Predicting Particle Critical Supersaturation from Hygroscopic Growth Measurements in the Humidified TDMA. Part II: Laboratory and Ambient Studies

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Abstract
Laboratory studies are used to test the method proposed in Part I for estimating the critical supersaturation of quasi-monodisperse, dry particles from measurements of hygroscopic growth at relative humidities below 100%. An advantage of the proposed technique is that it directly links dry particle size to cloud condensation nuclei (CCN) activity and simultaneously provides some information on particle chemical composition. Studies have been conducted on particles composed of NaCl, (NH4)2SO4, NH4HSO4, internally and externally mixed NaCl–(NH4)2SO4, and on ambient particles of unknown chemical composition. A modified form of the Köhler equation is fit to measurements from a humidified tandem differential mobility analyzer to derive two chemical composition–dependent parameters and the critical supersaturation for a given dry particle size. A cloud condensation nucleus counter is used to simultaneously observe the critical supersaturation of the same dry particles. Results show that for particles composed of single salts and for diameters between 32 and 57 nm, the average agreement between critical supersaturations derived from measurements of hygroscopic growth and theoretical values of Scrit is −13% (1 σ = 8.5%, n = 9). This agreement is similar to experimental uncertainties in critical supersaturations determined from laboratory studies on particles of known chemical composition. The agreement between values of Scrit predicted by the fit technique and CCN study-derived values is poorer (−6% to −65%) for ambient particles. This is likely due to both changes in ambient particle characteristics during the study and limitations in the modified Köhler model derived in this work.

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