Commentary on "Homogeneous nucleation of NAD and NAT in liquid stratospheric aerosols: insufficient to explain denitrification" by Knopf et al.

Abstract. In a recent published paper Knopf et a1. (2002) have suggested that the homogeneous freezing behavior of stratospheric aerosols, under polar winter conditions, can be simulated experimentally in large bulk phase-sized droplet samples (0.12-0.27 cm in diameter). Their hypothesis is based on the fact that a nucleus, which freezes the supercooled phase, forms within the bulk volume of a given sample, and therefore, if large bulk volumes don't freeze in the laboratory, then small volumes in particles most certainly remain unfrozen in the stratosphere. The important question to ask here is whether their initial hypothesis, which they have used to analyze their data, is even correct to begin with. For example, does a nucleus, which turns over the phase, forms within the bulk volume or on the surface of the supercooled phase? Some recent studies provide both experimental (Tabazadeh et al., 2002a, b) and theoretical (Djikaev et al., 2002, 2003) support for the formation of the nucleus at the surface of a supercooled droplet. If the homogeneous nucleation process initiates at the droplet surface, then the approach taken by Knopf. et al. to study this crystallization process may not be directly applicable to the stratospheric situation.