Absorption cross sections of 2?Nitrophenol in the 295?400 nm region and photolysis of 2?Nitrophenol at 308 and 351 nm

Article abstract of DOI:10.1021/acs.jpca.6b08961

2-Nitrophenol is an important component of “brown carbon” in the atmosphere. Photolysis is its dominant gas phase removal process. We have determined the gas phase absorption cross sections of 2-nitrophenol in the 295?400 nm region by using cavity ring-down spectroscopy. 2-Nitrophenol exhibits a broad absorption band over the wavelength region studied, with the peak absorption located at 345 nm. Absorption cross section values range between (2.86 ± 0.18) × 10^?18 and (2.63 ± 0.31) × 10^?20 cm²/ molecule over the 295?400 nm range. We have investigated the HONO, NO₂, and OH formation channels following the gas phase photolysis of 2-nitrophenol at 308 and 351 nm. Direct NO₂ formation was not observed. HONO and OH are direct products from 2-nitrophenol photolysis. The average OH quantum yields from the photolysis of 0.5, 1.0, and 2.0 mTorr of 2-nitrophenol are 0.69 ± 0.07 and 0.70 ± 0.07 at 308 and 351 nm. The average HONO quantum yields are 0.34 ± 0.09 and 0.39 ± 0.07 at 308 and 351 nm. The OH and HONO quantum yields are independent of nitrogen carrier gas pressure in the 20?600 Torr range. Oxidant formation rate constants from 2-nitrophenol photolysis have been calculated. Discussions have been made concerning the role of 2-nitrophenol gas phase photolysis in the formation of atmospheric oxidants in regions of high anthropogenic emissions.

Full text of DOI:10.1021/acs.jpca.6b08961

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Article
Absorption Cross Sections of 2-Nitrophenol in the 295-400 nm
Region, and Photolysis of 2-Nitrophenol at 308 and 351 nm
Manuvesh Sangwan, and Lei Zhu
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b08961 • Publication Date (Web): 29 Nov 2016
Downloaded from http://pubs.acs.org on December 7, 2016
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Absorption Cross Sections of 2ꢀNitrophenol in the 295ꢀ400 nm Region, and
Photolysis of 2ꢀNitrophenol at 308 and 351 nm
Manuvesh Sangwan and Lei Zhu*
Wadsworth Center, New York State Department of Health, and
Department of Environmental Health Sciences, University at Albany
Albany, NY 12201ꢀ0509
*
Corresponding author. Tel: 518ꢀ474ꢀ6846; fax: 518ꢀ473ꢀ2895; eꢀmail: zhul@wadsworth.org.
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Abstract
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2ꢀNitrophenol is an important component of “brown carbon” in the atmosphere.
Photolysis is its dominant gas phase removal process. We have determined the gas phase
absorption cross sections of 2ꢀnitrophenol in the 295ꢀ400 nm region by using cavity ringꢀdown
spectroscopy. 2ꢀNitrophenol exhibits broad absorption band over the wavelength region studied,
with peak absorption located at 345 nm. Absorption cross section values range between
(2.86±0.18)×10ꢀ18 and (2.63±0.31)×10ꢀ20 cm2/molecule over the 295ꢀ400 nm range. We have
investigated the HONO, NO2, and OH formation channels following the gas phase photolysis of
2ꢀnitrophenol at 308 and 351 nm. Direct NO2 formation was not observed. HONO and OH are
direct products from 2ꢀnitrophenol photolysis. The average OH quantum yields from the
photolysis of 0.5, 1.0, and 2.0 mTorr of 2ꢀnitrophenol are 0.69±0.07 and 0.70±0.07 at 308 and
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351 nm. The average HONO quantum yields are 0.34±0.09 and 0.39±0.07 at 308 and 351 nm.
The OH and HONO quantum yields are independent of nitrogen carrier gas pressure in the 20ꢀ
600 Torr range. Oxidant formation rate constants from 2ꢀnitrophenol photolysis have been
calculated. Discussions have been made concerning the role of 2ꢀnitrophenol gas phase
photolysis in the formation of atmospheric oxidants in regions of high anthropogenic emissions.
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1. Introduction
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The significantly elevated daytime nitrous acid (HONO) concentrations1ꢀ5 compared to
those predicted based on the photo stationary state between HONO sources and sinks lead to the
postulate that HONO is produced by photochemical sources. One proposed HONO source is gas
phase photolysis6 of 2ꢀnitrophenol (oꢀC6H4(NO2)OH) over the 300ꢀ500 nm region. 2ꢀ
Nitrophenol is emitted from the burning of coal and wood and from industrial sources such as
manufacturers of phenolꢀformaldehyde resins, dyes, explosives, and pharmaceutical drugs.7 It
can be formed from OH radicalꢀinitiated oxidation of phenol in the presence of NOx or reaction
of phenol with nitrate radicals.8ꢀ10 2ꢀNitrophenol has been observed in the gas, particle, and
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condensed phases of the environment.7,11ꢀ16
It is phytotoxic.17 Reported gas phase
concentrations18ꢀ23 of 2ꢀnitrophenol vary with geographic locations; they are in the range of 0.8ꢀ
350 ng/m3 (i.e., 0.14 to 62 pptv).
2ꢀNitrophenol in aqueous solution absorbs strongly in the near UV region.24 There have
been one reported study25 of the gas phase near UV absorption spectrum of 2ꢀnitrophenol in the
320ꢀ450 nm region using incoherent broadꢀband cavity enhanced absorption spectroscopy. But
the absorption cross section data are only qualitative in nature. Also, absorption cross section
values have not been obtained in the 290ꢀ320 nm region. Moreover, it is unclear why the
reported gas phase 2ꢀnitrophenol cross section values are 11ꢀ91 times larger than the gas phase
absorption cross sections of 2ꢀnitrobenzaldehyde,26 another nitroꢀaromatic compounds. Given
the key role 2ꢀnitrophenol gas phase photolysis likely plays in atmospheric HONO formation,
there is clearly a need to quantitatively determine the near UV absorption cross section values of
this compound so that contribution of gas phase 2ꢀnitrophenol photolysis to HONO formation
can be quantified. The ortho position of the OH and NO2 groups in 2ꢀnitrophenol enables strong
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intramolecular hydrogen bonding interaction to form aciꢀnitro isomer;27,28 the structure and
confirmation of which were identified29 by a combination of matrix isolation infrared
spectroscopy and densityꢀfunctional theory calculations. The aciꢀnitro isomer of 2ꢀnitrophenol
was postulated6 as an intermediate in HONO formation in the near UV photolysis of 2ꢀ
nitrophenol in the liquid30 and gas phase6. But the HONO quantum yield from the gas phase
photodissociation of 2ꢀnitrophenol and the variation of the HONO quantum yield with excitation
wavelength have not been reported. Formation of OH radical was observed by laser induced
fluorescence from gas phase photodissociation dynamics studies31,32 of 2ꢀnitrophenol at 266 nm,
355 nm, and in the 361ꢀ390 nm range. However, the corresponding OH quantum yields were not
measured. As a result of lacking quantitative product quantum yield information, it has not been
possible to assess the extent of oxidant formation from 2ꢀnitrophenol photolysis in the
atmosphere.
