Concepts were developed to establish relationships between the stable carbon
isotope ratios of nitrophenols in the atmosphere and the photochemical processing
of their precursors, light aromatic volatile organic compounds. These concepts
were based on the assumption that nitrophenols are formed dominantly from the
photo-oxidation of aromatic volatile organic compounds (VOCs). A mass balance model as well as various
scenarios based on the proposed mechanism of nitrophenol formation were
formulated and applied to derive the time-integrated exposure of the
precursors to processing by OH radicals (∫[OH]dt) from ambient
observations made between 2009 and 2012 in Toronto, Canada. The mechanistic
model included the possibility of isotopic fractionation during intermediate
steps, rather than only during the initial reaction step. This model takes
kinetic isotope effects for the reaction of the precursor VOC with
the hydroxyl radical and their respective rate constants into account, as well as carbon
isotope ratio source signatures. While many of these values are known, there
are some, such as the kinetic isotope effects of reactions of first- and
second-generation products, which are unknown. These values were predicted in
this study based on basic principles and published laboratory measurements of kinetic
carbon isotope effects and were applied to the mechanistic model. Due to the
uncertainty of the estimates based on general principles, three scenarios were
used with different values for isotope effects that were not known from
laboratory studies. Comparison of the dependence between nitrophenol carbon
isotope ratios and ∫[OH]dt with published results of laboratory
studies and ambient observations was used to narrow the range of plausible
scenarios for the mechanistic model. The results also suggests that mass-balance-based
models do not adequately describe the dependence between
nitrophenol carbon isotope ratios and ∫[OH]dt.
Introduction
Secondary organic aerosols (SOA) in the atmosphere formed from the
photo-oxidation of both anthropogenic and biogenic volatile organic
compounds (VOCs) are poorly understood. Products formed from these reactions
are only partly known, and apart from the composition of SOA little is known
about its atmospheric processing. It has been proposed that the use of
concentration measurements in conjunction with stable carbon isotope ratio
measurements can be used to gain insight into this topic (Goldstein and
Shaw, 2003; Rudolph, 2007; Gensch et al., 2014). The compounds of interest
in this study are nitrophenols, which have been proposed to be formed
specifically from the gas-phase photo-oxidation of aromatic VOCs (Forstner et
al., 1997; Atkinson, 2000; Jang and Kamens, 2001; Hamilton et al., 2005;
Sato et al., 2007). Once toluene, for example, is emitted from its
anthropogenic sources, it is expected to react according to the proposed
reaction mechanism (Forstner et al., 1997) to produce methylnitrophenols
(Fig. 1). As it is formed specifically from identified reactions, the stable
carbon isotope ratio of the product can be linked back to the precursor and
its source. Based on laboratory observations (Irei et al., 2011),
methylnitrophenols were found to have an average isotope ratio that is close
to the isotope ratio of the sum of all products calculated from the mass
balance. The aqueous phase production of nitrophenols, specifically
4-nitrophenol, has also been proposed to occur via a reaction
pathway involving the NO3 radical (Herrmann et al., 1995; Harrison et al.,
2005). This pathway is thought to be quite a significant source of
4-nitrophenol in the presence of clouds with a high liquid water content but
has been modelled to contribute less than 2 % when the liquid water
content is low (Harrison et al., 2005). Ambient measurements in the Toronto
area (Saccon et al., 2015) indicate that nitrophenols are second-generation
products due to their depletion in 13C relative to the carbon isotope
ratio of the precursor.
Proposed formation mechanism of 2-methyl-4-nitrophenol from
toluene oxidation (adapted from Forstner et al., 1997).
The carbon isotope ratio of a species, which will be referred to as the
delta value (δ13C), is defined using Eq. (1), where
(13C/12C)VPDB is the internationally accepted Vienna Peedee
Belemnite (VPDB) value of 0.0112372. As differences in isotope ratios
between species are small, delta values are expressed in per mille notation.
Limited studies using the carbon isotope ratio of atmospheric trace
components, including several aromatic VOCs and nitrophenols, have been
applied to differentiate between sources and to trace components back to
their precursors, respectively (Moukhtar et al., 2011; Kornilova et al., 2013;
Saccon et al., 2013). The concept of deriving information on photochemical
processing of trace constituents from isotope ratios is based on the kinetic
isotopic effect (KIE), which describes the dependence of the rate constant of
a reaction on the atomic mass of isotopologues. In this work, the KIE will
be referred to as ε (Eq. 2), where k12 and k13 are
the rate constants for 12C only and for one 13C containing
isotopologues, respectively. Normal KIE, which occurs when ε is
positive, is exhibited when a compound reacts in the atmosphere and the
remaining compound is enriched in heavier isotopes, for example 13C.
Like delta values, ε is also expressed in per mille notation.
1δ13C=(13C/12C)sample-(13C/12C)VPDB(13C/12C)VPDB×1000‰2ε=k12-k13k13×1000‰
Among other factors, the concentration of a species in the atmosphere depends
on its reactivity and the time over which it is exposed to reactants. For
many VOCs and semi-volatile organic compounds (SVOCs) the most important
reactant is the OH radical, and the time-integrated OH-radical concentration
is often referred to as the photochemical age (PCA). The combination of
laboratory experiments and ambient measurements can allow for the
determination of the PCA of a specific component in SOA. The PCA for an air
mass has previously been used to quantify the extent of the processing of a
precursor using the hydrocarbon clock and the mixing ratios of VOCs (Parrish
et al., 1992, 2007; Jobson et al., 1998, 1999; Kleinman et al., 2003). A more
recently developed method uses the carbon isotope ratio of an atmospheric VOC
with emissions as the only relevant source; the KIE and the isotope ratio
source signature are utilized to determine the PCA of this
VOC. This approach is often referred to as the isotope hydrocarbon clock (Rudolph and
Czuba, 2000; Rudolph et al., 2003; Thompson, 2003; Stein and Rudolph, 2007;
Kornilova, 2012; Kornilova et al., 2016). The PCA of a species can be
calculated using Eq. (3), where δ13Cpre is the
carbon isotope ratio of the measured ambient precursor,
δ13C0 is the carbon isotope ratio of the emissions, and
∫[OH]dt is the time-integrated OH concentration (PCA). It has been
shown that the concept of ascribing a PCA to an air mass (Parrish et al.,
1992) is only meaningful if all VOC emissions that contribute to the observed
VOC mixing ratios have been subject to an identical extent of processing. In
cases where air masses mix that contain VOC emissions which have been subject
to a different extent of processing, the concept of a PCA for an air mass has
to be replaced by the concept of a PCA for an individual VOC (Kornilova et
al., 2016). Rudolph and Czuba (2000) showed that VOC carbon isotope ratio
measurements can be used to determine the concentration weighted average of
the photochemical age of individual VOCs provided that the variability of the
carbon isotope ratio of the emissions is small compared to the change in the
carbon isotope ratio resulting from atmospheric removal reactions.
