Role of Methyl-2-nitrophenol Photolysis as a Potential Source of OH

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Role of Methyl-2-nitrophenol Photolysis as a Potential Source of OH Radicals in the Polluted Atmosphere: Implications from Laboratory Investigation Manuvesh Sangwan, and Lei Zhu J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b11235 • Publication Date (Web): 27 Jan 2018 Downloaded from http://pubs.acs.org on January 30, 2018

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The Journal of Physical Chemistry

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

Manuvesh Sangwan† and Lei Zhu* Wadsworth Center, New York State Department of Health, and Department of Environmental Health Sciences, University at Albany Albany, NY 12201-0509

*Corresponding author. Tel: 518-474-6846; fax: 518-473-2895; e-mail: [email protected]. †

Current address: Emergent Biosolutions, Lansing, MI 48906.

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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)×10-18, (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.


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1. Introduction Measured OH concentrations significantly higher than predicted values have been reported in the polluted suburban environment near Beijing and in the Pearl River Delta of China.1,2 Unidentified OH sources were invoked to explain such a discrepancy. Aromatic hydrocarbons are an important class of volatile organic compounds (VOCs) emitted by anthropogenic activities and are abundant in the polluted atmosphere.3 Aromatic hydrocarbons can undergo oxidation to form nitroaromatic compounds. Methyl-substituted 2-nitrophenols such as 4-methyl-2-nitrophenol (4M2NP) and 5-methyl-2-nitrophenol (5M2NP) are important components of “brown carbon” from biomass burning.4 They are released into the atmosphere by combustion sources5-9 and via in-situ oxidation of cresols10-14 by OH and NO3 radicals. Methyl-2-nitrophenols have been detected in the gas and aqueous phases of the atmosphere,5,6,9 with a higher percentage partitioned in the gas phase.5 Gaseous concentrations of 4-methyl-2-nitrophenol and 5-methyl-2-nitrophenol vary with geographic locations. 4-Methyl-2-nitrophenol concentrations9 up to 200 ng•m-3 (32 pptv) and 5-methyl-2-nitrophenol concentrations6 as high as 4.8 ng•m-3 (0.78 pptv) have been reported. Photolysis is the major gas phase removal process of methyl-2-nitrophenols.15 Recently, there is a renewed interest7 in the gas phase photolysis of 2-nitrophenol (2NP) and methyl-substituted 2-nitrophenols (M2NP) as their photolysis over the 300-500 nm region is a proposed photochemical source16 of HONO. HONO photolysis is a significant source of OH in the atmosphere17-24 and modeled HONO sources generally underestimate measured HONO mixing ratios.24 Our group has recently conducted quantitative determination of the gas phase near-UV absorption cross-sections, and measured OH and HONO quantum yields from the photolysis of 2nitrophenol.25 Gas phase absorption cross-sections of 2-nitrophenol were found to be 1.5×10-18, 2.9×10-18, and 2.6×10-20 cm2/molecule at 295, 345, and 400 nm, respectively. The average OH



quantum yields were 0.69±0.07 and 0.70±0.07 at 308 nm and at 351 nm, and the average HONO quantum yields were 0.34±0.09 and 0.39±0.07 at 308 nm and at 351 nm. The 2-nitrophenol photolysis rate constant estimated from our laboratory study results25 was about twice that of NO2. The large photolysis rate constant and the high OH and HONO quantum yields highlight the importance of 2-nitrophenol gas phase photolysis in forming OH and HONO in a polluted atmosphere rich in aromatic hydrocarbons. Theoretical studies26,27 have suggested that 2nitrophenol photolysis likely proceeds through intersystem crossing (ISC) from the S1 first excited singlet state to the T1 triplet state, with possible photolysis channels such as OH + nitrosophenoxy, HONO + phenyloxyl, and NO + hydroxyphenoxy. Photolysis of 4-methyl-2-nitrophenol and 5methyl-2-nitrophenol can also generate OH radicals. Formation of OH from the photolysis of these methyl-substituted 2-nitrophenol isomers has been suggested,15 but OH formation quantum yields have not been previously determined. To evaluate the impact of gas phase photolysis of methyl-substituted 2-nitrophenols as photochemical sources of OH and HONO in the polluted atmosphere, it is necessary to quantitatively determine their near-UV absorption cross-sections, and OH and HONO formation quantum yields. Gas phase near-UV absorption cross-sections of 5-methyl-2-nitrophenol have not been previously measured. One prior study reported near-UV absorption spectrum of 4-methyl2-nitrophenol in the 320-450 nm region,28 but absorption cross-section data have not been collected over the 290-320 nm range. HONO was reported as a product from the photolysis of 3methyl, 4-methyl-, and 5-methyl-2-nitrophenol in the presence of buffer gases in an environmental chamber.16 A HONO quantum yield of approximately 1.5×10-4 was estimated for 3-methyl-2nitrophenol using its absorption spectrum in liquid dichloromethane, and assuming the photolysis frequency was independent of wavelength over the wavelength range (300-500 nm) covered by


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flash lamps.16 However, the HONO quantum yields from the gas phase photolysis of methyl-2nitrophenols have not been directly measured; the variation of the HONO quantum yields with irradiation wavelength is not known. Determining gas phase photolysis of methyl-substituted 2nitrophenols and comparing the results obtained with those from 2-nitrophenol photolysis will allow elucidation of the effects of methyl-substitution on photolysis rates and products. The results presented in this paper were obtained from quantitative determinations of the gas phase absorption cross-sections of 4-methyl-2-nitrophenol and 5-methyl-2-nitrophenol in the 295-400 nm region using cavity ring-down spectroscopy29,30. We investigate the OH, HONO, and NO2 generation pathways after the 308 nm and 351 nm photodecomposition of these methyl-2nitrophenols, and then determine the OH and the HONO quantum yields. We calculate the OH and the HONO production rate constants from the gas phase photodissociation of these methylsubstituted 2-nitrophenols using experimental results obtained from the current study. Based on these findings, we discuss the role of daytime photolytic processes of these compounds as missing sources of OH and HONO in the polluted atmospheric environment. 2. Experimental Methods 2.1.

