AMTAtmospheric Measurement TechniquesAMTAtmos. Meas. Tech.1867-8548Copernicus PublicationsGöttingen, Germany10.5194/amt-9-4533-2016Measurement of isoprene nitrates by GCMSMillsGraham P.g.mills@uea.ac.ukHiatt-GipsonGlyn D.BewSean P.ReevesClaire E.https://orcid.org/0000-0003-4071-1926Centre for Ocean and Atmospheric Sciences, School of Environmental
Sciences, University of East Anglia, Norwich, NR4 7TJ, UKSchool of Chemistry, University of East Anglia, Norwich, NR4 7TJ,
UKGraham P. Mills (g.mills@uea.ac.uk)14September2016994533454525January201623March201629July201616August2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://amt.copernicus.org/articles/9/4533/2016/amt-9-4533-2016.htmlThe full text article is available as a PDF file from https://amt.copernicus.org/articles/9/4533/2016/amt-9-4533-2016.pdf
According to atmospheric chemistry models, isoprene nitrates play an important
role in determining the ozone production efficiency of isoprene; however this
is very poorly constrained through observations as isoprene nitrates have not
been widely measured. Measurements have been severely restricted largely due
to a limited ability to measure individual isoprene nitrate isomers. An
instrument based on gas chromatography/mass spectrometry (GCMS) and the
associated calibration methods are described for the speciated measurements
of individual isoprene nitrate isomers. Five of the primary isoprene nitrates
which formed in the presence of NOx by reaction of isoprene with the
hydroxyl radical (OH) in the Master Chemical Mechanism are identified using
known isomers on two column phases and are fully separated on the Rtx-200
column. Three primary isoprene nitrates from the reaction of isoprene with
the nitrate radical (NO3) are identified after synthesis from the
already identified analogous hydroxy nitrate. A Tenax adsorbent-based
trapping system allows the analysis of the majority of the known hydroxy and
carbonyl primary isoprene nitrates, although not the (1,2)-IN isomer, under
field-like levels of humidity and showed no impact from typical ambient
concentrations of NOx and ozone.
Introduction
On a global scale, isoprene is the most important biogenic volatile organic
compound (VOC) in the atmosphere, with its emissions accounting for a third
of the global total VOC emissions (i.e. natural and anthropogenic combined)
(Guenther et al., 2006). It is emitted primarily by vegetation and mostly
during the daytime. Being an unsaturated compound, it is readily oxidised by
the OH and NO3 radicals and ozone (O3). The reaction with OH
dominates during the daytime to produce eight isomeric hydroxyl peroxy
radicals, which like other organic peroxy radicals (RO2) react with
nitric oxide (NO). For isoprene-derived peroxy radicals, in common with most
RO2 radicals, the minor branch of the RO2+ NO reaction yields
stable organic nitrates (RONO2) as a product. At night when NO3
concentrations are higher, NO3 oxidation is believed to become the
dominant loss process for isoprene (Perring et al., 2009a) and because of the
higher estimated yields, reactions with NO3 are expected to contribute
more than 50 % of the total isoprene nitrates (INs) formed (Horowitz et
al., 2007; von Kuhlmann et al., 2004).
Isoprene-derived nitrates investigated in the CASMIN project. The
labelling scheme for hydroxy nitrates is the same as that of Lockwood et
al. (2010) and for the aldehydes as described in the text. NOA is acetone
nitrate.
The responses of different global chemistry transport models, in particular
the production of ozone in the models, are sensitive to the isoprene reaction
schemes (e.g. Fiore et al., 2005; Wu et al., 2007). Studies suggest that
the yields of the of isoprene nitrates (INs) (Squire et al., 2015), as well
as the proportion of the NOx tied up in them that is recycled as opposed to
lost (e.g. Emmerson and Evans, 2009) in the different models, are the main
factors in these discrepancies.
In addition to modelling studies, there have been a number of laboratory
studies which have used a variety of analytical methods to estimate the
total yield of isoprene nitrates from the oxidation of isoprene and reported
total yields range between 4.4 and 15 % (e.g. Chen et al., 1998;
Lockwood et al., 2010; Paulot et al., 2009; Sprengnether et al., 2002; Xiong et
al., 2015; Schwantes et al., 2015). There have also been reported
measurements of speciated isoprene nitrates in chamber experiments (Nguyen
et al., 2014; Schwantes et al., 2015).
There have been some field studies of the OH-oxidation-derived nitrates
(Schwantes et al., 2015; Perring et al., 2009b; Giacopelli et al., 2005;
Grossenbacher et al., 2001, 2004; Werner et al., 1999). However these
measurements and the laboratory experiments described above have all been
ultimately limited in their use for model improvement and evaluation by the
difficulty in unambiguously identifying and quantifying individual isomers
of INs, particularly in the field. Some progress in the identification,
quantification and measurement of reaction rates of individual isomers has
previously been reported (Lockwood et al., 2010; Lee et al., 2014; Jacobs et
al., 2015; Xiong et al., 2016). However, until recently, the limited
availability of pure isomers for calibrations, identifications and kinetic
studies as well as suitable methods for the accurate determination of the
isomeric distributions in complex mixtures have greatly restricted progress.
Here we present the results of the CASMIN project (Comprehensive Analytical
System for Measuring Isoprene-derived Nitrates) to synthesise pure isomers
of the primary INs, both OH- and NO3-derived, and to develop a
GCMS-based method for the calibrated, speciated measurement of IN isomers
using negative ion mass spectrometry which is highly sensitive to organic
nitrates (Worton et al., 2008; Tanimoto et al., 2000).
Synthesis
The compounds we have investigated in this study are shown in Fig. 1. For
the hydroxy nitrates, we follow the same naming convention as Lockwood et
al. (2010). The aldehydic nitrates are labelled similarly to the equivalent
hydroxy nitrate where the oxygen atom and nitrate are in the same position
in the molecule, except they have “–al” as a suffix.
Until the very recent report of the synthesis of an isoprene-derived
carbonyl nitrate (Xiong et al., 2016), the reported syntheses of IN isomers
were limited to three INs made in a non-specific process that produced
complex mixtures of the three INs and other products. The number of possible
reaction mechanisms responsible for the IN formation and the mixture
produced meant that post-separation identification of the INs was required.
In contrast, our general synthetic approach was to build the isoprene
nitrates in stages, which allowed us to design and assemble known carbon
skeletons and use protecting group strategies as well as late-stage
nitration under mild conditions to ensure selectivity in the location and
extent of nitration, thus yield individual, unambiguous isoprene nitrate
isomers. We report the synthesis and purification of (4,3)-IN, Z-(1,4)-IN,
E-(1,4)-IN, Z-(4,1)-IN, E-(4,1)-IN and acetone nitrate (NOA) in detail in
Bew et al. (2016; the NMR data are also included in the Supplement of this current paper) while the attempted syntheses of
(2,1)-IN, Z-(1,4)-al-IN, E-(1,4)-al-IN and Z-(4,1)-al-IN are described here.
