AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
The Influence of Molecular Structure and Photochemical Aging on Organic Films Coating Microscopic Aqueous Droplets
CHRIS RUEHL, Kevin Wilson, Lawrence Berkeley National Laboratory
Abstract Number: 466 Working Group: Aerosols, Clouds, and Climate
Abstract Although the surface activity of many atmospheric samples of particulate and dissolved organic matter has been well documented, the importance of surface activity in the prediction of cloud condensation nuclei (CCN) activity is still debated. We measured the CCN activity and high-RH (>99%) growth factors of both single-component organic particles and secondary organic aerosol (SOA) generated from either alpha-pinene or squalane. The partitioning of organic matter (OM) to the droplet surface is modeled in one of two ways: either as a “soluble” surfactant, in which OM exists in equilibrium between the surface and bulk phases as described by the Szyszkowski equation, or as an “insoluble” surfactant which exists almost completely in a film whose surface pressure depends on molecular area. In the latter case, droplet surface tension is insensitive to bulk concentration and therefore depends primarily on droplet surface area, while in the former case it depends on bulk surfactant concentration (i.e., droplet volume). The CCN activity of photo-oxidized squalane particles was best predicted when reduced surface tension was taken into account, equally well assuming the OM was soluble or insoluble surfactant; however, the scale bulk concentration in the Szyszkowski equation had to be reduced by several orders of magnitude to account for depletion in the bulk phase of microscopic droplets. The high-RH growth factors of individual compounds and alpha-pinene secondary organic aerosol, on the other hand, could only be modeled well as insoluble surfactants. For individual compounds, the film had properties similar to macroscopic observations of the same compounds, although no evidence for monolayer collapse was seen. For particles generated by ozonolysis of alpha-pinene, the modeled film thickness was 0.8 +/- 0.1 nm. When these particles were further photo-oxidized, modeled film thickness (i.e., surface concentration) decreased to ~0.5 nm.