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In this paper, we present results obtained from quantitative measurements of the gas
phase absorption cross sections of 2ꢀnitrophenol over the 295ꢀ400 nm range, by using cavity
ringꢀdown spectroscopy33,34. We have investigated the HONO, NO2, and OH formation channels
following the 308 and 351 nm photolysis of 2ꢀnitrophenol. The OH and the HONO quantum
yields have been obtained. We have estimated the atmospheric oxidant formation rate constants
following the gas phase photolysis of 2ꢀnitrophenol using 2ꢀnitrophenol near UV absorption
cross sections, and OH and HONO formation quantum yields obtained from the current study.
2. Experimental technique
Gasꢀphase absorption cross sections of 2ꢀnitrophenol in the 295ꢀ400 nm region were
measured in a 48 cm long ringꢀdown cavity. Both ends of the cavity cell were vacuumꢀsealed by
a pair of high reflectance cavity mirrors. Highꢀreflectance cavity mirrors with centering
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wavelengths at about 307 nm, 325 nm, 355 nm, and 373 nm were used; the corresponding
wavelength intervals for high reflectivity are 295ꢀ320 nm, 310ꢀ341 nm, 331ꢀ378 nm, and
339ꢀ407 nm. Detailed descriptions of our experimental setup can be found elsewhere.35ꢀ38 The
probe beam over the 295−400 nm range was generated by frequency doubling the output of a dye
laser (Continuum, ND6000) pumped by the second harmonic of a Nd:YAG laser (Continuum,
Surelite II). The probe beam entered the ringꢀdown cavity through the front cavity mirror. Dye
solutions of Rhodamin 610, Rhodamin 640, 4ꢀdicyanmethyleneꢀ2ꢀmethylꢀ6ꢀ(pꢀdimethylamino
styryl)ꢀ4Hꢀpyran (DCM), pyridine 1 (LDSꢀ698), LDS751, LDS 765 and LDS 821 were used.
Photon intensity decay inside the cavity was monitored by a photomultiplier tube (PMT) placed
behind the rear cavity mirror. The output of PMT was amplified, digitized, and sent to a
computer. The decay profile was fitted with a singleꢀexponential decay function to calculate the
ringꢀdown time constant (τ) and the total loss (Γ) per roundꢀtrip pass. The absorbance of 2ꢀ
nitrophenol was obtained by measuring cavity losses in the presence and absence of 2ꢀ
nitrophenol. The optical path length is 48 cm. The gas pressure inside the cell was monitored by
MKS Baratron capacitance manometers (0.1 and 1 Torr full scale; most of the measurements
were made using 0.1 Torr fullꢀscale pressure gauge). Before every experiment, the cavity cell
was pumped to 10ꢀ5 Torr. For absorption cross section measurements, pure 2ꢀnitrophenol was
used; the pressure range used varied between 5.0×10ꢀ4 and 1.5×10ꢀ2 Torr. The highest 2ꢀ
nitrophenol pressure used (1.5×10ꢀ2 Torr) was constrained by the strong near UV absorption of 2ꢀ
nitrophenol. Pressure measurement uncertainty is estimated in the range of 2.5%ꢀ0.5% for the 2ꢀ
nitrophenol pressure of 5.0×10ꢀ4 to 1.5×10ꢀ2 Torr when a 0.1 Torr fullꢀscale Baratron capacitance
monometer was used. The cell was evacuated to the initial vacuum conditions before a fresh
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sample of 2ꢀnitrophenol was added. Cross section measurements were made under static
conditions.
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Photolysis of 2ꢀnitrophenol was carried out by combining excimer laser photolysis at 308
nm and at 351 nm with cavity ringꢀdown spectroscopy. The output from an excimer laser
(Coherent COMPEX Pro102) was directed into the stainless steel reaction cell at a 150 angle
with the main cell axis through a side arm. With a 12mm wide and 6 mm tall apertured
photolysis beam, the illumination volume of the photolysis beam through the level arm is about
27.4 cm3. The probe beam entered the cell along the main cell axis. At the center of the cavity,
the pump beam and the probe beam overlapped geometrically. The overlap region of the
photolysis and the probe laser is a rectangular solid with its center overlapping that of the cell,
and its width and height defined by the dimension of the apertured photolysis beam; the length of
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ꢀ1
the rectangular solid is defined by (beam width) × (tan15°) , where 15° is the crossing angle
between the pump and the probe laser beams. The length of the photolysis/probe laser overlap
ꢀ1
region is defined by (beam width) × (sin15°) . The length of the photolysis/probe laser overlap
region is about 4.6 cm with a 12 mm wide photolysis beam. The delay time between the firing
of the photolysis laser and the firing of the probe laser was varied with a pulse/delay generator.
Formation of HONO and possible NO2 products from the 2ꢀnitrophenol photolysis was
monitored by their near UV absorptions39ꢀ46 in the 364 – 370 nm region. Production of OH from
the photolysis of 2ꢀnitrophenol was detected by OH radical’s characteristic sharp absorption
band47 in the 307.80ꢀ308.30 nm region. Absolute OH absorption was calibrated relative to that
from the 308 nm photolysis of nitric acid. Product absorptions from the photolysis of 2ꢀ
nitrophenol were obtained by measurement of the cavity losses in the presence of 2ꢀnitrophenol,
with and without photolysis. The photolysis beam energies prior to its entrance into the cell and
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after its exit from an empty cell were measured by a calibrated joulemeter. The incident beam
energy inside the cell was corrected for transmission loss of the photolysis beam at the front cell
window, and for reflection of the photolysis beam from the rear cell window.
