Atmos. Chem. Phys., 12, 7499–
7515
, 2012
www.atmos-chem-phys.net/12/7499/2012/
doi:10.5194/acp-12-7499-2012
© Author(s) 2012. CC Attribution 3.0 License.
Atmospheric
Chemistry
and Physics
Peroxy radical chemistry and OH radical production during the
NO
3
-initiated oxidation of isoprene
A. J. Kwan
1,*,****
, A. W. H. Chan
2,**
, N. L. Ng
2,***
, H. G. Kjaergaard
3
, J. H. Seinfeld
1,2
, and P. O. Wennberg
1,4
1
Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA 91125, USA
2
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA
3
Copenhagen Center for Atmospheric Chemistry, Department of Chemistry, University of Copenhagen,
2100 Copenhagen Ø, Denmark
4
Division of Geology and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
*
now at: Energy Storage Division, NEXT ENERGY EWE-Forschungszentrum f
̈
ur Energietechnologie e.V.,
26129 Oldenburg, Germany
**
now at: Department of Environmental Science, Policy, and Management, University of California, Berkeley, Berkeley,
CA 94720, USA
***
now at: School of Chemical and Biomolecular Engineering and School of Earth and Atmospheric Sciences,
Georgia Institute of Technology, Atlanta, GA 30332, USA
****
soon at (September 2012): United States Agency for International Development, Washington, DC, 20523 USA
Correspondence to:
A. J. Kwan (alan.j.kwan@gmail.com)
Received: 20 December 2011 – Published in Atmos. Chem. Phys. Discuss.: 24 January 2012
Revised: 22 June 2012 – Accepted: 7 July 2012 – Published: 17 August 2012
Abstract.
Peroxy radical reactions (RO
2
+ RO
2
) from the
NO
3
-initiated oxidation of isoprene are studied with both
gas chromatography and a chemical ionization mass spec-
trometry technique that allows for more specific speciation
of products than in previous studies of this system. We find
high nitrate yields (
∼
80 %), consistent with other studies.
We further see evidence of significant hydroxyl radical (OH)
formation in this system, which we propose comes from
RO
2
+ HO
2
reactions with a yield of
∼
38–58 %. An addi-
tional OH source is the second generation oxidation of the ni-
trooxyhydroperoxide, which produces OH and a dinitrooxye-
poxide with a yield of
∼
35 %. The branching ratio of the rad-
ical propagating, carbonyl- and alcohol-forming, and organic
peroxide-forming channels of the RO
2
+ RO
2
reaction are
found to be
∼
18–38 %,
∼
59–77 %, and
∼
3–4 %, respec-
tively. HO
2
formation in this system is lower than has been
previously assumed. Addition of RO
2
to isoprene is sug-
gested as a possible route to the formation of several isoprene
C
10
-organic peroxide compounds (ROOR). The nitrooxy, al-
lylic, and C
5
peroxy radicals present in this system exhibit
different behavior than the limited suite of peroxy radicals
that have been studied to date.
1 Introduction
The global emissions of isoprene (440–660 Tg yr
−
1
) (
Guen-
ther et al.
,
2006
) are larger than those of any other non-
methane hydrocarbon. Because of its high abundance and re-
activity towards atmospheric radicals, isoprene plays a major
role in the oxidative chemistry of the troposphere (
Chamei-
des et al.
,
1988
;
Williams et al.
,
1997
;
Roberts et al.
,
1998
;
Horowitz et al.
,
1998
;
Paulot et al.
,
2009a
) and is an impor-
tant precursor for secondary organic aerosol (SOA) (
Claeys
et al.
,
2004
;
Kroll et al.
,
2005
,
2006
;
Surratt et al.
,
2006
,
2010
;
Carlton et al.
,
2009
).
Nitrate radicals (NO
3
), which form primarily from the re-
action of NO
2
and O
3
, are likely the dominant oxidant of iso-
prene at night when photochemical production of hydroxyl
radicals (OH) ceases. Although nighttime isoprene emissions
are negligible (
Sharkey et al.
,
1996
;
Harley et al.
,
2004
), iso-
prene emitted late in the day, as OH concentrations drop, re-
mains in the nighttime atmosphere (
Starn et al.
,
1998
;
Stroud
et al.
,
2002
;
Warneke et al.
,
2004
;
Steinbacher et al.
,
2005
;
Brown et al.
,
2009
). The rate constant for isoprene’s reaction
with NO
3
is
∼
50 000 times higher than that of its reaction
Published by Copernicus Publications on behalf of the European Geosciences Union.
7500
A. J. Kwan et al.: RO
2
chemistry and OH production during isoprene + NO
3
reaction
with O
3
, the other major nighttime oxidant (
Atkinson
,
1997
).
Assuming an NO
3
mixing ratio of 10 ppt and an O
3
mixing
ratio of 40 ppb, oxidation of isoprene by NO
3
will proceed
more than an order of magnitude faster than that by O
3
. Mix-
ing ratios of NO
3
in the nighttime continental boundary layer
generally exceed 10 ppt, being in the range of 10–100 ppt
(
Platt and Janssen
,
1995
;
Smith et al.
,
1995
;
Heintz et al.
,
1996
;
Carslaw et al.
,
1997
), though concentrations on the or-
der of several hundred ppt have been reported (
Platt et al.
,
1981
;
von Friedeburg et al.
,
2002
;
Brown et al.
,
2006
;
Pen-
kett et al.
,
2007
).
During the day, NO
3
is efficiently destroyed by photoly-
sis and reaction with NO (
Wayne et al.
,
1991
), but signifi-
cant daytime concentrations have been measured under con-
ditions of sufficient O
x
(O
x
= O
3
+ NO
2
) and low actinic flux.
NO
3
has been shown to reach concentrations of
∼
1 ppt and
be responsible for
∼
10 % of total isoprene oxidation in the
daytime under clouds or in a forest canopy (
Brown et al.
,
2005
;
Forkel et al.
,
2006
;
Fuentes et al.