Experimental technique Absorption cross-sections of 4-methyl-2-nitrophenol and 5-methyl-2-nitrophenol were

measured in the 295-400 nm region by cavity ring-down spectroscopy. The cavity cell of 48 cm length was vacuum-sealed by a pair of high reflectance cavity mirrors. Several sets of highreflectance cavity mirrors were used to cover the 295-400 nm region; these mirrors have high reflectivity over the 295-320 nm, 310-341 nm, 331-378 nm, and 339-407 nm range. The experimental setup is described in detail elsewhere31-34; only a brief description critical for the current experiments is provided here. The probe beam generated from frequency-doubling of the



output of a dye laser (Continuum, ND6000) pumped by a 532 nm Nd:YAG laser (Continuum, Surelite II) entered the ring-down cavity through the front cavity mirror. The dye solutions used were made of Rhodamin 590, 610, 640, 4-dicyanmethylene-2-methyl-6-(p-dimethylamino styryl)4H-pyran (DCM), pyridine 1 (LDS-698), LDS 751, 765 and 821, all dissolved in methanol. The decay of photon intensity inside the cavity was monitored by a photomultiplier tube (PMT) placed behind the rear cavity mirror. The output signal of a PMT was amplified, digitized, and transferred to a computer. The photon intensity decay was fitted to a single-exponential decay function, which allowed extraction of the ring-down time constant (τ) and the total loss (Γ) per round-trip pass. The measured ring-down time constants for an evacuated cavity (τ0) were about 3.5-3.8 μs at 295, 380, and 385 nm; 2.9 μs at 300 nm; 1.5-2.1 μs over 305-330 nm and at 350, 360, 390, and 395 nm; 0.9-1.4 μs at 335, 345, 355, 365-375, and 400 nm; and 0.64 μs at 340 nm. By measuring the cavity losses with and without a methyl-substituted 2-nitrophenol inside the cavity, we obtained optical loss due to absorption by 4-methyl- or 5-methyl-2-nitrophenol. The gas pressure inside the cell was measured by an MKS Baratron capacitance manometer (0.1 or 1 Torr full scale). Absorption cross-section measurements were conducted in the absence of the carrier gas; the 0.1 Torr full scale pressure gauge was used to provide pressure reading in most experiments. Before each experiment, the stainless-steel cavity cell was evacuated to an absolute vacuum of 10-5 Torr. The room temperature vapor pressures of 4-methyl-2-nitrophenol and 5-methyl-2-nitrophenol are about 0.035 Torr and 0.025 Torr, respectively. Pressures of methyl-substituted 2-nitrophenol used in the cross-section measurements were in the range of 2.5×10-4 to 3.0×10-2 Torr for 4-methyl-2nitrophenol and in the range of 2.5×10-4 to 2.0×10-2 Torr for 5-methyl-2-nitrophenol. Estimated pressure measurement uncertainty is about 4.5%-0.5% for methyl-2-nitrophenol pressure of 2.5×10-4 to 3.0×10-2 Torr with the 0.1 Torr full-scale pressure readout.


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Photolysis studies were conducted by using cavity ring-down spectroscopy to probe products formed from photolyzing 4-methyl- and 5-methyl-2-nitrophenol using irradiation from an excimer laser operating at 308 nm and 351 nm. The pulsed output from the excimer laser (Coherent COMPEX Pro102) entered the stainless-steel reaction cell at a 150 angle with the main cell axis through a side arm. The ring-down probe beam was directed into the cell along the main cell axis. The photolysis beam and the probe beam overlapped geometrically at the center of the cell. The overlap region of the excitation and the probe beams is a rectangular prism with its center coincident with that of the cell, and its width and height defined by the dimension of the apertured photolysis beam; the length of the prism is defined by (beam width) × (tan15°)-1, where 15° is the crossing angle between the photolysis and the probe beams. The length of the pump/probe laser overlap region is (beam width)×(sin15°)-1. With a 12-mm wide photolysis beam, the length of the overlap region is about 4.6 cm. The time delay between the firings of the photolysis and the probe lasers was adjusted with a pulse/delay generator. Product absorptions from the photolysis of 4-methyl- and 5-methyl-2-nitrophenol were measured from the difference in the cavity losses in the presence of methyl-2-nitrophenol, but with and without photolysis. HONO and possible NO2 formation from the photolysis of methyl-substituted 2-nitrophenols was investigated by observing their UV absorptions35-42 in the 361 – 371 nm region. OH formation from the photolysis of 4methyl- and 5-methyl-2-nitrophenol was monitored by a characteristic sharp absorption band43 of OH in the 307.80-308.30 nm region. The gas phase nitric acid photolysis at 308 nm was used to obtain absolute OH absorption calibration. The photolysis beam energies before entering and after exiting an empty cell were measured by two calibrated joulemeters. The incident photolysis energy inside the cell was corrected for transmission loss at the front cell window, and for reflection of the photolysis beam from the rear cell window.



2.2.

Chemicals and Preparations 4-Methyl-2-nitrophenol and 5-methyl-2-nitrophenol were obtained as crystalline powder

from Sigma-Aldrich with stated purities of higher than 99% and 97%, respectively. They were degassed at liquid nitrogen temperature (77 K) for three times before they were used in the experiments. To condition the walls of the reaction cell before experimental runs, the cell was seasoned with 4-methyl- or 5-methyl-2-nitrophenol at a pressure of approximately 10 mTorr for 45 min, followed by complete removal of methyl-substituted 2-nitrophenols through extensive pumping. Conditioning of the cell wall was required to prevent methyl-substituted 2-nitrophenols from quickly depositing onto cell walls. Without such a procedure, we observed the decrease of methyl-2-nitrophenol pressure in the cell and the decrease of the methyl-2-nitrophenol near-UV absorption. After conditioning the cell wall, we could obtain a stable pressure reading of methylsubstituted 2-nitrophenols in the cell and reproducible UV absorption measurements. Partitioning of low vapor-pressure gases between the gas phase and the surface has recently been studied44 by several groups. Such interaction can depend on the nature of the surfaces. For example, we did not observe deposition of methyl-2-nitrophenols on cavity mirrors, perhaps because the surfaces of the cavity mirrors were superpolished. Experimental measurements of 4-methyl-2-nitrophenol and 5-methyl-2-nitrophenol were mostly conducted at different days with minimum pumping time of overnight to minimize cross contamination of results from different methyl-substituted 2nitrophenols. UHP grade nitrogen and oxygen carrier gases were obtained from Airgas and were used as received. Purified nitric acid was made by distilling in vacuum45 of a liquid mixture of sulfuric acid (98%; Mallinckrodt Baker) and nitric acid (70%; Mallinckrodt Baker) at volume ratio of 3:2 at 273 K. The distilled HNO3 was collected in a trap cooled at 195 K. A minimum of four repeated distillations were performed to purify an HNO3 sample. An additional distillation was


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done on the day of the experiments to further decrease NO2 impurity in an HNO3 sample. HONO was produced from the equilibrium reaction (NO2+NO+H2O = 2HONO) involving mixing reactants NO2, NO, and H2O at 1:1:2 mole ratios. We monitored the time dependence of NO2 absorption at 564.00 nm both immediately after the mixing and up to 60 hours after the mixing; we found that the NO2 absorption reached a minimum after we let the mixture to react and equilibrate for about 24 hours. From changes in NO2 concentration upon mixing and after 24 hours of reaction, we derived an equilibrium HONO concentration. NO2 was purchased from SigmaAldrich (≥99.5% purity). NO was obtained from Airgas (≥99.5% purity). Liquid water was drawn from a Barnstead nanopure ultrapure water system (Thermo Scientific, USA); it underwent triple freeze-pump-thawed cycles before gas output was used in the HONO preparation.