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Reagents and Bath Gas Used and Synthetic Processes of HNO3 and HONO
2ꢀNitrophenol was obtained as a yellow crystalline powder from SigmaꢀAldrich with
≥98% purity. It was stored in a glass bulb connected to the cavity cell and was pumped for about
1 hr. Before running the experiments, the 2ꢀnitrophenol pressure of about 0.1 Torr was
introduced into the cell; the sample was kept in the cavity cell for about 45 min before it was
completely evacuated. This procedure of conditioning the cell wall minimized the loss of gas
phase 2ꢀnitrophenol sample during cross section and photolysis product measurements. Nitrogen
carrier gas (Airgas; ≥99.999% purity) was used without further purification. Nitric acid was
prepared48 by vacuum distillation of a 3:2 mixture of sulfuric acid (98%; Mallinckrodt Baker)
and nitric acid (70%; Mallinckrodt Baker) at 273 K; the distilled HNO3 was collected into a trap
cooled to ethanol/dry ice temperature (195 K). At least four successive distillations were
conducted to purify the HNO3 sample. To further reduce NO2 impurity in HNO3, an extra
distillation of the HNO3 sample was done on the day of the experiment. HONO was prepared by
mixing 1:1:2 mixture of NO2:NO:H2O in a gas bulb and allowing for at least 24 hours for the
reaction NO2+NO+H2O = 2HONO to reach equilibrium. NO2 was obtained from SigmaꢀAldrich
(≥99.5% purity) and NO was obtained from Airgas (≥99.5% purity). Liquid H2O from a
Barnstead nanopure ultrapure water system (Thermo Scientific, USA) was freezeꢀpumpꢀthawed
for three times before water vapor was used in HONO synthesis. All experiments were
conducted at an ambient temperature of 295±2 K.
3. Results and discussion
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3.1. Gas phase absorption cross sections of 2ꢀnitrophenol in the 295ꢀ400 nm region.
We have obtained gas phase absorption cross sections of 2ꢀnitrophenol at 5 nm intervals
in the 295ꢀ400 nm region by using cavity ringꢀdown spectroscopy. Absorption cross section at
each wavelength was obtained by measurement of the roundꢀtrip absorbance by 2ꢀnitrophenol as
a function of the 2ꢀnitrophenol pressure in the cavity. Roundꢀtrip absorbance at a given 2ꢀ
nitrophenol pressure was determined from the differential measurement of cavity losses in the
absence and in the presence of 2ꢀnitrophenol. Shown in Figure 1 is roundꢀtrip absorbance
plotted against 2ꢀnitrophenol pressure at 300 nm, 330 nm, 355 nm, and 385 nm, respectively.
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25000
20000
15000
10000
5000
0
35000
30000
25000
20000
15000
10000
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0
300 nm
330 nm
-5000
0.000
-5000
0.000
0.001
0.002
0.003
0.004
0.005
0.006
0.001
0.002
0.003
0.004
0.005
0.006
P (Torr)
P (Torr)
30000
25000
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15000
10000
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0
20000
15000
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355 nm
385 nm
0.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016
0.000
0.002
0.004
0.006
0.008
0.010
P (Torr)
P (Torr)
Figure 1. Roundꢀtrip absorbance as a function of the 2ꢀnitrophenol pressure at 300 nm,
330 nm, 355 nm, and 385 nm. The solid lines are linear regression analysis of experimental
data from which absorption cross sections of 1.39×10ꢀ18, 1.74×10ꢀ18, 1.03×10ꢀ18, and
2.98×10ꢀ19cm2/molecule were extracted at 300 nm, 330 nm, 355 nm, and 385 nm, respectively.
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From the slope of the linear regression analysis, 2ꢀnitrophenol absorption cross sections of
(1.39±0.08)×10ꢀ18, (1.74±0.11)×10ꢀ18, (1.03±0.04)×10ꢀ18, and (2.98±0.08)×10ꢀ19 cm2/molecule
were extracted at 300 nm, 330 nm, 355 nm, and 385 nm. Listed in Table 1 and plotted in
Figure 2 are absorption cross sections of 2ꢀnitrophenol as a function of wavelength. 2ꢀ
Nitrophenol exhibits broad spectral absorption in the 295ꢀ400 nm region, with peak absorption
located at 345 nm. The near UV absorption spectrum of 2ꢀnitrophenol49 originates from the π →
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π* transition between the benzene ring and the nitro group. Absorption cross section values vary
between (2.86±0.18)×10ꢀ18 and (2.63±0.31)×10ꢀ20 cm2/molecule over the wavelength range
studied. Errors quoted (1σ) are relative errors derived from standard deviation of about 3
Table 1. Gas phase absorption cross sections (σ, in unit of cm2/molecule, base e) of 2ꢀnitrophenol as
a function of wavelength (λ).
λ (nm)
295
300
305
310
315
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325
330
335
340
345
350
355
360
365
370
375
380
385
390
395
400
σ (this work)a
σ (Chen et al.)
(1.47±0.11)×10ꢀ18
(1.36±0.08)×10ꢀ18
(1.10±0.12)×10ꢀ18
(1.44±0.10)×10ꢀ18
(1.77±0.19)×10ꢀ18
(1.80±0.15)×10ꢀ18
(1.60±0.18)×10ꢀ18
(1.98±0.17)×10ꢀ18
(2.03±0.19)×10ꢀ18
(2.23±0.12)×10ꢀ18
(2.86±0.18)×10ꢀ18
(2.29±0.21)×10ꢀ18
(1.12±0.05)×10ꢀ18
(9.37±0.29)×10ꢀ19
(8.16±0.38)×10ꢀ19
(7.24±0.25)×10ꢀ19
(6.26±0.29)×10ꢀ19
(3.73±0.16)×10ꢀ19
(2.88±0.12)×10ꢀ19
(9.29±0.38)×10ꢀ20
(3.10±0.10)×10ꢀ20
(2.63±0.31)×10ꢀ20
9.26×10ꢀ18
1.52×10ꢀ17
1.60×10ꢀ17
1.66×10ꢀ17
1.66×10ꢀ17
1.48×10ꢀ17
1.33×10ꢀ17
1.18×10ꢀ17
8.83×10ꢀ18
6.35×10ꢀ18
4.65×10ꢀ18
3.40×10ꢀ18
2.03×10ꢀ18
1.13×10ꢀ18
6.30×10ꢀ19
3.40×10ꢀ19
2.00×10ꢀ19
aErrors quoted are 1σ standard deviations in the precision of the measurements.
