Surface Reactions of Polycyclic Aromatic Hydrocarbons with Ozone on Secondary Organic Aerosol and their Effects on Evaporation

Polycyclic aromatic hydrocarbons (PAHs) are harmful byproducts from 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 and entrapped inside viscous SOA particles. The SOA shields them from evaporation and oxidation, thus enabling their long range transport. In turn, the presence of PAHs during SOA formation 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 surface of SOA particles, forming shells that alter the volatility of the resulting particles.

In this study, SOA was formed via ozonolysis of α-pinene, which was then exposed to gas-phase PAHs (anthracene or pyrene) and ozone for 1 hour. A shell composed of oxidized and unreacted PAHs formed on the particle surfaces, as evidenced from particle growth and the single particle mass spectra. Evaporation experiments on the resulting particles with core-shell morphology revealed that their volatility was lower compared to pure α-pinene SOA. Most notably, when evaporated under either dry (<5% RH) or wet ( 75% RH) conditions, a 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. These reactions affect the composition, morphology, and evaporation kinetics of these particles and can lead to longer than traditionally predicted atmospheric lifetimes of PAHs.