The

experimental studies were carried out at laboratory temperature of 295±2 K. 3. Results 3.1. Absorption Cross-Sections of 4-Methyl- and 5-Methyl-2-nitrophenol over 295-400 nm Range Gas phase absorption cross-sections of 4-methyl- and 5-methyl-2-nitrophenol were determined at every 5 nm over the 295-400 nm range. Each absorption cross-section was obtained from measurement of round-trip absorbance as a function of the methyl-2-nitrophenol pressure inside the cavity. Round-trip absorbance at a given methyl-2-nitrophenol pressure was determined from the difference in cavity losses with and without methyl-2-nitrophenol. Figure 1 shows plots of round-trip absorbance as a function of 4-methyl-2-nitrophenol pressure at 300 nm, 330 nm, 360 nm, and 385 nm along with linear regression fit to the data.



30000

20000

-6 Round-trip absorbance (10 )

300 nm Round-trip absorbance (10-6)

25000

20000

15000

10000

5000

0 0.000

0.002

0.004

0.006

330 nm 16000

12000

8000

4000

0 0.0000

0.008

0.0004

40000

0.0012

8000


360 nm 30000

20000

10000

0 0.000

0.0008

P (Torr)

P (Torr)


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0.001

0.002

0.003

0.004

385 nm 6000

4000

2000

0 0.000

0.005

0.005

P (Torr)

0.010

0.015

0.020

0.025

0.030

P (Torr)

Figure 1. Round-trip absorbance versus 4-methyl-2-nitrophenol pressure at 300, 330, 360, and 385 nm. Linear regression analysis of experimental data is also shown from which absorption cross-sections of 1.08×10-18, 5.02×10-18, 2.70×10-18, and 8.53×10-20 cm2/molecule were extracted at 300, 330, 360, and 385 nm, respectively.

Absorption cross-sections of 4-methyl-2-nitrophenol extracted from slopes of the fits were (1.08±0.04)×10-18, (5.02±0.18)×10-18, (2.70±0.08)×10-18, and (8.53±0.61)×10-20 cm2/molecule at 300 nm, 330 nm, 360 nm, and 385 nm, respectively. Absorption cross-sections of 4-methyl-210 ACS Paragon Plus Environment

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nitrophenol over the 295-400 nm range are listed in Table 1 and shown in Figure 2. As can be seen from Figure 2, 4-methyl-2-nitrophenol absorption cross-sections increased with wavelength in the 295-325 nm region; they were nearly independent of wavelength over the 330-355 nm range; and they decreased with increasing wavelength in the 360-400 nm region. Absorption crosssection values for 4-methyl-2-nitrophenol ranged from (1.01±0.07)×10-18 cm2/molecule at 295 nm, to (5.72±0.39)×10-18 cm2/molecule at 345 nm, and to (1.80±0.17)×10-20 cm2/molecule at 400 nm. Errors quoted (1σ) are measurement precision obtained from cumulative error analysis of the

Table 1. Gas Phase Absorption Cross-sections (σ, in unit of cm2/molecule, base e) of 4-Methyl-2nitrophenol (4M2NP) and 5-Methyl-2-nitrophenol (5M2NP) as a Function of Wavelength (λ).

λ (nm) 295 300 305 310 315 320 325 330 335 340 345 350 355 360 365 370 375 380 385 390 395 400

σ4M2NP (this work)a (1.01±0.07)×10-18 (1.14±0.07)×10-18 (1.48±0.11)×10-18 (1.67±0.13)×10-18 (1.99±0.09)×10-18 (2.53±0.16)×10-18 (4.68±0.35)×10-18 (5.07±0.22)×10-18 (5.43±0.08)×10-18 (5.58±0.32)×10-18 (5.72±0.39)×10-18 (5.05±0.94)×10-18 (5.27±0.06)×10-18 (2.71±0.07)×10-18 (1.57±0.05)×10-18 (1.29±0.06)×10-18 (5.24±0.13)×10-19 (2.00±0.29)×10-19 (8.07±0.41)×10-20 (5.77±0.31)×10-20 (4.41±0.02)×10-20 (1.80±0.17)×10-20

σ4M2NP (Ref. 28)

3.52×10-18 6.66×10-18 7.82×10-18 8.99×10-18 1.02×10-17 1.00×10-17 1.02×10-17 1.01×10-17 9.01×10-18 7.75×10-18 6.40×10-18 5.43×10-18 4.51×10-18 3.19×10-18 1.77×10-18 8.90×10-19 5.40×10-19

a

σ5M2NP (this work)a (9.04±0.77)×10-19 (1.54±0.06)×10-18 (1.46±0.12)×10-18 (1.78±0.16)×10-18 (3.15±0.46)×10-18 (3.77±0.39)×10-18 (4.00±0.34)×10-18 (4.55±0.23)×10-18 (5.38±0.18)×10-18 (5.40±0.27)×10-18 (5.89±0.54)×10-18 (4.55±0.18)×10-18 (5.18±0.21)×10-18 (3.07±0.36)×10-18 (2.88±0.14)×10-18 (1.19±0.17)×10-18 (1.39±0.13)×10-18 (1.75±0.09)×10-19 (1.34±0.08)×10-19 (9.76±0.35)×10-20 (6.26±0.67)×10-20 (2.81±0.14)×10-20

Errors quoted are 1σ standard deviations in the precision of the measurements. 11 ACS Paragon Plus Environment

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Cross Section (cm 2/molecule)


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10-16

10-17

10-18

10-19

10-20 280

300

320

340

360

380

400

420

Wavelength (nm)

Figure 2. Absorption cross-sections of 4-methyl-2-nitrophenol over the 295-400 nm range. Circles represent cross-section data obtained in the present study. Literature cross-section data28 are plotted as a solid line. standard deviation of about 3 independent cross-section measurements. Absolute uncertainties in pressure and absorbance measurements as well as absolute uncertainty in 4-methyl-2-nitrophenol gas phase concentration affected the absolute accuracy in cross-section values. The stated impurity for 4-methyl-2-nitrophenol powder is at most 1%. To minimize sample impurity, the 4-methyl-2nitrophenol powder was repeatedly degassed at 77 K for an extended period-of-time during the sample preparation; 4-methyl-2-nitrophenol powder in storage glassware was pumped before the vapor was introduced into the ring-down cavity. For comparison purposes, Figure 2 displays 12 ACS Paragon Plus Environment