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independent cross section measurements. Factors contributing to the systematic uncertainties
include absolute uncertainties in pressure and absorption measurements and accurate knowledge
of the 2ꢀnitrophenol concentration in the cell. An up to 2% impurity was specified for 2ꢀ
nitrophenol crystalline powder. Despite pumping the 2ꢀnitrophenol powder in a glass vessel for
an extended period of time before the experiments and pumping 2ꢀnitrophenol powder before
each fill, and quick sample filling time to minimize the effect of outgassing from the chamber
wall, the overall uncertainty of gas phase 2ꢀnitrophenol concentration is expected to be more
than 2%.
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10-16
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280
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Wavelength (nm)
Figure 2. Absorption cross section of 2ꢀnitrophenol as a function of wavelength
over the 295ꢀ400 nm range. Circles represent cross section data obtained in the present
work. The solid line denotes cross section data reported in Reference 25.
Shown in Figure 2 for comparison are previously reported25 gas phase absorption cross
sections of 2ꢀnitrophenol in the 320ꢀ400 nm region. Both the spectral shape and the absolute
cross section values differ between the literature study and the current study. Literature
absorption cross sections of 2ꢀnitrophenol25 vary from 9.26×10ꢀ18 cm2/molecule at 320 nm to
1.66×10ꢀ17 cm2/molecule at 335 nm, and to 2.00×10ꢀ19 cm2/molecule at 400 nm. Absorption
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cross sections of 2ꢀnitrophenol obtained in this study range from (1.80±0.15)×10ꢀ18 cm2/molecule
at 320 nm to (2.03±0.19)×10ꢀ18 cm2/molecule at 335 nm, and to (2.63±0.31)×10ꢀ20 at 400 nm.
Thus, literature absorption cross sections of 2ꢀnitrophenol are 5.1 times, 8.2 times, and 7.6 times
our cross section values at 320 nm, 335 nm, and 400 nm. Such large cross section differences
likely result from the lack of validation of an experimental method for low vapor pressure
species and lack of choosing proper experimental conditions in this literature study. Although
the literature study25 reported similar cross section values for benzaldehyde using incoherent
broadꢀband cavity enhanced absorption spectroscopy compared with those obtained previously
by our group26 using cavity ringꢀdown spectroscopy, the room temperature vapor pressure of 2ꢀ
nitrophenol (~0.1 Torr) is about one order of magnitude lower than that of benzaldehyde. Both
cavity ringꢀdown spectroscopy and cavity enhanced absorption spectroscopy are not backgroundꢀ
free absorption detection method. In order to obtain quantitative absorption values, it is
important that the measured absorption is distinguishable from the measured background cavity
loss. One set of cavity mirrors were used in the cavity enhanced spectroscopic measurement of
2ꢀnitrophenol absorption, with a reflectivity of up to 99% for both mirrors at the wavelengths of
highest reflectivity. We used several sets of mirrors; the mirror reflectivity at a wavelength
region of high reflectivity (99.8%ꢀ99.9%) is about one order of magnitude higher than that of the
literature study. The maximum mixing ratio of 90 ppbv of 2ꢀnitrophenol was used in the
literature study25, which would translate to the maximum 2ꢀnitrophenol partial pressure of
6.8×10ꢀ5 Torr. The minimum 2ꢀnitrophenol pressure we used was 5.0×10ꢀ4 Torr. The minimum
2ꢀnitrophenol pressure we used in the cross section determination is 7.3 times the maximum
pressure used in the prior study. The literature study did not provide Beer’s law absorption
calibration over a range of 2ꢀnitrophenol pressure at any representative wavelength. We
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obtained cross section values from Beer’s law plot at each wavelength. The combination of high
mirror loss, low 2ꢀnitrophenol pressure used, and small absorption value relative to the large
background mirror loss let us conclude this literature study is qualitative in nature.
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Gasꢀphase absorption cross sections of 2ꢀnitrophenol in the 295ꢀ400 nm region
determined in the current study are compared to those26 of 2ꢀnitrobenzaldehyde, the only other
multiꢀfunctional aromatic compound with nitro substitution, whose gas phase absorption cross
section data are available. Cross section values for both compounds are plotted in Figure 3.
Absorption cross sections of 2ꢀnitrophenol are larger than those of 2ꢀnitrobenzaldehyde over the
wavelength region studied. Since the origin of the near UV absorption band for 2ꢀnitrophenol49
is that of the π→π* transition from the NO2 group whereas the near UV absorption band for 2ꢀ
nitrobenzaldehyde results from the n→π* transitions from the NO2 and the CHO groups,50 the
larger near UV absorption cross section values for 2ꢀnitrophenol compared to those for 2ꢀ
10-17
10-18
10-19
10-20
ꢀꢀ 2-Nitrophenol (this work)
ꢀꢀ
ꢁꢁ 2-Nitrobenzaldehyde (Xiang et al., 2009)
ꢁꢁ
10-21
280
300
320
340
360
380
400
420
Wavelength (nm)
Figure 3. Absorption cross sections of 2ꢀnitrophenol and 2ꢀnitrobenzaldehyde as a
function of wavelength over the 295ꢀ400 nm range. Circles denote 2ꢀnitrophenol
cross section values obtained from the current study. Squares represent 2ꢀnitrobenzaldehyde
cross section data obtained by Xiang et al.26
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nitrobenzaldehyde can be rationalized in terms of the larger transition strength of the π→π*
transition for 2ꢀnitrophenol.
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3.2. Photolysis of 2ꢀnitrophenol at 308 and 351 nm.
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3.2.1. Timeꢀresolved study of OH radical formation from 2ꢀnitrophenol photolysis.
Formation of OH radical from the photolysis of 2ꢀnitrophenol was investigated by
combining excimer laser photolysis at 308 and 351 nm with cavity ringꢀdown spectroscopy.
Illustrated in the upper panel of Figure 4 are cavity ringꢀdown spectra plotted against wavelength
in the 307.80ꢀ308.30 nm region in the absence and presence of 2ꢀnitrophenol photolysis at 308
nm. A sharp absorption band peaking at 308.02 nm appeared immediately after the photolysis.