,
2007
). In Houston,
with large concentrations of both NO
x
and O
3
, NO
3
con-
centrations between 5–30 ppt in the hours before sunset have
been measured (
Geyer et al.
,
2003a
).
The reaction of isoprene and NO
3
can be significant to at-
mospheric carbon and nitrogen budgets – and subsequently
ozone formation – particularly on a regional scale. Globally,
it is estimated that the isoprene + NO
3
reaction is responsi-
ble for
∼
6–7 % of total isoprene oxidation (
Horowitz et al.
,
2007
;
Ng et al.
,
2008
) and
∼
15 % of oxidized nitrogen con-
sumption (
Brown et al.
,
2009
). Field studies in the north-
eastern United States, which has a mix of NO
x
and isoprene
sources, find that
∼
22 % of isoprene oxidation in the residual
daytime boundary layer,
∼
40 % of isoprene oxidation in air-
masses advected offshore within the marine boundary layer,
and
∼
73 % of NO
3
consumption can be attributed to this re-
action (
Warneke et al.
,
2004
;
Brown et al.
,
2009
). In addition,
the isoprene + NO
3
reaction is likely an important source of
isoprene nitrates, which are significant NO
x
-reservoir com-
pounds affecting regional ozone formation (
von Kuhlmann
et al.
,
2004
;
Fiore et al.
,
2005
;
Horowitz et al.
,
1998
,
2007
).
The oxidation mechanism and products of the iso-
prene + NO
3
reaction have been the subject of numerous
studies (
Jay and Stieglitz
,
1989
;
Barnes et al.
,
1990
;
Skov
et al.
,
1992
;
Kwok et al.
,
1996
;
Berndt and Boge
,
1997
;
Suh et al.
,
2001
;
Zhang and Zhang
,
2002
;
Fan and Zhang
,
2004
;
Ng et al.
,
2008
;
Perring et al.
,
2009
;
Rollins et al.
,
2009
). The initial step in the reaction is NO
3
addition to
one of the double bonds, followed by addition of O
2
to
make a nitrooxyalkyl peroxy radical (RO
2
). The RO
2
radi-
cals then react with NO
2
(to make short-lived peroxynitrate
compounds), NO
3
, HO
2
, NO
2
, or another RO
2
, leading to a
variety of 1st generation products (Fig.
1
). We neglect RO
2
reactions with NO as NO concentrations are generally very
low at night in the remote environments where this reaction is
most likely to occur (and low under our experimental condi-
tions (Sect. 2) due to the rapid reaction NO
3
+ NO
→
2NO
2
).
O
2
NO
3
O
2
NO
OO
HO
2
O
2
NO
OOH
(RO
2
)
O
2
NO
O
(RO)
O
2
O
2
NO
O
+
HO
2
isomerization
See Figs 2 & 3
dissociation
See Fig. 3
RO
2
NO
3
O
2
NO
OH
O
2
NO
O
+
O
2
NO
OO
ONO
2
NO
2
-NO
2
O
2
NO
OONO
2
nitrooxyhydroperoxide
m/z 248
ROOR
m/z 377 and 393
nitrooxycarbonyl
m/z 230
hydroxynitrate
m/z 232
nitrooxycarbonyl
m/z 230
Fig. 1.
Generalized reaction mechanism of the isoprene +
NO
3
reaction. Boxed compounds are detected by
CIMS instrument as
CF
3
O
−
adducts at the indicated
m/z
values.
31
Fig. 1.
Generalized reaction mechanism of the isoprene + NO
3
re-
action. Boxed compounds are detected by CIMS instrument as
CF
3
O
−
adducts at the indicated
m/z
values.
In a previous study (
Ng et al.
,
2008
), we show that the
SOA yield from the reaction of isoprene with NO
3
radicals
is higher when experimental conditions favor RO
2
+ RO
2
re-
actions over RO
2
+ NO
3
reactions. This phenomenon is ex-
plained in part by the formation of low vapor pressure C
10
-
organic peroxides (ROOR), a product channel that had pre-
viously been considered insignificant. In light of the poten-
tial importance of RO
2
+ RO
2
reactions, we present here a
detailed product study of the RO
2
+ RO
2
reactions from the
NO
3
-initiated oxidation of isoprene.
Our study also requires analysis of RO
2
+ HO
2
reactions,
which inevitably occur in this system. Such reactions are
generally considered to form peroxides (ROOH), but there
is a growing body of work showing that, for certain RO
2
,
other product channels are significant, in particular the chan-
nel leading to the formation of hydroxyl radical (OH) (
Has-
son et al.
,
2004
,
2012
;
Jenkin et al.
,
2007
,
2008
,
2010
;
Dillon
and Crowley
,
2008
;
Birdsall et al.
,
2010
;
Birdsall and El-
rod
,
2011
). Since isoprene + NO
3
reactions occur when there
is limited photochemical production of OH, such a channel
may play an important role in determining the oxidative ca-
pacity of the nighttime atmosphere.
2 Experimental
This work presents a detailed product study of the “excess
isoprene” experiment discussed in
Ng et al.
(
2008
). The ther-
mal decomposition of N
2
O
5
serves as the source of NO
3
radicals. N
2
O
5
is synthesized by mixing streams of nitric
oxide (
≥
99.5 %, Matheson Tri Gas) and ozone in a glass
bulb, which forms N
2
O
5
via the following reactions (
David-
son et al.
,
1978
):
NO
+
O
3
→
NO
2
+
O
2
(R1)
NO
2
+
O
3
→
NO
3
+
O
2
(R2)
Atmos. Chem. Phys., 12, 7499–
7515
, 2012
www.atmos-chem-phys.net/12/7499/2012/
A. J. Kwan et al.: RO
2
chemistry and OH production during isoprene + NO
3
reaction
7501
NO
2
+
NO
3
↔
N
2
O
5
(R3)
Ozone is generated by flowing oxygen through an ozonizer
(OREC V10-0); its mixing ratio is found to be
∼
2 % as mea-
sured by a UV/VIS spectrometer (Hewlett-Packard 8453).