Retention times (minutes) for isoprene nitrates on two different
stationary phases. Both columns are 30 m, 0.32 mm ID, 1 µm phase
thickness. The flow and oven conditions are the same for both columns. A
constant flow of He carrier of 4.0 mL min-1 was used and the oven
program was 35 ∘C (hold 3 min), increasing by
15 ∘C min-1 to 70 ∘C (hold 1 min),
+3 ∘C min-1 to 110 ∘C (hold 15 min),
+5 ∘C,́min-1 to 180 ∘C (hold 7 min).
As reported in Bew et al. (2016), the attempted synthesis of (2,1)-IN failed
to produce a purified nitrate despite the synthesis of a promising precursor.
This was due to the surprisingly low reactivity of the precursor to
nitration, despite a number of methods being applied, and the difficulties in
separating the reaction products after nitration. Despite these problems, the
initial post-nitration mixture was subjected to further steps in an attempt
to produce (2,1)-IN. The synthetic methods, analytical data and
identification reasoning are included in the Supplement. Because separation
of the products was not achieved, interpretable NMR spectra were impossible
to obtain but the headspace of this mixture, analysed using a
chemiluminescence (CL) system (see Sect. 5), showed the presence of NOy,
whilst analysis with the GCMS showed a single volatile component that
displays fragment ions with m/z 46 and m/z 62 with NI and a large
m/z 46 with EI, suggesting that the observed component is nitrated. Since
identification is ambiguous, this compound will be referred to as species X.
The carbonyl nitrates in Fig. 1 were produced by oxidation of the
corresponding hydroxy isoprene nitrate with manganese dioxide in
acetonitrile, with purification by flash chromatography on silica gel. Based
on GCMS headspace analysis, E-(1,4)-al-IN was produced as a single isomer
from E-(1,4)-IN whilst the oxidation of Z-(1,4)-IN (probably as a mixture
with E-(1,4)-IN) produced Z-(1,4)-al-IN as a mixture with E-(1,4)-al-IN. A
synthesis of Z-(4,1)-al-IN was performed for identification, but no further
attempts to purify it from its parent alcohol were undertaken.
We were unable to obtain NMR data of purified Z-(1,4)-al-IN and
Z-(4,1)-al-IN isomers as they appear to decompose or polymerise rapidly,
either during purification or in the NMR tube. The resulting spectra were
broad and uninterpretable. We did, however, manage to obtain the 1H NMR
of the purified E-(1,4)-al-IN isomer (included in the Supplement)
and it is in excellent agreement with that of the same isomer
very recently synthesised by Xiong et al. (2016).
Identification of isoprene nitrates via GC-MSChromatography
The retention times and mass spectra of the INs were determined by injecting
approximately 20 µL of diluted headspace vapour directly onto the
column using a stainless steel 6-port Valco valve and a sample loop made
from Sulfinert capillary tubing. Our initial characterisation attempts used
a 60 m Rtx-1701 column. However, we had great difficulty in observing the
E-(1,4)-IN and Z-(1,4)-IN isomers. The chromatograms of these two isomers,
when peaks were visible at all, showed signs of on-column decomposition such
as baseline distortions and very asymmetric peak shapes, neither of which
were observed with other isomers. Lowering the column temperature improved
the peak shapes and reproducibility somewhat. The faster elution of these
isomers on the 60 m Rtx-200 column also improved results. The use of shorter
columns reduced this problem yet further, with the isomers eluting after
spending less time on column and at a lower average temperature than on the
longer columns. All identification, separation and calibrations reported
here were thus performed with 30 m long, 0.32 mm ID silica columns with
1 µm stationary phase thickness.
The (4,3)-IN, E-(1,4)-IN, E-(4,1)-IN, Z-(4,1)-IN, species X, E-(1,4)-al-IN
isomers and acetone nitrate (NOA) retention times and mass spectra were
recorded using individual isomers. The others were determined from mixtures
of two isomers, one of which was already identified from a pure isomer. The
retention time measurements were performed on two columns of the same length,
diameter and film thickness, but with different column phases. Both columns
were operated under the same constant flow of He carrier of
4.0 mL min-1 and the same oven program of 35 ∘C (hold
3 min), increasing by 15 ∘C min-1 to 70 ∘C (hold
1 min), +3 ∘C min-1 to 110 ∘C (hold 15 min),
+5 ∘C min-1 to 180 ∘C (hold 7 min). The retention
times on both columns are shown in Table 1.
Mass spectra (EI left, NI right) of isoprene nitrates
(4,3)-IN (a, b), Z-(1,4)-IN (c, d),
E-(1,4)-IN (e ,f), Z-(4,1)-IN (g, h),
E-(4,1)-IN (i, j), species X (k, l),
Z-(4,1)-al-IN (m, n), E-(1,4)-al-IN (o, p),
Z-(1,4)-al-IN (q, r), acetone nitrate (s, t). Mass axes are
limited to m/z below 150 for clarity; however scans were to higher
m/z but no ions above this m/z were observed for any compounds. EI and NI
mass spectra for all the synthesised INs, including more annotated versions of
those shown here, are in the Supplement.
Mass spectra
Electron capture NI mass spectra (240 eV and argon buffer gas) were recorded
in scan mode up to m/z 250, EI mass spectra (70 eV) were obtained up to
m/z 250. The exception to this was compound X where the EI and NI mass
spectra were measured up to m/z 400. These mass limits were chosen to be
above the maximum expected mass of likely synthesis impurities, such as
partially reacted precursors and dinitrates, whilst still retaining good
sensitivity and scan rates. Figure 2 shows the EI and NI mass spectra for
four INs which represent the range of mass spectra obtained in this study.
Full EI and NI mass spectra for all the compounds synthesised in this study
are shown in Fig. 2. No ions above m/z 150 were observed for any of the
synthesised compounds in this study.
