Isomer distributions of molecular weight 247 and 273 nitro-PAHs in ambient samples, NIST diesel SRM, and from radical-initiated chamber reactions

Molecular weight (mw) 247 nitrofluoranthenes and nitropyrenes and mw 273 nitrotriphenylenes (NTPs), nitrobenz[a]anthracenes, and nitrochrysenes were quantified in ambient particles collected in Riverside, CA, Tokyo, Japan, and Mexico City, Mexico. 2-Nitrofluoranthene (2-NFL) was the most abundant nitro-polycyclic aromatic hydrocarbon (nitro-PAH) in Riverside and Mexico City, and the mw 273 nitro-PAHs were observed in lower concentrations. However, in Tokyo concentrations of 1- + 2-NTP were more similar to that of 2-NFL. NIST SRM 1975 diesel extract standard reference material was also analyzed to examine nitro-PAH isomer distributions, and 12-nitrobenz[a]anthracene was identified for the first time. The atmospheric formation pathways of nitro-PAHs were studied from chamber reactions of fluoranthene, pyrene, triphenylene, benz[a]anthracene, and chrysene with OH and NO3 radicals at room temperature and atmospheric pressure, with the PAH concentrations being controlled by their vapor pressures. Sampling media were spiked with deuterated PAH to examine heterogeneous nitration. Comparing specific nitro-PAH ratios in ambient and diesel particles with those from our chamber experiments suggests that the low 2-NFL/NTPs ratios in Tokyo particulate matter are not a result of gas-phase radical-initiated chemistry since both gas-phase OH and NO3 radical-initiated reactions result in high 2-NFL/NTPs ratios. Comparisons of the relative formation of deuterated nitro-PAHs on the sampling media suggest that heterogeneous reactions with N2O5 on ambient particle surfaces also do not explain the nitro-PAH profiles of Tokyo particles. Thus, the source of NTPs in Tokyo remains unidentified.

► Relative to 2-nitrofluoranthene (2NF), nitrotriphenylenes (NTPs) were high in Tokyo.
► Relative to 2NF, NTPs were low in Riverside, California and Mexico City particles.
► Chamber studies show that atmospheric formation of NTPs is likely to be minor.
► High concentrations of NTPs in Tokyo remain unexplained.