10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Insights into Isoprene-Epoxydiol SOA Chemistry in Cloud Droplets using WRF-Chem and HI-SCALE Field Observations
MANISHKUMAR SHRIVASTAVA, Alla Zelenyuk, David Bell, 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: 895 Working Group: Aerosol Modeling
Abstract Multiphase chemistry of isoprene epoxydiols (IEPOX) is an important pathway resulting in the formation of IEPOX-SOA. A number of studies report substantial IEPOX-SOA formation on wet sulfate aerosols, at many locations globally. However, IEPOX-SOA formation in clouds has not been reported previously. Measurements during recent HI-SCALE field Campaign provide direct evidence for IEPOX-SOA formation in cloud droplets.
To simulate this process, we implemented IEPOX-SOA chemistry module in WRF-Chem and applied this chemistry to both wet interstitial aerosols and cloud droplets based on the simpleGamma multiphase kinetic model [Woo and McNeill, 2015]. WRF-Chem simulations are conducted at high spatial resolution (1.3 km grids). We analyze model results over the southeast part of our modeling domain that abounds in biogenic isoprene emissions. Simulations show that at cloud level, average concentration of gas-phase IEPOX that dissolves in cloud droplets (40 ng m-3) is an order of magnitude higher compared to dissolved IEPOX in wet aerosols (3 ng m-3) due to significantly larger cloud droplet water content as compared to interstitial aerosols. Further reaction of dissolved IEPOX results in the formation of IEPOX-SOA. However, since reaction kinetics are significantly slower in more dilute cloud droplets as compared to interstitial aerosols, previous studies often neglect the role of cloud chemistry in IEPOX-SOA formation. In contrast, our simulations indicate that formation of IEPOX-SOA in clouds is 3 times greater than IEPOX-SOA in interstitial aerosols. Simulations indicate that conversion of dissolved IEPOX to IEPOX-SOA is indeed slower in cloud droplets compared to interstitial aerosols (10% in cloud droplets, 50% in interstitial aerosols). However, this slow conversion is compensated by large amounts of dissolved IEPOX available in cloud droplets compared to interstitial aerosols. Model results are in good agreement with measurements by the single particle mass spectrometer, miniSPLAT, which indicates preferential formation of IEPOX-SOA in cloud droplets. Simulations also indicate that when cloud droplets evaporate, cloud-phase IEPOX-SOA becomes part of interstitial aerosols, increasing interstitial IEPOX-SOA by a factor of 2. Future work will investigate factors affecting observed regional variations in IEPOX-SOA such as variations in aerosol and cloud properties, biogenic emissions, and their aging in the atmosphere.