When organic gases undergo oxidation in the atmosphere, semi and low volatility products that can partition into either the solid or liquid aerosol phase are created. These products are referred to as secondary organic aerosol (SOA). These resulting particles can then uptake water from their surroundings and form cloud droplets. When aerosols act as the seeds on to which water condenses, they are referred to as cloud condensation nuclei (CCN). The hygroscopicity, or ability of a particle to activate is traditionally quantified by a single parameter, kappa. Here we examined the hygroscopicity of SOA generated by the photooxidation of two biogenic compounds, isoprene and longifolene, using the University of California - Riverside, College of Engineering - Center for Environmental Research and Technology (UCR CE-CERT) Environmental Chamber. A TSI SMPS ran in parallel with a Droplet Measurement Technologies (DMT) Cloud Condensation Nuclei Counter (CCNC) was used to observe the hygroscopicity of resulting secondary organic aerosol (SOA). Traditional CCN analysis is performed by dividing the number of particles activated (CCN) by the total number of particles (condensation nuclei, CN). The point at which the CCN to CN ratio equals 0.5 is used to identify the critical supersaturation at which 50% of the particles activate for a given diameter. Alternatively, the critical diameter at which 50% of particles for a given supersaturation can be identified. This critical supersaturation and the corresponding dry diameter are then used to calculate kappa. However, this calculation assumes that the hygroscopicity of particles stays constant for a given precursor gas. The CCN to CN ratio at given diameters and supersaturations was calculated and plotted against photooxidation time to observe if this assumption holds true, and a novel method of observing CCN activity is proposed.

Critical activation time of cloud condensation nuclei for biogenic precursors

251st National Meeting (2016)

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