Parahalogenated phenols accelerate the photochemical release of nitrogen oxides from frozen solutions containing nitrate

Article abstract of DOI:10.1021/jp210249t

The photolysis of nitrate anion (NO₃^-) contained in surface ice and snow can be a regionally significant source of gas-phase nitrogen oxides and affect the composition of the planetary boundary layer. In this study, the photochemical release of nitrogen oxides from frozen solutions containing NO₃^- in the presence of organic compounds was investigated. Gas-phase nitrogen oxides were quantified primarily by NO-O ₃ chemiluminescence detection of NO and NO_y (=NO + NO ₂ + HONO + HNO₃ + -PAN + -AN...) and cavity ring-down spectroscopy of NO₂ and total alkyl nitrates (-AN). The photochemical production of gas-phase NO_y was suppressed by the presence of formate, methanesulfonate, toluene, or phenol. In contrast, para-halogenated phenols (in the order of Cl > Br > F) promoted the conversion of NO ₃^- to gas-phase NO_y, rationalized by acidification of the ice surface.

Full text of DOI:10.1021/jp210249t

Article
pubs.acs.org/JPCA
Parahalogenated Phenols Accelerate the Photochemical Release of
Nitrogen Oxides from Frozen Solutions Containing Nitrate
Otman Abida† and Hans D. Osthoff*
Department of Chemistry, University of Calgary, 2500 University Drive Northwest, Calgary, Alberta T2N 1N4, Canada
S
* Supporting Information
ABSTRACT: The photolysis of nitrate anion (NO3−) contained in
surface ice and snow can be a regionally significant source of gas-phase
nitrogen oxides and affect the composition of the planetary boundary
layer. In this study, the photochemical release of nitrogen oxides from
frozen solutions containing NO3− in the presence of organic
compounds was investigated. Gas-phase nitrogen oxides were
quantified primarily by NO−O3 chemiluminescence detection of NO
and NOy (=NO + NO2 + HONO + HNO3 + ∑PAN + ∑AN ...) and
cavity ring-down spectroscopy of NO2 and total alkyl nitrates (∑AN).
The photochemical production of gas-phase NOy was suppressed by
the presence of formate, methanesulfonate, toluene, or phenol. In contrast, para-halogenated phenols (in the order of Cl > Br >
F) promoted the conversion of NO3− to gas-phase NOy, rationalized by acidification of the ice surface.
interpretation of polar ice core data, in particular because this
chemistry is accompanied by isotopic fractionation.20,21 Third,
photolysis of NO3−, NO2−, and HONO yields the hydroxyl
radical (OH), the most important oxidant in the atmosphere.
Production of OH from NO3− photolysis in the condensed
phase has consequences on, among others, the breakdown of
persistent organic pollutants and has been shown to lead to
photonitration of aromatic compounds to generate toxic and
carcinogenic nitroaromatics in both the liquid22−31 and ice32
phases. Further, OH radicals produced from NO3− photolysis
can release active halogens from the condensed phase.33
Because of its importance to atmospheric chemistry,
photolysis of NO3− in ice has been the subject of numerous
laboratory studies.34−49 Many factors are known to affect the
yield of nitrogen oxides in the gas phase, including temperature,
ionic strength, and acidity. For example, in a previous study in
our laboratory, we found the production of gas-phase nitrogen
oxides to be promoted by acidic conditions.49 To explain higher
yields of gas-phase NO2 at low pH, we suggested that
peroxynitric acid (HOONO), formed in a minor channel in
the reaction of OH with NO2, acts as a volatile intermediate
under acidic conditions.48,49 Furthermore, the extent of
accumulation of NO3− ions and other ice solute molecules in
either a liquid or quasi-liquid phase on the ice surface and/or in
so-called micropockets or microveins upon freezing is
undoubtedly an important factor.50−52
1. INTRODUCTION
Atmospheric chemistry is driven to a very large part by
photochemical reactions in the gas phase. Important photo-
chemical reactions also take place on surfaces,1−3 in liquids
such as clouds,4 and in terrestrial snow and ice,5−7 where
photochemically initiated transformations of inorganic and
organic substances can have a profound influence on the
composition of the planetary boundary layer.8−12 The nitrate
anion (NO3−) is one of the major anions present in the polar
snowpack,13 and its photolysis is of particular importance to the
polar boundary layer. It is generally accepted that photolysis of
NO3− in ice occurs mainly in a thin liquid-like surface layer;7
hence, its photochemistry closely resembles that encountered in
aqueous solutions where photolysis of NO3− in the relatively
weak n → π* band proceeds via two main channels.14,15
−
−
−
+
NO + hv → NO + O
O + H → OH
(1)
(2)
3
2
NO3 + hv → NO2 + O(3P)
−
−
In ice, the quantum yields of reactions 1 and 2 are of similar
magnitude (e.g., at 263 K, Φ1 ≈ 0.0040 and Φ2 ≈ 0.0015).7
The nitrite anion (NO2−), produced by reaction 2, readily
photodissociates to produce NO.
