Photostability of Nitro-Polycyclic Aromatic Hydrocarbons on Combustion Soot Particles in Sunlight

Abstract

Little is known about the stability of nitro-polycyclic aromatic hydrocarbons (NPAH) on atmospheric aerosols. In this study, the photostability of particle-associated NPAH was investigated under natural sunlight. Deuterated and native NPAH along with diesel exhaust or wood smoke particles were added to a 190-m3 outdoor smog chamber and permitted to age under sunlight in cold and warm temperatures. Ozone (O3), nitrogen oxides (NOx), and volatile hydrocarbons in the gas phase were monitored. A sampling train consisting of an annular denuder filter plus another denuder was used for the collection of gas- and particle-phase PAH and NPAH. Rapid degradation of deuterated and native NPAH was observed in sunlight, over a temperature range of −19 to +38 °C. Deuterated 1-nitropyrene (d9-1NP) displayed the same behavior as native 1-nitropyrene (1NP), which indicated that it was reasonable to use deuterated NPAH as substitutes for native NPAH. The photolysis rate of NPAH was referenced to the NO2 photolysis rate in order to relate the observed decay of NPAH to the changing solar radiation.To model the decay of NPAH on diesel particles, an average rate constant of kNPAH = (0.04 ± 0.01) × kNO2 was used for nitropyrenes (NPs), and a average rate of kNPAH = (0.025 ± 0.005) × kNO2 was needed to model the behavior of nitrofluoranthenes (NFs). A higher rate, kNPAH = (0.050 ± 0.005) × kNO2, was needed to model the decay of NFs and NPs decay on wood smoke. A photolysis rate of NO2 (kNO2 = 8.3 × 10-3 s-1) at noon on June 15, 1994, gave half-lives of 0.8 h for 1NP and 2-nitropyrene (2NP) and 1.2 h for 2-nitrofluoranthene (2NF), d9-3-, and d9-8-nitrofluoranthene (d9-3NF and d9-8NF) on diesel soot particles. The half-life was 0.5 h for d9-1NP, d9-3NF, and d9-8NF on wood soot particles. These results showed that photodecay was the main loss pathway for NPAH on diesel soot and wood smoke and that photodecomposition of NPAH was dependent on the solar radiation and the chemical and physical properties of the substrates. The effect of temperature was not significant.