POLYNUCLEAR AROMATIC HYDROCARBON DEGRADATION BY HETEROGENEOUS REACTIONS WITH N2O5 ON ATMOSPHERIC PARTICLES

The degradation of particulate polynuclear aromatic hydrocarbons (PAH) on atmospheric soot particles in the presence of gas phase dinitrogen pentoxide (N2O5) was explored. Dilute diesel and wood soot particles containing PAH were reacted with ∼10 ppm of N2O5 in a 200 ℓ continuous stirred tank reactor (CSTR). To provide a stable source of particles for reaction in the CSTR, diesel or wood soot particles were injected at night into a 25 m3 Teflon outdoor chamber. The large chamber served as a reservoir for the feed aerosol, and the aerosol could then be introduced at a constant flow rate into the CSTR. PAH-N2O5 heterogeneous rate constants for wood soot at 15°C ranged from 2 × 10-18 to 5 × 10-18 cm3 molecules-1 s-1. For diesel soot the rate constants at 16°C were higher and ranged from 5 × 10-18 to 30 × 10-18 cm3 molecules-1 s-1. Comparisons with other studies suggest that sunlight is the most important factor which influences PAH decay. This is followed by ozone, NO2, N2O5 and nitric acid. The rate constants of nitro-PAH formation from a parent PAH and N2O5 were of the order of 1 × 10-19-1 × 10-18 molecules-1s-1. The uncertainty associated with all of these rate constants is ± a factor of 3. Given, however, the small magnitude of the rate constants and the low levels of N2O5 present in the atmosphere, we concluded that PAH heterogeneous reactions with gas phase N2O5 degrade particle-bound PAH or to form nitro-PAH from PAH are not very important. (Direct application of the specific rate constants derived in this study to ambient atmospheres should not be undertaken unless the ambient particle size distributions and chemical composition of the particles are similar to the ones reported in this study.). © 1990.