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

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) X 10(-18), (5.72 +/- 0.39) X 10(-18), and (1.80 +/- 0.17) X 10(-20) cm(2)/molecule at 295, 345, and 400 nm, where errors quoted represent 1 sigma measurement uncertainty. Cross-section values for 5-methyl-2-nitrophenol were (9.04 +/- 0.77) X 10(-19), (5.89 +/- 0.54) X10(-18), and (2.81 +/- 0.14) X 10(-20) cm(2)/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 1 sigma 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 X 10(6) and 3.0 X 10(6) molecules cm(-3) s(-1) at 16.9 degrees zenith angle.