Laboratory studies of the efficiency of selected organic aerosols as CCN

Experiments were conducted in the large (600 m3) expansion chamber of the Calspan Corporation to test the efficiency of selected organic aerosols as cloud condensation nuclei (CCN). Solutions of pinonic acid (PA), ammonium sulfate (AS) (as a benchmark), and a mixture of both were nebulized and used for initial CCN spectra, as were products of cyclohexene and α-pinene oxidation by ozone. Measurements of the resulting aerosols were made with both particle-sizing instruments and a cloud condensation nucleus (CCN) counter. Activated cloud drop number concentrations (CDNC) were determined during chamber expansion by standard laser-scattering probes. The results support the contention that the presence of sparingly soluble organic components in aerosols significantly affect the rate at which these aerosols activate to form cloud drops. However, even the organic aerosols eventually fully activated to the extent expected due to size assuming complete solubility. It is only the time necessary to activate them which is altered and the atmospheric significance of this kinetic effect of organics is not yet clear.