Heterogeneous Reactions of Polycyclic Aromatic Hydrocarbons on Atmospheric and Terrestrial Surfaces

Abstract:

The heterogeneous reactions of five higher molecular weight polycyclic aromatic hydrocarbons (PAHs), benzo[a]pyrene-d₁₂ (BaP-d₁₂), benzo(k)fluoranthene-d₁₂ (BkF-d₁₂), benzo[g,h,i]perylene-d₁₂ (BghiP-d₁₂), dibenzo(a,i)pyrene-d₁₄ (DBaiP-d₁₄), and dibenzo[a,l]pyrene (DalP), with NO₂, NO₃/N₂O₅, O₃, and OH radicals were investigated in a 7000 L indoor Teflon chamber. Quartz fiber filters (QFF) were used as the reaction surface and substrate and the analyses of parent PAHs and Nitro-PAH (NPAH) products was conducted using electron impact gas chromatographic mass spectrometry (GC/MS) and negative chemical ionization GC/MS. In parallel to the laboratory experiments, a theoretical study was conducted to assist in determining the formation of NPAH isomers based on the OH-radical initiated reaction. The thermodynamic stability of OH-PAH intermediates was used to indicate the position of highest electron density and the most stable NPAH products were synthesized to confirm their identity. NO₂ and NO₃/N₂O₅ were the most effective oxidizing agents in transforming PAHs deposited on filters to NPAHs, under the experimental conditions. Reaction of BaP-d₁₂, BkF-d₁₂ and BghiP-d₁₂ resulted in the formation of several mono-nitro PAH isomer product, while the reaction of DalP and DaiP-d₁₄ resulted in the formation of only one mono-nitro PAH isomer product. The direct-acting mutagenicity of the products increased the most after NO₃/N₂O₅ exposure, particularly for BkF-d₁₂ in which the formation of dinitro- PAHs was observed.

In addition, the degradation of particulate matter (PM)-bound PAHs by heterogeneous reaction with OH radicals, O₃, NO₃/N₂O₅ was also studied. Ambient PM samples collected from Beijing, China and Riverside, California were exposed in an indoor chamber under simulated trans-Pacific atmospheric transport conditions and the formation of NPAHs was studied. NPAHs were most effectively formed during the NO₃/N₂O₅ exposure and, for all exposures, there was no significant formation of 2-nitrofluoranthene or 2-nitropyrene, suggesting that heterogeneous reactions predominated. The importance of this research with respect to atmospheric long-range transport of PM-bound PAHs and heterogeneous reaction of PAHs on terrestrial surfaces will be discussed.