AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
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Growth and Evaporation Kinetics of Secondary Organic Aerosol as a Function of Relative Humidity
RAHUL ZAVERI, John Shilling, Alla Zelenyuk, Maria Zawadowicz, Kaitlyn J. Suski, Swarup China, Daniel Veghte, Alexander Laskin, Pacific Northwest National Laboratory
Abstract Number: 720 Working Group: Aerosol Chemistry
Abstract Low bulk diffusivity inside viscous semisolid atmospheric secondary organic aerosol (SOA) can prolong equilibration timescale as well as influence aerosol growth kinetics and the associated size distribution evolution. We present here quantitative insights into the effects of bulk diffusivity on the growth and evaporation kinetics of isoprene SOA as a function of relative humidity (RH). A series of chamber experiments of SOA formation from photooxidation of isoprene were carried out in the presence of bimodal aerosol seeds consisting of Aitken mode composed of potassium sulfate and accumulation mode composed of aged a-pinene SOA. Evaporation kinetics indicate that isoprene SOA is composed of several semivolatile organic compounds (SVOCs). Model analysis shows that while liquid-like bulk diffusivities can be used to fit the observed evaporation kinetics, they fail to explain the growth kinetics of bimodal aerosol by significantly under-predicting the evolution of the Aitken mode. In contrast, lower bulk diffusivity values corresponding to viscous semisolids can successfully reproduce both evaporation and growth kinetics, with the interpretation that hindered partitioning of SVOCs into large viscous particles effectively promotes the growth of smaller particles that have shorter diffusion timescales. The estimated effective bulk diffusivity was found to experience only a 4-fold increase as the RH increased from 0 to 75%, suggesting that aged a-pinene SOA was quite viscous even at 75% RH—contrary to published literature. These results have important implications for modeling SOA formation and growth of ultrafine particles under ambient conditions.