Some Observations on Times to Equilibrium for Semivolatile Polycyclic Aromatic Hydrocarbons

Abstract

The relative time scales at which semivolatile gas and particle PAH approach equilibrium under both moderate and cool temperatures were investigated. Combustion particles were added directly from a diesel car and a wood stove to a 190 m^3 outdoor Teflon film chamber. The rate of migration of a gas-phase semivolatile PAH to combustion particles at a warm outdoor temperature was explored by volatilizing solid deuterated pyrene (d10-py) in a hot injector (200 °C) into the rural background air in the chamber atmosphere. After 2 h, diesel exhaust was added to the chamber. Results show an initial rapid migration of d10-py from the gas to the particle phase in an attempt to re-establish equilibrium. The relative closeness to equilibrium was monitored by calculating the equilibrium constant Kp overtime as measured by PAH_(part)/(PAH_(gas) x TSP). As gas-phase PAH concentrations changed in the chamber, due to wall losses, particle off-gassing occurred so that Kp was reasonably constant over time. Under cool outdoor conditions (-1 to -4 °C), PAH loss from the particle phase could not keep up with gas-phase PAH wall losses, and the system departed from equilibrium. Kinetic simulations suggested that tens of hours would be required to reestablish 90% of equilibrium concentrations for compounds like phenanthrene and pyrene once they had departed from equilibrium in the particle phase by 34 and 18%, respectively.