Heterogeneous Nucleation Experiments of n-Butanol Vapor at Varying Nucleation Temperature

Recent experiments on the temperature dependence of heterogeneous nucleation have yielded quite unexpected results. While the nucleation of water vapor on silver (Ag) particles has shown a theoretically unpredicted maximum in the onset saturation ratio as a function of temperature (Kupc et al. (2013) Aerosol Sci. Technol. 47, i), the nucleation of n-propanol on sodium chloride (NaCl) particles in the temperature range between 262K and 287K has even revealed a reversed trend compared to the Kelvin equation (Schobesberger et al. (2010) Chem. Phys. Chem. 11, 3874). The close chemical similarity of propanol and butanol suggests that such behavior depending on the nucleation temperature may also play a role in the detection efficiency of condensation particle counters (CPC) using butanol as working fluid. In fact, Ankilov et al., Atmos. Res. 62, 209 (2002) and Hermann et al., J Aerosol Sci. 38, 674 (2007) reported significantly lower counting efficiencies for NaCl compared to Ag particles using butanol CPCs.

In this study we report measurements of the heterogeneous nucleation of butanol at variable nucleation temperature. To this end, monodisperse NaCl and Ag particles were passed into an expansion type CPC to measure heterogeneous nucleation at well-defined supersaturations. In parallel we operated a commercial TSI 3776 butanol CPC whose counting efficiency was analyzed with respect to changing condenser and saturator temperature. It is notable that by reducing the nucleation temperature only by 8 K we find counting efficiencies for NaCl particles exceeding those of Ag particles. The heterogeneous nucleation of butanol on NaCl seed particles thus indicates a strong dependence on nucleation temperature. This finding is of immediate relevance for nanoparticle detection in CPCs and raises questions on the fundamental mechanisms leading to this behavior.