SOME OBSERVATIONS ON TIMES TO EQUILIBRIUM FOR SEMIVOLATILE POLYCYCLICAROMATIC-HYDROCARBONS

The relative time scales at which semivolatile gas and particle PAH ap proach equilibrium under both moderate and cool temperatures were inve stigated. Combustion particles were added directly from a diesel car a na 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 deuterat ed pyrene (d10-py) in a hot-injector(200 degrees C) into the rural bac kground air in the chamber atmosphere. After 2 h, diesel exhaust was a dded 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 equi librium. The relative closeness to equilibrium was monitored by calcul ating the-equilibrium c onstant K-p over time as measured by PAH(part) / (PAH(gas) x TSP). As gas-phase PAH concentrations changed in the cha mber, due to wall losses, particle off-gassing occurred so that K-p wa s reasonably constant over time. Under cool outdoor conditions (-1 to -4 degrees 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 ten of hours would be required to r eestablish 90% of equilibrium concentrations for compounds like phenan threne and pyrene once they had departed from equilibrium in the parti cle phase by 34 and 18%, respectively.