The carbon isotope ratios of the emissions of many important anthropogenic
VOC precursors have previously been measured, and their uncertainty is typically
below 1 ‰ (Czapiewski et al., 2002; Rudolph et al.,
2002; Rudolph, 2007; Gensch et al., 2014). The change in the VOC mixing ratios
due to reaction with OH radicals can be described by Eq. (4), where
χamb and χ0 are the mixing ratios of the ambient precursor
and the mixing ratio that would be observed in the absence of reaction with
OH radicals, respectively. Consequently, a combination of carbon isotope
ratio measurements and concentration measurements allows for separation between
the impact of atmospheric reactions and changes in source strength or
atmospheric mixing and dilution.
3δ13Cpre=δ13C0+k12ε∫OHdt4χamb=χ0exp(-k12∫OHdt)
Comparison of the difference between χ0 and χamb with
the ambient concentrations of the products of the photochemical reactions of
VOCs will provide insight into the yield of the secondary pollutants from the
reactions. In the case of mixing air masses containing VOCs with different
PCAs, Eq. (3) is still a very good approximation for the concentration
weighted average ∫[OH]dt of the precursor VOC (Rudolph and Czuba,
2000). However, Eq. (4) is only a valid approximation if the variability in the PCAs
is small compared to the average PCA. Otherwise Eq. (4) will underestimate
the average of χ0 (Rudolph and Czuba, 2000). Consequently, yield
estimates derived from combining results derived from Eqs. (3) and (4) with
measured ambient concentrations of reaction products will be an upper limit.
In principle this limitation can be avoided by using the carbon isotope ratio
of the reaction product to derive ∫[OH]dt, as the average of ∫[OH]dt derived from the carbon isotope ratios of the reaction products
will be weighted according to the concentration of the products, which will
accumulate as a result of the photochemical reaction of the precursor.
However, due to the simultaneous formation and removal of the reaction
products, the relation between the carbon isotope ratio of the reaction
product and the extent of photochemical processing is more complex than the
simple relation between the carbon isotope ratio of the precursor and ∫[OH]dt as described by Eq. (3). The reported carbon isotope ratio measurements of nitrophenols
in the solid and the gas phases from Saccon et al. (2015) showed that,
although the average isotope ratios are consistent with laboratory studies, a
significant number of delta values are between 2 ‰ and
3 ‰ lower than predicted from mass balance. This difference cannot
be explained by the uncertainty of the carbon isotope ratios of precursor
emission or measurement errors. However, a simple mass balance only considers
the KIE for the first step of the reaction mechanism (shown in Fig. 1). It
must be accepted that further fractionation can occur in reaction steps
following the initial reaction of the aromatic VOC with the OH radical. In
this work, we will present calculations using a mechanism based on the
formation and removal of nitrophenols from the atmosphere that describe the
dependence between the photochemical processing of light aromatic
hydrocarbons and the carbon isotope ratio of nitrophenols, which are products
of atmospheric reactions of light aromatic hydrocarbons. Different scenarios
using a range of isotope fractionation effects for a simple mechanistic model
will be discussed using the carbon isotope ratios of nitrophenols published
by Saccon et al. (2015) and the laboratory studies published by Irei et
al. (2015). From this comparison, the magnitude of isotope fractionation
effects following the initial reaction of light aromatic VOCs with OH
radicals can be constrained. PCAs derived from nitrophenol carbon isotope
ratios will be compared with PCAs derived from carbon isotope ratios of the
precursors (Kornilova et al., 2016).
Materials and method
The experimental method used in this work is described in detail by Saccon et
al. (2013), which is based on methods developed by Moukhtar et al. (2011) and
Irei et al. (2013). The results of the carbon isotope ratio measurements have
been presented by Saccon et al. (2015); therefore, we will only briefly
describe the experimental procedure. Sample collection was carried out at
York University in Toronto, Canada, using 20.32 cm × 25.4 cm
quartz-fiber filters (Pallflex®
Tissuquartz™ filters – 2500 QAT – Pall
Gelman Sciences) on high-volume air samplers (TE-6001 from Tisch
Environmental Inc.) equipped with PM2.5 heads. Uncoated quartz filters
were used to collect particulate matter (PM) alone, with an average sampling
time of 1–3 days, and filters coated with
XAD-4™ resin were used for the collection of
both the gas phase and PM, with an average sampling time of 1 day. Filter
samples were collected between March 2009 and August 2012. The analysis of
the filters included extraction in acetonitrile, and HPLC separation and
solid-phase extraction were used as sample clean-up steps. Concentration
measurements were undertaken using a HP 5890 GC equipped with a HP 5972 mass
spectrometer; carbon isotope ratio measurements were carried out using an
Micromass IsoPrime isotope ratio mass spectrometer (Isomass Scientific,
Inc.). Method performance characteristics are given in Saccon et
al. (2013, 2015).
Determination of PCA
Laboratory experiments studying the carbon isotope ratios of secondary
particulate organic matter (POM) formed by the gas-phase oxidation of
toluene showed that the δ13C value of total secondary POM can
be approximated by mass balance (Irei et al., 2006, 2011). However, compound-specific
measurements also indicate that in some cases detailed mechanistic
considerations are required to explain the observed δ13C values
of secondary phenols that are lower than expected from mass balance alone
(Irei et al., 2015).
PCA from mass balance
Mass balance calculations allow a straightforward determination of the
dependence between the δ13C of the total of secondary POM and the PCA.
This requires the assumption that the carbon isotope ratio
of the gas-phase reaction products in the atmosphere is identical to the carbon isotope ratio
of secondary POM observed in laboratory studies (Irei et al., 2006,
2011). Furthermore, for compound-specific carbon isotope ratio measurements
it also has to be assumed that the carbon isotope ratio of the individual
products is representative of the carbon isotope ratio of all secondary
POM. In this case the dependence between the PCA of the precursors and the
product isotope ratio (Eq. 5) can be derived from Eqs. (3) and (4). Here,
kOH is the averaged rate constant of all isotopomers of the precursor
reacting with OH and, for practical purposes, is equal to k12.
δ13Cprod=δ13Co-exp(-kOH∫OHdt)1-exp5(δ13Co+k12OHε∫OHdt)(-kOH∫OHdt)
It should be noted that, similar to the conventional hydrocarbon clock, in
the case of mixing of air masses with different PCAs, the PCA from Eq. (5) is
a combination of the PCAs of the individual air masses, which is not always
easy to interpret. Equation (5) also neglects possible isotope fractionation
resulting from the loss of secondary POM. The values for rate constants, the carbon
isotope ratio of precursor VOC emissions, and the kinetic isotope effects used
are listed in Table 1.
Parameters, including the rate constant of the precursor with the
OH radical (kOH), the KIE, and the carbon isotope ratio of emissions
(δ13C0) used for the determination of the PCA by Eq. (5).
The uncertainty of the parameter is given in parentheses.
a Calvert et al. (2002), uncertainty not included as the uncertainty
resulting from the error in the rate constants is small compared with
uncertainties derived from the error in the carbon isotope ratio and KIE. b
Anderson et al. (2004). c Rudolph et al. (2002); for m-xylene the value
reported for p+m-xylene is given.