Hydroxy nitrates
The EI mass spectra of the hydroxy nitrates are all similar and show
fragmentation patterns that resemble pentenols, though with no sign of the
molecular ion. The m/z 71 ion is most likely associated with the loss of
the vinyl CH2ONO2 group – a common bond cleavage amongst the
structurally analogous pentenols. Further fragmentation of the m/z 71 ion
by loss of CH3, H, OH and H2O would result in the ions of masses
56–53. The masses 84, 83 and 82 are likely the result of NO2 loss
followed by further fragmentation of the mass 101 fragment by loss of OH,
H2O and of both OH and H2 respectively or by loss of NO3 and
subsequent loss of H atoms. The m/z 46 ion is common to all the IN
investigated in this study, presumably the [NO2]+ ion, though
species X has a much higher abundance than the other compounds. Attempts to
observe the molecular ion using positive chemical ionisation (PCI) with
methane as reagent gas resulted in poor sensitivity and mass spectra that
were very similar to those from 70 eV EI. Using 10 eV EI gave fewer
fragments and yielded slightly higher abundances of larger fragments but as
for PCI, showed no detectable molecular ion. Similarly NI mass spectra of the
different IN isomers generally show the same ion fragments, with m/z 46
(NO2-) usually being the most abundant, behaviour which is comparable
to simple alkyl nitrates (Worton et al., 2008). The m/z 99 and 101 ions are
also prominent and are probably the organic fragments formed following
NO2 loss. The high abundance of these ions is entirely consistent with
alkyl nitrate NI mass spectra in which the organic fragments from larger
nitrates tend to have higher abundances. For simple alkyl nitrates, ions of
m/z corresponding to [RO]- fragments are rarely observed, although
fragments with m/z corresponding to [RO-H2]- are commonly
observed (Worton et al., 2008) – usually presumed as elimination of H2
from the α/β hydrogens. In contrast, both [RO]- and
[RO-H2]- ions are observed in INs, even in (Z)- and (E)- isomers
where H2 elimination pathways are not obvious. The proportions of
m/z 99 and m/z 101 for E-(1,4)-IN are quite different to Z-(1,4)-IN,
Z-(4,1)-IN and E-(4,1)-IN, despite having no more of an obvious source of H2
loss than the others. Likewise (4,3)-IN produces mainly m/z 101, despite
having suitable hydrogens available for elimination.
The EI mass spectrum of species X is very similar to the known INs, although
it shows m/z 76 as a minor ion, presumably CH2ONO2+, whilst
the other synthesised isomers do not show this ion at all. They only show a m/z 71
fragment that would result from the same bond homolysis. As noted above, the
relative abundance of m/z 46 is much higher in species X than the other
nitrates. The NI mass spectrum of species X is quite different to the other
INs in that it shows only two significant ions, presumably NO2-
(m/z 46) and NO3- (m/z 62), with m/z 99 and 98 making up less
than 1 % of the ions formed and m/z 101 not detectable. It is worth
noting that Schymanski et al. (2009) report that prediction of detailed mass
spectra based on structure alone is not reliable.
Carbonyl nitrates
The EI mass spectrum of NOA shows m/z 43 as by far the most abundant ion
resulting from the typical carbonyl α-cleavage to yield the
CH3CO+ ion. It is likely that m/z 76 is CH2ONO2+, the other
possible ion from the same bond rupture. Unlike the mass spectrum of acetone
itself, no molecular ion (M) was observed.
The aldehydes typically show masses for [M- NO2]+ (m/z 99) and
[M-NO3]+ (m/z 83). These ions may also lose CO to yield m/z 71
and 55 which are both common ions. Losses of one or two hydrogen atoms from these
ions would yield m/z 98, 97, 82, 81, 69 and 53, the other commonly observed
fragments. As for the hydroxy nitrates, PCI and 10 eV EI mass spectra showed
no evidence of a molecular ion.
The NI mass spectra of NOA, as is typical of low molecular mass nitrates,
yields m/z 46 as the major ion with only low abundances of the organic
fragment from the same bond rupture (m/z 73). The m/z 71 fragment is
presumably the result of further H atom or molecular H2 loss from the
m/z 73 fragment, although the nature of this loss process and the resulting
ion are unclear. Unlike the small alkyl nitrates, m/z 62 is seen, albeit at
low abundance.
The most distinctive characteristic of the NI mass spectra of the aldehydes
are the high abundances of m/z 62 and a high proportion of m/z 98. The
m/z 98 ion is most likely to be either the result of further H atom loss
from the m/z 99 ion resulting from NO2 loss, or by direct elimination
of HNO2 from the molecular ion, which itself is not seen.
Sample matrix and photochemistry experiments
To test the potential for separation of mixtures of isomers in complex
matrices with each column, a number of qualitative photolysis experiments
were performed using 10 L Tedlar sample bags. Several mixtures containing
isoprene (approx. 100–1000 ppb) NO (approx. 50 ppb), humid synthetic air
(50 %RH) and 20–100 ppb (ppb; nmol mol-1) of a radical source
(ethyl nitrite t-butyl nitrite) were prepared in the Tedlar bags and exposed
to ambient sunlight (clear, winter midday conditions for southern UK) for
approximately 5 min. The contents of the bags were sampled (50 mL samples
trapped using the heated capillary inlet and sampling methodology described
in Sect. 4) and analysed by NI-GCMS in SIM (selected ion monitoring) mode
with only a small number of ions monitored. The different nitrites used as
radical sources had no observable impact on the experiments. Similar mixtures
containing no isoprene were also prepared and exposed to ambient sunlight as
controls for comparison. No particular effort was made to control or quantify
the concentrations of components in the bags or to measure the solar
irradiance.
The chromatograms obtained without isoprene present contained only a few
components and, with the exception of a small number of peaks on m/z 46,
the retention window of the INs showed no significant peaks containing any of
the m/z 62, 98, 99 and 101 ions. The chromatograms obtained when isoprene
was present during photolysis are shown in Fig. 3 and contained many
additional peaks throughout the chromatogram. Most of these new peaks had
m/z 46 as their sole ion (of the ions we monitored), suggesting that they
were products that contained the - NO2 moiety, thus are likely to be
some kind of nitrated organic compound. Figure 3a shows the total ion
chromatogram (TIC) calculated from the sum of all monitored ions. The complex
chromatogram means it is impossible to reliably identify any particular
isoprene nitrate using the TIC (or even only the m/z 46 ion which in this
case constitutes the majority of the TIC signal) under these conditions. The
use of IN fragment ions other than m/z 46 gave significantly simpler
chromatograms with far fewer peaks, and Figs. 3b and 4 show composite
chromatograms comprised of these ions for analyses of similar photochemistry
experiments on the two column phases used in this study. It is clear from
these figures that, using selected ions, isoprene nitrates can be identified
in a complex mixture with either column, although it is worth noting that the
use of the Rtx-200 column allows the separation of the E-(4,1)-IN and the
Z-(1,4)-IN isomers which co-elute on the Rtx-1701.
Results of a bag photochemistry experiment analysed on a 30 m,
0.32 mm ID Rtx-200 column using NI mass spectrometry. (a) Total Ion
Chromatogram. (b) Composite extracted-ion chromatogram of the data
in (a). The small peak marked (*) has the same retention time and
ions as the small impurity in the synthesised species X.
Composite extracted-ion chromatogram showing INs formed during a bag
photochemistry experiment and analysed on a 30 m, 0.32 mm ID, Rtx-1701
column. The conditions for the photolysis are similar to those used in
Fig. 3, and the analytical conditions are identical to those used for the
Rtx-200 column used in Fig. 3. The E-(4,1) and Z-(1,4) isomers are not
separated under the column conditions used. The peak marked (*) is the
same retention time and major ions as the impurity in the synthesised species
X. MSD (mass selective detector) is the mass spectrometer.
The ions used in Figs. 3b and 4 for each compound were chosen to give the
least ambiguous identification and quantification of each IN within the
samples from the bag photochemistry experiments. To estimate the relative
proportions of isomers present in the bag, the quantifying ion's fraction of
the total mass spectrum for that isomer was used as a scaling factor to
correct for sensitivity differences. The results from the different
experiments and columns are slightly different, but in both cases we observe
that (4,3)-IN represents ≥ 50 % of the observed INs. In addition to
the hydroxy nitrates, it is also evident that aldehydic nitrates are formed
under the conditions employed and in the Master Chemical Mechanism (MCM) such
nitrates are represented by a lumped species (NC4CHO), which is only
formed from NO3 addition to isoprene.
