10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Direct Observations of the Formation of Isoprene-derived Secondary Organic Aerosol in Ambient Cloud Droplets
ALLA ZELENYUK, David Bell, ManishKumar Shrivastava, Jerome Fast, Joel A. Thornton, Dan Imre, Kaitlyn J. Suski, Larry Berg, John Shilling, Jiumeng Liu, Fan Mei, Jason Tomlinson, Jian Wang, Pacific Northwest National Laboratory
Abstract Number: 1002 Working Group: Aerosol Chemistry
Abstract Multiphase chemistry of isoprene photooxidation products has been shown to be a major source of secondary organic aerosol (SOA) in the atmosphere, yet many models apply simplified treatments of organic aqueous chemistry. A number of recent studies indicate that aqueous aerosol phase provides a medium for reactive uptake of isoprene photooxidation products, and in particular, isomeric isoprene epoxydiols (IEPOX). Studies have shown that the reaction rates and yields of IEPOX-SOA formation depend on aerosol acidity, water content, sulfate concentration, and the presence of other organics. However, very few studies focused on chemistry occurring within actual cloud droplets.
We will present data acquired during recent ARM Holistic Interactions of Shallow Clouds, Aerosols, and Land Ecosystems (HI-SCALE) Campaign, which provide direct evidence for IEPOX-SOA formation in cloud droplets. Single particle mass spectrometer, miniSPLAT, and a high-resolution, time-of-flight aerosol mass spectrometer were used to characterize the composition of aerosol particles and cloud droplet residuals, while a high-resolution, time-of-flight chemical ionization mass spectrometer (HR-ToF-CIMS) was used to characterize gas-phase compounds, including isoprene and IEPOX.
We find that the size and composition of cloud droplet residuals were markedly different than those of aerosol particles sampled outside the cloud. Cloud droplet residuals were larger than particles outside the cloud and comprised of individual particles with high relative fractions of sulfate and nitrate and significant fraction of particles with mass spectra that are nearly identical to those of laboratory-generated IEPOX-SOA particles. Moreover, we show that the observed cloud-induced formation of IEPOX-SOA particles was accompanied by simultaneous decrease in measured concentrations of IEPOX and other gas-phase isoprene photooxidation products.
Finally, we use these HI-SCALE field observations of cloud residuals, interstitial aerosol particles, and gas-phase species to develop and evaluate model treatment of aqueous-phase isoprene SOA formation. We show that the addition of aqueous cloud chemistry to WRF-Chem qualitatively reproduces the observed concentrations of biogenic volatile organic compounds and the impact of aqueous-phase chemistry on cloud-borne organic mass.