This new absorption band is attributed to that of 2ꢀnitrophenol photodissociation product. The
OH radical has wellꢀknown A2Σ+, v’=0 ← X2Π, v”=0 absorption band47 around 308 nm. To
verify whether the product absorption band at 308.02 nm is that of OH, we measured the OH
radical absorption spectrum from the 308 nm photolysis of HNO3.51 Shown in the lower panel of
Figure 4 are cavity ringꢀdown spectra with and without 308 nm photolysis of HNO3. The
spectral feature of the product absorption band formed from 2ꢀnitrophenol photolysis matches
that of OH formed from the HNO3 photolysis. This gives clear indication that the OH radical is
formed from 2ꢀnitrophenol photolysis at 308 nm. The OH radical was also observed as a product
from the 351 nm photolysis of 2ꢀnitrophenol. By tuning the wavelength of the ringꢀdown probe
beam to OH absorption maximum at 308.02 nm and varying the delay time between the firing of
the photolysis laser and the firing of the probe laser, absorption of OH formed from 2ꢀ
nitrophenol photolysis was recorded as a function of time. Figure 5 shows an OH radical roundꢀ
trip absorption profile from 308 nm photolysis of 1.0 mTorr of 2ꢀnitrophenol as a function of
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P
= 1 mTorr
2-nitrophenol
2.0 mTorr 2-Nitrophenol
18000
16000
14000
12000
9
With 308 nm photolysis
w photolysis
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No photolysis
w/o photolysis
307.7
307.8
307.9
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308.1
308.2
308.3
Wavelength (nm)
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10000
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2.0 Torr HNO3
w photolysis
w/o photolysis
307.7
307.8
307.9
308.0
308.1
308.2
308.3
Wavelength (nm)
Figure 4. Upper panel: cavity ringꢀdown spectra plotted against wavelength in the
307.80ꢀ308.30 nm region in the absence and presence of 308 nm photolysis of
2ꢀnitrophenol at 2.0 mTorr pressure. Photolysis/probe laser delay was set at 15 ꢀs.
Wavelength scan was conducted at 0.01 nm interval. Lower panel: cavity ringꢀdown
spectra with and without 308 nm photolysis of HNO3 at 2.0 Torr pressure.
time. OH radical absorption at 308.02 nm increased immediately after the photolysis pulse, and
it subsequently decreased rapidly with photolysis/probe laser delay time in the 20ꢀ80 ꢀs range.
The measured initial OH absorption values at 15 µs are higher for higher 2ꢀnitrophenol
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1.0 mTorr 2-nitrophenol
308 nm photolysis
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t (ꢀꢀs)
ꢀꢀ
Figure 5. Transient OH radical roundꢀtrip absorption profile from the 308 nm
photolysis of 1.0 mTorr of 2ꢀnitrophenol measured at 308.02nm.
pressures. The decay rate of OH radical absorption in the 15ꢀ60 ꢀs range does not increase with
increasing 2ꢀnitrophenol pressure for 2ꢀnitrophenol pressures of 0.5 mTorr, 1.0 mTorr, and 2.0
mTorr. Such results are consistent with literature OH/2ꢀnitrophenol reaction rate constant52 of
9.0×10ꢀ13 cm3moleculeꢀ1sꢀ1 and the limited pressure range we varied 2ꢀnitrophenol pressure,
which was constrained by strong near UV absorption of the probe beam by the background 2ꢀ
nitrophenol molecules. The OH decay profiles from the photolysis of 2ꢀnitrophenol at 0.5
mTorr, 1.0 mTorr and 2.0 mTorr pressures were used to extract OH diffusion coefficient in the
presence of 2ꢀnitrophenol at mTorr pressure; it is about 5.9×104 cm2/s. Diffusion out of the
probe beam likely plays an important role in the decay of OH radical absorption in the 15ꢀ80 ꢀs
range for OH temporal absorption profile shown in Figure 5. Temporal absorption measurement
of OH shows that OH radical absorption formed from 2ꢀnitrophenol photolysis in the
photolysis/probe laser overlap region decays with photolysis/probe laser delay time, and we need
to use OH absorption at 15 ꢀs delay time to obtain nascent OH absorption formed from 2ꢀ
nitrophenol photolysis.
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The OH radical quantum yield from the photolysis of 2ꢀnitrophenol was derived from the
ratio of the OH concentration produced in the photolysis/probe laser overlap region to the
absorbed photon density in the same region. The absorbed photolysis photon density in the
photolysis and detection laser overlap region was calculated from the difference between the
transmitted photolysis beam energies entering (Ein) and leaving (Eout) the overlap region, the
individual photolysis photon energy (hc/λ), and the volume (v) of the overlap region. The
photolysis beam energy before and after the photolysis/probe laser overlap region was obtained
from the incident photolysis beam energy entering the cell (E0), the absorption cross section (σ)
and the concentration (n) of 2ꢀnitrophenol in the cell, and the absorbing path length, by applying
the BeerꢀLambert law:
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Ein = E0•exp(ꢀσnl1)
Eout = E0•exp(ꢀσnl2
(1)
(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 photon fluence used in the OH quantum yield determination was ~0.04 J/cm2 at 308
nm and ~0.006 J/cm2 at 351 nm. Absorbed photolysis photon densities in the pump/probe laser
overlap region from the 308 nm photolysis of 0.5, 1.0, and 2.0 mTorr of 2ꢀnitrophenol were
estimated to be 1.4×1012, 2.8×1012, and 5.5×1012 molecules/cm3, respectively. Absorbed
photolysis photon densities from 351 nm photolysis of 2ꢀnitrophenol at 0.5, 1.0, and 2.0 mTorr
pressure were 3.2×1011, 6.4×1011, and 1.3×1012 molecules/cm3. The OH radical concentration
after the 308 or 351 nm photolysis of a 2ꢀnitrophenol sample was obtained by measurement of
OH absorption at 308.02 nm at a photolysis and a probe laser delay of 15 ꢁs. Calibration of the
absolute OH concentration from the 2ꢀnitrophenol photolysis was made by measurement of OH
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absorption from the 308 nm HNO3 photolysis for which OH formation quantum yield of unity
was reported.51 Absorbed photon densities following the 308 nm photolysis of 0.5 Torr, 1.0
Torr, and 2.0 Torr HNO3 and the roundꢀtrip OH absorptions at 308.02 nm after the photolysis of
HNO3 were determined. Combining experimentally obtained round trip OH absorption for a
given HNO3 pressure with literature OH quantum yield of 1 from the 308 nm photolysis of
HNO3 and the estimated absorbed photolysis photon density by HNO3 allows us to obtain
absolute OH absorption calibration at 308.02 nm. The average OH quantum yields from
photodissociation of 2ꢀnitrophenol at 0.5, 1.0, and 2.0 mTorr pressures are 0.69±0.07 at 308 nm
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and 0.70±0.07 at 351 nm, where errors quoted (1σ) represent experimental scatter. Systematic
errors in the OH quantum yield measurements include absolute uncertainties in the determination
of: OH radical absorption cross section (~13%), 2ꢀnitrophenol concentration (~4%) and
absorption cross section (~9% at 308 nm and 351 nm), and photolysis pulse energy (~5% at 308
nm and ~10% at 351 nm). Considering both random and systematic errors, the overall
uncertainties in the determination of the OH quantum yields from 2ꢀnitrophenol photolysis are
about 41% at 308 nm, and about 46% at 351 nm. The OH quantum yields are very similar from
the 308 nm and 351 nm photolysis of 2ꢀnitrophenol, suggesting that the photon energies at these
wavelengths are much higher than the dissociation threshold for 2ꢀnitrophenol to form
OH+•C6H4NO2.