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literature absorption cross-sections of 4-methyl-2-nitrophenol over the 320-400 nm range, obtained using incoherent broad-band cavity enhanced absorption spectroscopy.28 Absorption cross-sections of 4-methyl-2-nitrophenol obtained by Chen et al.28 vary from 3.52×10-18 cm2/molecule at 320 nm to 9.01×10-18 cm2/molecule at 360 nm, to 5.43×10-18 cm2/molecule at 375 nm, and to 5.40×10-19 cm2/molecule at 400 nm. The current study reports 4-methyl-2-nitrophenol absorption cross-sections of (2.53±0.16)×10-18 cm2/molecule at 320 nm, (2.71±0.07)×10-18 cm2/molecule at 360 nm, (5.24±0.13)×10-19 cm2/molecule at 375 nm, and (1.80±0.17)×10-20 cm2/molecule at 400 nm. The differences in 4-methyl-2-nitrophenol absorption cross-section values between the study by Chen et al.28 and the current study are 1.4 fold at 320 nm, 3.3 fold at 360 nm, 10 fold at 375 nm, and 30 fold at 400 nm. We speculate that the large differences in crosssections at 375 nm and longer wavelengths between the Chen et al. study28 and the current study likely result from making absorbance measurements at about or below the detection limit in the Chen et al. study. To obtain quantitative absorbance measurements, the measured absorbance should be distinguishable from the background cavity losses, irrespective of whether the measurement technique is cavity ring-down spectroscopy or cavity-enhanced absorption spectroscopy. The Chen et al. study used one set of cavity mirrors with reflectivity for each mirror at best of 99% (i.e., 10,000 ppm minimum transmission loss per mirror) at the wavelengths of highest reflectivity. In other words, the background cavity loss was a minimum of 20,000 ppm for two mirrors. Assuming 1% error in the cavity loss measurement for cavity enhanced absorption spectroscopy, we estimated the detection sensitivity for absorbance measurement at approximately 200 ppm. The 4-methyl-2-nitrophenol maximum mixing ratio of 42 ppbv or equivalent maximum partial pressure of 3.2×10-5 Torr was used in the Chen et al. study. The estimated round-trip absorbance owing to 3.2×10-5 Torr of 4-methyl-2-nitrophenol for the 4.62 m long cavity are 2426



ppm, 2599 ppm, 503 ppm, and 17 ppm at 320, 360, 375, and 400 nm, respectively, which are above detection sensitivity at 320 nm and 360 nm, about detection sensitivity at 375 nm, and below detection sensitivity at 400 nm at the highest pressure used in this study. Since the measured absorbance for the highest pressure used in the Chen et al. study28 was at about or below the detection limit at the longer wavelength region, this may explain why the cross-section difference between the Chen et al. and the current study increases with wavelength, with a difference of 1.4 fold at 320 nm and 30 fold at 400 nm. Additionally, the Chen et al. cross-section determination28 was performed in the presence of air while accuracy in cross-section values could be affected by accuracy in 4-methyl-2-nitrophenol concentration in a mixture. In the present study, we used pure 4-methy-2-nitrophenol in the cross-section determination, and employed several sets of high reflectivity cavity mirrors. We simultaneously monitored 4-methyl-2-nitrophenol absorbance and its pressure inside the cavity. Our measured 4-methyl-2-nitrophenol absorbance at each pressure was clearly distinguishable from the background cavity loss. We used a minimum 4-methyl-2nitrophenol pressure of 2.5×10-4 Torr in the present study to achieve accurate absorbance measurements. We found no evidence for deposition of 4-methyl-2-nitrophenol on cavity mirrors, perhaps because the cavity mirrors were super-polished. If there were adsorption of 4-methyl-2nitrophenol on cavity mirrors, we would observe non-zero intercept in Figure 1, which was not what we observed experimentally. Deposition of 4-methyl-2-nitrophenol on cavity mirrors would be manifested by the cavity loss increase between the clean cavity mirrors and the cavity mirrors that were exposed to 4-methyl-2-nitrophenol but were subsequently evacuated from the cavity. 5-Methyl-2-nitrophenol absorption cross-sections were also determined in the 295-400 nm region using similar procedures as those described in the determination of 4-methyl-2-nitrophenol absorption cross-sections. They are listed in Table 1 and plotted in Figure 3.


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Cross section (cm2/molecule)

Page 15 of 40

10-17

10-18

10-19

10-20 280

300

320

340

360

380

400

420

Wavelength (nm) Figure 3. Wavelength-dependent absorption cross-sections of 5-methyl-2-nitrophenol (circles) and 4-methyl-2-nitrophenol (solid line) determined in this study. For comparison, Figure 3 also shows absorption cross-section values for 4-methyl-2-nitrophenol obtained in the current study. Absorption cross-sections of 5-methyl-2-nitrophenol and 4-methyl2-nitrophenol are of similar size over the 295-400 nm range. The peak absorption cross-sections for 4-methyl-2-nitrophenol and 5-methyl-2-nitrophenol are around 5.7×10-18 and 5.9×10-18 cm2/molecule, respectively, while that for 2-nitrophenol25 is about 2.9×10-18 cm2/molecule. As the nature of the near-UV absorption band peaking around 345 nm for 2-nitrophenol and methylsubstituted 2-nitrophenols is that of the π(benzene ring) → π*(nitro group) transition46 and methyl radical is an electron-donor, methyl-substitution will likely lower the energy of the π* level, and thus increase the probability of the π→π* transition. This may result in larger peak absorption cross-sections for methyl-substituted 2-nitrophenols. 15 ACS Paragon Plus Environment


3.2. Photolysis of 4-Methyl- and 5-Methyl-2-nitrophenol at 308 and 351 nm 3.2.1. OH Radical Formation from the Photolysis of 4-Methyl- and 5-Methyl-2-nitrophenol We also investigated OH radical formation from the 308 and 351 nm photolysis of 4methyl- and 5-methyl-2-nitrophenol. The top panel of Figure 4 shows cavity ring-down absorption spectra of 5-methyl-2-nitrophenol with and without 308 nm photolysis in the 307.80-308.30 nm region. A prominent absorption band appeared at 308.02 nm immediately after the photolysis. The similarity of this product absorption band to that of the OH radical from the 308 nm photolysis of HNO3 (see bottom panel of Figure 4) suggests that OH radical is a product of the 308 nm photolysis of 5-methyl-2-nitrophenol. The OH radical is also a product from the 351 nm photolysis of 5-methyl-2-nitrophenol, and from the 308 and 351 nm photolysis of 4-methyl-2-nitrophenol. The ring-down probe beam was tuned to the prominent OH absorption peak at 308.02 nm; temporal absorption profiles of OH from photolyzing 4-methyl- and 5-methyl-2-nitrophenol at several pressures in the mTorr range were determined as a function of pump/probe laser delay time. Figure 5 presents a transient OH absorption profile from the 308 nm photolysis of 5-methyl-2nitrophenol at 3.0 mTorr pressure. Absorption at 308.02 nm appeared immediately after the photolysis pulse, and it decreased quickly with delay time in the 15-80 μs range. The ring-down time scale with 3.0 mTorr of 5-methyl-2-nitrophenol inside the cell was about 0.18 μs, which is much shorter than the OH radical decay time. Absolute OH absorption values at 15 µs delay time were larger at higher 5-methyl-2-nitrophenol pressures. A rate constant15 of 6.72×10-12 cm3molecule-1s-1 has been reported for the reaction of OH with 5-methyl-2-nitrophenol. The fast OH absorption decay at a pump/probe laser delay time of less than 80 μs is unlikely to be caused by the reaction between OH and 5-methyl-2-nitrophenol at several mTorr pressure; it is instead attributed to diffusion of OH out of the probe beam. We cannot increase 5-methyl-2-nitrophenol