−
−
−
+
NO + hv → NO + O
O + H → OH
(3)
2
The photolysis of NO3− in ice is of interest for several
reasons. First, the nitrogen oxide photoproducts (i.e., NOx =
NO + NO2 and, under acidic conditions,16 HONO) are volatile
and partition to the gas phase. There, NOx catalyzes the
photochemical production of ozone,11 an important green-
house gas, and ultimately changes the oxidative balance of the
polar boundary layer.17−19 Second, photolysis of NO3− removes
it from snow and ice and thus needs to be considered in the
Surprisingly, there have been relatively few studies
investigating how the presence of organic impurities might
Special Issue: A. R. Ravishankara Festschrift
Received: October 25, 2011
Revised: December 19, 2011
Published: January 3, 2012
© 2012 American Chemical Society
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affect the production of nitrogen oxides from photolysis of
NO3− in ice. For example, Dubowski et al.36 and Minero et al.47
have shown that the yield of NO2− from NO3− photolysis can
be increased up to 5-fold by the presence of organic
compounds such as formate, but we are not aware of a study
investigating how the presence of organic compounds affects
the yield of nitrogen oxides in the gas phase.
In this study, we investigated the production of gas-phase
nitrogen oxides from NO3− photolysis in ice under actinic
conditions (i.e., λ > 300 nm) in the presence of the organic
compounds formate (HCOO−), methanesulfonate (MSA−,
CH3SO3−), methanesulfonic acid (MSA, CH3SO3H), the
neutral aromatic compounds toluene and phenol, and para-
halogenated phenols (4-XP) such as 4-chlorophenol (4-CP).
Chlorinated phenols are ubiquitous in the environment as they
are introduced by a multitude of anthropogenic activities, such
as water disinfection, waste incineration, use of pesticides and
herbicides, and the bleaching of paper pulp with chlorine.53
Further, a considerable amount of literature exists on both the
direct54−60 and indirect photodegradation of 4-CP, for example,
by OH produced from NO3− photolysis in aqueous solution,23
from H2O2 photolysis in ice,61 or from NO2− photolysis in
ice,62,63 making it a suitable model organic compound to
include in this study.
laser cavity ring-down spectrometer (CRDS) as described
earlier.48 The CL instrument is equipped with a heated Mo
catalyst; even though this instrument is marketed as a NOx
analyzer, the catalyst was found to reduce various components
of NOy (=NO + NO2 + ∑PN + ∑AN + HNO3 ...) to NO. In a
separate experiment, we verified that the CL NOx analyzer was
sensitive to HONO and HNO3. A thermal decomposition
CRDS was used for selective measurement of either NO2, NO2
plus total peroxy nitrates (∑PAN), or NO2 + ∑PAN + total
alkyl nitrates (∑AN).65,66 The TD-CRDS was connected to
the photoreactor via PFA Teflon tubing and compression
fittings; the exhaust of the TD-CRDS was then sampled by the
CL analyzer. Potential interferences in the blue diode laser
CRDS are discussed in the Supporting Information (SI). In
some experiments, iodide ion chemical ionization mass
spectrometry (CIMS) was used as described by Abida et al.48
to confirm the evolution of HNO2 and HNO3. For selected
experiments, organic photoproducts formed were monitored by
HPLC (see the SI).
3. RESULTS
3.1. Photochemical Conversion of NO3− to NOy in the
Presence of Organic Modifiers. Figure 1 shows representa-
2. EXPERIMENTAL METHODS
2.1. Chemicals. All reagents were of the purest
commercially available grade and used as received. Sodium
nitrate, phenol (4-HP), 4-fluorophenol (4-FP), 4-bromophenol
(4-BP), 4-CP, sodium MSA−, MSA, sodium formate (HCOO−),
toluene, and isopropanol were obtained from Sigma-Aldrich.
All aqueous solutions were prepared with deionized water
(Barnstead Easypure II, 18 MΩ cm). The pH of the solutions
was measured prior to freezing. Dilute (0.05 M) buffer
solutions were prepared by mixing the appropriate amounts
of sodium carbonate and bicarbonate (Sigma-Aldrich; pH 9.5)
or sodium acetate and acetic acid (Sigma-Aldrich; pH 4.5).
