Predicting particle critical supersaturation from hygroscopic growth measurements in the humidified TDMA. Part II: Laboratory and ambient studies

Laboratory studies are used to test the method proposed in Part I for estim ating the critical supersaturation of quasi-monodisperse, dry particles fro m measurements of hygroscopic growth at relative humidities blow 100%. An a dvantage of the proposed technique is that it directly links dry particle s ize to cloud condensation nuclei (CCN) activity and simultaneously provides some information on particle chemical composition. Studies have been condu cted on particles composed of NaCl, (NH4)(2)SO4, NH4HSO4, internally and ex ternally mixed NaCl-(NH4)(2)SO4, and on ambient particles of unknown chemic al composition. A modified form of the Kohler equation is fit to measuremen ts from a humidified tandem differential mobility analyzer to derive two ch emical composition-dependent parameters and the critical supersaturation fo r a given dry particle size. A cloud condensation nucleus counter is used t o simultaneously observe the critical supersaturation of the same dry parti cles. Results show that for particles composed of single salts and for diameters between 32 and 57 nm, the average agreement between critical supersaturatio ns derived from measurements of hygroscopic growth and theoretical values o f S-crit is -13% (1 sigma = 8.5%, n = 9). This agreement is similar to expe rimental uncertainties in critical supersaturations determined from laborat ory studies on particles of known chemical composition. The agreement betwe en values of S-crit predicted by the fit technique and CCN study-derived va lues is poorer (-6% to -65%) for ambient particles. This is likely due to b oth changes in ambient particle characteristics during the study and limita tions in the modified Kohler model derived in this work.