PCA from detailed mechanistic concepts
The reaction sequence resulting in the formation of nitrophenols from the
oxidation of toluene proposed by Forstner et al. (1997) is shown in Fig. 1.
It does not include the details of the various branching reactions and alternate
pathways resulting in other products or isotope fractionation due to loss
reactions of secondary POM. While in many cases branching ratios are known,
there is little direct knowledge on isotope fractionation resulting from
branching reactions. Nevertheless, isotope effects for specific pathways can
be estimated from the type of reaction and known principles of isotope
fractionation. For example, after the formation of the cresol first-generation
product (Fig. 1), the probability that an OH radical is added to the ring is
92 % (Atkinson et al., 1980), whereas reaction at the phenolic OH group,
which is expected to result in formation of nitrophenols, occurs in 8 %
of all reactions. The reaction of the phenolic OH group is expected to
result in negligible carbon isotope fraction as it is a secondary isotope
effect, whereas the OH-radical addition will have a carbon isotope effect
similar to that of other OH addition reactions to aromatic rings. Similarly,
the main gas-phase loss process of nitrophenols is expected to be via
reaction with the OH radical, which occurs through an OH addition to the ring
more than 80 % of the time (Bejan et al., 2007). As an OH radical
is added to the ring, fractionation typical for reaction at the six carbon
atoms of an aromatic ring is expected to occur for 80 % of the loss
reactions. Reaction at the alkyl group or the phenolic OH results in a much
lower KIE than for addition to the aromatic ring and therefore their
contribution to the KIE is negligible.
Another complication is the distribution of nitrophenols between the gas
phase and PM. It is assumed that there are no chemical losses when
nitrophenols partition into PM, and that there is equilibrium between the gas
and particle phases. As phase distribution processes typically have very
small isotope effects, partitioning between the gas phase and PM is expected
to have only a marginal impact on the carbon isotope ratio. This is
consistent with the findings of Saccon et al. (2015). However, partitioning
will influence the loss rate for SVOCs, as it is assumed that there is little
to no chemical loss in the PM phase. It has been reported that phenols can be
oxidized in aqueous solutions under conditions somewhat similar to
atmospheric conditions in fog or cloud water (Yu et al., 2016). However,
these reactions are very slow compared to the reaction of nitrophenols with
OH radicals in the gas phase (for details see Sect. S1 of the Supplement).
Uncertainty in phase partitioning will only result in minor uncertainties of
the photochemical nitrophenol loss rate, as it has been reported that only
approximately 20 % of the nitrophenols partition into the particle phase
(Saccon et al., 2013, 2015). There is no information available that would
allow for the estimation of the rate of exchange of nitrophenols between the
gas phase and PM. It is assumed that phase partitioning is fast compared to
gas-phase reactions. There are indications that in some cases exchange
between the gas phase and PM is slower than the formation or loss reactions
of SVOCs in the gas phase (Saccon et al., 2015), but the reported average of
the difference in carbon isotope ratios between the gas phase and PM is
negligible (0.3±0.5 ‰).
Reaction rate constants for which no laboratory measurements are available
are estimated on the following principles. The rate constant for
3-methyl-4-nitrophenol is estimated based on the position of its
substituents on the aromatic ring relative to other isomers that have known
rate constants, such as 3-methyl-2-nitrophenol and 4-methyl-2-nitrophenol.
For 2,6-dimethyl-4-nitrophenol
it is assumed that loss reactions due to the
addition of the OH radical to the aromatic ring are negligible as the
nitro and hydroxyl substituents both direct reactions to positions that
are already occupied by other substituents; furthermore, reactions at positions three
or five are unlikely to occur. It should be noted that reaction at the phenolic
OH group of 2,6-dimethyl-4-nitrophenol will not directly impact the carbon
isotope ratio as this secondary carbon isotope effect will be negligible,
independent of the rate of this reaction.
Using these assumptions, a set of differential equations is derived that
describe the change in the concentration of the isotopologues of the
precursor, the first-generation product (phenols), and the second-generation
product (nitrophenols) (Eqs. 6 to 8). Here,
12Cprod/int/pre and 13Cprod/int/pre
are the concentrations of each of the 12C and 13C
isotopologues of the second-generation nitrophenol, first-generation phenolic
compound, and aromatic precursor, respectively. int12Y and
int13Y are the yields of the first-generation product from
reaction of the precursor, and prod12Y and
prod13Yaretheyieldsof nitrophenols from the
reaction of the phenol. The rate constants k12 and k13 for
different isotopologues are calculated from constants
and the KIE.
6ad12Cprod=-12Cprodkprod12OHdt+prod12Y12Cintkint12OHdt6bd13Cprod=-13Cprodkprod13OHdt+prod13Y13Cintkint13OHdt7ad12Cint=-12Cintkint12OHdt+int12Y12Cprekpre12OHdt7bd13Cint=-13Cintkint13OHdt+int13Y13Cprekpre13OHdt8ad12Cpre=-12Cprekpre12OHdt8bd13Cpre=-13Cprekpre13OHdt
Details regarding the numerical integration of these coupled differential equations
are described in Sect. S2. The largest uncertainty arises
from the possible isotope dependence of the yields in Eqs. (6) and (7), but
uncertainty in the kinetic isotope effects will also contribute to uncertainty
in the calculated carbon isotope ratio. Equations (6)–(8) only describe the
fractionation relative to the carbon isotope ratio of the precursor
emissions. In order to determine carbon isotope ratios that can be compared
with observations we use the carbon isotope ratios for emissions reported by
Rudolph et al. (2002). To obtain insight into the possible impact of
uncertainties in model parameters we utilize different scenarios.
As the yields of nitrophenols from the reaction of light aromatic VOCs are
small, the feedback of the differences in yields for isotopologues of the
product on the carbon isotope ratio of the first-generation product is very
small. It cannot be distinguished if the isotope fractionation occurs during
the formation of the first- or the second-generation product. The consequence is
that at low PCA values the reaction channel specific isotope
fractionation for the formation of nitrophenols is determined by the
following equation:
KIEFor=prod13Ykint13int13Ykpre13prod12Ykint12int12Ykpre12,
where KIEFor represents the total isotope fractionation during the
formation of nitrophenols following the initial reaction of the precursor
with the OH radical (for details see Sect. S3). This greatly reduces the
number of scenarios that need to be considered.
The basic parameters used for solving these differential equations are
listed in Table 2. The rate constants are for 298 K for consistency with
data available for comparison from published literature. Rate constants for this
temperature were used in the determination of the PCA from carbon isotope ratios
of light aromatic VOCs (Kornilova et al., 2016). The laboratory studies of
the formation of methylnitrophenols (Irei et al., 2015) from toluene were
conducted at room temperature. To understand uncertainties arising from the
assumptions made to estimate the KIEs, which were not determined
experimentally, different scenarios are used. In the first scenario
(Scenario 1) it is assumed that formation of nitrophenols occurs entirely via the abstraction
of a hydrogen atom from the phenolic OH group and that, as it is a
secondary KIE, there is no isotope fractionation from this reaction step. It
is also assumed that there is no reaction channel specific isotope
fractionation for the formation of the phenolic intermediate from the
precursor. This is equivalent to the assumption that int12Y=int13Yprod12Ykint12=prod13Ykint13 and int12Y=int13Y.