The dependence of the OH quantum yields from the 308 nm and 351 nm photolysis of 2ꢀ
nitrophenol on nitrogen carrier gas pressure was examined. The OH quantum yields at these
photolysis wavelengths were found to be independent of the nitrogen carrier gas pressure within
experimental error limit in the 20ꢀ600 Torr range. Since 2ꢀnitrophenol has a large number of
vibrational degrees of freedoms (3Nꢀ6 = 39), its intramolecular vibrational and rotational
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relaxation is much faster than intermolecular collisional quenching by nitrogen.
If
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intramolecular vibrational and rotational relaxation occurs following 308 and 351 nm excitation
of 2ꢀnitrophenol but still above its dissociation threshold, OH quantum yield is not expected to
be dependent on nitrogen carrier gas pressure.
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3.2.2. Monitoring HONO and possible NO2 formation from the 2ꢀnitrophenol photolysis.
Formation of HONO and possible NO2 products from the 2ꢀnitrophenol photolysis at 308
nm and 351 nm was investigated by comparing the 2ꢀnitrophenol absorption spectra in the
presence and absence of photolysis with the standard HONO and NO2 absorption spectra in the
364ꢀ370 nm region. Shown in Figure 6 are absorption spectra of 2ꢀnitrophenol in the 364ꢀ370
nm region with and without photolysis at 308 nm.
A product absorption band peaking at ~368.5ꢀ369.0 nm appeared after photolyzing 2ꢀ
nitrophenol at 308 nm. Also displayed in Figure 6 is a standard HONO absorption spectrum
recorded in the 364ꢀ370 nm region. HONO reference absorption spectrum in the 364ꢀ370 nm
region was obtained by measuring the spectral absorption of the equilibrated
NO2/NO/H2O/HONO mixture and subtracting the spectral absorption of the unreacted NO2.
Shown in the inset is a net product spectrum after 308 nm photolysis of 2ꢀnitrophenol and a
HONO absorption spectrum in the 367.5ꢀ370.0 nm region. After comparing the net production
absorption spectrum from the photolysis at 308 nm with the standard HONO spectrum, it is
apparent that HONO is formed from the 2ꢀnitrophenol photolysis at 308 nm. HONO is also a
product from 2ꢀnitrophenol photolysis at 351 nm. On the other hand, a comparison of the 2ꢀ
nitrophenol absorption spectra following 308 nm and 351 nm photolysis with the standard NO2
spectrum indicates NO2 is not formed directly from the 2ꢀnitrophenol photolysis.
The HONO quantum yields formed from the 2ꢀnitrophenol photolysis at 308 nm and 351
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nm were determined by tuning the probe laser to HONO absorption peak around 368.5 nm and
measuring the cavity losses in the absence and presence of photolysis of 2ꢀnitrophenol at 2ꢀ
nitrophenol pressures of 1.0 mTorr, 2.0 mTorr, 3.0 mTorr, and 4.0 mTorr. By dividing the
HONO concentration formed from the 2ꢀnitrophenol photolysis per photolysis pulse by the
absorbed photolysis photon density, we obtained the HONO quantum yield formed from the
photolysis of 2ꢀnitrophenol at a given pressure. The HONO quantum yield was found to be
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Wavelength (nm)
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3000
2-nitrophenol w/o photolysis
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NO2
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HONO
0
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Wavelength (nm)
Figure 6. Cavity ringꢀdown absorption spectra of 2.0 mTorr of 2ꢀnitrophenol
with and without 308 nm photolysis along with absorption spectra for HONO
and for NO2. Spectra have been vertically shifted for ease of viewing.
The spectral scan was conducted at 0.50 nm interval. Absorption by HONO standard was
obtained by subtracting absorption by unreacted NO2 from the measured total
absorption by an equilibrated NO2/NO/H2O/HONO mixture. Shown in inset are net
2ꢀnitrophenol photolysis product absorption spectrum, obtained by subtracting
2ꢀnitrophenol absorption spectrum in the absence of photolysis from that
in the presence of photolysis, and HONO spectrum in the 367.5ꢀ370.0 nm region.
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independent of 2ꢀnitrophenol pressure. The average HONO quantum yields from the 308 nm
and 351 nm photolysis of 2ꢀnitrophenol are 0.34±0.09 and 0.39±0.07, where error quoted (1σ)
represents experimental scatter. The magnitude of HONO yields is similar from the 308 and 351
nm photolysis of 2ꢀnitrophenol to within the experimental error limit.
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To derive HONO concentration in the HONO quantum yield measurements, HONO
absorptions from 2ꢀnitrophenol photolysis at 308 nm and 351 nm were determined by averaging
absorption measurements over 10 photolysis pulses. One may ask if 308 nm and 351 nm
photolysis pulses could photolyze HONO product formed in the data acquisition and data
averaging processes. HONO does not have known absorption39 at 308 nm. HONO is not
expected to be photolyzed by the 308 nm photolysis beam. The 351 nm photon flux used in the
study was low (~0.006 J/cm2), HONO absorption cross section39 at 351 nm is 12 times smaller
than that of 2ꢀnitrophenol, and HONO concentration is about two orders of magnitude smaller
than 2ꢀnitrophenol concentration. Photodissociation of HONO formed by each successive 351
nm photolysis pulse is estimated to decrease HONO yield by about 0.2%. Because the effect is
so small, we did not make correction to the HONO yield at 351 nm.
