The first part of this seminar addresses characterizations of the maximum steady state supersaturation in the University of Wyoming thermal diffusion cloud condensation nuclei (CCN) instrument.  This work is based on studies performed using nearly-monodisperse test aerosol composed of sodium chloride and ammonium sulfate.  The fraction of test particles activating to form cloud droplets was evaluated over a range of dry particle diameters.  Values of the particle size corresponding to an activation fraction equal to 0.5 (50% CCN activation) were derived and used to evaluate the maximum steady state chamber supersaturation.  Further, uncertainties associated with the instrument’s plate temperature and light scattering measurement systems, and their propagation into the derived supersaturation, are discussed. These studies reveal three significant findings.  First, the maximum steady state chamber supersaturation is ~40% smaller than values based on temperature measured at the top and bottom of the diffusion chamber.  Second, using deliquesced test particles, mobility selected at a prescribed relative humidity, we show that the 40% bias is not the result of test particle asphericity.  Third, the size width of the activation fraction function is substantially larger than the width derived using measured test aerosol size distributions.  Such instrument characterizations are essential for the studies discussed in the second part of this two-part seminar.

http://www-das.uwyo.edu/~jsnider/snider_2005.pdf

The second part of this seminar addresses characterizations of surrogates for high-molecular-weight humic-like substances, making up a substantial fraction of the carbonaceous aerosol mass.  Two topics are addressed.  First, measurements of the hygroscopic growth of laboratory-generated aerosol, composed of commercially available polymers, are presented.    Their hygroscopicity is larger than expected for their high molecular weights and is not a strong function of the degree of polymerization. Flory-Huggins theory, which resolves the entropy of mixing for polymers, is used to model the water activity for these solutions, explaining both perceived discrepancies with Köhler theory and the insensitivity of hygroscopic growth factors to compound molecular weight. Second, mechanisms of deliquescence- and miscibility-controlled activation are discussed. Deliquescence-controlled activation has been observed for a number of pure compounds, and also for laboratory generated secondary organic aerosol.  Flory-Huggins theory predicts miscibility-limits, and these may lead—analogous to deliquescence-controlled activation—to non-classical cloud droplet formation. Not accounting for these phenomena may result in discrepancies between predicted and measured particle activation characteristics, even when particle composition and size are well-constrained.

http://www-das.uwyo.edu/~jsnider/petters_tellus_2005.pdf