Parameters used to determine the PCA for individual products. Units
of k (rate constant) are in cm3 molecules-1 s-1. As 80 %
of the phenols are in the gas phase (Saccon et al., 2013), the rate constant
for the product loss was adjusted to 80 % of the gas-phase rate constant.
If available, the uncertainty of the parameter is given in parentheses.
a Reaction rate constant taken from Calvert et al. (2002). b
Kinetic isotope effects taken from Anderson et al. (2004). c Carbon
isotope ratio of emissions taken from Rudolph et al. (2002) and Kornilova et al. (2016);
for m-xylene the value reported for p+m-xylene is given.
d Reaction occurs via OH abstraction (Atkinson et al., 1992) and the
secondary carbon isotope effect is assumed to be negligible. e Rate
constant from Grosjean (1991). f Estimated based on the loss reaction
proceeding mostly by addition to the aromatic ring (Grosjean, 1991) and the
carbon kinetic isotope effects reported by Anderson et al. (2004). g
Estimated on the assumption that reaction proceeds primarily through an
addition pathway (Atkinson et al., 1980). The kinetic isotope effect for the
addition of OH radicals to an aromatic ring are based on the kinetic isotope
effects reported by Anderson et al. (2004). h Rate constant from Bejan
et al. (2007). i Estimate based on the carbon kinetic isotope effects
for reactions of aromatic VOCs reported by Anderson et al. (2004). j
Rate constant assumed to be the average of the rate constants for 3-me-2-NP
(3.69×10-12 cm3 molecules-1 s-1) and 4-me-2-NP
reported by Bejan et al. (2007). k Rate constant estimated to be
identical to the rate constant reported for 4-me-2-NP by Bejan et al. (2007).
l Reaction rate constant from Atkinson and Aschmann (1990).
m Assumed to have no loss reaction that results in carbon isotope
fractionation (see text).
Another scenario (Scenario 2) is based on the assumptions that the isotope
fractionation for the formation of nitrophenols from the intermediate is
identical to the fractionation for all reactions of the intermediate and
that there is no reaction channel specific isotope fractionation. It should
be noted that for the formation of nitrophenol from the reaction of benzene,
the two scenarios will be identical as the reaction of phenol with the
OH radical occurs predominantly via abstraction from the OH group (Atkinson
et al., 1992); it is also assumed that this secondary carbon isotope effect is
negligible (kint12=kint13). For the
reactions of toluene and xylene, these two scenarios represent an estimate
for the upper and lower limit of carbon isotope fractionation resulting from
reactions of the first-generation product.
The third scenario (Scenario 3) is based on laboratory studies of the
formation of nitrophenols from gas-phase reactions of toluene in the presence
of NO2 (Irei et al., 2011, 2015) and the lower end of atmospheric
observations of nitrophenol carbon isotope ratios reported by Saccon et
al. (2015). Details of how these observations can be used to constrain the
isotope fractionation during the formation of nitrophenols from aromatic VOCs
and the uncertainties of these constraints are given in Sect. S4. Scenario 3
uses an overall isotope fractionation that is 3 ‰ greater than in
Scenario 1. It should also be noted that this does not necessarily imply a
specific process for the formation of nitrophenols from phenols.
The results of the numerical integration are plotted in Fig. 2 for
2,6-dimethyl-4-nitrophenol, 4-nitrophenol, and 2-methyl-4-nitrophenol in
addition to predictions from mass balance. For comparison, the median, the
10th and the 90th percentiles, and the lowest and highest
carbon isotope ratios reported by Saccon at al. (2015) are also shown. For
2-methyl-4-nitrophenol the results of laboratory studies reported by Irei et
al. (2015) are included.
Dependence between the carbon isotope ratio and the PCA (∫[OH]dt) for several nitrophenols calculated for different scenarios using a
mechanistic model and mass balance. Also shown are the median, the 10th and
90th percentiles, and the lowest and highest carbon isotope ratios measured
by Saccon at al. (2015) in an urban area. The triangles and squares
represent the carbon isotope ratios of 3-methyl-4-nitrophenol and
2-methyl-4-nitrophenol, respectively, reported by Irei et al. (2015) for
laboratory studies.
The predictions from the mechanistic model are very similar for the
methylnitrophenol isomers (see the example in Fig. S1), and for a wide range of
PCAs the difference in predicted carbon isotope ratios between the isomers is
less than the estimated accuracy of 0.5 ‰ for carbon
isotope ratio measurements of methylnitrophenols (Saccon et al., 2013).
For methylnitrophenols and 2,6-dimethyl-4-nitrophenol in all three scenarios
the shape of the functions describing the dependence between the carbon isotope
ratio and ∫[OH]dt is very similar; the difference in the
intercept with the y axis is determined by the isotope fractionation
specific for the reaction channel resulting in the formation of nitrophenols and
the kinetic isotope effect for the reaction of the precursor as well as the
carbon isotope ratio of precursor emissions.
The steep increase in δ13C at low values of ∫[OH]dt
is the result of the high reactivity of the phenolic first-generation
product and the resulting rapid increase in its carbon isotope ratio (Fig. 3).
The exception is Scenario 1 for nitrophenol which assumes that the
kinetic isotope effect for the reaction of phenol with the OH radical is
negligible.
Dependence between the carbon isotope ratio and the PCA (∫[OH]dt) for 2-methyl-4-nitrophenol, its precursor (toluene), and the
phenolic intermediate calculated for Scenario 3. Also shown are the median,
the 10th and 90th percentiles, and the lowest and highest carbon isotope
ratios for toluene reported by Kornilova et al. (2016) for an urban area in
Toronto, Canada.
At high PCAs the dependence between the carbon isotope ratio and the PCA is nearly
linear, representing conditions where the first-generation phenol is in
quasi-steady state between formation from the precursor and loss reactions
(Fig. 3). The PCA (∫[OH]dt) for the transition between the initial
steep increase and the nearly linear range depends primarily on the reaction
rate constants of the phenolic intermediates. At high values of ∫[OH]dt the slope of the dependence between δ13C and ∫[OH]dt is mainly determined by the rate constant and the kinetic isotope effect
for the reaction of nitrophenol with the OH radical as most of the
aromatic precursor has been consumed (Fig. 3). Nevertheless, due to the
continuing formation of nitrophenols from the precursor and the increase in
δ13C of the precursor this slope is slightly steeper than
predicted by the rate constant and the kinetic isotope effect for the reaction
of the nitrophenol alone. It should be noted that the carbon isotope ratios
of the precursor predicted by our mechanistic model are fully consistent
with the range of carbon isotope ratios of aromatic VOCs in the atmosphere
reported by Kornilova et al. (2016) (Fig. 3).
