Role of Methyl-2-nitrophenol Photolysis as a Potential Source of OH Radicals in the Polluted Atmosphere: Implications...

Methyl-substituted 2-nitrophenols are important components of "brown carbon" from biomass burning. Photolysis is their major gas-phase degradation pathway. To determine the extent of light absorptions by 4-methyl-2-nitrophenol and 5-methyl-2-nitrophenol, we obtained their absorption cross sections in the 295-400 nm region by using cavity ring-down spectroscopy. Cross-section values for 4-methyl-2-nitrophenol were (1.01 ± 0.07) × 10^-18, (5.72 ± 0.39) × 10^-18, and (1.80 ± 0.17) × 10^-20 cm²/molecule at 295, 345, and 400 nm, where errors quoted represent 1σ measurement uncertainty. Cross-section values for 5-methyl-2-nitrophenol were (9.04 ± 0.77) × 10^-19, (5.89 ± 0.54) × 10^-18, and (2.81 ± 0.14) × 10^-20 cm²/molecule at 295, 345, and 400 nm. The HONO, NO₂, and OH formation channels following 308 and 351 nm photolysis of methyl-2-nitrophenols were investigated. The OH quantum yields at 308 and 351 nm were obtained as the ratio of the OH concentration generated in pump/probe laser overlap region to the photon density absorbed by methyl-substituted 2-nitrophenol in the same region; they were 0.066 ± 0.021 and 0.031 ± 0.017 for 4-methyl-2-nitrophenol and 0.078 ± 0.038 and 0.042 ± 0.015 for 5-methyl-2-nitrophenol, where uncertainties represent 1σ precision. The average HONO quantum yields at 308 and 351 nm were 0.26 ± 0.06 and 0.26 ± 0.03 for 4-methyl-2-nitrophenol and 0.37 ± 0.05 and 0.35 ± 0.06 for 5-methyl-2-nitrophenol. Estimated OH production rates from photolyzing 10 pptv of 4-methyl- and 5-methyl-2-nitrophenol are 2.3 × 10⁶ and 3.0 × 10⁶ molecules·cm^-3·s^-1 at 16.9° zenith angle.