The flow rate of nitric oxide into the glass bulb is adjusted
until the brown color in the bulb disappears. The N
2
O
5
is
trapped for 2 h in an acetone-dry ice bath at approximately
−
80
◦
C, cold enough to trap N
2
O
5
but not O
3
, as condensed
O
3
can explode upon warming. After synthesis, the bulb con-
taining the N
2
O
5
, a white solid, is stored in a liquid nitrogen
dewar.
Experiments are performed in the Caltech dual 28 m
3
Teflon chambers (
Cocker et al.
,
2001
;
Keywood et al.
,
2004
). O
3
(Horiba, APOA 360), NO and NO
2
(Horiba,
APNA 360), and temperature and relative humidity (RH)
(Vaisala, HMP 233) are continuously monitored. The cham-
bers are maintained in the dark at room temperature (
∼
20–
21
◦
C) under dry conditions (RH
<
10 %). Prior to an experi-
ment, the chambers are continuously flushed for at least 24 h.
The N
2
O
5
is removed from the liquid nitrogen and vaporizes
into an evacuated 500 ml glass bulb, the pressure in which is
continuously monitored by a capacitance manometer (MKS).
Once a sufficient pressure of N
2
O
5
has been achieved in the
bulb, the bulb’s contents are flushed into the chamber with a
5 l min
−
1
air stream. After waiting
∼
1 h to allow the N
2
O
5
to
become well-mixed in the chamber, a known volume of iso-
prene (Aldrich, 99 %) is injected into a glass bulb and flushed
into the chamber with a 5 l min
−
1
dry air stream, which initi-
ates the reaction.
The amount of isoprene added corresponds to a mixing
ratio in the chamber of
∼
800 ppb, while the N
2
O
5
concen-
tration is
∼
150 ppb. The large excess of hydrocarbon with
respect to N
2
O
5
maximizes peroxy radical self- and cross-
reactions and minimizes NO
3
reactions with both peroxy rad-
icals and stable first generation products (i.e., species other
than isoprene). This excess is magnified by adding the hydro-
carbon after the N
2
O
5
is well-mixed in the chamber: within
the injected plume, hydrocarbon concentrations will be much
greater than 800 ppb.
An Agilent 6890N gas chromatograph with flame ion-
ization detector (GC-FID) measures isoprene and the ox-
idation products methyl vinyl ketone, methacrolein, and
3-methylfuran. The GC-FID, equipped with a bonded
polystyrene-divinylbenzene based column (HP-Plot Q,
15 m
×
.53 mm, 40 μm thickness, J&W Scientific), is held at
60
◦
C for 0.5 min, then ramped at 35
◦
C min
−
1
to 200
◦
C, af-
ter which the temperature is held steady for 3.5 min.
The other gas phase products reported here are moni-
tored with a custom-modified Varian 1200 chemical ion-
ization mass spectrometer (CIMS) (
Ng et al.
,
2007
;
Paulot
et al.
,
2009b
;
St. Clair et al.
,
2010
), which selectively
clusters CF
3
O
−
with compounds having a high fluo-
ride affinity (e.g., acids, peroxides, and multifunctional
nitrooxy- and hydroxy-compounds), forming ions detected at
m/z
MW + 85 (
Crounse et al.
,
2006
). The quadrupole mass
filter scans from
m/z
50 to
m/z
425, with a dwell time of
0.5 s per mass. The CIMS enables more specific speciation
of organic nitrates than other techniques that have been em-
ployed to study the isoprene + NO
3
system: Fourier trans-
form infrared (FT-IR) (
Barnes et al.
,
1990
;
Skov et al.
,
1992
;
Berndt and Boge
,
1997
), thermal dissociation-laser induced
fluorescence (TD-LIF) (
Perring et al.
,
2009
;
Rollins et al.
,
2009
), and proton transfer reaction mass spectrometry (PTR-
MS) (
Kwok et al.
,
1996
;
Perring et al.
,
2009
;
Rollins et al.
,
2009
). FT-IR and TD-LIF measure the amount of a certain
functionality (e.g., nitrates), but in complex mixtures it is
difficult to distinguish compounds sharing a common func-
tional group (e.g., nitrooxycarbonyls and hydroxynitrates).
The PTR-MS allows for identification of individual com-
pounds, but does so with significant fragmentation and water
clustering, which leads to complex mass spectra and an in-
creased probability of mass analogs. In contrast, the CIMS
technique does not lead to significant fragmentation or water
clustering under these experimental conditions, which sim-
plifies interpretation of mass spectra.
Because authentic standards for the major products are un-
available, we estimate the sensitivity of the CIMS to these
products using the empirical method of
Su and Chesnavich
(
1982
). This method estimates the collision rate of CF
3
O
−
and an analyte based on the analyte’s dipole moment and
polarizability. We calculate the conformationally averaged
dipole moment and polarizability of the analytes with the
Spartan06 quantum package using molecular structures op-
timized with the B3LYP/6-31G(d) method. While this theo-
retical approach compares favorably with experimentally de-
rived sensitivities for many compounds (
Garden et al.
,
2009
;
Paulot et al.
,
2009b
,
a
), it represents the largest source of un-
certainty (
±
25 %) for the CIMS data.
3 Results and discussion
Because the isoprene + NO
3
reaction is rapid, the low time
resolution of our measurements (one measurement every
∼
12 min for the GC-FID and
∼
8 min for the CIMS) allows
us to determine only the final product distribution (Table
1
).
The molar yields in Table
1
vary slightly from those re-
ported in
Ng et al.
(
2008
) due to refinements in the estimated
CIMS sensitivity, but these changes do not significantly al-
ter the conclusions drawn in our earlier work. Due to the
computational cost of estimating the conformationally aver-
aged dipole and polarizability of large molecules, we have
assumed that the CIMS has the same sensitivity to all of the
C
9
and C
10
compounds.
