THE EFFECT OF ORGANIC COATINGS ON THE CLOUD CONDENSATION NUCLEI ACTIVATION OF INORGANIC ATMOSPHERIC AEROSOL

Atmospheric aerosols have mixed chemical composition, with a variety o f inorganic (e.g., sulfate, nitrate, ammonium, and sodium) and organic species often present in a single particle. In the present study, we investigate experimentally the cloud condensation nuclei (CCN) activat ion of submicron aerosol consisting of an inorganic core (e.g., ammoni um sulfate) coated by an organic film, at typical atmospheric supersat urations. We use two types of organic coatings on the (NH4)(2)SO4 part icles. The first is glutaric acid, a CCN active organic found in the a tmosphere, and the second species is dioctylphthalate (DOP), a nonhygr oscopic organic. The CCN activation of (NH4)(2)SO4-glutaric acid parti cles was measured at a supersaturation of 0.3%, for different inorgani c core sizes and organic film thickness. We found that a coating of gl utaric acid increases the CCN activation of an (NH4)(2)SO4 particle an d that this behavior can be predicted by Kohler theory. The deviation from Kohler theory for the mixed aerosol was determined by comparing t heoretical and experimental CCN activation diameters for the particles and was found to be within experimental error. A thick coating of DOP (at least 70% by mass) did not hinder the activation of (NH4)(2)SO4 p articles at supersaturations of 0.5 and 1.0%. The values for the measu red activation diameters for the DOP coated (NH4)(2)SO4 particles were within the experimental error determined by the pure inorganic experi ments, indicating that DOP was most likely acting as inert mass during activation.