The dependence of HONO quantum yields on nitrogen carrier gas pressure was
examined. Nitrogen pressure was varied between 20 and 600 Torr. The HONO yields from 2ꢀ
nitrophenol photolysis at 308 and 351 nm were found to be independent of nitrogen pressure to
within the experimental error limit.
3.3. Oxidant formation rate constants from gas phase 2ꢀnitrophenol photolysis and
atmospheric implications of the results.
Atmospheric photolysis rate constant of 2ꢀnitrophenol (J2-Nitrophenol) was calculated using
the following equation:
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J2-Nitrophenol = Σσ(λ)·ϕ(λ)·F(λ)ꢂλ
(3)
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obtained in the present study, and ϕ(λ) is the photolysis quantum yield for 2ꢀnitrophenol. F(λ)ꢂλ
values reported by Demerjian et al.53 were used. Photolysis rate constants were estimated for
two scenarios of photolysis quantum yields. One scenario assumes photolysis quantum yield to
be unity in the 295ꢀ400 nm region. The other scenario assumes photolysis quantum yield to be
unity in the 295ꢀ355 nm region and zero in the 360ꢀ400 nm region. The first scenario and the
second scenario provide upper and lower limits of the 2ꢀnitrophenol photolysis rate constant.
Our experimental results have already shown that the sum of the OH and HONO quantum yields
of about unity both at 308 nm and 351 nm. As OH radical was observed by laser induced
fluorescence from gas phase photodissociation dynamics studies31,32 of 2ꢀnitrophenol in the 361ꢀ
390 nm range, there is 2ꢀnitrophenol photolysis in the 360ꢀ390 nm region as well. The
photolysis rate constants for 2ꢀnitrophenol were modeled as a function of the zenith angle for
cloudless conditions, at sea level, and for bestꢀestimate albedo54; they are plotted in Figure 7.
The modeled photolysis rate constants for 2ꢀnitrophenol are 1.9×10ꢀ2 to 1.0×10ꢀ2 sꢀ1 for 0ꢀ60º
zenith angles at the upper limit, which translate to photolysis lifetimes of 0.9 to 1.7 min. The
modeled photolysis rate constants for 2ꢀnitrophenol are 1.4×10ꢀ2 to 6.9×10ꢀ3 sꢀ1 for 0ꢀ60º zenith
angles at the lower limit, which correspond to photolysis lifetimes of 1.2 to 2.4 min. Gas phase
reaction of 2ꢀnitrophenol with OH has a reported rate constant52 of 9×10ꢀ13 cm3moleculeꢀ1sꢀ1.
Using 12 h daily average OH concentration55 of 1.6×106 moleculeꢃcmꢀ3, the atmospheric OH
radical reaction lifetime of 2ꢀnitrophenol is about 193 h. Even at 86º zenith angle, the modeled
photolysis rate constants for 2ꢀnitrophenol are 6.2×10ꢀ4 and 4.0×10ꢀ4 sꢀ1 at the upper and the
lower limit, which correspond to photolysis lifetimes of 27 min and 42 min. Photolysis is a
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Upper limit
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Lower limit
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Zenith angle (degree)
Figure 7. Modeled atmospheric photodissociation rate constants of 2ꢀnitrophenol as a function of
solar zenith angle under cloudless conditions, at sea level, and for bestꢀestimate albedo. Circles:
upper limit of the photolysis rate constants by assuming photolysis quantum yield of 1 in the 295ꢀ
400 nm region. Squares: Lower limit of the photolysis rate constants by assuming photolysis
quantum yield of 1 over the 295ꢀ355 nm range and 0 over the 360ꢀ400 nm range.
dominant gas phase degradation process for 2ꢀnitrophenol from the atmosphere at all zenith
angles. Since the oxidant formation quantum yields from 308 nm and 351 nm photolysis of 2ꢀ
nitrophenol are about unity, the lower limit of oxidant formation rate constants from 2ꢀ
nitrophenol gas phase photolysis are estimated in the range of 1.4×10ꢀ2 to 6.9×10ꢀ3 sꢀ1 for 0ꢀ60º
zenith angles.
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The modeled 2ꢀnitrophenol photolysis rate constant based upon current laboratory study
results is two orders of magnitude faster than that of 1.4%×photolysis rate constant of NO2
reported by Bardini.56 For example, the upper and lower limits of our estimated 2ꢀnitrophenol
photolysis rate constants at 60º zenith angle are 1.0×10ꢀ2 sꢀ1 and 6.9×10ꢀ3 sꢀ1. Bardini’s 2ꢀ
nitrophenol photolysis rate constant is about 6.3×10ꢀ5 sꢀ1 at 60° zenith angle. Our calculated 2ꢀ
nitrophenol photolysis rates are 110ꢀ159 times faster than that reported by Bardini in 2006. The
same group published25 qualitative 2ꢀnitrophenol gas phase absorption cross section data in
2011. Using the photolysis rate reported by Bardini and the 2ꢀnitrophenol absorption cross
section data from the same group would yield a 2ꢀnitrophenol spectrallyꢀaveraged photolysis
quantum yield25 of 0.1%. Our direct experimental measurements of OH and HONO yields has
shown the 2ꢀnitrophenol photolysis quantum yields to be unity at 308 nm and 351 nm. We
speculate that the photolysis rate constant reported by Bardini is a gross underestimate.
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The modeled lower limit of HONO formation rate constant from 2ꢀnitrophenol gas phase
photolysis at 60° zenith angle using our 2ꢀnitrophenol absorption cross section and HONO
quantum yield data is about 2.6×10ꢀ3 sꢀ1. Formation rate constant of HONO from the gas phase
2ꢀnitrophenol photolysis of 2.9×10ꢀ5 sꢀ1 was reported6 in a photoꢀreactor with broadband 300ꢀ500
nm irradiation by Bejan et al. Our calculated HONO formation rate constant from 2ꢀnitrophenol
photolysis is about two orders of magnitude faster than that reported by Bejan et al. The stability
of the nitrophenol source varied significantly during the experiments in the literature study.6
The variable and lack of calibration of absolute 2ꢀnitrophenol concentration in air mixture in the
prior study may have contributed to the large difference between the estimated HONO formation
rate constant based on the results of the current study and the HONO formation rate constant
reported6 by Bejan et al. The literature study was conducted in the time frame of several hours.