The only species for which we see time dependent sig-
nals are the ROOR C
10
-organic peroxide compounds (CIMS
m/z
332, 377, and 393), which reach peak signals 1–3 h af-
ter the reaction is initiated, followed by a slow decay. This
behavior is likely because these compounds have low vapor
www.atmos-chem-phys.net/12/7499/2012/
Atmos. Chem. Phys., 12, 7499–
7515
, 2012
7502
A. J. Kwan et al.: RO
2
chemistry and OH production during isoprene + NO
3
reaction
Table 1.
Products detected by GC-FID and CIMS.
Compound
Method
m/z
(CIMS)
Final concentration
Percent Yield
(ppb)
a
(%)
b
C
4
non-nitrate compounds
MACR
GC-FID
–
3
2.3
MVK
GC-FID
–
6
4.7
C
4
-hydroxycarbonyl
CIMS
171
<
0.5
∼
0
C
5
Nitrates
C
5
-nitrooxycarbonyl
CIMS
230
45.7
35.6
C
5
-hydroxynitrate
CIMS
232
27.5
21.4
C
5
-nitrooxyhydroperoxide
CIMS
248
12.5
9.7
C
4
/C
5
Isomerized nitrates
C
5
-nitrooxyhydroxycarbonyl
CIMS
246
5.5
4.3
C
5
-nitrooxydiol
CIMS
248
3.3
2.6
C
5
-nitrooxyhydroxyhydroperoxide
CIMS
264
2.1
1.6
C
4
-nitrooxycarbonyl
CIMS
216
0.6
0.5
C
5
Hydroxy compounds
C
5
-hydroxycarbonyl
CIMS
185
2.6
2.0
C
5
-diol
CIMS
187
2.3
1.8
C
5
-hydroxyhydroperoxide
CIMS
203
4.2
3.3
C
5
Isomerized hydroxy compounds
C
5
-dihydroxycarbonyl
CIMS
201
1.5
1.2
C
5
-triol
CIMS
203
1.3
1.0
C
5
-dihydroxyhydroperoxide
CIMS
219
<
0.5
∼
0
Organic peroxides
C
10
-dinitrooxy ROOR
CIMS
377
1.0
0.8
C
10
-isomerized dinitrooxy ROOR
CIMS
393
0.6
0.5
C
10
-nitrooxycarbonyl ROOR
CIMS
330
<
0.5
∼
0
C
10
-hydroxynitrate ROOR
CIMS
332
0.6
0.5
C
10
-nitrooxyhydroperoxide ROOR
CIMS
348
<
0.5
∼
0
C
9
-nitrooxycarbonyl ROOR
CIMS
316
<
0.5
∼
0
Other
3-MF
GC-FID
–
4.5
3.5
hydroxyacetone
CIMS
159
0.5
0.4
hydrogen peroxide
CIMS
119
5.5
4.3
glycolaldehyde
CIMS
145
0.9
0.7
Total
c
128.4
a
Products with small but non-zero signals are noted as
<
0.5 ppb.
b
Molar yield.
c
Sum of all products except hydrogen peroxide and minor signals. C
10
compounds are counted twice as they comprise two isoprene
molecules.
pressures and thus interact significantly with instrument tub-
ing or condense into secondary organic aerosol (
∼
10 μg m
−
3
of SOA forms rapidly in this experiment). For these com-
pounds, the reported values are the peak mixing ratios seen
during the experiment.
3.1 Nitrate yield
C
5
-nitrooxycarbonyls, hydroxynitrates, and nitrooxyhy-
droperoxides, the major products of the isoprene + NO
3
re-
action, are detected by the CIMS at
m/z
230, 232, and 248,
respectively. In addition, we see compounds appearing at
m/z
216, 246, and 264, which are consistent with nitrate
Atmos. Chem. Phys., 12, 7499–
7515
, 2012
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A. J. Kwan et al.: RO
2
chemistry and OH production during isoprene + NO
3
reaction
7503
products resulting from the isomerization of the alkoxy (RO)
radical originating from the
δ
-nitrooxyperoxy radical formed
by (1,4) or (4,1) addition (the notation (x,y) indicates NO
3
addition to the x carbon and subsequent O
2
addition to the y
carbon) (Fig.
2
). Previous studies have shown that (1,4) addi-
tions are dominant in this system (
Skov et al.
,
1992
;
Berndt
and Boge
,
1997
;
Suh et al.
,
2001
). Isomerization also leads
to a nitrate product at
m/z
248, the same mass as the ni-
trooxyhydroperoxide. To estimate the ratio of these two iso-
baric species, we assume that the alkoxy radical yield from
RO
2
+ RO
2
reactions is identical for both the non-isomerized
and isomerized nitrooxyperoxy radical (the branching ratio
of RO
2
+ RO
2
is discussed further in Sect. 3.4). Finally, we
see C
10
-organic peroxides at
m/z
332, 377, and 393 (further
discussed in Sect. 3.6). Summing the concentrations of these
nitrates (and noting that the ROOR compounds at
m/z
377
and 393 sequester two nitrates), we find a total organic nitrate
concentration of
∼
100 ppb.
We can express the nitrate yield with respect to both re-
acted nitrogen or carbon. For the nitrogen-based yield, we
divide the nitrate concentration by the amount of NO
3
radi-
cal consumed, which is equivalent to the loss of N
2
O
5
during
this reaction. Lacking a quantitative measurement of N
2
O
5
,
we use the change in NO
2
concentration after the addition of
isoprene (
∼
125 ppb) as a proxy. Every conversion of N
2
O
5
to NO
3
releases NO
2
, but the total change in NO
2
may be
an overestimate of total NO
3
reacted because NO
2
can also
be released in the formation of methyl vinyl ketone (MVK),
methacrolein (MACR), 3-methylfuran (3-MF), and the C
5
hydroxycarbonyl (Fig.