2.2. Photolysis Setup. The photolysis setup is shown in
Figure 1 of Abida et al.48 Briefly, the experiments were
conducted using a custom-built Pyrex photoreactor (internal
volume 275 cm3). Samples of frozen solutions were prepared
by placing 4 mL of precooled aqueous solution onto the
bottom of the photoreactor tube, which was temperature-
controlled using a circulating chiller/circulator. The ice
temperature was 253 K in all experiments unless stated
otherwise. The reactor was continuously (i.e., during sample
freezing and irradiation) flushed with ultrapure (“zero”) air
(Praxair) at a flow rate of (1.4 0.1) L min−1. The zero air
entered the reaction chamber near the top and exited via a side
arm located 1 cm above the ice surface, minimizing the
residence time of evolving gases in the chamber. Upon freezing,
polycrystalline ice pellets with an average thickness of about 2
mm and an area of approximately 19.6 cm2 were obtained.
The photolysis light source was a 150 W xenon arc lamp
(ORIEL 96000) equipped with an IR filter (ORIEL 81096) and
a digital exposure controller (ORIEL 68951). Its output was
directed through a Pyrex window (λ > 300 nm) at the top of
reactor. The incident light flux was measured offline by
ferrioxalate actinometry64 to be 1.5 × 1016 photon s−1 cm−2.
2.3. Quantification of Photolysis Products. Concen-
trations of gas-phase nitrogen oxides were primarily monitored
using a commercial NO/NOy chemiluminescence (CL)
analyzer (Thermo 42TL), calibrated against a blue diode
Figure 1. Production of gas-phase NOy upon irradiation of frozen (T =
−20 °C) solutions containing 1.0 M NaNO3 and the organic additive
at the concentration indicated. Irradiation was commenced at t = 0.
While most organic compounds suppress, MSA and 4-CP enhance
NOy production.
tive time series of NOy released from irradiated frozen solutions
containing 1.0 M NaNO3. Irradiation was commenced at t = 0.
In the absence of organic additives, approximately 2.1 parts-per-
billion by volume (ppbv, 10−9) of NOy were observed. The
addition of HCOO−, phenol, toluene, or MSA− lowered the
amount of NOy produced, with the lowest amount (400 parts
per trillion by volume, or 400 pptv) observed in the presence of
formate. When MSA (pKa ≈ −0.6;67 the pH of a 10 mM
solution prior to freezing was measured at 3.1) or 4-CP (10
mM) was added, more NOy was evolved than was in their
absence. In the case of MSA, the yield of NOy approximately
doubled to 4.0 ppb, whereas the addition of 4-CP increased the
yield of NOy by a factor of approximately six to about 12 ppbv.
3.2. Photochemical Conversion of NO3− to NOy in the
Presence of para-Halogenated Phenols. Figure 2 shows a
comparison of NOy observed upon irradiation of ice samples
containing 1.0 M NaNO3 and 10 mM 4-HP, 4-FP, 4-BP, or 4-
CP. The shaded areas show the amounts of NO2 observed by
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experiments, in which the TD-CRDS sampled the effluent of
the photoreactor after about 1/2 h of irradiation. When 4-CP
or 4-BP were present in the ice, the TD-CRDS signal increased
until a plateau was reached at an inlet temperature of about
400−420 °C. This temperature profile is consistent with the
presence of alkyl nitrates (∑AN) in the effluent.65,68 We
verified the absence of nitryl chloride (ClNO2), which
dissociates at the same inlet temperature as ∑AN,69 offline
by measuring the effluent using chemical ionization mass
spectrometry (Figure S-2, SI). In contrast to 4-BP and 4-CP,
we found no evidence for alkyl nitrate formation in the
presence of 4-FP (Figure 3) or in the absence of the other
organic additives (data not shown). Table 1 summarizes the
mixing ratios of the NOy components for the data shown in
Figures 2 and 3.
Figure 2. Comparison of photochemical production of NOy from
photolysis of 1.0 M NaNO3 in the presence of 10 mM 4-CP, 4-BP, 4-
FP, or 4-HP at −20 °C. The shaded areas indicate the fraction of NOy
due to NO2 measured by CRDS. The lamp was switched off after 1/2
h.