Page 16 of 40

Page 17 of 40

12200

Absorption (10-6)

12000

2 mTorr 5M2NP With 308 nm photolysis

11800 11600 11400 11200 11000

Without photolysis

10800 10600 307.8

307.9

308.0

308.1

308.2

Wavelength (nm)

14000

2 Torr HNO3

Absorption (10-6)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


With 308 nm photolysis

13000

12000

11000

Without photolysis 307.8

307.9

308.0

308.1

308.2

Wavelength (nm)

Figure 4. Top Panel: Cavity ring-down absorption spectra in the 307.80-308.30 nm region before and after 308 nm photolysis of 5-methyl-2-nitrophenol at 2 mTorr pressure. Spectra were recorded at a pump/probe laser delay of 15 μs while scans were made at 0.01 nm interval. Bottom Panel: Cavity ring-down absorption spectra before and after 308 nm photolysis of HNO3 at 2 Torr pressure. 17 ACS Paragon Plus Environment


800

3.0 mTorr 5M2NP -6

OH absorption (10 )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 40

308 nm photolysis

600

308.02 nm detection

400

200

0 0

50

100

150

200

250

300

350

400

t (μs) Figure 5. Temporal round-trip absorption profile of OH from the 308 nm photolysis of 3.0 mTorr of 5-methyl-2-nitrophenol detected at 308.02 nm. pressure much higher than several mTorr to decrease OH diffusion, as background absorption by 3.0 mTorr of 5-methyl-2-nitrophenol is already about 14,800 ppm at 308.02 nm. Since OH radical diffusion out of the probe beam is fast for methyl-2-nitrophenol at pressure levels of several mTorr, it is necessary to use OH radical absorption at 15 μs after the photolysis to obtain transient OH concentration produced from the photolysis of 5-methyl-2-nitrophenol and 4-methyl-2nitrophenol. The OH quantum yield from the photolysis of a methyl-substituted 2-nitrophenol was obtained as the ratio of the OH concentration generated in the pump/probe laser overlap region to 18 ACS Paragon Plus Environment

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the photon density absorbed by methyl-substituted 2-nitrophenol in the same region. Absorbed photolysis photon density in the overlap region of two lasers was calculated from the difference between the transmitted photolysis beam energies entering (Ein) and leaving (Eout) this region, the energy of a photolysis photon (hc/λ), and the volume (v) of the overlap region. The photolysis beam energy before and after the two-laser overlap region was derived from the incident photolysis beam energy entering the cell (E0), the absorption cross section (σ) and the concentration (n) of 4methyl- or 5-methyl-2-nitrophenol in the cell, and the absorbing path length using the BeerLambert law: Ein = E0•exp(-σnl1)

(1)

Eout = E0•exp(-σnl2)

(2)

,

where l1 is the distance between the photolysis beam entrance and the beginning of the overlap region, and l2 is the distance between the photolysis beam entrance and the end of the overlap region. The photolysis fluences were measured by two calibrated joulemeters and they were ~0.034 J cm-2 at 308 nm and ~0.009 J cm-2 at 351 nm. We did not focus the 308 nm and the 351 nm photolysis beam between the laser output and the cell entrance. The photolysis fluence at 351 nm was weak. We varied the 308 nm photolysis fluence in the range of 0.025 J•cm-2 and 0.049 J•cm-2, and found linear dependence of OH absorption on photolysis fluence from 308 nm photolysis of 4-methyl-2-nitrophenol and 5-methyl-2-nitrophenol. Multiphoton absorption by methyl-2-nitrophenols was unlikely at both photolysis wavelengths. The photolysis photon densities absorbed by 5-methyl-2-nitrophenol at 308 nm in the two-laser overlap region were about 2.7×1012, 6.0×1012, and 9.3×1012 molecules/cm3 at 5-methyl-2-nitrophenol pressures of 1.0, 2.2, and 3.4 mTorr, respectively. The respective photolysis photon densities absorbed by 5-methyl-2nitrophenol at 351 nm in the laser overlap region were 2.4×1012, 4.1×1012, and 5.6×1012



molecules/cm3 at pressures of 1.0, 1.7, and 2.3 mTorr. The concentration of OH generated by 308 or 351 nm photolysis of 5-methyl-2-nitrophenol was acquired by determining OH absorption at 308.02 nm at a pump/probe laser delay of 15 μs. Absolute OH concentration was calibrated relative to OH formed from the 308 nm HNO3 photolysis for which the OH quantum yield of unity47 is established. Round-trip OH absorptions from the 308 nm photolysis of HNO3 at 2.0 and 4.0 Torr pressures were measured at 308.02 nm. Literature OH quantum yield from the 308 nm photolysis of HNO3 was used in conjunction with experimentally-determined round-trip OH absorption at a given HNO3 pressure and the estimated photolysis photon density absorbed by HNO3 to derive absolute OH absorption calibration at 308.02 nm. Once the OH absorption calibration was performed at 308.02 nm, we either measured OH absorptions from the 308 nm photolysis of 5methyl-2-nitrophenol, or switched the photolysis wavelength to 351 nm (but kept the same probe wavelength as was used in OH absorption calibration) and determined OH absorptions from the 351 nm photolysis of 5-methyl-2-nitrophenol. We compared the OH quantum yields obtained from the photolysis of methyl-2-nitrophenols both in the absence and in the presence of nitrogen or oxygen carrier gas. After correcting for pressure broadening of the OH ro-vibrational line48 at the probe wavelength by the carrier-gas pressure (correction factor: 7% at 100 Torr, 15% at 300 Torr, and 25% at 600 Torr), we obtained similar OH quantum yields from the photolysis of pure methyl-2-nitrophenol and methyl-2-nitrophenol in mixture with nitrogen (20-600 Torr) or oxygen (1-200 Torr) carrier gas. As the effect of diffusion is much reduced in the presence of a carrier gas, such a comparison seems to indicate that diffusion correction to OH absorption value at 15 μs after the photolysis of a pure methyl-2-nitrophenol sample is not needed. The OH quantum yields after the 308 nm and 351 nm photolysis of 4-methyl-2-nitrophenol and 5-methyl-2-nitrophenol are independent of nitrogen carrier gas pressure in the 20-600 Torr range, within the experimental