The chromatograms of the bag photochemistry experiments also clearly show a
component that has the same retention time and major ions as the synthesised
species X and which is formed only in the experiments when isoprene and NOx
are present, suggesting that species X is indeed an isoprene-derived nitrate.
The peak marked (*) in Figs. 3 and 4 is formed during the bag
experiments, and the retention time and ions are identical to a small impurity
in our synthesised species X, which has a very similar EI mass spectrum (in
the Supplement) and an identical NI mass spectrum to species X.
Sample trapping and conditioning
Because of the reactive nature and low volatility of INs, all sample lines,
valves and transfer lines were heated to 120 ∘C to reduce
adsorptive losses and memory effects. Additionally, where possible, sections
of GC columns were used as transfer lines to keep as many surfaces as inert
as possible. Because of the improved separation and faster elution times
afforded by the Rtx-200 column, all the sample trapping and conditioning
tests were performed using the Rtx-200 column.
The sample trap used to collect the data in this study was a 1/4′′ glass
thermal desorption (TD) tube packed with 3 cm of 60/80 Tenax TA adsorbent,
held in the tube with a glass wool plug. During sample trapping the trap was
held at 35 ∘C and desorption off the trap onto the column was
achieved with a reversed gas flow at 150 ∘C for 4 min. All samples
were trapped at a constant flow of 40 mL min-1 controlled by a mass
flow controller. No additional sample conditioning was used other than to
flush the trap with dry nitrogen for 1 min before injection to reduce the
oxygen in the tube. The TD tube was held between two Siltek-treated stainless
steel unions with PFA ferrules and was connected to a 6-port, stainless steel
Valco valve with 15 cm of 0.53 mm ID MXT-1701 column maintained at
120 ∘C.
Injection temperatures of 150 ∘C were used for a period of more than
3 min as it was found that, below these limits, re-heating the
sample trap a second time yielded observable quantities of IN, indicating
that complete desorption had not occurred on the first heating. Higher
injection temperatures were not studied because, as mentioned earlier,
Z-(1,4)-IN and E-(1,4)-IN show signs of significant decomposition on the
60 m Rtx-1701 column at 150 ∘C (albeit they are exposed to those
temperatures for considerably longer during separation on the column than
during an injection). This trapping method is similar to that used by
Grossenbacher et al. (2001, 2004) in the use of non-cooled Tenax adsorbents,
although the trapping and desorption temperatures reported here are an
intrinsic part of our different methodology for dealing with oxidants and to
accommodate the thermal decomposition characteristics of INs.
To test the trapping method, materials and conditions for linearity and
potential breakthrough of INs on the trap, six samples with volumes between
120 and 480 mL of a mixture of INs and ethylhexyl nitrate (C8 alkyl
nitrate) were extracted from a temperature-controlled drum (100 L aluminium
drum, containing approximately 1 ppb of Z-(4,1)-IN and less than 100 ppt
(ppt; pmol mol-1) of the minor components, then trapped and injected
onto the column. Over the range covered by the test volumes, the observed
mass spectrometer signal was linearly proportional to the trapped volume for
the INs (figure included in Supplement), which indicated that the breakthrough
volumes at 35 ∘C are greater than 480 mL. The linearity with
trapping volume also indicates that the compounds do not decompose on the
adsorbent to any significant degree over the trapping period. Confirmation
that decomposition of the adsorbed INs was not significant over the trapping
period was obtained when 160 mL samples trapped and held on the trap at
35 ∘C for a further 20 min were indistinguishable from those
trapped and injected immediately.
The reproducibility of the trapping and injection method was assessed by
analysing seven consecutive 200 mL samples of the same mixture from the
temperature-controlled drum (graph in Supplement). Excluding species X, the
standard deviation of the seven measurements range between 3 % for the
C8 alkyl nitrate and 6.6 % for E-(4,1)-IN with a mean of 4.9 %
for the five isoprene nitrates. Species X has a much larger standard
deviation than the other components (10.3 %), despite it behaving in a
similar manner to the other IN during the linearity tests. The reactive
nature of INs means that conditioning of surfaces in the system is a
potential issue that may impact on the precision and uncertainty in real
measurements at low mole fractions. Repeated measurements of the INs at low
absolute abundances would show evidence of any such conditioning occurring
within the system. To test this, six 50 mL samples of the IN mixture in the
temperature-controlled drum were trapped and injected onto the GCMS system
after it had been left unused for two weeks. We did not observe any increase
in the GCMS response with the increasing number of sample injections. Nor did
we see different responses for 50 mL samples analysed before and after the
sampling of much larger volumes.
Oxidant impacts
Isoprene nitrates contain both an unsaturated C=C bond and a hydroxyl
group, which provide sites for attack by oxidants such as NO2 and
O3. To investigate the potential impacts of sampling INs in an
oxidant-rich environment, 200 mL samples from a mixture of IN (and a C8
alkyl nitrate) were collected normally and then, before injection, an equal
volume of one of NO2, O3 (at 100 ppb) or clean synthetic air was
sampled on to the same trap. The trap contents were then flushed for 1 min
with nitrogen to remove any air or oxidant from the sample prior to injection
and the sample injected normally. Two consecutive samples of the IN mixture
plus air were compared to two consecutive samples of the IN mixture plus
oxidant for each oxidant. At a trap temperature of 35 ∘C, there was
no discernible difference between the samples. In contrast, at a trapping
temperature of -15∘C, there were reductions of approximately
25 % in the INs (and no corresponding change in the C8 alkyl nitrate
in the mixture) when NO2 or O3 were trapped compared to the
synthetic air controls. This indicates that at 35 ∘C the oxidants
are not co-trapped with the INs and have little impact on their analysis and
that prior conditioning of the sample is unnecessary.
Impact of humidity
Lee et al. (2014) report that when trapping three IN isomers directly onto
the analytical column at -20∘C, they observed evidence of
heterogeneous reactions as a result of co-trapping of water, something which
will be more important for field measurements than chamber studies in dry
air. In this work, we trap at above-ambient temperatures on hydrophobic
adsorbents which will prevent the concentration of water, thus preventing hydrolysis
of the IN on the trap. On the very few occasions where water (m/z 18) was
monitored on the EI mass spectrometer it remained at background levels
throughout the chromatogram, suggesting that we do not collect water on the
trap. It remains a slight possibility that any water possibly
concentrated on the trap has eluted very quickly and cleanly in the time
allowed for residual air from the trap to leave the column before the mass
spectrometer is turned on.
