summarized in Table 2. In panel c of Figure 7 the equivalent diameter deq(T) is presented, which is the size of a spherical droplet at which the predicted number of freezing events due to surface and volume nucleation are equal, defined as: 11,14,15 ( ) ( ) = ( ) ( ) Table 2). Panel (c) shows deq(T), the diameter at which surface and volume freezing are equivalent (eq.…”

“…Therefore, the overall freezing rate R(T) (units of s -1 ) should be described as the sum of the freezing rates due to each component in the system: 11,14,15,19,23 ( ) = ( ) ( ) + ( ) ( ) =…”

“…[11][12][13][14][15][16][17] Surface nucleation may become more important in smaller droplets, in which the surface to volume ratio is larger.…”

“…Analysis of droplet freezing data for a range of droplets sizes has suggested that for supercooled water droplets with water-air interfaces the size where the probability of surface and volume nucleation are equal is less than 10 µm diameter at around 235 -237 K. 14,15 The rate of surface nucleation is likely to depend on the nature of the interface, hence it may depend on the presence of air, surfactants, oils, solid surfaces or charge. Regardless of the nature of the interface, it is important to quantify the relative contribution of surface and volume nucleation in any experiment where the objective is to study homogeneous nucleation of small droplets.…”