Table 1. Yields of NOy species (in ppbv) Observed after 1/2
h of Irradiation for the Experiment Shown in Figure 2
species
4-HP
4-FP
0.4
4-BP
4-CP
NO (obs)
NO2 (obs)
0.1
0.4
n/d
0.3
2.6
1.3
3.2
7.4
0.6
3.8
a
3.7
CRDS at 405 nm (see the SI for a discussion of potential
positive interferences). NO data and the NO2 data for 4-FP are
shown in Figure S-1 (SI). Considerable differences are seen in
the yield of nitrogen oxides. The highest amounts of NOy and
NO2 were observed in the presence of 4-CP, followed by 4-BP,
4-FP, and 4-HP. In the case of 4-HP (and likely also 4-FP; see
the SI), most (∼85%) of the NOy observed was in the form of
NO2. In the presence 4-BP and 4-CP, a high NO2 content was
observed in the early phase of the irradiation, which stabilized
over the course of a few minutes. In both cases, mixing ratios of
NOz (=NOy − NOx) continued to increase during the
irradiation experiment. Using CIMS, we identified (see Figure
S-2 (SI) for an example) the NOz species as HONO (95%) and
HNO3 (5%). When the lamp was switched off after 1/2 h of
irradiation, the 405 nm CRDS absorption signal returned to the
baseline relatively quickly (except for 4-FP; see the SI), whereas
NOz decayed more slowly. The latter behavior is identical to
our previous observations of HONO in highly acidic
samples.48,49
∑AN (obs)
HONO (+ HNO3)
0.0
2.2
b
3.6
NOy (obs)
0.7
3.7
10.2
a
The upper limit of the absorption measurement at 405 nm is possibly
b
affected by an interference; see SI. Calculated by the difference
(=NOy − NO − NO2 − ∑AN).
Figure 4 shows the amount of NOy observed after 1/2 h of
irradiation as a function of the 4-XP concentration added to the
Figure 3 shows the TD-CRDS signal observed as a function
of the TD-CRDS inlet temperature for a set of three
Figure 4. Production of gas-phase NOy from an irradiated frozen
solution containing 1.0 M NaNO3 as a function of para-halogenated
phenol concentration at −20 °C. The trend lines are fits of observed
NOy against the square root of concentration and are intended as
visual guidelines only without physical meaning.
ice samples. The amount of NOy produced is dependent on the
identity of the halogen atom, X, and the concentrations of 4-
XP. The latter relationship is nonlinear.
3.3. Roles of OH and H+ in the Photoconversion of
NO3− to NOy in the Presence of 4-CP. To probe the
mechanism of the photochemistry of NO3− in the presence of
4-CP, isopropanol was added as a OH radical trap. Figure 5
shows the effect of adding 0.5% v/v isopropanol to an ice
sample containing 1.0 M NaNO3 and 10 mM 4-CP prior to
freezing. The addition of isopropanol decreased the amount of
Figure 3. TD-CRDS inlet temperature scan to measure the total alkyl
nitrate (∑AN) emitted from irradiated ice samples containing 1.0 M
NaNO3 and 10 mM 4-CP, 4-BP, or 4-FP. The increase in NO2 signal
between inlet temperatures of ∼250 and ∼400 °C is consistent with
thermal dissociation of compounds of molecular formula RONO2,
where R is an alkyl group.
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presence of organic molecules. The majority of the compounds
tested in this work lowered the amount of gas-phase NOy
generated photochemically (Figure 1), which at neutral to basic
pH is mainly in the form of NO2.49
The lowest NOy (and NO2) mixing ratios were observed in
the presence of 10 mM formate (Figure 1). Dubowski et al.36
investigated the yields of condensed-phase NO2− from NO3−
photolysis and rationalized higher NO2− yields in the presence
of formate by scavenging of OH radicals by organic compounds
to reduce the rate of NO2− to NO2 conversion and by reaction
of NO2 with the superoxide anion (O2−):
−
−
NO + OH → NO + OH
(4)
(5)
2
−
2
−
O2 + NO2 → O2 + NO2
Figure 5. Effect of adding isopropanol (0.5% v/v) on the
photochemical production of NOy from 1.0 M NaNO3 in ice in the
presence of 10 mM 4-CP at −20 °C. The NaNO3 only case (see
Figure 1) is shown for comparison.
The superoxide anion is generated by the following reaction
sequence:
−
−
HCO2 + OH → H2O + CO2
(6)
(7)
−
−
NOy released from the irradiated ice surface to a level below
that in the absence of organic additives.
CO2 + O2 → O2 + CO2
−
HO2 ⇌ H+ + O2
pKa = 4.8
The pH was measured for two samples containing 20 mM 4-
CP and 1.0 M NaNO3 before freezing and for thawed solutions
after the irradiation; before freezing and irradiation, the sample
pHs were 7.20 and 7.33, respectively. After 2 h of irradiation,
the pH values of the thawed samples were 4.72 and 5.18,
respectively. pH values observed in irradiated samples
containing only NaNO3 were above 5.5.
