Using a Particle-Resolved Model to Design Cloud Chamber Experiments for IEPOX-SOA Formation

MANISHKUMAR SHRIVASTAVA, Jie Zhang, Steven Krueger, John Shilling, Alla Zelenyuk, Yuzhi Chen, Richard Flagan, Mikhail Ovchinnikov, Raymond Shaw, Will Cantrell, Tania Gautam, Gourihar Kulkarni, Mikhail Pekour, Pacific Northwest National Laboratory

     Abstract Number: 407
     Working Group: Aerosol Chemistry

Abstract
Isoprene is the largest non-methane organic gas emitted by vegetation globally. Over the past decade, extensive laboratory and modeling studies have provided important insights into the formation of isoprene epoxydiol secondary organic aerosol (IEPOX-SOA). However, most laboratory measurements have been limited to flow tubes or environmental chambers focused on aerosol chemistry, while direct measurements of in-cloud chemical formation of IEPOX-SOA remain rare and have not yet been conducted in laboratory cloud chambers.

In this study, we use our detailed particle-resolved 1D EMPM-Chem model to design experiments in the Michigan Tech cloud chamber aimed at maximizing IEPOX-SOA formation. The model resolves turbulence down to ~1 mm (Kolmogorov scales) and performs cloud chemistry calculations on every droplet and haze particle. It tracks each particle’s history as it undergoes repeated cycles of activation and evaporation.

Simulations show that in weakly forced conditions in the atmosphere (like a small updraft velocity in a shallow cumulus cloud parcel)—producing smaller, longer-lived cloud droplets—are favorable for IEPOX-SOA formation via cloud chemistry, as larger droplets are lost more quickly due to gravitational settling. Within the laboratory cloud chamber, forcing can be reduced by lowering the temperature difference between the bottom and top plates that lowers the mean supersaturation and its variability, enhancing droplet lifetimes and in-cloud reaction times. Maintaining acidic conditions in cloud droplets also accelerates IEPOX-SOA formation and favors production of non-volatile products like organosulfates and methyltetrol oligomers over semi-volatile methyltetrols, which can be lost during sampling.

We use these modeling insights to guide actual IEPOX-SOA experiments in the cloud chamber. This talk will present key lessons learned about cloud chemical kinetics of IEPOX-SOA and demonstrate how state-of-the-science modeling tools can be used to design more effective laboratory experiments that improve our understanding of real atmospheric processes.