Page 20 of 40

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error limit. We also did not observe obvious dependence of OH quantum yields on oxygen carrier gas pressure in the 1-200 Torr range. These observations are consistent with absence of significant quenching of excited 4-methyl-2-nitrophenol and 5-methyl-2-nitrophenol by nitrogen or oxygen carrier gas following 308 nm and 351 nm photoexcitation. Based upon these experimental findings, we deemed it suitable to use OH quantum yields after the photolysis of pure methyl-2nitrophenol and its mixture in a carrier gas to derive OH quantum yield from the photolysis of a methyl-2-nitrophenol. The average OH quantum yields from the photolysis of 5-methyl-2nitrophenol were 0.078±0.038 and 0.042±0.015 at 308 nm and 351 nm, respectively, with relative errors (1σ) quoted. Absolute uncertainties in the OH quantum yield measurements are affected by uncertainties in the determination of the following factors: OH radical absorption cross section (~13%), 5-methyl-2-nitrophenol concentration (~4.5%) and absorption cross-section (~9% at 308 nm and ~4% at 351 nm), and photolysis pulse energy (~7% at 308 nm and ~15% at 351 nm). When the relative and the absolute errors are summed, the overall uncertainties in the measurements of the OH quantum yields from 5-methyl-2-nitrophenol photolysis are estimated around 83% at 308 nm, and around 73% at 351 nm. Similar data collection procedures and data analysis algorithms were used to derive OH quantum yields from the photolysis of 4-methyl-2-nitrophenol at 308 nm and 351 nm. The respective average OH quantum yields from the 308 nm and 351 nm photolysis of 4-methyl-2nitrophenol were 0.066±0.021 and 0.031±0.017, where errors quoted are 1σ measurement uncertainty. Bejan et al.15 determined the relative reaction rate constants of OH with 4-methyl-2nitrophenol and 5-methyl-2-nitrophenol. They used isoprene as an OH radical scavenger and observed a small amount of OH formation from the photolysis of these methyl-2-nitrophenols. OH quantum yields of less than 10% from the 308 nm and 351 nm photolysis of 4-methyl-2-



nitrophenol and 5-methyl-2-nitrophenol obtained in the current study are consistent with small OH formation reported by Bejan et al.15 3.2.2. HONO and Possible NO2 Formation from the Photolysis of 4-Methyl- and 5-Methyl-2nitrophenol HONO and possible NO2 products generated from the photolysis of 4-methyl- and 5methyl-2-nitrophenol at 308 nm and 351 nm were monitored by comparing absorption spectra of these compounds with and without photolysis to those of HONO and NO2 standard in the 361-371 nm region. Figure 6 presents absorption spectra of 5-methyl-2-nitrophenol before and after 308 nm photolysis in the 361-371 nm region. As shown, a somewhat broad product absorption maximum appeared at ~369.0-369.8 nm following the 308 nm photolysis of 5-methyl-2nitrophenol. A standard HONO absorption spectrum is also plotted in Figure 6. The HONO reference spectrum was obtained by recording the absorption spectrum of an equilibrated mixture of NO2/NO/H2O/HONO and subtracting the absorption spectrum of unreacted NO2 from the mixture spectrum. A comparison of the absorption spectrum of 5-methyl-2-nitrophenol after 308 nm photolysis with the standard HONO spectrum suggests HONO was formed from the 308 nm photolysis of 5-methyl-2-nitrophenol. HONO also appeared to be a product of 351 nm photolysis of 5-methyl-2-nitrophenol and of 308 and 351 nm photolysis of 4-methyl-2-nitrophenol. NO2 did not appear to be a product of 308 and 351 nm photolysis of both 4-methyl- and 5-methyl-2nitrophenol, and we deduced an upper limit to the NO2 yield of 5%. The HONO absorptions generated by photolyzing 4-methyl- and 5-methyl-2-nitrophenol at 308 nm and 351 nm were obtained by adjusting the wavelength of the ring-down probe beam to HONO absorption maximum and determining cavity losses in the absence and presence of photolysis of mTorr level methylsubstituted 2-nitrophenol. The HONO quantum yield from the 308 or 351 nm photolysis of a


Page 22 of 40

Page 23 of 40

9500

9000

Absorption (ppm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5M2NP after 308 nm photolysis

8500

5M2NP before photolysis 8000

HONO 7500

NO2 7000

6500 362

364

366

368

370

Wavelength (nm)

Figure 6. Absorption spectra of about 1 mTorr of 5-methyl-2-nitrophenol before and after 308 nm photolysis, and absorption spectra for HONO and NO2 standard. Absorption spectra of HONO and NO2 have been vertically shifted for viewing purposes. The spectral scan was carried out at 0.75 nm interval. Absorption spectrum of HONO standard was acquired by subtraction of unreacted NO2 absorption spectrum from

the

measured

absorption

spectrum

of

an

equilibrated

mixture

of

NO2/NO/H2O/HONO. methyl-substituted 2-nitrophenol was derived from the ratio of HONO concentration generated per photolysis pulse to the photolysis photon density absorbed by this methyl-substituted 2nitrophenol. The dependence of the HONO quantum yields on nitrogen buffer gas pressure was examined. The HONO quantum yields were found to be independent of nitrogen carrier gas pressure in the 20-600 Torr range, within the experimental error limit. The HONO quantum yields were also independent of oxygen carrier gas pressure. Representative HONO quantum yields from 23 ACS Paragon Plus Environment


0.5

308 nm photolysis

HONO Quantum Yield

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 40

0.4

0.3

0.2

0.1

Circles: 5M2NP Squares: 5M2NP in nitrogen Triangles: 5M2NP in oxygen

0.0 0

200

400

600

800

P (Total)

Figure 7. Circles represent HONO quantum yields from the 308 nm photolysis of 0.49, 1.6, and 2.6 mTorr of 5-methyl-2-nitrophenol; squares denote HONO quantum yields from the 308 nm photolysis of 0.59, 1.7, and 2.5 mTorr 5-methyl-2-nitrophenol in the presence of 100, 300, and 600 Torr nitrogen carrier gas; triangles are HONO quantum yields from the 308 nm photolysis of 0.52, 1.7, and 2.6 mTorr 5-methyl-2-nitrophenol in the presence of 100 and 300 Torr oxygen carrier gas.

the 308 nm photolysis of pure 5-methyl-2-nitrophenol and its mixture in nitrogen or oxygen carrier gas are shown in Figure 7. The average HONO quantum yields from the 308 nm and 351 nm photolysis 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, where errors quoted (1σ) represent measurement precision. 4. Discussion 4.1. Implications for Atmospheric Photolysis Rates of 4-Methyl- and 5-Methyl-2-nitrophenol and Lifetimes Photolysis rate constants of 4-methyl- and 5-methyl-2-nitrophenol (J) were estimated using the following formula: 24 ACS Paragon Plus Environment

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J = Σσ(λ)·ϕ(λ)·F(λ)Δλ

(3)

