Cloud Condensation Nuclei properties of model and atmospheric HULIS

Humic like substances (HULIS) have been identified as a major fraction of the organic component of atmospheric aerosols. These large multifunctional compounds of both primary and secondary sources are surface active and water soluble. Hence, it is expected that they could affect activation of organic aerosols into cloud droplets. We have compared the activation of aerosols containing atmospheric HULIS extracted from fresh, aged and pollution particles to activation of size fractionated fulvic acid from an aquatic source (Suwannee River Fulvic Acid), and correlated it to the estimated molecular weight and measured surface tension. A correlation was found between CCN-activation diameter of SRFA fractions and number average molecular weight of the fraction. The lower molecular weight fractions activated at lower critical diameters, which is explained by the greater number of solute species in the droplet with decreasing molecular weight. The three aerosol-extracted HULIS samples activated at lower diameters than any of the sizefractionated or bulk SRFA. The Köhler model was found to account for activation diameters, provided that accurate physico-chemical parameters are known.