Observationally Constrained Modeling of the Reactive Uptake of IEPOX under Elevated RH and Varying Acidity of Seed Aerosol Conditions

JIE ZHANG, ManishKumar Shrivastava, Alla Zelenyuk, Rahul Zaveri, Jason Surratt, Matthieu Riva, David Bell, Marianne Glasius, Pacific Northwest National Laboratory

     Abstract Number: 72
     Working Group: Aerosol-Ecosystem Interactions

Abstract
Isoprene is the non-methane volatile organic compound (VOC) emitted in the largest amounts to the atmosphere, and it is a significant source of secondary organic aerosol (SOA) mass. The uptake of isoprene oxidation products followed by multiphase chemistry in fine particles is the key pathway to form isoprene epoxydiol-derived SOA (IEPOX-SOA). However, many parameters that relate to the diffusion and reaction of IEPOX in the particle phase remain uncertain, since reaction kinetics previously measured in bulk aqueous phase solutions might be different from atmospheric aerosols. Here, we use simultaneous environmental chamber measurements of multiple parameters governing IEPOX-SOA formation at timescales of ~hours: particle size distribution, composition, and volatility of IEPOX-SOA to constrain the key parameters governing IEPOX-SOA formation under humid (i.e., 50% relative humidity, RH) and varying seed aerosol acidity conditions. Compared to their application in previous models, reducing the 2-methyltetrol (tetrol) reaction rate constants by a factor of 4 brings the model predictions in agreement with the IEPOX-SOA measurements with acidified ammonium bisulfate seed aerosols. For less acidic ammonium sulfate aerosols, we find that both the organosulfate (OS) and tetrol reaction rate constants need to be reduced to bring model predictions closer to chamber observations. Using the measured non-volatile content of IEPOX-SOA we constrain the oligomerization timescale of tetrols. We find that the oligomerization timescale is 4 hours with acidified seed aerosols, but a much longer timescale of 24 hours is needed for non-acidified aerosols, indicating that the aerosol acidity greatly affects the oligomerization rate of tetrols. Our results imply that IEPOX reactive uptake kinetics used in regional and global models, which are based on bulk aqueous solution measurements have likely overpredicted IEPOX-SOA formation from aqueous chemistry in the atmosphere.