3
), though in Sect. 3.2 we discuss al-
ternative formation pathways for these compounds. Subtract-
ing these additional NO
2
sources to get a lower limit for NO
3
consumption leads to an NO
3
consumption range of 109–
125 ppb and a corresponding nitrate yield of
∼
80–90 % (all
percentage yields in this work are calculated on a molar ba-
sis).
This high yield suggests that the NO
3
radical reacts with
isoprene predominantly, if not exclusively, via addition to
a double bond. The CIMS does not see a detectable rise in
HNO
3
, indicating that hydrogen abstraction is not a signifi-
cant pathway for this reaction (our sensitivity to HNO
3
, how-
ever, is hampered by a large background – probably from im-
purities in the N
2
O
5
or reaction of N
2
O
5
with trace water on
the surface of the chamber). Assuming most of the 16.1 ppb
of MVK, MACR, 3-MF, and the C
5
-hydroxycarbonyl orig-
inates from nitrooxyperoxy radicals, we can account for
∼
100 % of the reacted NO
3
. Additionally, although our ex-
perimental design seeks to minimize reactions of NO
3
with
species other than isoprene, there are possible (likely small)
losses of NO
3
from reaction with other radicals or first gen-
eration products, or heterogeneously to the chamber walls or
SOA.
The measured nitrate yield with respect to NO
3
is consis-
tent with the substantial yields determined by other studies:
∼
95 % (under NO-free conditions) (
Berndt and Boge
,
1997
),
O
2
NO
3
O
2
NO
RO
2
O
2
/RO
2
O
2
/RO
2
1,5 H shift
OO
O
2
NO
O
O
2
NO
OH
isomerization
O
2
NO
OH
O
2
/RO
2
O
2
NO
OH
O
O
2
NO
OH
OH
O
2
NO
OH
O
O
2
NO
O
O
2
O
2
NO
OH
OO
O
2
/HO
2
O
2
NO
OH
OOH
HO
2
O
2
NO
OH
OOH
RO
2
O
2
NO
OH
O
RO
2
O
2
NO
OH
O
O
2
/ -HO
2
+ CH
2
O
+ HO
2
dissociation
RO
2
O
2
NO
OH
OH
nitrooxyhydroxy hydroperoxide
m/z 264
nitrooxyhydroxy carbonyl
m/z 246
nitrooxyhydroxy diol
m/z 248
C
4
nitroxycarbonyl
m/z 216
nitrooxyhydroxy diol
m/z 248
nitrooxyhydroxy carbonyl
m/z 246
nitrooxyhydroxy hydroperoxide
m/z 264
Fig. 2.
Formation mechanism of compounds resulting from the isomerization of alkoxy radicals and measured
by the CIMS at
m/z
216, 246, 248, and 264. This figure assumes initial
NO
3
attachment to the 1-carbon and
formation of an (E)-
δ
-peroxy radical, but other isomers are possible.
32
Fig. 2.
Formation mechanism of compounds resulting from the
isomerization of alkoxy radicals and measured by the CIMS at
m/z
216, 246, 248, and 264. This figure assumes initial NO
3
at-
tachment to the 1-carbon and formation of an (E)-
δ
-peroxy radical,
but other isomers are possible.
57
±
11 % (
Perring et al.
,
2009
), and 70
±
8 % (
Rollins et al.
,
2009
). Variance in yields with different experimental meth-
ods is not surprising because they depend on the relative con-
centrations of different radicals, as well as physical loss and
mixing processes, which are unique to each work. Further-
more, the final product distribution is a strong function of
the distribution of peroxy radical isomers:
δ
-nitrooxyperoxy
radicals tend to maintain their nitrate functionality (with the
exception of the possible formation of hydroxycarbonyl or
3-MF), while
β
-nitrooxyperoxy radicals, if they become ni-
trooxyalkoxy radicals, are likely to lose the nitrate to form
MVK or MACR (
Vereecken and Peeters
,
2009
).
Berndt and
Boge
(
1997
) and
Peeters et al.
(
2009
) suggest that peroxy
radical isomers formed from isoprene oxidation are continu-
ously interconverting. If this is true, the degree of intercon-
version is affected by the rate at which RO
2
become stable
products relative to the interconversion rate, i.e., the magni-
tudes of
k
1
and
k
2
with respect to
k
int1
and
k
int2
in Fig.
4
.
These rates are specific to the unique experimental condi-
tions of each study, such as temperature, pressure, the de-
gree of mixing, and hydrocarbon and oxidant concentrations.
Therefore, the distribution of isomers – which defines the fi-
nal product distribution – may be sensitive to specific exper-
imental conditions.
To calculate the nitrate yield with respect to carbon, we di-
vide the concentration of nitrates by the amount of isoprene
reacted. Because a portion of the isoprene reacts immediately
upon introduction into the chamber, we do not know the exact
starting isoprene concentration. Therefore, we assume that
each of the products listed in Table 1 comes from one iso-
prene molecule, with the exception of the ROOR compounds
(which comprise two isoprene molecules) and hydrogen per-
oxide (which comprises zero). This leads to an estimate of
∼
130 ppb of isoprene reacted, and a nitrate yield of
∼
80 %.
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7515
, 2012
7504
A. J. Kwan et al.: RO
2
chemistry and OH production during isoprene + NO
3
reaction
O
2
NO
3
O
2
NO
RO
2
OO
O
2
NO
O
O
+ CH
2
ONO
2
CH
2
O + NO
2
O
2
NO
3
RO
2
O
H
+ CH
2
ONO
2
CH
2
O + NO
2
ONO
2
OO
ONO
2
O
O
2
NO
3
RO
2
O
2
NO
OO
O
2
NO
O
O
2
NO
OH
O
OH
+ NO
2
1,5-H shift
a)
b)
d)
O
2
NO
3
RO
2
O
2
NO
OO
O
2
NO
O
c)
cyclization
O
+ H
2
O + NO
2
Fig. 3.
Formation mechanisms of methyl vinyl ketone
(a)
, methacrolein
(b)
, 3-methylfuran
(c)
, and hydrox-
ycarbonyl
(d)
, leading to release of
NO
2
. The exact mechanism of 3-methylfuran formation is still uncertain
(Francisco-Marquez et al., 2005).