Other trapping methods
Cryogenic trapping at -150∘C using empty Silcosteel tube and at
-50∘C using sections of MXT column and Sulfinert traps with Rtx-1
coated column packing material were also tested as potential trapping
materials and methods in this study. Cryogenic trapping gave comparable
precision to that obtained with Tenax traps when analysing the same nitrate
mixture multiple times, but instrument sensitivities determined with
different IN dilutions using cryogenic trapping gave poor reproducibility,
probably due to the different NO2 concentrations (presumably from
decomposition of the IN in the drum) in each IN dilution. In addition, water
would also be effectively trapped, meaning that scrubbers for NO2,
O3 and water would be required for reliable quantitative analysis of
INs. The reactive nature and low volatility of the INs would mean finding
suitable scrubbers would be very difficult or impossible. For example,
manganese dioxide (MnO2), a common O3 scrubber, is utilised in our
syntheses of the aldehydic nitrates to oxidise the corresponding
OH-containing IN to the aldehyde, so is highly unlikely to be a suitable
O3 scrubber. The use of Rtx-1 coated column packing material (10 cm
1/8′′ Sulfinert loop packed with 20 % Rtx-1 on 100/120 Silcoport W,
Restek) was also investigated. At 30 ∘C, there was little impact
from NO2 and O3 on the sampled IN; however the breakthrough volumes
of the more volatile INs (such as (4,3)-IN) were found to be 100=-200 mL.
Trapping at -15∘C improved the breakthrough volume to
> 500 mL, but NO2 and O3 were then found to adversely affect
the analysis.
Calibration methodology
Our initial attempts to calibrate INs utilised a vacuum line equipped with a
calibrated volume to inject pure isoprene nitrate vapour at measured
temperature and pressure either onto the GC column directly or into a known
dilution volume (glass, polyethene or aluminium), a method that has worked
well for stable compounds. However, this proved to be impossible due to the
reactive nature of the isoprene nitrates. The measured vapour pressures of
the pure isomers was typically below 0.05 mbar at 25 ∘C, which would
have allowed a single step dilution to ppt (10-12 mol mol-1) mole
fractions; however the observed pressure in the system continually increased
in the presence of the nitrates. Furthermore, the results of either direct
injection to the column or sampling from the dilution drum gave very variable
results. It is highly likely that adsorption and decomposition in the vacuum
line prevented the production of accurate and repeatable IN concentrations
with this method.
We thus used the dilution of single isomers with synthetic air in drums to
produce indeterminate mole fractions which were then accurately measured by
thermal decomposition to NO2 and a chemiluminescence (CL) measurement
of the NO2 as the basis of our calibrations.
Chemiluminescence detector and measurements
Thermal decomposition at approximately 400 ∘C in a quartz tube has
been used to convert organic nitrates into NO2 for detection by a number
of methods (Day et al., 2002; Lockwood et al., 2010; O'Brien et al., 1998;
Paul et al., 2009) and our system does not differ fundamentally from these.
The quartz loop used in this study was 2 mm ID and the heated length was
approximately 10 cm. At our sample flow rate of 30 mL min-1 the
residence time in the heated zone was in the order of 500 ms, sufficient
time to give quantitative decomposition of organic nitrates to NO2 (Day
et al., 2002). The resulting NO2 was determined by a simple and
well-established luminol-based CL method (Maeda et al., 1980; Kelly et al.,
1990). The NOy content of the drum was determined from the difference in
CL signals obtained when the sample passed through the same quartz tube when
it was heated and when it was at room temperature, assuming that all the
organic nitrates thermally decompose to yield NO2 quantitatively. The CL
detector was itself calibrated against a NO2 gas standard (9.5×10-6 mol mol-1 in nitrogen, BOC Spectra-Seal) diluted with
synthetic air (BTCA 178, BOC) to mole fractions between 5 and 100 ppb. The
measurement precision of the CL detector based on signal variability and
sensitivity during repeated calibrations was 5 % with a minimum overall
uncertainty of 98 ppt, requiring that calibrations were performed on low ppb
mixing ratios of nitrates rather than the low ppt levels we expect in the
real atmosphere. The overall uncertainty of the mole fraction of NOy
determined by the CL method, including the certified accuracy of the NO2
standard, is estimated at ±11.7 % and the average overall uncertainty
for the measurement of INs (including the GCMS precision and calibration
uncertainties) is ±14 %.
IN calibration results
For the calibrations a glass cube with 30 cm sides was constructed from
laminated glass, with the contents mixed with a small brushless fan. The size
of the cube allowed rapid internal mixing and allowed us to heat the cube in
an oven to aid cleaning. During experiments, the cube's contents were
shielded from light with aluminium foil on all sides.
Sub-µL quantities of an isoprene nitrate were introduced into the
cube by wetting the tip of a fine stainless steel wire with the isomer and
placing the tip inside the volume for 1–10 min and allowing it to
evaporate. It was left for at least 2 h to equilibrate before it was
analysed by CL. Further flushing with BTCA air and re-equilibration were
performed until the observed NOy mole fraction was approximately the
desired value. Experiments with 2-ethylhexyl nitrate showed that complete
mixing occurred in less than 10 min within the cube. However, it was found
that the isoprene nitrate levels dropped rapidly even after this 10 min
mixing period and continued to decrease slowly for several days, although
after 2 h the rate of change was low enough that it was effectively constant
over the time required for concentration determinations and GCMS sampling of
the cube; therefore it did not adversely impact the measurements. The rapid
initial losses and the subsequent slow decrease in IN concentrations occurred
in 100 L aluminium drums and 15 L polyethene drums as well as the glass
cube.
After CL analysis, the cube's contents were analysed immediately by GCMS
(using the Tenax trap as described in Sect. 4), then were reanalysed by CL
for NOy content to quantify the effects of sample removal and
decomposition during GCMS sample collection. Typically less than 1 % of
the cube's volume was removed during CL and GCMS analysis, and over the
10 min period between the two CL analyses no difference in NOy was
observed within the precision of the CL measurement. As for the tests in
Sect. 4, all GCMS calibrations of the INs used the Rtx-200 column because of
its improved performance compared to the Rtx-1701.
The cube's contents were sampled by the GCMS at 25 mL min-1 for
2–4 min through a heated capillary column inlet (0.53 mm ID Rtx-200 at
120 ∘C), 5 cm of which was inserted through the cube's inlet port
to avoid sampling the cubes contents through an unheated fitting – the same
sampling method used during the CL measurements.
GCMS response and calculated GCMS sensitivity for Z-(4,1)-IN at
different mole fractions (measured by CL). The error bars shown are the
combined CL and GCMS measurement precisions (±8.3 %) as discussed in
Sect. 4.
