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

Abstract

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, (5.72 ± 0.39) × 10, and (1.80 ± 0.17) × 10 cm/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, (5.89 ± 0.54) × 10, and (2.81 ± 0.14) × 10 cm/molecule at 295, 345, and 400 nm. The HONO, NO, 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 × 10 and 3.0 × 10 molecules·cm·s at 16.9° zenith angle.

The formation of nitro-aromatic compounds under high NO<sub><i>x</i></sub> and anthropogenic VOC conditions in urban Beijing, China

“…This was more likely associated with the fact that particle-phase NPs only account for a minor part of the atmospheric NP abundance due to the high vapor pressure of NPs (Table S1). The abundance of particulate NP could largely depend on gas-to-particle partitioning, which is strongly affected by temperature, as well as their gas-phase loss pathways (e.g., photolysis) (Bejan et al, 2007;Yuan et al, 2016;Sangwan and Zhu, 2018). NC and MNC were mainly present in the particle phase (Table S1).…”

Direct observations indicate photodegradable oxygenated VOCs as larger contributors to radicals and ozone production in the atmosphere

“…Yuan et al (2016) have reported that photolysis was the most efficient loss pathway for nitrophenol in the gas phase. Different values of absorption cross sections and quantum yields for nitrophenol have been reported (Chen et al, 2011;Sangwan and Zhu, 2018;Bejan et al, 2006). In this study, we used the values from Chen et al (Chen et al, 2011), which can reproduce well the observed concentrations of nitrophenol and methyl nitrophenol during the measurement period.…”

The promotion effect of nitrous acid on aerosol formation in wintertime Beijing: possible contribution of traffic-related emission

“…Visibility and planetary boundary layer (PBL) height were measured using a visibility sensor (PWD22, Vaisala) and a ceilometer (CL51,Vaisala), respectively 2.2 HONO budget calculation. Multiple sources of ambient HONO have been identified, such as emission from soil (Esoil) (Oswald et al, 2015;Meusel et al, 2018) and vehicle exhaust (Evehicle) (Trinh et al, 2017), production through homogeneous reaction between NO and OH (PNO-OH) in the atmosphere, photolysis of nitrate (Pnitrate) (Bao et al, 2018), nitrous acid (PHNO3) and nitrophenenol (Pnitrophenol) (Sangwan and Zhu, 2018), heterogeneous reaction of NO2 on aerosol surface (Paerosol) and ground surface (Pground) (Liu et al, 2019c;Li et al, 2018;Wang et al, 2017b).…”