Comparison of predicted carbon isotope ratios with
laboratory and ambient measurements
For 2,6-dimethyl-4-nitrophenol and methylnitrophenols, the lower end of the
mass balance predictions is substantially heavier than the lower end of
ambient observations; however, for methylnitrophenols mass balance predicts
carbon isotope ratios well within the range of the laboratory results
reported by Irei et al. (2015). The lower end of carbon isotope ratios
predicted by Scenario 1 for 2,6-dimethyl-4-nitrophenol and methylnitrophenols
is 3 ‰ to 4 ‰ heavier than the lower end of ambient observations reported by
Saccon et al. (2015). Furthermore, six out of the seven carbon isotope ratios
of methylnitrophenols observed in laboratory studies by Irei et al. (2015)
are more than 2 ‰ lighter than predictions based on Scenario 1.
For 4-nitrophenol at small values of the precursor's PCA (∫[OH]dt≤1011 s molecules cm-3),
Scenario 3 predicts that the carbon isotope ratios are lower than the lower limit of ambient observations in an
urban area of Toronto (Saccon et al., 2015). Similarly, the methylnitrophenol
carbon isotope ratios predicted by Scenario 2 for a ∫[OH]dt of
less than 3×1010 s molecules cm-3 are lighter than the
lowest ambient observations (Saccon et al., 2015). Kornilova et al. (2016)
report that 25 % of PCAs derived from carbon isotope ratio measurements
of benzene and toluene are below 1.1×1011 and
1.6×1010 s molecules cm-3, respectively.
However, it has to be taken into account that the PCA derived from carbon
isotope ratios of the precursor and the reaction product-based PCA are not
necessarily identical for mixing air masses with different PCAs (see
Sect. 3.5).
For high PCAs mass balance predicts a substantially lower slope for the
dependence between the PCA and carbon isotope ratios than all three scenarios
based on a mechanistic model. This is due to the conceptual limitation of
the mass balance, which does not include the change in the carbon isotope ratio
resulting from the atmospheric reaction of nitrophenols; consequently, it
cannot be expected that a mass balance can correctly predict carbon isotope
ratios at high PCAs.
Most of the observed nitrophenol carbon isotope ratios correspond to PCAs at
the lower end of the PCAs predicted by Scenario 3 (Fig. 2). For this range a
linear approximation can be used (Sect. S5). The estimated accuracy of the
nitrophenol carbon isotope ratio measurements published by Saccon et
al. (2015) is ±0.5 ‰. This corresponds to
uncertainty in ∫[OH]dt in the range from 6×109 to 9×109 s molecules cm-3. This is similar to the sensitivity of ∫[OH]dt derived from the measurement of carbon isotope ratios of toluene
(Kornilova et al., 2016). However, for PCAs derived from nitrophenol carbon
isotope ratios, the uncertainty of model predictions will also contribute to
the overall uncertainty. The overall uncertainty can be described as an
uncertainty independent of the PCA and a contribution proportional to the
PCA. Detailed estimates of uncertainty are given in Sect. S6.
PCA determined from carbon isotope ratios of nitrophenols
Based on the dependence between the PCA and the carbon isotope ratio of VOC reaction
products (Fig. 3), ∫[OH]dt can be determined from measured carbon
isotope ratios of ambient nitrophenols under the assumption of a uniform PCA
for the observed nitrophenols. In Table 3, the average PCA determined from product carbon
isotope ratios is compared with ∫[OH]dt values
calculated directly from precursor isotope ratios, which were published
by Kornilova (2012) and Kornilova et al. (2016). The nitrophenol-derived PCA
is based on Scenario 3. It should be noted that, although collected at
locations only 3 km apart, precursor and product samples were
not collected simultaneously in most cases, and in some cases they were even collected in different years.
Nevertheless, the substantial number of samples in most of the data sets and
the low uncertainty of the mean PCA justify a comparison of averages and the
distribution of precursor-derived PCA with ∫[OH]dt values
calculated from second-generation product carbon isotope ratios.
Averages and uncertainty of the mean PCA for nitrophenols in both
PM and the gas phase in addition to PM calculated for Scenario 3. Also shown are the
average carbon isotope ratios. The number of data points used is shown in
parentheses. For comparison, the PCA calculated from the carbon isotope ratios of
the precursor VOCs reported by Kornilova et al. (2016) for Toronto are
included.
Average PCA∗AverageAverage PCAPrecursor(1011 s molecules cm-3)Productδ13C (‰ )(1011 s molecules cm-3)Benzene3.1±0.54-NP-33.5±0.34.7±0.3(43)(58)Toluene0.85±0.11Methylnitrophenols-33.1±0.10.42±0.02(73)(120)p,m-Xylene0.34±0.062,6-dime-4-NP-33.4±0.50.11±0.04(56)(19)
∗ The average carbon isotope ratio and PCA determined by Kornilova (2012)
and Kornilova et al. (2016).
Similar to the precursor carbon isotope ratio-based PCAs the product isotope
ratio-derived PCAs increase substantially with decreasing precursor
reactivity. This was explained by Kornilova et al. (2016) in terms of the mixing of
air masses with different PCAs, which results in a lower weight for VOCs with
high reactivity in aged air due to the faster photochemical removal. However,
the weighting of contributions from different air masses differs between
precursor isotope ratio-derived PCAs and product isotope ratio-derived PCAs.
Details will be discussed in Sect. 3.5.
All precursor carbon isotope ratio-derived PCAs differ significantly from the
PCAs determined from nitrophenol carbon isotope ratios. Toluene and xylene
precursor-derived PCAs are lower than second-generation product-derived PCAs
by approximately 4×1010 and
3×1010 s molecules cm-3, respectively. The average PCA
derived from 4-nitrophenol carbon isotope ratios is approximately 50 %
higher than the average PCA calculated from benzene carbon isotope ratios.
The uncertainty in the calculated average PCA can result from uncertainty in
the parameters used to calculate PCAs from carbon isotope ratios. The 10th
and 90th percentiles of the second-generation product carbon
isotope ratios range from approximately -36 ‰ to -31 ‰.
For this range, errors in the rate constants and the kinetic
isotope effects for reactions of the precursors or the second-generation
product only have a small impact on the dependence between the PCA and carbon
isotope ratio (Figs. S2 and S3); therefore, these factors cannot explain the difference
in average PCA. However, uncertainties in the carbon isotope ratios of VOC
emissions as well as the isotope fractionation for reactions or branching of
the intermediates in the reaction sequence resulting in nitrophenol
formation can have a significant impact on PCAs calculated from nitrophenol
carbon isotope ratios (Tables S5, S6).