Figure 5 shows the calibration results using this method at four different
mole fractions of Z-(4,1)-IN, and it is evident that the GCMS response is
linearly proportional to the NOy mole fraction measured by CL, and
that the GCMS sensitivity determined at each of these very different mole
fractions is the same within the measurement precision of the instrument.
Relative sensitivities of different INs to n-butyl nitrate, using
different quantification ions. Relative sensitivity is calculated as peak
area per unit mole fraction IN (ion) / peak area of n-butyl nitrate
(m/z 46) per unit mole fraction n-butyl nitrate. It should be noted that
the signal noise is typically a factor of 2 to 3 larger for m/z 46 than for
the other ions.
IsopreneSensitivity: Peak area / ppt of IN of analysis Nitrateion relative to n-butyl nitrate m/z 46 m/z 46m/z 101m/z 99m/z 62(4,3)-IN1.431.710.250.08Z-(4,1)-IN1.890.481.060.43NOA2.84––0.29
The GCMS sensitivity of three isoprene nitrates and n-butyl nitrate were
determined from at least three sets of measurements, and the results, shown
in Table 2, indicate that the NI mass spectrometer is typically more
sensitive to the INs than a simple alkyl nitrate. This is consistent with the
findings of Lockwood et al. (2010), who showed that a GC electron capture
detector (ECD) system was more sensitive to INs than n-butyl nitrate.
Typically the limit of detection for n-butyl nitrate on our mass
spectrometer-based system is < 0.05 ppt for 500 mL samples, and the
higher S/N observed for the INs suggests that a similar LoD is, in
principle, possible, although measurements of real air at a location and
during a season where isoprene is abundant have not yet been made to verify
this.
IN mixtures – stability
During the development of the calibration and trapping methods, it was
observed that dilution drums (glass, aluminium and polyethylene) left for
several weeks still contained appreciable concentrations of the INs unless
they had been cleaned at elevated temperatures. It was also noted that the
observed concentration of INs from the same drum was very sensitive to the
temperature of the drum (see Supplement for figure and details) suggesting
that some form of equilibrium between adsorbed and gas phase INs had
established. In light of this temperature dependence, a 100 L aluminium drum
was subsequently insulated and controlled at 31 ∘C (warmer than room
temperature and cooler than trapping temperature) and this
temperature-controlled drum was used in the precision measurements and
sampling tests (Sect. 4).
For those tests, a number of isoprene nitrates and a stable C8 alkyl
nitrate (ethylhexyl nitrate) were introduced into the drum and left for
3 weeks to equilibrate before being analysed by GCMS. Based on our GCMS
sensitivity to Z-(4,1)-IN, a crude estimate of the mole fractions of the
isomers in the drum after the initial equilibration period were 1 ppb for
Z-(4,1)-IN and less than 100 ppt for the minor components. The drum mixture
was sampled repeatedly over 3 weeks during repeated analytical tests, then
again 1 month and 4 months later. The results of measurements made under
identical conditions to the initial measurement are shown in Fig. 6. Over the
initial measurement period, the ratio of most of the INs with C8 nitrate
show a slight decrease, the obvious exception being E-(1,4)-al-IN, which
shows a larger decrease in the ratio, presumably as a result of continued
decomposition of the aldehyde. Figure 6 also indicates that the
IN : C8 nitrate ratio continues to decrease over time, suggesting that
the INs are still decomposing slowly after 4 months.
Ratios of INs to a stable C8 alkyl nitrate in a
temperature-controlled, fan-mixed 100 L aluminium drum measured over a
period of 4 months using the Tenax trapping method described in Sect. 4. The
drum contents were left for 3 weeks to equilibrate before the first sample.
Possible field sampling and calibration methodology
To calibrate the system during short, laboratory-based studies we can use
individual INs and drum dilutions or aged, temperature controlled and slowly
changing dilutions of mixtures of INs that have had the mole fractions of the
components individually determined. For fieldwork, a less cumbersome and more
practical solution would be desirable. One method would be to generate and
measure a mixture of INs in a drum dilution and to sample the drum with a
number of Tenax adsorption tubes identical to the main sample trap. Many of
the pure isoprene nitrates we synthesised are stable at -25∘C for
at least several months which suggests it may be possible to store the tubes
at -25∘C and use them in place of the main trap as an in-field
multi-component standard. However this has yet to be confirmed
experimentally. An alternative and complementary approach is to determine the
sensitivity of the mass spectrometer to each IN relative to that of a stable
organic nitrate (as we have done against n-butyl nitrate in this work) and
then use an organic nitrate standard to calibrate for the INs indirectly.
The fact we use a Tenax adsorption tube as our sample trap without any need
for sample pre-treatment means that such tubes should be suitable for general
sample collection, provided the collected samples are stored at
-25∘C to prevent decomposition.
Comparison with earlier work
Until the very recently reported synthesis of Z-(1,4)-al-IN (Xiong et al.,
2016) and our reported synthesis of five of the hydroxy isoprene nitrates by
unambiguous means in Bew et al. (2016), the only published syntheses of
isoprene nitrates were those of three IN isomers separated from mixtures
reported in detail by Lockwood et al. (2010) and Lee et al. (2014). Both
groups used the same starting material but differ in the particular but
similar nitration conditions employed, and it is quite probable that they
synthesised the same isomers, although they identify them differently. This
highlights the potential ambiguity that results from identifying components
separated from complex synthetic mixtures. The NMR spectra obtained for our
synthesised E-(1,4)-IN, Z-(1,4)-IN and (4,3)- IN are identical to those
obtained by Lee et al. (2014). As reported in Bew et al. (2016), Z-(1,4)-IN
isomerises rapidly to E-(1,4)-IN, resulting in a mixture of the two isomers,
a situation also reported by Lee et al. (2014) who found that samples of
Z-(1,4)-IN contained approximately 15 % of the E-(1,4)-IN. Similarly,
Lockwood et al. report similar behaviour for two of their isomers which are
clearly separated by HPLC yet are mixtures when analysed by GC.
Our attempted synthesis of (2,1)-IN produced species X, a compound that has
many characteristics expected from an isoprene nitrate and was produced from
a starting material that should yield very few products. However, Nguyen et
al. (2014) report that (2,1)-IN elutes before (4,3)-IN on the Rtx-1701
column, whereas our species X elutes much later. The Nguyen study used a
synthesised standard (although the synthesis and analytical data are as yet
unpublished) to determine the retention order, thus it precludes species X
being the (2,1)-IN isomer. It is possible that species X is an
isoprene-derived nitrate of some sort, possibly a conversion product of
(2,1)-IN (or something else altogether), although without either the identity
of species X being known or a pure sample of (2,1)-IN, this remains
speculation.