33
Fig. 3.
Formation mechanisms of methyl vinyl ketone
(a)
,
methacrolein
(b)
, 3-methylfuran
(c)
, and hydroxycarbonyl
(d)
, lead-
ing to release of NO
2
. The exact mechanism of 3-methylfuran for-
mation is still uncertain (
Francisco-Marquez et al.
,
2005
).
As with the nitrogen-based yield, this result too is consis-
tent with other studies:
∼
80 % (
Barnes et al.
,
1990
),
∼
90 %
(
Berndt and Boge
,
1997
), 70
±
8 % (
Rollins et al.
,
2009
), and
65
±
12 % (
Perring et al.
,
2009
).
Much of the discrepancy between our estimates of iso-
prene and NO
3
consumption is likely due to our lack of an
empirical calibration for the CIMS. Some of it, however, is
due to an additional loss process of isoprene besides oxida-
tion by NO
3
, which we discuss in the following section.
3.2 Hydroxyl radical (OH) formation
The CIMS detects the formation of products at
m/z
185,
187, 203, and 201, which are indicative of compounds at
MW 100, 102, 118, and 116, respectively. These compounds
are analogous to those depicted in Figs.
1
and
2
, only with
oxidation initiated by the hydroxyl radical (OH) (Fig.
5
)
(
Surratt et al.
,
2010
); the relative contribution of isobaric
species is determined in the same manner as in Sec. 3.1.
Some of the signal at
m/z
201 may also be attributable to C
5
-
hydroperoxyaldehydes, which have recently been reported to
result from OH oxidation (
Crounse et al.
,
2011
).
Perring et al.
(
2009
) report PTR-MS signals at
m/z
101, 103, 119, and
117, which could be the protonated clusters of these com-
pounds, though they attribute the latter three
m/z
to water
clusters of other major product ions. Under the dry conditions
of our experiment, however, we do not typically observe wa-
ter clusters with, or significant fragmentation of, our product
ions, so we are confident that the signals on the CIMS in fact
represent hydroxy compounds. OH formation may also con-
tribute to some or all of the MVK and MACR produced in
our system, though it is likely that most of the 3-MF comes
β
‐RO
2
δ
‐RO
2
Stable products
(mostly non‐nitrates)
Stable products (mostly nitrates)
k
1
k
2
k
int2
k
int1
Fig. 4.
Schematic of the relationship between the interconversion of peroxy radical isomers and nitrate yields
34
Fig. 4.
Schematic of the relationship between the interconversion of
peroxy radical isomers and nitrate yields.
O
H
O
O
H
O
H
O
H
OOH
O
O
H
O
H
O
H
O
H
O
H
OOH
O
H
O
H
hydroxycarbonyl
diol
hydroxyhydroperoxide
m/z 187
dihydroxycarbonyl
triol
dihydroxyhydroperoxide
m/z 201
m/z 203
m/z 219
m/z 185
m/z 203
Fig. 5.
Products detected by CIMS that may result from the OH-initiated oxidation of isoprene. Other isomers
are possible.
35
Fig. 5.
Products detected by CIMS that may result from the OH-
initiated oxidation of isoprene. Other isomers are possible.
from isoprene + NO
3
reactions because its yield in the iso-
prene + OH system is low (
Ruppert and Becker
,
2000
;
Paulot
et al.
,
2009b
).
We evaluate five possible routes to OH formation in our
system: reactions of (i) O
3
and isoprene (
Neeb and Moort-
gat
,
1999
), (ii) HO
2
and O
3
(
Sinha et al.
,
1987
), (iii) HO
2
and NO (
Seeley et al.
,
1996
), (iv) HO
2
and NO
3
(
Mellouki
et al.
,
1993
), and (v) RO
2
and HO
2
(
Hasson et al.
,
2004
,
2005
;
Jenkin et al.
,
2007
,
2008
,
2010
;
Dillon and Crow-
ley
,
2008
). Routes (i) and (ii) are unlikely to be significant
sources of OH in our experiments. Not only does our O
3
monitor not detect any ozone during the experiment (limit
of detection
∼
2 ppb), but we also see no evidence in the
CIMS data of significant organic acid or peroxide formation,
which would result from the reaction of O
3
with isoprene
(
Hasson et al.
,
2001
;
Orzechowska and Paulson
,
2005
). Fur-
thermore, for route (ii) to be feasible, HO
2
+ O
3
reactions
(
k
=
1
.
9
×
10
−
15
cm
3
molec
−
1
s
−
1
at 298 K,
Sander et al.
,
2011
) must be significantly faster than HO
2
+ HO
2
reac-
tions (
k
=
2
.
3
×
10
−
12
cm
3
molec
−
1
s
−
1
at 1 atm and 298 K,
Sander et al.
,
2011
), which produce ppb levels of H
2
O
2
in
the system (Table
1
). This would require O
3
to be more
than three orders of magnitude more abundant than HO
2
, i.e.,
Atmos. Chem. Phys., 12, 7499–
7515
, 2012
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A. J. Kwan et al.: RO
2
chemistry and OH production during isoprene + NO
3
reaction
7505
Table 2.
Reactions considered for box model assessment of OH sources in the isoprene + NO
3
system.
No.
Reaction
Rate constant
a
Source
1
b
NO
3
+ isoprene
→
RO
2
+ HO
2
6.6
×
10
−
13
Atkinson
(
1997
)
2
RO
2
+ RO
2
→
products
1
×
10
−
13
Atkinson et al.
(
2006
), and references therein
3
RO
2
+ NO
3
→
products
3
×
10
−
12
Biggs et al.
(
1994
);
Daele et al.
(
1995
);
Canosa-Mas et al.
(
1996
);
Vaughan et al.
(
2006
)
4
RO
2
+ HO
2
→
products
2.2
×
10
−
11
Atkinson et al.