Experiments were conducted in which the proton concen-
tration of samples containing 10 mM 4-CP and 1.0 M NaNO3
were buffered using acetic acid/acetate at pH 3.6 and sodium
bicarbonate and carbonate at pH 9.6 (measured prior to
freezing). Figure 6 shows the effect on the photoconversion of
(8)
The pKa is from Buxton et al.70 Reaction 5 converts the volatile
NO2 to nonvolatile NO2− and is thus predicted to suppress
NO2 formation. The low rate of NO2 production from NO3−
photolysis in the presence of formate observed in this work is
consistent with the above mechanism. Interestingly, the
photochemical production of NOy in the presence of formate
does not appear to achieve a steady state, which is in contrast to
what is observed in the presence of other species, for which the
NO2 production stabilizes within a few minutes after the lamp
has been switched on (Figure 1). This result implies that a
secondary product, whose concentration builds up relatively
slowly during the photolysis experiments, suppresses NO2
formation by reducing NO2 to NO2− via the CO2−/O2−
pathway (reactions 5−8).
Dubowski et al.36 also suggested that removal of OH by the
organic molecule reduces the rate of reaction 4, that is, the
conversion of NO2− to NO2, and could explain the higher yield
of NO2− in their experiments. In a follow-up study, Minero et
al.47 pointed out that if reaction 4 was dominant, the NO2−
yield should scale with the product of the OH loss-rate
coefficient of the organic additive (Table 2) and its
Table 2. Reaction Rates of Selected Organic Species with
OH in Aqueous Solution at Room Temperature70
−1
species
phenol
rate constant (M−1 s )
(0.65−1.8) × 1010
Figure 6. Effect of buffering on the photochemical production of NOy
from 1.0 M NaNO3 in the presence of 10 mM 4-CP at −20 °C.
4-CP
7.6 × 109
formate
toluene
isopropanol
MSA−
3.2 × 109
3.0 × 109
NO3−. When the proton concentration is buffered, less NOy is
evolved from the ice than that under unbuffered conditions.
Under basic conditions, evolution of NOy is lower than that
under unbuffered conditions, consistent with the results that we
reported earlier.49
1.9 × 109
1.3 × 109
concentration. Studying the yield of NO2− from NO3−
photolysis in aqueous solution, Minero et al.47 observed a
lower rate of NO2− production in the presence of MSA− than
that in the presence of formate, even when MSA− was present
at concentrations several-fold that of formate. From this
observation, the authors concluded that reaction 4 is a minor
pathway.
4. DISCUSSION
4.1. Suppression of Gas-Phase NOy Production from
NO3− Photolysis by OH Radical Quenchers. This study has
shown that the rate of photochemical recycling of ice-phase
NO3− to gas-phase nitrogen oxides is strongly affected by the
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Figure 7 gives a graphical overview of extent of change in
NOy, ΔNOy, observed in this work plotted against the rate
yield in the presence of MSA− is slightly lower than for pure
NaNO3 in ice. The amount of NOy observed under acidic
condition is consistent with the trend of NOy observed as a
function of acidity that we reported earlier using oxalic acid/
oxalate, phosphate, or acetic acid/acetate buffers (see Figure 2
in ref 49).
This observation suggests that the nature of the organic
modifier and differences in the rates of O2−/HO2 formation
play minor roles in acidified samples. Under acidic conditions,
O2−, generated from oxidation of the organic compound, is
protonated to HO2, which does not convert NO2 to NO2−.
Thus, suppression of NO2 and NOy formation via reaction 5 is
not expected in acidified samples.
−
4.3. Promotion of NOy Production from NO3
Photolysis by para-Halogenated Phenols. The efficient
production of NOy in the presence of 4-CP and 4-BP (Figure
2) is surprising, in particular because the presence of 4-HP
suppresses NOy formation. It is clear that this effect is
accompanied by acidification of the ice sample as the pH of the
thawed solution dropped considerably during photolysis and
buffering of the ice reduced the amount of NOy produced
(Figure 6). It is also clear from the isopropanol experiment
(Figure 5) that radical species are key intermediates. The
questions arise why the ice samples become acidic upon
irradiation and why the 4-CP promoted production of NOy is
suppressed by isopropanol.