The bag photochemistry experiments in this study, while poorly constrained,
produced isomer distributions that were very different to those typically
produced by models (e.g. Paulot et al., 2009) which predict the (1,2)-IN to
be the major isomer. We observed (4,3)–IN as by far the most abundant isomer
and we do not see any peaks near (4,3)-IN in the chromatograms that are
strong candidates for an isoprene nitrate. Both Nguyen et al. (2014) and
Xiong et al. (2015) using CIMS detection report that (1,2)-IN (identified
from a mix of isomers generated in chamber photolysis experiments) elutes
before (4,3)-IN on the Rtx-1701 column (though again there is no currently
published synthesis or analytical data for the (1,2)-IN isomer). The (1,2)-IN
isomer is reported to be more reactive than the other IN isomers
(J. D. Crounse, personal communication, 2016) and the most likely explanation
for our unexpected yields is that under the particular trapping and/or
analytical conditions used here the (1,2)-IN isomer is lost before detection.
The observation of what is believed to be the (1,2)-IN isomer using a direct
injection method, cooled short column and no metal parts (see Supplement for
details) supports this conclusion.
Our observed elution order on the Rtx-1701 column for the hydroxy isoprene
nitrates synthesised in this study are entirely in agreement with those
reported by Schwantes et al. (2015) using the same column phase. Our observed
elution window between the (4,3) and the Z-(4,1)-IN isomers and the elution
order of the three aldehydic isoprene nitrates that were synthesised in this study
agree well with those reported by Schwantes et al. That study predicted
elution order from those of the analogue alcohols and peroxides as well as
theoretically derived yields, whilst we identified ours based on direct
production from known synthesised hydroxy nitrate isomers. The positive
identification of E-(1,4)-al-IN by NMR increases our confidence in the
identities of the other two aldehydes since they were synthesised by the same
method, although without NMR there will remain some uncertainty in their
identity.
Conclusions and further work
In the CASMIN project, we have synthesised five isoprene hydroxy nitrates
using controlled synthesis routes and developed a simple GCMS method for
their analysis in complex air matrices, including those with realistic
relative humidity, although further improvements are needed to overcome our
problems observing the (1,2)-IN isomer. We have identified the nitrates on
two different columns and have shown that the use of an Rtx-200 column
allows the separation of two isomers (E-(4,1)-IN and Z-(1,4)-IN) which
co-elute on the more widely used Rtx-1701 column. In addition we have
synthesised and identified three carbonyl isoprene nitrates, one of which is
unambiguously identified by NMR and that confirms the results of recent
work. Furthermore, we have demonstrated that they can be separated and
measured as individual isomers in photochemistry experiments.
The unidentified species X, produced during an attempt at synthesising
(2,1)-IN, is also formed during the same photochemistry experiments in which
the INs were observed. The limited evidence available suggests that it is a
nitrated species and since it is only observed in photochemistry experiments
when isoprene is present, it may be an isoprene-derived nitrate of some
sort, although its identity and how it is formed (e.g. decomposition of
(2,1)-IN) are entirely unknown.
Data availability
Data used in the paper is available upon request to the authors as the raw
data is not publicly archived.
The Supplement related to this article is available online at doi:10.5194/amt-9-4533-2016-supplement.
Acknowledgements
We would like to acknowledge funding from the National Environmental
Research Council (NERC) under grant number NE/J008389/1.
Edited by: A. Hofzumahaus
Reviewed by: J. D. Crounse and two anonymous referees
ReferencesBew, S. P., Hiatt-Gipson, G. D., Mills, G. P., and Reeves, C. E.: Efficient
syntheses of climate impacting isoprene nitrates and (1R,5S)-(-)-myrtenol
nitrate, Beilstein J. Org. Chem., 12, 1081–1095, 10.3762/bjoc.12.103,
2016.Chen, X. H., Hulbert, D., and Shepson, P. B.: Measurement of the organic
nitrate yield from OH reaction with isoprene, J. Geophys. Res., 103,
25563–25568, 10.1029/98JD01483, 1998.Day, D. A., Wooldridge, P. J., Dillon, M. B., Thornton, J. A., and Cohen, R.
C.: A thermal dissociation laser-induced fluorescence instrument for in situ
detection of NO2, peroxy nitrates, alkyl nitrates, and HNO3, J.
Geophys. Res., 107, 4046, 10.1029/2001JD000779, 2002.Emmerson, K. M. and Evans, M. J.: Comparison of tropospheric gas-phase
chemistry schemes for use within global models, Atmos. Chem. Phys., 9,
1831–1845, 10.5194/acp-9-1831-2009, 2009.Fiore, A. M., Horowitz, L. W., Purves, D. W, Levy, H., Evans, M. J., Wang, Y.
X., Li, Q. B., and Yantosca, R. M.: Evaluating the contribution of changes in
isoprene emissions to surface ozone trends over the eastern United States, J.
Geophys. Res., 110, D12303, 10.1029/2004JD005485, 2005.Giacopelli, P., Ford, K ., Espada, C., and Shepson, P. B.: Comparison of the
measured and simulated isoprene nitrate distributions above a forest canopy,
J. Geophys. Res., 110, D01304, 10.1029/2004JD005123, 2005.Grossenbacher, J. W., Couch, T., Shepson, P. B., Thornberry, T., Witmer-Rich,
M., Carroll, M. A., Faloona, I., Tan, D., Brune, W., Ostling, K., and
Bertman, S.: Measurements of isoprene nitrates above a forest canopy, J.
Geophys. Res., 106, 24429–24438, 10.1029/2001JD900029, 2001.Grossenbacher, J. W., Barket, D. J., Shepson, P. B., Carroll, M. A., Olszyna,
K., and Apel, E.: A comparison of isoprene nitrate concentrations at two
forest-impacted sites, J. Geophys. Res., 109, D11311,
10.1029/2003JD003966, 2004.Guenther, A., Karl, T., Harley, P., Wiedinmyer, C., Palmer, P. I., and Geron,
C.: Estimates of global terrestrial isoprene emissions using MEGAN (Model of
Emissions of Gases and Aerosols from Nature), Atmos. Chem. Phys., 6,
3181–3210, 10.5194/acp-6-3181-2006, 2006.Horowitz, L. W., Fiore, A. M., Milly, G. P., Cohen, R. C., Perring, A.,
Wooldridge, P. J., Hess, P. G., Emmons, L. K., and Lamarque, J. F.:
Observational constraints on the chemistry of isoprene nitrates over the
eastern United States, J. Geophys. Res., 112, D12S08,
10.1029/2006JD007747, 2007.Jacobs, M. I., Burke, W. J., and Elrod, M. J.: Kinetics of the reactions of
isoprene-derived hydroxynitrates: gas phase epoxide formation and solution
phase hydrolysis, Atmos. Chem. Phys., 14, 8933–8946,
10.5194/acp-14-8933-2014, 2014.Kelly, J., Spicer, C. W., and Ward, G. F.: An assessment of the luminol
chemiluminescence technique for measurement of NO2 in ambient air,
Atmos. Environ., 24, 2397–2403, 1990.Lee, L., Teng, A. P., Wennberg, P. O., Crounse, J. D., and Cohen, R. C.: On
Rates and Mechanisms of OH and O3 Reactions with Isoprene-Derived
Hydroxy Nitrates, J. Phys. Chem A, 118, 1622–1637, 10.1021/jp4107603,
2014.Lockwood, A. L., Shepson, P. B., Fiddler, M. N., and Alaghmand, M.: Isoprene
nitrates: preparation, separation, identification, yields, and atmospheric
chemistry, Atmos. Chem. Phys., 10, 6169–6178, 10.5194/acp-10-6169-2010,
2010.Maeda, Y., Aoki, K., and Munemori, M.: Chemiluminescence method for the
determination of nitrogen dioxide, Anal. Chem., 52, 307–311,
10.1021/ac50052a022, 1980.Nguyen, T. B., Crounse, J. D., Schwantes, R. H., Teng, A. P., Bates, K. H.,
Zhang, X., St. Clair, J. M., Brune, W. H., Tyndall, G. S., Keutsch, F. N.,
Seinfeld, J. H., and Wennberg, P. O.: Overview of the Focused Isoprene
eXperiment at the California Institute of Technology (FIXCIT): mechanistic
chamber studies on the oxidation of biogenic compounds, Atmos. Chem. Phys.,
14, 13531–13549, 10.5194/acp-14-13531-2014, 2014.O'Brien, J. M., Czuba, E., Hastie, D. R., Francisco, J. S., and Shepson, P.
