Cloud droplet activation of secondary organic aerosol

Measurements of hygroscopicity and cloud condensation nuclei (CCN) activity were conducted on secondary organic aerosol (SOA) formed in a smog chamber. SOA precursors included α‐pinene, β‐pinene, Δ3‐carene, and toluene, representative of both naturally and anthropogenically emitted organic species. Measured CCN activation was comparable for all of the species studied and occurred at humidity conditions which are readily attained in the atmosphere. Further, there was little variation in hygroscopic growth between compounds. However, measured droplet activation conditions were inconsistent with hygroscopicity measured below water saturation and Köhler theory expressions based on Raoult’s law for several parameterizations for water activity. In the atmosphere, SOA may compose a large fraction of atmospheric particulate matter and will often exist internally mixed with inorganic species. Using the current results, we compare SOA to insoluble organic species to calculate CCN activation from mixed organic‐sulfate particles for a range of atmospheric conditions. We find that droplet activation behavior of mixed particles containing SOA is the same as that of mixed particles for which the organic component is nonhygroscopic, except for cases in which there are low particle concentrations, low updraft velocities, and the aerosol composition is dominated by organics.