,

where F(λ), σ(λ), and ϕ(λ) represent solar actinic flux, absorption cross section, and photolysis quantum yield at wavelength λ. Literature F(λ)Δλ values49 were used. We estimated the 4-methyl2-nitrophenol photolysis rate constant by assuming photolysis quantum yield of 0.33 in the 290310 nm region, 0.29 at 355 nm, linearly variable from 0.33 at 310 nm to 0.29 at 355 nm, and zero in the 360-400 nm region (the sum of OH and HONO quantum yields are 0.33 at 308 nm and 0.29 at 351 nm for 4-methyl-2-nitrophenol). Similarly, we assumed the 5-methyl-2-nitrophenol photolysis quantum yield of 0.45 in the 295-310 nm region, 0.39 at 355 nm, linearly variable from 0.45 at 310 nm to 0.39 at 355 nm, and zero in the 360-400 nm region. Table S1 listed estimated photolysis rate constants for 4-methyl-2-nitrophenol and 5-methyl-2-nitrophenol as a function of the zenith angle for cloudless conditions at sea level and for best-estimate50 albedo. The estimated photolysis rate constants for 4-methyl-2-nitrophenol were 9.6×10-3, 5.0×10-3, and 3.0×10-4 s-1 at 0°, 60º, and 86º zenith angles; these values correspond to photolytic lifetimes of 1.7, 3.3, and 56 min, respectively. Photolysis rate constants for 5-methyl-2-nitrophenol were 1.3×10-2, 6.6×10-3, and 3.9×10-4 s-1 at 0°, 60º, and 86º zenith angles, which correspond to respective photolytic lifetimes of 1.3, 2.5, and 43 min. Reported rate constants15 for reactions of OH with 4-methyl-2nitrophenol and 5-methyl-2-nitrophenol are 3.59×10-12 and 6.72×10-12 cm3•molecule-1•s-1. Gas phase OH radical reaction lifetimes are about 26 h for 4-methyl-2-nitrophenol and about 14 h for 5-methyl-2-nitrophenol, with a 12 h daily average OH concentration51 of about 3.0×106 molecule·cm-3 in New York City in summer. Therefore, photolysis is the dominant tropospheric gas phase removal process for methyl-substituted 2-nitrophenols. 4.2. Comparison of OH and HONO Quantum Yields from Methyl-2-nitrophenol and 2Nitrophenol Photolysis and Mechanistic Implications.



Page 26 of 40

It is interesting to compare OH and HONO quantum yields from the photolysis of 4methyl- and 5-methyl-2-nitrophenol with those from the photolysis of 2-nitrophenol. The OH quantum yields from the 308 nm and 351 nm photolysis of 4-methyl-2-nitrophenol and 5-methyl2-nitrophenol are a factor of 9-22 lower than those of 0.69±0.07 and 0.70±0.07 from the 308 nm and 351 nm photolysis of 2-nitrophenol (see Table 2). Table 2. Quantum Yields of OH and HONO from the Photolysis of 4-Methyl-2-nitrophenol (4M2NP), 5-Methyl-2-nitrophenol (5M2NP), and 2-Nitrophenol (2NP). Wavelength

4M2NP

5M2NP

2NP

ϕOH

ϕHONO

ϕOH

ϕHONO

ϕOH

ϕHONO

308

0.066±0.021

0.26±0.06

0.078±0.038

0.37±0.05

0.69±0.07

0.34±0.09

351

0.031±0.017

0.26±0.03

0.042±0.015

0.35±0.06

0.70±0.07

0.39±0.07

(nm)

On the other hand, the HONO quantum yields from the 308 and 351 nm photolysis of 4-methyland 5-methyl-2-nitrophenol were similar in size to those of 0.34±0.09 and 0.39±0.07 for 2nitrophenol at 308 and 351 nm photolysis wavelengths, suggesting HONO formation pathway is not sensitive to methyl-substitution on the benzene ring. The ortho position of the OH and the NO2 group in both 2-nitrophenol and methyl-substituted 2-nitrophenols can facilitate intramolecular hydrogen bonding to form aci-nitro isomer.52-54 Such an isomer was proposed as an intermediate for HONO formation from the near-UV photolysis of 2-nitrophenol in the liquid55 and gas phase16. Similar HONO quantum yields from the photolysis of both 2-nitrophenol and methyl-substituted 2-nitrophenols provide experimental support for such a postulate. Theoretical studies26,27 have also suggested OH to be a product from dissociation of aci-nitro isomer of 226 ACS Paragon Plus Environment

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nitrophenol. Since there have been no theoretical calculations of the potential energy surfaces for methyl-substituted 2-nitrophenols (M2NP), we provide in Figure 8 a scheme for OH and HONO formation from the photolysis of these compounds based upon theoretical studies of OH and HONO formation from the photolysis of 2-nitrophenol.

Figure 8. Scheme for OH and HONO formation from the photolysis of methylsubstituted 2-nitrophenol (M2NP) drawn in analogy to those calculated26,27 for OH and HONO formation from the photolysis of 2-nitrophenol. Absorption of a UV photon excites methyl-substituted 2-nitrophenol from the ground state (S0) to the first-excited single state (S1) which can undergo rapid intersystem crossing to form excited triplet state (T1) of M2NP and 3aci-M2NP. The much lower OH quantum yields from the 308 nm and 351 nm photolysis of 4-methyl-2-nitrophenol and 5-methyl-2-nitrophenol compared to those from the photolysis of 2-nitrophenol seems to suggest that methyl-substitution on the benzene ring may have increased the barrier height for the OH formation from the dissociation of 3aci-M2NP. Further theoretical studies are needed to understand the mechanism of 2-nitrophenol and methylsubstituted 2-nitrophenol photolysis. 27 ACS Paragon Plus Environment


4.3. OH and HONO Production Rates from Methyl-2-nitrophenol Photolysis and Potential Implications on Missing OH Sources from Field Studies. Based on our study results, the estimated HONO formation rates from the gas phase photolysis of 4-methyl-2-nitrophenol and 5-methyl-2-nitrophenol at 16.9° zenith angle were about 7.9×10-3 and 1.1×10-2 s-1, respectively, while 16.9° zenith angle corresponds to 40°N latitude for July 1 at noontime. Literature HONO formation rates of 1.1×10-5 and 2.4×10-5 s-1 were reported by Bejan et al.16 from the photolysis of 4-methyl-2-nitrophenol and 5-methyl-2-nitrophenol in a flow tube photoreactor with broadband 300-500 nm irradiation. Literature HONO formation rates from the photolysis of 4-methyl- and 5-methyl-2-nitrophenol under atmospheric condition at 16.9° zenith angle were around 4.7×10-5 and 1.0×10-4 s-1 by scaling up the HONO formation rates measured in the photoreactor16 by the ratio of NO2 photolysis rates15 measured in the atmosphere to that measured in the photoreactor. Our estimated HONO formation rates for 4-methyl-2nitrophenol and 5-methyl-2-nitrophenol were 168 and 110 times faster than those reported by Bejan et al. The stability of the methyl-2-nitrophenol sources varied significantly in the Bejan et al. study16 while FTIR was not sensitive enough to monitor changes in methyl-2-nitrophenol concentrations. It is unknown how absolute methyl-2-nitrophenol concentration was calibrated in this prior study and the accuracy of such calibration. It is also not clear what OH concentrations were used by Bejan et al.16 and whether OH could have removed significant amount of HONO. It should be noted that the Bejan et al. study16 and the current study were carried out using very different experimental setups, which may have contributed to the difference in findings. In our study, we found it was crucial to treat the chamber wall to prevent the loss of methyl-2-nitrophenol to the chamber wall from affecting methyl-2-nitrophenol gas phase concentration. We want to point out that the photolysis volume in our study is about 27 cm3, which is a small fraction of the


Page 28 of 40

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gas chamber volume of about 2.1 liter.