Figure 7. Plot of suppression of NOy production relative to “pure” 1.0
M NaNO3, ΔNOy, as a function of the OH rate constant of the
organic molecule added.
constants of the organic modifiers with respect to reaction with
OH in aqueous solution. In the interpretation of this plot, the
following four simplifying assumptions are made: (1) that the
organic additive concentrations are equal as the freezing ice
concentrates them near the surface, (2) that the organic
additives do not themselves absorb light and initiate photo-
chemistry, (3) that rate coefficients for reaction of OH with
organics in the ice’s surface layer correlate with those observed
in aqueous solution at room temperature, and (4) that
reactions with secondary species do not dominate the
chemistry. Some of these assumptions are questionable, as
recent studies by the Donaldson group71−73 and the greater
suppression of NO2 by formate after prolonged irradiation (see
above) have shown. However, relevant data, for example, on
OH rate coefficients in the liquid-like ice surface layer, are
sparse or do not exist, and Figure 7 at least allows for a
qualitative analysis.
4.3.1. Direct Photolysis of para-Halogenated Phenols. One
conceivable explanation for the observed acidification of the ice
samples is direct photoexcitation of the para-halogenated
phenols. Phenols are known to be fairly acidic in their excited
singlet states. The pKa* of 4-CP, for example, is approximately
3.2, whereas its ground-state pKa is 9.42.74 However, the pKa*'s
of 4-HP, 4-FP, and 4-BP, respectively, equal 4.0, 4.4, and 3.1.74
Thus, the pKa* values of excited-state phenols are inconsistent
with the trend shown in Figure 2. On the other hand, direct
photolysis of 4-CP (at 266 nm) is known to yield para-
benzoquinone (BQ) and the strong acid HCl (pKa ≈ −8)67 as
an elimination product.54,75 In this work, BQ was observed as a
photoproduct by HPLC (see Figure S-3 in the SI). Production
of HCl in the surface would produce very acidic conditions, in
particular in a thin aqueous surface layer, and promote the
production of NOy. Similarly, photolysis of 4-BP yields the
strong acid HBr55 (pKa ≈ −9)67 and promotes NOy formation,
as observed. Because the acidity of strong acids in aqueous
media is limited to that of the hydronium cation, H3O+, one
might expect the photolysis of 4-CP to have the same effect on
pH as the photolysis of 4-BP. However, Durand et al.55 have
evaluated the relative photoreactivities of halogenated phenols
and reported that 4-CP is more reactive (judged by the amount
of halogenated phenol remaining at the end of the photolysis
experiment) than 4-FP, which in turn is more reactive than 4-
BP. Even though 4-FP is more reactive than 4-BP, photolysis of
4-FP “only” produces the weak acid HF (pKa ≈ +3.2),67 and
the NOy yield is consequently lower. Phenol does not eliminate
an acid and thus has the lowest NOy yield. Thus, the order
observed (4-CP > 4-BP > 4-FP > 4-HP) is consistent with what
one might expect based on the existing literature.
If reaction 4 were dominant, one would predict a larger yield
of NO2 with a more efficient OH scavenger. Figure 7 generally
shows the opposite trend. Neglecting the para-halogenated
phenols and MSA (both are discussed below), the slope
observed is consistent with reaction 5 as a dominant factor
because reaction of OH with organic compounds in the
presence of O2 produces HO2/O2−. Thus, the trends in gas-
phase NO2 production observed in this work are consistent
with the conclusions by Minero et al.47 based on observations
of NO2− in the condensed phase.
−
4.2. Promotion of NOy Production from NO3
Photolysis by Acidic Conditions. It is well established that
the acidity plays a key role in the photochemical volatilization
of nitrogen oxides from ice. Under acidic conditions, NO2−,
peroxynitrite (ONOO−), and peroxynitrate (O2NO2−) are
protonated to form HONO, HO2NO2, and HOONO, which
partition to the gas phase.16,48 Furthermore, acidic conditions
promote the formation of NO2, which has been rationalized by
formation of HOONO as a volatile intermediate.49 The
observations in the presence of 10 mM MSA− at either neutral
or acidic conditions (Figure 1) clearly demonstrate that
acidification of the ice promotes the photochemical evolution
of NOy from NO3− photolysis. In the acidified sample, the yield
of NOy increases by a factor of approximately 2, whereas the
One could argue that it is unfeasible that direct photo-
excitation of para-halogenated phenols takes place in our
experiments because the absorption spectra of para-halogen-
ated phenols in aqueous solution do not overlap with the
lamp’s emission spectrum (Figure S-5, SI). However, the
Donaldson group recently showed that the absorption spectra
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of simple aromatic compounds in the ice surface layer can be
significantly red shifted to wavelengths above 300 nm,
rationalized by self-association of the aromatic rings.73
́
Furthermore, Klanova et al. reported that the absorption
spectrum of 4-CP in ice overlaps slightly with the actinic
spectrum.76 To investigate if direct photolysis of 4-CP was a
significant factor in our experiments, photolysis experiments on
ice containing only 4-CP were conducted. After >60 min of
irradiation in the absence of NO3−, no 4-CP degradation
products were observed by HPLC (see SI). Furthermore, a
direct photolysis mechanism would not explain the results of
the radical quenching experiment (Figure 5). Thus, we
conclude that it is unlikely that direct photolysis of para-
halogenated phenols is mainly responsible for the observed
promotion of NOy formation.