However, PCAs derived from precursor's carbon isotope ratio measurements also
strongly depend on the carbon isotope ratios of the emissions (Kornilova et
al., 2016). For a 1 ‰ decrease in the emission isotope ratios,
the PCAs derived from the carbon isotope ratios of benzene,
toluene, and m-xylene increase by 0.9×1011, 0.3×1011,
and 0.09×1011 s molecules cm-3, respectively. Consequently, a 1 ‰ decrease in
the carbon isotope ratio of emissions would reduce the difference between the
precursor- and product-derived PCAs for benzene, toluene, and
m-xylene by approximately 0.7×1011, 0.15×1011 s molecules cm-3, and less than 0.01×1011 s molecules cm-3, respectively. A decrease of approximately
3 ‰ in the carbon isotope ratio of toluene emissions would be able
to explain the difference in PCAs derived from toluene carbon isotope ratios
and methylnitrophenol carbon isotope ratios. Similarly, a 2.5 ‰
increase in the carbon isotope ratio of benzene emissions would eliminate the
difference between benzene and 4-nitrophenol-derived PCAs. However, a
2–3 ‰ error in the carbon isotope ratio of emissions is
substantially larger than the uncertainty derived from VOC emission studies
(Rudolph et al., 2002; Rudolph, 2007; Kornilova et al., 2016). Moreover, a
carbon isotope ratio of benzene emissions that is 2.5 ‰ heavier than
the value used in our calculations (Table 2) would not be compatible with the
lower end of ambient benzene carbon isotope ratios reported by Kornilova et
al. (2016). The discrepancies between the m,p-xylene and
2,6-dimethyl-4-nitrophenol-derived PCA cannot be explained by the uncertainty
of the carbon isotope ratios of xylene emissions. However, it should be noted
that the precursor-based PCA is derived from ambient observations of the
combined isotope ratios of p-xylene and m-xylene, whereas
only m-xylene is a precursor of 2,6-dimethyl-4-nitrophenol.
An increase in the carbon isotope fractionation specific for the formation
of nitrophenols from the intermediate phenol of approximately 3 ‰ would result in very good agreement between precursor
and second-generation product-derived PCA for toluene and xylene. However,
for the conditions of the laboratory studies reported by Irei et al. (2015),
a model with an additional isotope fractionation for the formation of
nitrophenols from reaction of the intermediate such as this would predict carbon isotope
ratios that were 2.5 ‰ lighter on average than the measured
values. Based on the reported average experimental uncertainty of less than
1 ‰ this difference is significant at a confidence level higher than 99.9 %.
For the formation of 2,6-dimethyl-4-nitrophenol from m-xylene, no
laboratory results are available that would allow for the constraint of the
carbon isotope fractionation for reactions of the intermediate
phenol. However, it is unlikely that carbon isotope fractionation for reactions of
the intermediate dimethyl phenol are substantially larger than for the cresol
intermediates.
The formation of 4-nitrophenol from the atmospheric oxidation of benzene
proceeds via phenol, which reacts with OH radicals, in contrast to methyl
substituted phenols, which primarily form by H-abstraction from the phenol
group. Consequently, a reaction channel specific carbon isotope fractionation
substantially different from that for reactions of methyl-substituted phenols
cannot be ruled out. However, a model scenario that would result in good
agreement between precursor and second-generation product-derived average
PCAs for benzene also predicts that the lowest carbon isotope ratio for
4-nitrophenol exceeds approximately 30 % of the measured ambient carbon
isotope ratios reported by Saccon et al. (2015) by more than the measurement
uncertainty.
In addition to the formation of nitrophenols via OH-radical initiated
oxidation, the reaction of the intermediate cresol with NO3 also has to be
considered as a possible reaction pathway for the formation of the
methylnitrophenols (Carter et al., 1981). Here, it was proposed that at
NOx levels greater than 20 ppb and O3 levels much larger than NO
levels, the reaction with NO3 would dominate over the proposed reaction
with OH radicals. However, as [OH] and [NO3] each exhibit very
pronounced diurnal cycles, with [OH] peaking during the day and [NO3]
peaking at night (due to its fast photolysis during daytime), reactions with
NO3 during the day can be ignored. The proposed reaction pathway of the
cresol +NO3 reaction is via an addition reaction, which results in a
similar estimated KIE to the addition of the OH group. Consequently, the
carbon isotope ratio of nitrophenols formed via this reaction pathway will
not depend significantly on the formation pathway. However, due to the
possible nighttime processing of the phenolic intermediate in the presence
of NO3 this may create a difference between the true value for
∫[OH]dt and the PCA derived from the carbon isotope ratio of the
nitrophenol. To determine this possible bias Scenario 3 was modified. At a
value for ∫[OH]dt corresponding to the average carbon isotope
ratios reported by Saccon et al. (2015), the OH-radical concentration was set
to zero and replaced by a mechanism representing the reaction of the
intermediate at 1 pmol mol-1 of NO3 until the phenolic intermediate
was nearly completely depleted. The resulting average bias in ∫[OH]dt corresponds to less than 0.2 ‰ in the carbon isotope
ratio for all of the methylnitrophenol isomers when compared with the
unmodified Scenario 3.
The reactions of cresols with OH radicals are substantially faster than the
formation of cresols from the reaction of toluene with OH radicals. This does
not allow for the build-up of high concentrations of cresols during the day,
and limits the possible role of the NO3 reaction pathway. For the
same reason it is unlikely that the NO3 reaction pathway plays a
substantial role in the formation of 4-nitrophenol or
2,6-dimethyl-4-nitrophenol.
Figure 4 shows the frequency distributions for PCAs determined from
the carbon isotope ratios of 4-nitrophenol (Fig. 4a) and methylnitrophenols
(Fig. 4b) using Scenario 3. For comparison, percentiles for PCAs derived from
carbon isotope ratios of benzene (Fig. 4a) and toluene (Fig. 4b) reported by
Kornilova et al. (2016) are also shown. Consistent with the difference in
the average PCA (Table 3), PCAs derived from 4-nitrophenol carbon isotope ratios
are shifted approximately 2×1011 s molecules cm-3
towards higher values than PCAs derived from benzene carbon isotope ratios,
but the width of the two PCA distributions are very similar (Fig. 4a). The
PCA-independent uncertainty for the 4-nitrophenol carbon isotope ratio-derived
PCA is only 7×1011 s molecules cm-3 (Table S5), which
cannot explain the difference in the average PCA. The PCA-dependent relative
uncertainty is 32 % (Table S5). Combined with the PCA-independent
uncertainty this could explain the difference in the average PCA.
However, such a scenario also predicts a more than 30 % narrower
distribution for 4-nitrophenol-derived PCAs than the best estimate. A
distribution such as this would be substantially narrower than the distribution of
PCAs derived from benzene carbon isotope ratios.
Frequency distribution of PCAs determined from the carbon isotope
ratios of 4-nitrophenol (a) and methylnitrophenols (b) using Scenario 3. For
comparison the median (dotted line), the 75th and 25th percentiles (dashed line), and
the 10th and 90th percentiles (solid line) determined by Kornilova et al. (2016)
from carbon isotope ratios of benzene (a) and toluene (b) are included.
The PCA distribution derived for methylnitrophenols is, compared to the
toluene-derived distribution, not only shifted to lower values, but also
much narrower (Fig. 4b). This discrepancy cannot be explained by the
uncertainty of PCAs derived from methylnitrophenol carbon isotope ratios.