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Significant HONO loss is expected when HONO formed from 2ꢀnitrophenol photolysis was
photolyzed in a photoꢀreactor for several hours. The OH radical formed from 2ꢀnitrophenol
photolysis could also react with HONO. These secondary losses could decrease the observed
HONO concentrations and resulted in much lower HONO formation rate constant as reported in
this prior study.
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The importance of the gas phase 2ꢀnitrophenol photolysis to HONO and OH formation in the
atmosphere is not included in the current atmospheric chemistry model, as there was no prior
quantitative laboratory near UV absorption cross section and product quantum yield information
which would suggest this process to be important. Yuan et al.23 measured ambient concentrations
of phenol, nitrophenols (2ꢀnitrophenol and 4ꢀnitrophenol), methylꢀnitrophenols, and
dinitrophenols in the Uintah Basin, UT in winter as a result of oil and gas operations. They
found phenol is mainly formed from OH radical oxidation of benzene during the daytime, while
2ꢀnitrophenol is formed by subsequent oxidation of phenol both during the daytime and the night
time. They used 2ꢀnitrophenol photolysis rate constant56 reported by Bardini in the box model
simulation. But the box model cannot simultaneously fit their measured 2ꢀnitrophenol and
phenol data. If the measured phenol concentrations were used as a constraint in the box model
simulation of their data, they would predict much higher nitrophenol concentrations than the
measurement values. The supplement of Yuan et al. paper showed that the modeled 2ꢀ
nitrophenol level could be up to 150 ppt when the measured nitrophenol level is ~10ꢀ15 ppt,
Given that our study has shown that the calculated 2ꢀnitrophenol photolysis rate is much faster
than that reported by Bardini, inclusion of faster 2ꢀnitrophenol photolysis rate constant in the box
model will likely reduce discrepancy in the simultaneous fitting of phenol and nitrophenol data,
and explain why the modeled nitrophenol concentrations are always higher than the
measurement values for both daytime and night time when 2ꢀnitrophenol photolysis rate constant
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reported by Bardini was used. Assuming photolysis rates for nitrophenols, methylꢀnitrophenols,
and dinitrophenols were the same as that reported by Bardini, and assuming HONO yields were
100% from the photolysis of these compounds, Yuan et al. found the steadyꢀstate HONO
concentration would only increase by 0.5 ppt in the morning and 0.2 ppt during the noontime
period from the gas phase photolysis of these compounds. These additional HONO
concentrations are a small fraction of the measured HONO concentrations57 (50ꢀ100 ppt) during
UBWOS2014, which led Yuan et al. to conclude that the photolysis of nitrophenols, methylꢀ
nitrophenols, and dinitrophenols to be insignificant in forming HONO in the polluted
atmosphere. Contrary to the conclusion by Yuan et al., if we use the calculated 2ꢀnitrophenol
photolysis rate constants determined in the current study, and the HONO yield of ~0.34 from 2ꢀ
nitrophenol gas phase photolysis, and assuming photolysis rate constants and HONO quantum
yields of 4ꢀnitrophenol, methylꢀnitrophenols and dinitrophenols are similar to those for 2ꢀ
nitrophenol, we estimated the total HONO concentration from the gas phase photolysis of
nitrophenols, methylꢀnitrophenols, and dinitrophenols to be around 18ꢀ26 ppt in the morning and
7ꢀ10 ppt during the noontime period. It is not difficult to see that HONO formation from the gas
phase photolysis of these nitrophenol compounds can account for a significant fraction of the
HONO concentration in the UBWOS 2014. The coꢀproduct of the HONO formation channel
from the gas phase 2ꢀnitrophenol photolysis can extract an Hꢀatom to form phenoxy radical
(C6H5O•); subsequent reaction58 of phenoxy radical with NO2 in the atmosphere can regenerate
2ꢀnitrophenol. Results of the present laboratory study highlight the importance of 2ꢀnitrophenol
photolysis in the formation of OH and HONO in the atmosphere. Its contribution to the oxidation
capacity of the atmosphere should not be overlooked.
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In addition to gas phase degradation processes, wet deposition is generally thought to
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play an important role in the removal of nitrophenol compounds from the atmosphere. The
partition between the atmospheric gas and the liquid phase is different for different nitrophenols.
For example, 2ꢀnitrophenol is much more hydrophobic than 4ꢀnitrophenol,59 which can displace
2ꢀnitrophenol towards the gas phase and leave the less volatile 4ꢀnitrophenol being preferentially
present in the aqueous phase.7,21,22 For 2ꢀnitrophenol, its Henry’s law constant in the aqueous
phase59 is small (81 M atmꢀ1 at 298 K). For a typical tropospheric cloud water volume fraction
of ~10ꢀ6 at ~280 K, an equilibrium partitioning of ~1% into the aqueous phase is predicted from
the Henry’s Law coefficient for 2ꢀnitrophenol. Given that the current study indicates that gas
phase 2ꢀnitrophenol photolysis is several orders of magnitude faster than its reaction with OH
radical in the gas phase, 2ꢀnitrophenol gas phase photolysis is likely much more important in
removing 2ꢀnitrophenol from the atmosphere than its aqueous phase removal.
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Conclusions
2ꢀNitrophenol is an important component of “brown carbon” in the atmosphere. The
concentration of 2ꢀnitrophenol is expected to be high in polluted urban areas where there are
increased emissions of aromatic hydrocarbons. To assess the air quality impacts of pollutant
emission, it is important to know oxidant formation potential of the emitted species. Although
photodissociation dynamics studies31,32 of 2ꢀnitrophenol have reported OH formation at
photolysis wavelengths of 266 nm, 355 nm, and over the 361ꢀ390 nm range, and HONO was
observed as a product6 from 2ꢀnitrophenol photolysis in smog chamber, the lack of quantitative
absorption cross section and product quantum yield information has prevented quantitative
assessment of the extent of oxidant formation from 2ꢀnitrophenol photolysis in the atmosphere.
The current work has served the purpose of filling this missing knowledge gap, without which
the importance of the gas phase photolysis of 2ꢀnitrophenol in the formation of atmospheric
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oxidants cannot be assessed with any certainty. Given that there are multitudes of pollutants in
the atmosphere and there are many factors that could affect ambient oxidant concentrations, it is
often difficult to identify important processes that contribute to atmospheric oxidant formation
based upon field study results alone. Our experimental work has demonstrated why careful
laboratory studies are still needed to address the complex problem of atmospheric oxidant
formation.
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Acknowledgments
We are grateful for the support provided by the National Science Foundation under grant
#AGSꢀ1405610.
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