4.3.2. Reaction of para-Halogenated Phenols with OH. An
alternative explanation for the observed acidification of the
irradiated ice sample and promotion of NOy formation is that
the para-halogenated phenols react with OH produced from
NO3− photolysis and eliminate halogen acids, promoting the
formation of volatile nitrogen oxides. The reaction of 4-CP with
OH in aqueous solution has been studied both experimen-
tally23,77−84 and theoretically85,86 and has been reviewed by
Burrows et al.87 The products of this reaction in aerated
aqueous solution are 4-chlorocatechol (4-chloro-1,2-dihydrox-
ybenzene; CC) and hydroquinone (1,4-dihydroxybenzene,
HQ); usually, only trace amounts of BQ have been
observed.77,78 In ice at −20 °C with H2O2 as the OH source,
́
Klanova et al. observed CC as the major product and trace
levels of HQ and coupling products.61 Schedel et al. used NO3−
photolysis to generate OH in aqueous solution and reported
formation of (among others) CC, BQ, 2-nitrohydroquinone
(2NHQ), and 2-hydroxybenzoquinone (2HBQ) with relative
yields that depend on the NO3− concentration; at a NO3−
concentration of 0.05 M, a greater amount of BQ was observed
than that in the presence of 0.01 M.23 In this work, BQ and CC
were observed as the major organic products, and 2NHQ was
observed as the minor organic product by HPLC (Figure S-3,
SI). HQ was not observed; however, we cannot rule out that
HQ was present and coeluted with another (more abundant)
species. The relatively high yield of BQ is consistent with the
results by Schedel et al.23 because the NO3− concentration in
the ice’s liquid-like surface layer was likely higher than the 1.0
M solution prior to freezing.
Figure 8 gives an overview of the reactions that are likely
taking place in our experiments. The first step is production of
OH by photolysis of NO3− (there is also secondary production
from NO2− photolysis, which is neglected here). Then, one of
several reactions may happen. The hydroxyl radical can add at
the 2 position of the aromatic ring to form the 4-chloro-1,2-
dihydroxycyclohexadienyl radical (4C1,2D•), which converts to
CC and HQ. Formation of HQ has been rationalized by
dismutation reactions between 4-CP• and 4-chlorodihydrox-
ycyclohexadienyl radicals (4CD•).78 Alternatively, reaction of
4CD• with O2 can lead to ring opening, loss of aromaticity, and
HO2 production (not shown).80,82 An alternative (minor)
reaction is abstraction of the phenolic hydrogen by OH to form
the 4-chlorophenoxy radical (4-CP•), which is believed to react
further to generate HCl and nonaromatic products.87 A third
possibility is that OH adds to the aromatic ring at the 4
position, forming the 4-chloro-1,4-dihydroxycyclohexadienyl
radical (4C1,4D•), which eliminates HCl to form the
benzosemiquinone radical (BSQ•).78,80 In studies that did not
Figure 8. Proposed chemical transformation pathways during nitrate-
induced photo-oxidation of 4-CP.
use NO3− as the source of OH, BSQ• was reduced to HQ. To
account for the formation of BQ in the presence of NO3− in
this study and the work by Schedel et al.,23 we propose that BQ
is formed by oxidation of HQ and/or BSQ• and speculate that
this oxidation is accompanied by reduction of NO3− or O2.
In the above mechanism, HCl is eliminated from 4-CP and
acidifies the ice’s surface. This acidification is primarily
responsible for the observed high yield of gas-phase nitrogen
oxides in the presence of 4-CP, as is evident from the
suppression of NOy production in the presence of a mildly
basic buffer (Figure 5). However, additional volatile nitrogen
oxides may be produced by reduction of NO3− by HQ or BSQ•
or other intermediate species such as 4CD• or BSQ•. It is also
possible that the species being reduced is NO3−*, as suggested
by Minero et al.47 For example, one could envision that NO3−*
undergoes an electron transfer to 4-CP; halogenated phenols
are generally good electron acceptors. Clearly, more studies will
be needed to determine if any of these pathways are significant.