(
2006
), and references therein
5
RO
2
+ NO
2
→
RO
2
NO
2
5
×
10
−
11
Sander et al.
(
2011
), and references therein
6
RO
2
NO
2
→
RO
2
+ NO
2
20
Sander et al.
(
2011
), and references therein
7
NO
3
+ HO
2
→
OH + NO
2
+ O
2
3.5
×
10
−
12
Sander et al.
(
2011
), and references therein
8
NO
2
+ NO
3
→
N
2
O
5
6.7
×
10
−
12
Sander et al.
(
2011
), and references therein
9
N
2
O
5
→
NO
2
+ NO
3
2.2
×
10
−
1
Sander et al.
(
2011
), and references therein
10
HO
2
+ HO
2
→
H
2
O
2
2.3
×
10
−
12
Sander et al.
(
2011
), and references therein
11
HO
2
+ NO
2
→
HO
2
NO
2
2.8
×
10
−
12
Sander et al.
(
2011
), and references therein
12
HO
2
NO
2
→
HO
2
+ NO
2
1.8
×
10
−
1
Sander et al.
(
2011
), and references therein
13
NO
3
+ NO
2
→
NO + NO
2
+ O
2
6.6
×
10
−
16
Sander et al.
(
2011
), and references therein
14
NO
3
+ NO
→
2NO
2
2.6
×
10
−
11
Sander et al.
(
2011
), and references therein
15
HO
2
+ NO
→
NO
2
+ OH
8.0
×
10
−
12
Sander et al.
(
2011
), and references therein
a
At 1 atm and 298 K. Units are cm
3
molec
−
1
s
−
1
, except
k
6
,
k
9
and
k
12
, which are s
−
1
. Reaction rates involving RO
2
are approximated
from values found in the literature.
b
HO
2
yield is an upper limit to facilitate model analysis.
at ppm levels that cannot come from trace contamination of
the chamber.
To examine the remaining hypotheses, we create a box
model incorporating the major reactions in the system for
developing a qualitative understanding of which processes
may be important for the final product yield. Table
2
lists
the parameters of this box model; for rate constants that have
not been experimentally determined, we use estimates based
on the rate constants of similar reactions found in the lit-
erature, but caution that the actual rate constants may dif-
fer significantly. Initial conditions reflect the nominal con-
centration of reagents in the chamber: [isoprene] = 800 ppb,
[N
2
O
5
] = 125 ppb, and [NO
2
] = 50 ppb (the NO
2
likely re-
sults from decomposition of N
2
O
5
prior to isoprene injec-
tion). In reality, though, the isoprene concentration is higher
than 800 ppb during the reaction because of our injection
method. As discussed later (Sect. 3.4), there are major un-
certainties in the HO
2
sources and magnitudes, so for the
purposes of assessing possible OH sources, we assume as an
upper limit that the formation rate of HO
2
is the same as that
of RO
2
in Eq. (1) of Table
2
; our final concentration of perox-
ides (i.e., [ROOH] + 2
×
[H
2
O
2
]) is
∼
29 ppb, much less than
the
∼
109–125 ppb of RO
2
that is formed (Sect. 3.1), suggest-
ing that the formation of HO
2
is significantly less than that
of RO
2
.
The box model shows that the NO levels in the chamber
are too low to sustain substantial OH formation via route (iii).
The NO
x
monitor measures
<
1 ppb of NO throughout our
experiment, and any NO that may exist prior to the exper-
iment (or as a trace impurity in the N
2
O
5
) reacts quickly
with NO
3
after N
2
O
5
injection; the NO lifetime is
∼
1 s with
our N
2
O
5
loading. Although NO may be generated as a mi-
nor channel of the NO
2
+ NO
3
reaction, the rapid reaction
of NO and NO
3
limits the steady state concentration of NO
to
<
∼
4 ppt; at this concentration, NO cannot compete with
other radicals reacting with HO
2
(i.e., RO
2
, HO
2
, NO
3
, and
NO
2
). Therefore, HO
2
+ NO is unlikely to contribute signifi-
cantly to the
∼
12–21 ppb of OH that is formed in our system.
The box model also suggests that route (iv) is not feasi-
ble because of the substantial difference in the rates of the
NO
3
+ isoprene and NO
3
+ HO
2
reactions, both of which are
well established experimentally. Under the base conditions of
our box model in Table
2
, which significantly overestimates
the prevalence of HO
2
and underestimates the concentration
of isoprene in the plume, less than 1 % of the NO
3
reacts
with HO
2
, while 94 % reacts with isoprene and the rest with
RO
2
. Therefore, while there is significant uncertainty with
the RO
2
+ HO
2
, RO
2
+ RO
2
, and RO
2
+ NO
3
rate constants,
the frequency of the NO
3
+ HO
2
reaction predicted by the
model is very insensitive to these rates because NO
3
reac-
tivity is dominated by its reaction with isoprene. Even if we
favor NO
3
+ HO
2
reactions by reducing the RO
2
+ HO
2
and
RO
2
+ NO
3
rate constants by a factor of 100, we only obtain
∼
5 ppb of OH formation; in contrast, lowering the NO
3
+
isoprene rate constant would lead to significantly more pro-
duction of OH via NO
3
+ HO
2
(Fig.
6
). These simulations are
consistent with the observation of
Atkinson et al.
(
1988
) dur-
ing hydrocarbon + NO
3
kinetics studies that there is OH for-
mation when slower reacting hydrocarbons are studied. The
reaction of isoprene with NO
3
is sufficiently fast under our
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Atmos. Chem. Phys., 12, 7499–
7515
, 2012
7506
A. J. Kwan et al.: RO
2
chemistry and OH production during isoprene + NO
3
reaction
0
1
2
3
4
5
6
7
8
9
10
0
5
10
15
20
time (minutes)
OH formed (ppb)
base model
10x slower radical
100x slower radical
10x slower hydrocarbon
100x slower hydrocarbon
Fig. 6.
