Ozonolysis of Polycyclic Aromatic Hydrocarbons on the Surfaces of Secondary Organic Aerosol Particles and its Effects on Particle Properties
ALLA ZELENYUK, Kaitlyn J. Suski, ManishKumar Shrivastava, Simeon Schum, Lynn Mazzoleni, Pacific Northwest National Laboratory
Abstract Number: 532
Working Group: Aerosol Chemistry
Abstract
Polycyclic aromatic hydrocarbons (PAHs) are toxic byproducts of combustion that can undergo long-range transport into remote regions of the world. Previously we have shown that when secondary organic aerosol (SOA) particles are formed in the presence of gas-phase PAHs, PAHs become incorporated entrapped inside viscous SOA particles, shielded from evaporation and oxidation, thus enabling their long range transport. In turn, the presence of PAHs during SOA formation increases SOA number concentration, mass loadings, and makes these particles less volatile and more viscous. In contrast, PAHs deposited on the surface of SOA particles, evaporate quickly and do not alter the SOA volatility.
In this work we show that in the presence of ozone PAHs can undergo heterogeneous oxidation on the surfaces of SOA particles, forming shells that alter the evaporation kinetics of the resulting particles. SOA was formed via ozonolysis of α-pinene and was exposed to gas-phase PAHs (anthracene or pyrene) and ozone for 1 hour, during which a shell composed of unreacted and oxidized PAHs was formed on the particle surfaces, as evident from particle growth and the mass spectra. Evaporation experiments on the resulting particles with core-shell morphology revealed that their volatility is lower compared to that of pure α-pinene SOA. Most notably, when evaporated under either dry (<5% RH) or wet (~75% RH) conditions, significant fraction of the PAHs and their oxidation products remained in the particles after a day.
These results suggest that PAHs can become incorporated into SOA particles at any time during their atmospheric evolution via adsorption and surface reactions with ozone, affecting the composition, morphology, and mass-transfer processes, including SOA evaporation kinetics. The data show that surface reactions of PAHs with ozone can result in higher loadings of SOA, PAHs and PAH oxidation products, leading to their longer than predicted atmospheric lifetimes.