Impacts on Air Quality due to Photosensitized Production of Excited State O2 (1Δg) by PAHs and Oxy-PAHs in the Lower Atmosphere: An Experimental and Computational Modeling Approach

Abstract:

Complex reactions between hydroxyl radicals (OH) and volatile organic compounds (VOCs) in the lower atmosphere have a high impact on the formation/fates of airborne toxic chemicals, polycyclic aromatic hydrocarbons (PAHs), and particulate matter.¹ Recently, air quality models have been implemented to identify OH sources, but have underpredicted OH concentrations. Studies suggest that O₂ (¹Δ_g) is produced via an energy transfer (ET) mechanism initiated by the electronic excitation of PAH and oxygenated-PAH. Energy transfer involves the formation of triplet excited state PAH which is then quenched by the surrounding ground state O₂ (³∑_g) resulting in excited state O₂ (¹Δ_g) formation. Excited state O₂ (¹Δ_g) is known to readily react with mono-olefins to produce organic hydroperoxides.^2,3 Furthermore, the organic hydroperoxide can photodegrade to yield OH. In this study, a Nd:YAG laser coupled to a time-resolved near infrared detector was used to obtain quantum yields of O₂ (¹Δ_g) production by irradiating PAHs and oxy-PAHs at both 355 nm and 532 nm in different solvents. Select PAHs, primarily emitted by combustion engines (e.g. pyrene and benzo[a]pyrene), and their oxygenated forms (oxy-PAHs) have been identified as highly efficient O₂ (¹Δ_g) photosensitizers. For example, the measured quantum yield for pyrene in toluene was 0.90 ± 0.02. The measured quantum yields were used to calculate the photochemical rate constants for O₂ (¹Δ_g) production via ET from electronically excited PAHs and oxy-PAHs. These results were incorporated into the University of California, Irvine-California Institute of Technology (UCI-CIT) model to assess the impact on OH concentrations and the overall air quality of the South Coast Air Basin of California.

References

¹ Finlayson-Pitts, B.J., and J. N. Pitts (1997), Science, 276(5315),1045–1052.

² Foote, C. S. (1968), Accts. Chem. Res., 1, 104-110; Gollnick, K. (1968), Adv. Photochem., 6, 1-112; Kearns, D. R. (1971), Chem. Rev., 71, 395-427.

³ Frimer, A. A. (1985), Singlet O₂, CRC, Boca Raton, FL.