2014, Wex, H., Demott, P.J., Tobo, Y., Hartmann, S., Rösch, M., Clauss, T., Tomsche, L., Niedermeier, D., Stratmann, F.

Kaolinite particles from two different sources (Fluka and Clay Minerals Society (CMS)) were examined with respect to their ability to act as ice nuclei (IN). This was done in the water-subsaturated regime where often deposition ice nucleation is assumed to occur, and for water-supersaturated conditions, i.e., in the immersion freezing mode. Measurements were done using a flow tube (the Leipzig Aerosol Cloud Interaction Simulator, LACIS) and a continuous-flow diffusion chamber (CFDC). Pure and coated particles were used, with coating thicknesses of a few nanometers or less, where the coating consisted of levoglucosan, succinic acid or sulfuric acid. In general, it was found that the coatings strongly reduced deposition ice nucleation. Remaining ice formation in the water-subsaturated regime could be attributed to immersion freezing, with particles immersed in concentrated solutions formed by the coatings. In the immersion freezing mode, ice nucleation rate coefficients het from both instruments agreed well with each other, particularly when the residence times in the instruments were accounted for. Fluka kaolinite particles coated with either levoglucosan or succinic acid showed the same IN activity as pure Fluka kaolinite particles; i.e., it can be assumed that these two types of coating did not alter the ice-active surface chemically, and that the coatings were diluted enough in the droplets that were formed prior to the ice nucleation, so that freezing point depression was negligible. However, Fluka kaolinite particles, which were either coated with pure sulfuric acid or were first coated with the acid and then exposed to additional water vapor, both showed a reduced ability to nucleate ice compared to the pure particles. For the CMS kaolinite particles, the ability to nucleate ice in the immersion freezing mode was similar for all examined particles, i.e., for the pure ones and the ones with the different types of coating. Moreover, het derived for the CMS kaolinite particles was comparable to het derived for Fluka kaolinite particles coated with sulfuric acid. This is suggestive for the Fluka kaolinite possessing a type of ice-nucleating surface feature which is not present on the CMS kaolinite, and which can be destroyed by reaction with sulfuric acid. This might be potassium feldspar.

2010, Poulain, L., Wu, Z., Petters, M.D., Wex, H., Hallbauer, E., Wehner, B., Massling, A., Kreidenweis, S.M., Stratmann, F.

The influence of varying levels of water mixing ratio,r during the formation of secondary organic aerosol (SOA) from the ozonolysis of α-pinene on the SOA hygroscopicity and volatility was investigated. The reaction proceeded and aerosols were generated in a mixing chamber and the hygroscopic characteristics of the SOA were determined with the Leipzig Aerosol Cloud Interaction Simulator (LACIS) and a Cloud Condensation Nuclei counter (CCNc). In parallel, a High-Resolution Time-of-Flight Aerodyne Aerosol Mass Spectrometer (HR-ToF-AMS) located downstream of a thermodenuder (TD) sampling from the mixing chamber, to collect mass spectra of particles from the volatile and less-volatile fractions of the SOA. Results showed that both hygroscopic growth and the volatile fraction of the SOA increased with increases in r inside the mixing chamber during SOA generation. An effective density of 1.40 g cm-3 was observed for the generated SOA when the reaction proceeded with <1 g kg-1. Changes in the concentrations of the fragment CO2+ and the sum of CxH+y(short name CHO) and CxH+y (short name CH) fragments as measured by the HR-ToF-AMS were used to estimate changes in the oxidation level of the SOA with reaction conditions, using the ratios CO2 + to CH and CHO to CH. Under humid conditions, both ratios increased, corresponding to the presence of more oxygenated functional groups (i.e., multifunctional carboxylic acids). This result is consistent with the α-pinene ozonolysis mechanisms which suggest that water interacts with the stabilized Criegee intermediate. The volatility and the hygroscopicity results show that SOA generation via ozonolysis of α-pinene in the presence of water vapour (r <16.9 g kg-1) leads to the formation of more highly oxygenated compounds that are more hygroscopic and more volatile than compounds formed under dry conditions. © 2010 Author(s).