Preliminary experiments show that 4-CP is able to promote
the photoreduction of NO2− to NO (Figure S-7, SI). However,
an even greater effect was seen in the presence of phenol. The
reversed order is rationalized in part by the lack of a reaction
corresponding to reaction 5, as O2− does not react with NO as
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Article
it does with NO2, and the resulting lack of suppression of
volatile nitrogen oxides formation from NO2− photolysis. More
studies are needed to elucidate the details of the photochemical
transformation of NO2− in the presence of para-substituted
phenols in ice.
Finally, there may be secondary photochemistry to consider.
For example, many of the secondary products formed from 4-
CP have absorption spectra (e.g., Figure S-4, SI) that at least
partially overlap with the lamp’s excitation spectrum, making
them prone to photoexcitation. However, given the short
irradiation times and corresponding low conversion rates in this
study, we believe such secondary photochemistry did not
impact the results.
study and improved ice preparation techniques, such as the
shock-freezing technique to produce artificial snow
grains.43,92−94 Future experiments also need to take control
of the acidity of the ice, if possible. Potential effects of repeated
freeze/thaw cycles (which concentrate species near the
surface)50,51 should be considered. Finally, the effects of adding
mixtures of organic compounds, including organic photo-
degradation products, on the photochemical conversion of
NO3− to NOy in snow and ice should be investigated. These
mixtures of the organic compounds should not only resemble
those occurring in pristine environments but also those found
in polluted environments in cold climates.
4.4. Photonitration. The photoconversion of nitrate in the
presence of aromatic compounds can, in principle, lead to
formation of nitroaromatic compounds, which are generally
carcinogenic. Several groups23,29 have studied the photo-
transformation of NO3− in the presence of 2-, 3-, and 4-CP
and found minor nitration products; there is also evidence for
proton-concentration-dependent photonitration.28 In this
study, we observed trace amounts of 2NHQ. The mechanism
for 2NHQ formation is not obvious. We speculate that 2NHQ
is formed from reaction of BSQ• with NO2.
We also observed evidence for significant formation of
volatile alkyl nitrates (Figure 3). We are certain that the
observed organic nitrates are alkyl nitrates as nitroaromatic
compounds are more thermally stable.88 It is well documented
that 4-CP readily ring opens upon photo-oxidation to form
nonaromatic adducts.82 Many of the intermediate products
formed are unsaturated and are prone to addition reactions
(e.g., of the NO3 radical). Furthermore, many of the
intermediates include alkoxy and organic peroxy radicals,
which can form adducts with NO2.89 Photochemical formation
of alkyl nitrates in natural snow and ice has been observed in
field measurements.90 The formation of alkyl nitrates from 4-
CP observed in this work is likely one of many pathways for
their generation.
ASSOCIATED CONTENT
■
S
* Supporting Information
Discussion of potential interferences with CRDS NO2
measurements and description of photoproduct analysis by
HPLC. Six figures: Time series of NOy, NO2, and NO mixing
ratios. Sample chemical ionization mass spectrum and HPLC
chromatogram with PDA spectra. Lamp emission and
absorption spectra of nitrate and 4-CP. Photochemical
production of NOy as a function of temperature. Photochemical
production of NO and NOy from frozen solutions containing
nitrite. This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: hosthoff@ucalgary.ca. Phone: 403-220-8689. Fax:
403-289-9488.
Notes
†Present Address: Wadsworth Center, New York State
Department of Health, Department of Environmental Health
and Toxicology, State University of New York, Albany, NY
12201−0509, United States.
ACKNOWLEDGMENTS
■
5. CONCLUSIONS
This work was financially supported by the Canadian
Foundation for Climate and Atmospheric Sciences (CFCAS,
Grant # GR-7054).
It is well established that the photochemical recycling of NO3−
to volatile nitrogen oxides has consequences on the
composition of the overlying atmosphere. Here, the rate of
photochemical conversion of NO3− from snow and ice to
nitrogen oxides that partition to the gas phase was found to be
strongly affected by the presence of organic compounds and
the ice’s acidity. While most of the organic compounds tested
lowered the yield of NOy, OH-induced photodegradation of 4-
CP and 4-BP increased the yield of NOy, rationalized by
acidification due to elimination of HCl and HBr. In addition,
we demonstrated that NO3−-induced photochemistry could
result in photonitration of aromatic compounds, which is of
health concerns. For example, p-nitrobenzoquinone was
identified during the irradiation of nitrate in the presence of
4-CP. Finally, we showed that volatile alkyl nitrates can be
generated from NO3− photolysis in snow and ice.
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