B.: Determination of the Hydroxy Nitrate Yields from the Reaction of
C2-C6 Alkenes with OH in the Presence of NO, J. Phys. Chem. A, 102,
8903–8908, 1998.Paul, D., Furgeson A., and Osthoff, H. D.: Measurements of total peroxy and
alkyl nitrate abundances in laboratory-generated gas samples by thermal
dissociation cavity ring-down spectroscopy, Rev. Sci. Instrum., 80, 114101,
10.1063/1.3258204, 2009.Paulot, F., Crounse, J. D., Kjaergaard, H. G., Kroll, J. H., Seinfeld, J. H.,
and Wennberg, P. O.: Isoprene photooxidation: new insights into the
production of acids and organic nitrates, Atmos. Chem. Phys., 9, 1479–1501,
10.5194/acp-9-1479-2009, 2009.Perring, A. E., Wisthaler, A., Graus, M., Wooldridge, P. J., Lockwood, A. L.,
Mielke, L. H., Shepson, P. B., Hansel, A., and Cohen, R. C.: A product study
of the isoprene+NO3 reaction, Atmos. Chem. Phys., 9, 4945–4956,
10.5194/acp-9-4945-2009, 2009a.Perring, A. E., Bertram, T. H., Wooldridge, P. J., Fried, A., Heikes, B. G.,
Dibb, J., Crounse, J. D., Wennberg, P. O., Blake, N. J., Blake, D. R., Brune,
W. H., Singh, H. B., and Cohen, R. C.: Airborne observations of total
RONO2: new constraints on the yield and lifetime of isoprene nitrates,
Atmos. Chem. Phys., 9, 1451–1463, 10.5194/acp-9-1451-2009, 2009b.Schwantes, R. H., Teng, A. P., Nguyen, T. B., Coggon, M. M., Crounse, J. D.,
St. Clair, J. M., Zhang, X., Schilling, K. A., Seinfeld, J. H., and Wennberg,
P. O.: Isoprene NO3 Oxidation Products from the RO2+ HO2
Pathway, J. Phys. Chem. A., 119, 10158–10171,
10.1021/acs.jpca.5b06355, 2015.
Schymanski, E. L., Meringer, M., and Brack, W.: Matching Structures to Mass
Spectra Using Fragmentation Patterns: Are the Results As Good As They Look?,
Anal. Chem., 81, 3608–3617, 2009.Sprengnether, M., Demerjian, K. L., Donahue, N. M., and Anderson, J. G.:
Product analysis of the OH oxidation of isoprene and 1,3-butadiene in the
presence of NO, J. Geophys. Res., 107, 4269, 10.1029/2001JD000716, 2002.
Squire, O. J., Archibald, A. T., Griffiths, P. T., Jenkin, M. E., Smith, D.,
and Pyle, J. A.: Influence of isoprene chemical mechanism on modelled changes
in tropospheric ozone due to climate and land use over the 21st century,
Atmos. Chem. Phys., 15, 5123–5143, 10.5194/acp-15-5123-2015, 2015.Tanimoto, H., Hirokawa, J., Kajii, Y., and Akimoto, H.: Characterization of
gas chromatography/negative ion chemical ionization mass spectrometry for
ambient measurement of PAN: Potential interferences and long-term sensitivity
drift, Geophys. Res. Lett., 27, 2089–2092, 10.1029/1999GL011284, 2000.von Kuhlmann, R., Lawrence, M. G., Pöschl, U., and Crutzen, P. J.:
Sensitivities in global scale modeling of isoprene, Atmos. Chem. Phys., 4,
1-17, 10.5194/acp-4-1-2004, 2004.Werner, G., Kastler, J., Looser, R., and Ballschmiter, K.: Organic nitrates
of isoprene as atmospheric trace compounds, Angewandte Chemie-Int. Ed., 38,
1634–1637,
10.1002/(SICI)1521-3773(19990601)38:11<1634::AID-ANIE1634>3.0.CO;2-C,
1999.Worton, D. R., Mills, G. P., Oram, D. E., and Sturges, W. T.: Gas
chromatography negative ion chemical ionization mass spectrometry:
application to the detection of alkyl nitrates and halocarbons in the
atmosphere, J. Chromatogr. A, 1201, 112–119,
10.1016/j.chroma.2008.06.019, 2008.Wu, S. L., Mickley, L. J., Jacob, D. J., Logan, J. A., Yantosca, R. M., and
Rind, D.: Why are there large differences between models in global budgets of
tropospheric ozone?, J. Geophys. Res., 112, D05302, 10.1029/2006JD007801,
2007.Xiong, F., McAvey, K. M., Pratt, K. A., Groff, C. J., Hostetler, M. A.,
Lipton, M. A., Starn, T. K., Seeley, J. V., Bertman, S. B., Teng, A. P.,
Crounse, J. D., Nguyen, T. B., Wennberg, P. O., Misztal, P. K., Goldstein, A.
H., Guenther, A. B., Koss, A. R., Olson, K. F., de Gouw, J. A., Baumann, K.,
Edgerton, E. S., Feiner, P. A., Zhang, L., Miller, D. O., Brune, W. H., and
Shepson, P. B.: Observation of isoprene hydroxynitrates in the southeastern
United States and implications for the fate of NOx, Atmos. Chem. Phys.,
15, 11257–11272, 10.5194/acp-15-11257-2015, 2015.Xiong, F., Borca, C. H., Slipchenko, L. V., and Shepson, P. B.: Photochemical
degradation of isoprene-derived 4,1-nitrooxy enal, Atmos. Chem. Phys., 16,
5595–5610, 10.5194/acp-16-5595-2016, 2016.