Average PCA and mixing of air masses
Based on the measurement of carbon isotope ratios of several aromatic VOCs,
Kornilova et al. (2016) concluded that the mixing ratios and average PCAs of
aromatic VOCs in Toronto are typically determined by the mixing of air masses
with VOCs of different origin and different PCAs. While ∫[OH]dt
values determined from the carbon isotope ratios of aromatic VOCs represent, for all
practical purposes, the correct concentration weighted average PCA for the
VOC studied (Rudolph and Czuba, 2000; Kornilova et al., 2016), the situation
is different for PCAs derived from carbon isotope ratios of VOC reaction
products such as nitrophenols. In the case of atmospheric mixing of VOCs and VOC
reaction products the PCAs derived from product carbon isotope ratios can
differ from the ∫[OH]dt calculated for VOC isotope ratios for several
reasons.
For nitrophenols with different PCAs the decrease in the sensitivity of the
PCA–carbon isotope ratio dependence outside of the linear range will reduce
the apparent PCA derived from nitrophenols compared to the VOC-derived PCA.
Conversely, with increasing PCA the VOC precursor concentrations will
not only decrease due to atmospheric dilution but also due to chemical
reactions, which reduce their weight for the average PCA. In contrast,
nitrophenols are formed as a result of precursor reactions, which will
counteract the effect of atmospheric dilution. However, in opposition to light
aromatic hydrocarbons the polar nitrophenols are water-soluble and are found
in both the particle and gas phase (Saccon et al., 2013). Consequently, they
will be removed not only by chemical reactions, but also by wet and dry
deposition. Carbon isotope fractionation resulting from physical removal
processes is much smaller than isotope fractionation during chemical
reactions and therefore will have little direct impact on the carbon isotope
ratio of nitrophenols. However, physical removal processes will reduce the
contribution of aged air masses to nitrophenol concentrations and
therefore reduce the weight of aged air in samples representing air masses
with different PCAs. Combined, these effects have the potential to create a
complex situation with substantial differences in PCAs derived from precursor
carbon isotope ratios compared with nitrophenol-derived PCAs.
Consequently, the mixing of aged air with fresh emissions of light aromatic
VOCs can result in discrepancies between precursor carbon isotope ratio-derived
PCAs and benzene carbon isotope ratio-derived PCAs. Lower values for precursor-derived PCAs can be expected if fresh emissions are mixed with aged air
masses under conditions which allow for the accumulation of reaction products
(Fig. S7a). The accumulation of polar low-volatility reaction products is not only
limited by gas-phase reactions, but also by deposition (an example of the
principle of the impact of deposition on PCAs is given in Sect. S7 in the Supplement).
While conceptually, mixing of two air masses with different PCAs explains the
difference in the frequency of observations of between precursor- and product-derived PCAs,
it can be expected that a range of PCAs will be
a more realistic situation for urban sites. For the average precursor-derived PCA the
distribution for individual PCA observations is known (Kornilova et al.,
2016). We use these distributions to calculate the PCA distribution for
4-nitrophenol and understand the source of differences in the average PCA.
Figure 5 shows the resulting PCA distributions calculated for different
depositional loss rates of 4-nitrophenol. With increasing loss by
deposition, the centres of the distributions shift towards lower PCAs and
become narrower, which is the consequence of decreasing contributions of
air masses with high PCAs. The centre of the distribution resulting from a
depositional loss rate five times faster than loss due to reaction with the
OH radical has its maximum at a value for [OH]dt of approximately
5×1011 s molecules cm-3, which is close to the average
of PCAs derived from observed 4-nitrophenol carbon isotope ratios (Table 3).
PCA distributions calculated for different depositional loss rates
of 4-nitrophenol. The depositional loss rates are given as multiples of the
chemical loss rate of 4-nitrophenol due to reaction with OH radicals. For
comparison, the PCA distribution determined from the precursor carbon isotope
ratio distribution (Kornilova et al., 2016) is also shown (solid line). The
numbers on the graph indicate the OH-loss rates (similar to Fig. 6).
A comparison of the calculated distributions with the carbon isotope
ratio-derived PCA distributions shows that not only the averages but also the
widths of the distributions agree for depositional loss rates of
4-nitrophenol between three and seven times faster than reaction with the OH
radical (Fig. 6) within the statistical errors of the observations. Based on
an average OH-radical concentration of 106 radicals cm-3 the
4-nitrophenol loss by deposition corresponds to a lifetime in the range of 6
days to 2 weeks. This is at the lower end of the atmospheric residence time
of PM. However, only a small fraction of atmospheric 4-nitrophenol is found
in the particle phase (Saccon et al., 2013), which explains the fact that the
atmospheric residence time of 4-nitrophenol exceeds the average residence
time of PM in the lower troposphere. Isotopic evidence does not allow for
differentiation between different processes unless the isotope fractionation
resulting from these processes differs. There are, consequently, no or very small
isotope fractionation effects. To our knowledge there are no published values
for wet or dry deposition rates of 4-nitrophenol. Consequently, we cannot
identify the contribution of specific types of physical deposition processes.
Based on current knowledge, chemical reactions in the condensed phase are too
slow to contribute to the atmospheric loss of 4-nitrophenol (see the detailed
estimate in Sect. S1 in the Supplement).
Comparison of the PCA distributions calculated for different
depositional loss rates of 4-nitrophenol. The depositional loss rates are
given as multiples of the chemical loss rate of 4-nitrophenol due to
reaction with OH radicals. For comparison, the PCA distributions determined
from the 4-nitrophenol carbon isotope ratios reported by Saccon et
al. (2015) are also shown. The error bars represent the statistical uncertainty
resulting from the limited number of observations.
The contribution of an air mass with a given PCA derived from 4-nitrophenol
carbon isotope ratios depends on the deposition rate relative to the rate of
the reaction of 4-nitrophenol and the benzene precursor with the OH radical
(Sect. S7). However, there is no direct connection between
deposition rates and the reaction rate with OH radicals; therefore,
the ratio of the depositional loss rate over the impact
of OH-radical chemistry can vary substantially for individual observations. For example, during rain
events it is expected that deposition will be faster than average,
whereas removal as well as formation of 4-nitrophenol due to reaction with
OH radicals will be slower.
Indeed, rain has a substantial impact on the atmospheric concentrations of
nitrophenols in the particle phase as well as in the gas phase. Substantial
precipitation during sampling or on the day before sampling, reduces the
nitrophenol concentrations by a factor of between 3 and 6 (Fig. 7a). In
contrast, precipitation has no significant impact on the PCA (Fig. 7b).
Changes in the PCA are within the uncertainty of the averages for different
precipitation conditions and, except for 4-methyl-2-nitrophenol, are below
25 %. Precipitation during or immediately before sampling reduces contributions
from air masses with different PCAs independent of the PCAs of the air
masses. This reduces the atmospheric concentrations, but does not
significantly impact the average PCA.
Average nitrophenol concentrations (a) and PCA (b)
determined from carbon isotope ratios reported by Saccon et al. (2015) using
Scenario 3 for different precipitation conditions during and before sampling.