Although 4-methyl-2-nitrophenol and 5-methyl-2-

nitrophenol on the photolysis beam path were photolyzed quickly, the photolysis and the probe beam only overlapped at the center of the cell and there were unphotolyzed 4-methyl-2-nitrophenol and 5-methyl-2-nitrophenol on the probe beam path even at 60 min after the photolysis (see Figure S1). Formation of oxidants from the gas phase photolysis of methyl-substituted 2-nitrophenol isomers has not previously been included in the atmospheric chemistry models such as Master Chemical Mechanism56. Assuming for simplicity of estimation that the abundance of 4-methyl-2nitrophenol and 5-methyl-2-nitrophenol is at 10 pptv each in the polluted environment, the primary and secondary (i.e., use HONO as secondary OH source) formation rates of OH from the gas phase photolysis of 4-methyl-2-nitrophenol are on the order of 2.3×106 and 1.2×106 molecules•cm-3•s-1 at 16.9° and 60° zenith angles. The primary and secondary OH production rates from the gas phase photolysis of 5-methyl-2-nitrophenol are on the order of 3.0×106 and 1.7×106 molecules•cm-3•s-1 at 16.9° and 60° zenith angles. Mao et al. reported57 a peak OH production rate of 1.8×107 molecules•cm-3•s-1 from HONO photolysis in a New York City field campaign, a peak OH production rate of 1.8×107 molecules•cm-3•s-1 from formaldehyde photolysis in a Mexico City field campaign, and a peak OH production rate of 1.4×107 molecules•cm-3•s-1 from ozone photolysis and subsequent O(1D)+H2O reaction in a La Porte, TX field campaign. The OH production rates of 2.3×106 and 3.0×106 molecules•cm-3•s-1 from photolyzing 10 pptv of 4-methyl- and 5-methyl2-nitrophenol at 16.9° zenith angle are significant compared to the fresh OH production rates from photolyzing HONO, formaldehyde, and ozone reported in field campaigns.57 4-Methyl-2-nitrophenol and 5-methyl-2-nitrophenol can be formed from the nighttime reaction of NO3 with p-cresol and m-cresol, cumulated overnight, and photolyzed in the early



morning hours to form OH and HONO. Gas phase reaction of nitrate radical with p-cresol has a reported rate constant13 of 1.1×10-11 cm3molecule-1s-1 at 296 K and a 4-methyl-2-nitrophenol yield of 0.74. Gas phase reaction of nitrate radical with m-cresol has a reported rate constant of 9.7×10-12 cm3molecule-1s-1 and a 5-methyl-2-nitrophenol yield of 0.20. To maintain 4-methyl-2nitrophenol or 5-methyl-2-nitrophenol concentration of 10 pptv throughout the day, similar daytime production rate and photolysis removal rate would be required to maintain steady-state in the atmosphere. 4-Methyl-2-nitrophenol can be formed from OH radical-initiated oxidation of pcresol with a rate constant3 of 5.0×10-11 cm3molecule-1s-1 and a yield11 of 7.6%. 5-Methyl-2nitrophenol can be formed from OH radical-initiated oxidation of m-cresol with a rate constant of 6.8×10-11 cm3molecule-1s-1 and a yield of 4.4%. A highly polluted environment is likely needed to sustain daytime 4-methyl-2-nitrophenol and 5-methyl-2-nitrophenol sources. In the metropolitan region of Beijing (China), ambient concentrations of methyl-nitrophenols up to 50 pptv have been detected58 in a haze. In the VOC-rich polluted atmosphere such as the polluted suburban environment near Beijing and the Pearl River Delta region of China, the measured OH concentrations are significantly higher than predicted values, even after accounting for the contributions of the known OH sources1,2. The larger OH production rate than the OH loss rate and the high aromatic concentrations in the polluted region in China may indeed suggest the existence of unidentified OH sources that can be formed in the atmosphere and photolyzed to generate OH radicals. Our prior laboratory study results on the photolysis of 2-nitrophenol and current results on the photolysis of methyl-substituted 2-nitrophenols have provided quantitative experimental demonstrations as to why the contribution of these gas phase photolytic processes should be included in closing the OH and the HONO budgets in the polluted atmosphere.


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5. Conclusions Methyl-substituted 2-nitrophenol isomers and 2-nitrophenol are important constituents of atmospheric “brown carbon”. Investigations of the gas phase photolysis of these compounds are not only crucial for understanding the molecular level chemistry of these photolysis processes, but also of practical importance in identifying and quantifying atmospheric oxidant sources and levels. The fact that the HONO quantum yields are similar from the 308 nm and 351 nm photolysis of 2nitrophenol and methyl-substituted 2-nitrophenol isomers while OH quantum yields are very different indicates the involvement of different potential energy surfaces in forming OH and HONO products. The current study has provided quantitative absorption cross-section and product quantum yield information needed for objective assessments of the extent of oxidant formation from the photolysis of methyl-substituted 2-nitrophenol isomers in the atmosphere. Despite many decades of research, the atmospheric chemistry problem of missing OH sources and missing HONO sources in the polluted atmosphere has not been resolved. Results of our study illustrate why gas phase photolysis of 2-nitrophenol and methyl-substituted 2-nitrophenols can drive atmospheric oxidant formation, and why they should be included in models such as Master Chemical Mechanism56 to close the gap in budgets of OH and HONO for the polluted atmosphere. Gas phase photolysis of 2-nitrophenol and methyl-substituted 2-nitrophenols is also a renoxification process that can convert 2-nitrophenol and methyl-2-nitrophenols from a NOx reservoir into photochemically active forms of HONO and NO, affecting not only the HOx cycles but also the NOx cycles in the atmosphere. Supporting Information Table S1 provides estimated photolysis rate constants of 4-methyl-2-nitrophenol and 5methyl-2-nitrophenol as a function of solar zenith angle. Figure S1 shows the dependence of 4-



methyl-2-nitrophenol and 5-methyl-2-nitrophenol absorption at 305 nm as a function of their residence time in the chamber (t = 0-60 min) with and without 308 nm photolysis. Acknowledgments Insightful comments and suggestions by Dr. Jingqiu Mao, and helpful discussions with Drs. Liang T. Chu, Robert Keesee, and Xianliang Zhou are acknowledged. We thank Dr. Kimberly McClive-Reed for editing the manuscript. We are grateful for the support provided by the National Science Foundation under grant #AGS-1405610. References 1.

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TOC Graphic


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Role of Methyl-2-nitrophenol Photolysis as a Potential Source of OH

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