10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Dependence of Heterogeneous Nucleation of N-Butanol Vapor on Temperature and Humidity for improved detection of Nanoparticles
CHRISTIAN TAUBER, Gerhard Steiner, Paul M. Winkler, University of Vienna
Abstract Number: 327 Working Group: Aerosol Physics
Abstract Commercial Condensation Particle Counters (CPCs) are mainly used to measure the number concentration of airborne particles (McMurry, P. H. (2000). Atmospheric Environment, 34(12-14):1959-1999). Most ultrafine continuous flow CPCs use n-butanol as working fluid and typically have lower particle detection limits in the range between 2.5 and 10 nm (Stolzenburg, M. R., & McMurry, P. H. (1991). Aerosol Science and Technology, 14(1):48-65). Given the fact that n-butanol CPCs are operated at fixed temperatures, the performance towards the detection of even smaller particles may be optimized by choosing appropriate temperature settings.
Until now the temperature dependence of heterogeneous nucleation of n-butanol vapor on nanoparticles in a size-range down to 2.5 nm was not investigated. A topic, however, that is worth examining since it gives information on the saturation ratio which is needed to activate particles in this size range. Results of earlier experiments with water vapor nucleating on silver particles have shown a theoretically unpredicted maximum in the onset saturation ratio as a function of temperature (Kupc et al. (2013). Aerosol Science and Technology, 47(9):i-iv). Further studies on the nucleation of n-propanol on sodium chloride particles in the temperature range from 262K to 287K indicate a reverse trend of the onset saturation ratio compared to the Kelvin equation (Schobesberger et al. (2010). ChemPhysChem, 11:3874–3882). Due to the close chemical similarity of n-propanol and n-butanol, and the common use of n-butanol as a working fluid in commercial CPCs, the temperature dependence of heterogeneous nucleation of n-butanol on Ag and NaCl particles is investigated in this study.
We have measured the onset saturation ratio of n-butanol depending on nucleation temperature with the size analyzing nuclei counter SANC (Wagner et al. (2003). Phys. Rev. E67:021605-1). NaCl and Ag seeds generated by a Scheibel-Porstendörfer (1983) aerosol generator in the size range from 2.5 to 9.0 nm were used as condensation nuclei. We found an inverse temperature dependence for the heterogeneous nucleation of n-butanol on sodium chloride particles similar to the n-propanol data from Schobesberger. To rule out humidity effects the air supply of the tube furnace was exchanged, dried compressed air was replaced by synthetic air with < 3ppm-mol humidity. Thereby the found inverse temperature trend disappeared. This indicates that the driving force for the inverse temperature dependence is the increased relative humidity at lower temperature.
Additional to the SANC measurements, we determined the cut-off diameter at variable condenser temperature and air humidity, with constant ΔT between saturator and condenser, for a commercial n-butanol CPC. Thereby, lower D50 cut-off diameter for NaCl and Ag particles at reduced temperatures could be observed. Accordingly, by lowering the condenser temperature and hence nucleation temperature of a butanol CPC, the counting efficiency can be increased significantly for both, NaCl and Ag seeds – with a stronger response for NaCl particles. Furthermore, the effect on detection efficiency of different relative humidity was investigated; results of earlier studies have indicated different humidity dependence for sodium chloride and silver nanoparticles (Gilmore, J. S. (2002). Atmospheric Research, 62(3–4):267-294).
By merging the experimental results of the SANC and commercial condensation particle counters combined with theoretical predictions we are able to examine the heterogeneous nucleation of n-butanol on different seeds in greater detail. The findings are of immediate relevance for nanoparticle detection in CPCs and raise questions on the fundamental mechanisms leading to this behavior.