No rain refers to a total precipitation of less than 1 mm on the day of
sampling and the day before; light rain refers to between 1 and 10 mm
precipitation on the day of sampling or a total of >4 mm on the day of
sampling and the day before; and heavy rain refers to >20 mm
precipitation on the day of sampling or >10 mm on the day of sampling and
>20 mm on the day before. Historical meteorological data were acquired
from Environment Canada, Toronto North York site
(http://climate.weather.gc.ca/historical_data/search_historic_data_e.html,
last access: 23 April 2019).
For the precursor of methylnitrophenols, toluene, the PCA distribution is
very different from the distribution observed for benzene, the precursor of
4-nitrophenol (Kornilova et al., 2016). The average PCA for toluene is
only approximately one third of the benzene PCA, and the distribution peaks
at a PCA close to zero, indicating a strong influence from very recent toluene
emissions. The different behaviour of benzene and toluene is explained by the
difference in reactivity and the different geographical distribution of
emission sources (Kornilova et al., 2016). There are substantial sources of
toluene within the Toronto metropolitan area, whereas most major sources
of benzene are located in the surrounding regions.
The low average PCA derived from methylnitrophenol carbon isotope ratios is
consistent with a dominant role of local emissions of toluene and
demonstrates that air masses containing methylnitrophenols with high PCAs are
of limited importance in determining the methylnitrophenol-derived PCAs.
This is supported by the dependence of methylnitrophenol concentrations,
carbon isotope ratios, and PCAs on wind speed shown in Figs. 8 and 9.
Plot of concentrations (black diamonds, left axis) and carbon
isotope ratios (open diamonds, right axis) of 2-methyl-4-nitrophenol as a
function of the maximum wind speed during sampling (Environment Canada,
Historical Weather Data, Toronto North York site). Points were sorted in
order of increasing wind speed, and each point is an average of 10 filter
samples; the samples collected while there was precipitation were excluded.
Error bars are the errors of the mean.
The PCA of 2-methyl-4-nitrophenol as a function of the maximum
wind speed during sampling (Environment Canada, Historical Data, Toronto
North York site). Points were sorted in order of increasing wind speed and
each point is an average of 10 filter samples; the samples collected while there
was precipitation were excluded. Error bars are the errors of the mean.
Figure 8 indicates that when the maximum wind speed over the sampling period
is lowest, concentrations for 2-methyl-4-nitrophenol are highest and the
corresponding carbon isotope ratios are lowest, indicating that
methylnitrophenols may be dominantly produced from local emissions with
limited mixing. This is consistent with the observed PCA (Fig. 9), which is
lowest when the wind speed is lowest and increases with increasing wind
speed. This can be explained by a decrease in the impact of local emissions
resulting in a larger relative contribution of aged 2-methyl-4-nitrophenol
originating from further away. A similar trend is observed for
3-methyl-4-nitrophenol, whereas 4-methyl-2-nitrophenol and
2,6-dimethyl-4-nitrophenol were not considered due to the small number of
samples. 4-nitrophenol did not show any systematic trend. This is consistent
with the lower reactivity of benzene compared to toluene, the 4-nitrophenol
precursor, and the lower local emission rates for benzene (Kornilova et al.,
2016). Both factors will greatly diminish the role of local emission and local
photochemistry on the average PCA derived from 4-nitrophenol carbon
isotope ratios.
Summary and conclusions
Similar to primary emissions of VOCs for secondary pollutants PCAs derived
from carbon isotope ratios decrease with increasing reactivity of the
precursor. However, for the nitrophenols studied here the reactivity of the
secondary pollutant is highly correlated with the reactivity of the primary
pollutant. Consequently, the available experimental evidence does not allow
for a distinction between the impacts of the reactivity of primary or
secondary pollutants. However, this evidence does allow for probing of the
atmospheric processing of pollutants at different timescales and,
consequently, differentiation between impacts from local emissions and
long-range transport. In principle carbon isotope ratios of secondary organic
pollutants provide better insight into the formation of secondary products
than carbon isotope ratios of precursors. However, the use of carbon isotope
ratios of secondary organic pollutants is currently limited by uncertainties
and gaps in the understanding of formation mechanism and carbon isotope
fractionation during the reaction sequence.
Available ambient observations of the carbon isotope ratios of precursor and second-generation products
provide constraints for the parameters and their
uncertainty in a mechanistic model, describing the dependence between the carbon
isotope ratio and the PCA of second-generation products formed by the photo-oxidation of light aromatic VOCs. Predictions by this mechanistic model are
consistent with the results of laboratory experiments studying the formation of
methylnitrophenols from the photo-oxidation of toluene.
Mixing of air masses with nitrophenols that have different ∫[OH]dt values plays an important role in determining their carbon isotope ratios
and needs to be considered in the interpretation of carbon isotope ratios of
secondary organic pollutants and in the relation between concentrations and
carbon isotope ratios. Loss processes such as physical processes based on
diffusion, solubility, or chemical reactions, such as secondary isotope
effects that cause only very small isotope fractionation, can still have a
strong indirect impact on the carbon isotope ratio of nitrophenols if they
play a major role in determining their atmospheric residence time.
Consequently, the dependence between the atmospheric residence time and the carbon
isotope ratios of nitrophenols results in a strong dependence between
the average nitrophenol PCA and the deposition rate. The dependence of the deposition
rate on factors only weakly related to the photochemical reactivity of the
atmosphere can explain the absence of a significant dependence between the
concentration of nitrophenols and their carbon isotope ratios. Similarly,
dispersion in the atmosphere has an indirect but visible impact not only on
the concentration of nitrophenols but also on their carbon isotope ratios.
These results are based on observations in a major urban area with
substantial local and regional nitrophenol precursor emissions. Due to the
increasing uncertainty of the predictions of the mechanistic model with
increasing PCAs and the non-linearity of the dependence between nitrophenol
carbon isotope ratios and PCAs any extrapolation of these results to regions
without substantial emission sources for the light aromatic compounds may be
highly uncertain.
Data availability
The underlying data are published in the cited paper
(Saccon et al., 2013, 2015, and Kornilova et al., 2016). Historical
meteorological data were acquired from Environment Canada, Toronto North York
site
(http://climate.weather.gc.ca/historical_data/search_historic_data_e.html,
last access: 23 April 2019).
The supplement related to this article is available online at: https://doi.org/10.5194/acp-19-5495-2019-supplement.
Author contributions
MS conducted the nitrophenol isotope ratio and concentration
measurements, developed the mechanistic model, conducted the model
calculations, and led the writing and discussion. AK conducted the
concentration and isotope ratio measurements for atmospheric light aromatic
VOCs and contributed to the discussions and writing. LH contributed to and
supervised the carbon isotope ratio measurements. JR contributed to the
discussions, writing, and the mechanistic model development and
calculations.
Competing interests
The authors declare that they have no conflict of
interest.
Acknowledgements
The authors would like to thank Darrell Ernst and Wendy Zhang from
Environment & Climate Change Canada for their technical support. This
research was financially supported by the Natural Sciences and Engineering
Research Council of Canada (NSERC) and the Canadian Foundation for Climate
and Atmospheric Sciences (CFCAS).
Edited by: Rupert Holzinger
Reviewed by: three anonymous referees
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