Box model simulations for OH production in isoprene +
NO
3
system. Blue: base case described in
Table 2; Red:
RO
2
+
HO
2
and
RO
2
+
NO
3
rate constants reduced by factor of 10; Green:
RO
2
+
HO
2
and
RO
2
+
NO
3
rate constants reduced by factor of 100; Pink: isoprene +
NO
3
rate constant reduced by factor of
10; Light Blue: isoprene +
NO
3
rate constant reduced by factor of 100. Initial conditions: 150 ppb
N
2
O
5
,
800 ppb isoprene, 50 ppb
NO
2
.
36
Fig. 6.
Box model simulations for OH production in iso-
prene + NO
3
system. Blue: base case described in Table 2;
Red: RO
2
+ HO
2
and RO
2
+ NO
3
rate constants reduced by factor
of 10; Green: RO
2
+ HO
2
and RO
2
+ NO
3
rate constants reduced by
factor of 100; Pink: isoprene + NO
3
rate constant reduced by factor
of 10; Light Blue: isoprene + NO
3
rate constant reduced by factor
of 100. Initial conditions: 150 ppb N
2
O
5
, 800 ppb isoprene, 50 ppb
NO
2
.
experimental conditions, however, that such behavior should
not occur.
We therefore suggest that formation of OH radicals most
likely results from the reaction of RO
2
and HO
2
radicals.
Quantifying the branching ratio of the RO
2
+ HO
2
reaction,
however, is not trivial. There are four documented pathways
for the RO
2
+ HO
2
reaction:
RO
2
+
HO
2
→
ROOH
+
O
2
(R4)
RO
2
+
HO
2
→
ROH
+
O
3
(R5)
RO
2
+
HO
2
→
RO
+
OH
+
O
2
(R6)
RO
2
+
HO
2
→
R
′
CHO
+
H
2
O
+
O
2
(R7)
Channel (R4) can be quantified with CIMS measurements of
peroxides. We neglect channel (R5), first because we don’t
see any evidence for ozone formation, and also because this
channel is believed to proceed via a hydrotetroxide interme-
diate that only yields O
3
if RO
2
is an acylperoxy radical
(RC(O)OO) (
Hasson et al.
,
2005
). To quantify channel (R6),
we can use the sum of OH products as a tracer, but MVK,
MACR, and the C
5
-hydroxycarbonyl can come from either
OH or NO
3
, which leads to uncertainty in this quantity. Sim-
ilarly, the nitrooxycarbonyl can come directly from chan-
nel (R7), indirectly from the RO formed in channel (R6),
or from RO
2
+ RO
2
. Because multiple pathways share com-
mon products, and lacking more knowledge about these in-
dividual pathways, we cannot unambiguously constrain the
RO
2
+ HO
2
branching ratios with the available data.
Recognizing the uncertainties, we estimate the OH yield
from RO
2
+ HO
2
but emphasize that our assumptions and re-
sults must be verified by further studies. We assume chan-
nel (R7) is negligible, as well as OH from RO
2
+ HO
2
reac-
tions where the RO
2
originates from isoprene + OH (
Paulot
et al.
,
2009a
). We thus constrain the range of OH forma-
tion to 9–20.5 ppb, with the upper limit incorporating all
the hydroxy products plus MVK and MACR, and the lower
limit being the upper limit minus MVK, MACR, and the
hydroxycarbonyl. We estimate channel (R4) by the con-
centration of the nitrooxyhydoperoxides at
m/z
248 and
m/z
264, so obtain a range for (R6)/[(R6) + (R4)] of be-
tween 9/(9 + 12.5 + 2.1) and 20.5/(20.5 + 12.5 + 2.1), or 38–
58 %. Because this analysis assumes that RO
2
+ HO
2
reac-
tions are the exclusive source of OH radicals and also ignores
channel (R7), this yield should be considered an upper limit.
Also, this yield is for the isomeric mix of RO
2
in this system,
which is dominated by RO
2
from (1,4) additions, but also
contains other isomers.
To our knowledge, this is the first study that has attempted
to quantify the OH yield from RO
2
+ HO
2
reactions involving
the nitrooxyperoxy radicals in our system. Thus far, signif-
icant OH yields (15–80 %) have been found for acylperoxy
(RC
(
O
)
OO), methoxymethylperoxy (CH
3
OCH
2
OO), and
α
-
carbonylperoxy (RC
(
O
)
CH
2
OO) radicals, and evidence for
OH formation also exists for bicyclic hydroxyperoxy radicals
derived from toluene; in contrast, alkylperoxy and hydrox-
yalkylperoxy radicals have exhibited minimal yields (
Has-
son et al.
,
2004
,
2012
;
Jenkin et al.
,
2007
,
2008
,
2010
;
Dillon
and Crowley
,
2008
;
Birdsall et al.
,
2010
;
Birdsall and El-
rod
,
2011
). For the peroxy radicals in this study, the high
OH yields may result from the presence of the electron-
withdrawing nitrooxy group conjugated through the dou-
ble bond, which may stabilize (i.e., lower the enthalpy of)
the alkoxy radical formed by the radical propagating chan-
nel (R6), thereby making this channel more thermodynami-
cally favored.
3.2.1 OH formation from 2nd generation
dinitrooxyepoxide formation
While this study focuses on the first generation products from
the isoprene + NO
3
reaction, another nighttime source of OH
in the atmosphere would be the further oxidation of the ni-
trooxyhydroperoxide, which can produce a dinitrooxyepox-
ide and OH (
Paulot et al.
,
2009a
). In another experiment de-
scribed in detail in
Ng et al.
(
2008
), we first add 179 ppb
of isoprene to the chamber followed by three additions of
N
2
O
5
(
∼
120, 50, and 210 ppb). After the first two additions,
isoprene is completely consumed, so the third aliquot leads
primarily to the formation of second generation products;
some second generation products may be oxidized by this
third addition, but the amount of N
2
O
5
added is similar to the
concentration of first generation products (which is roughly
equal to the starting isoprene concentration), so such tertiary
chemistry is likely to be minimal. After this third addition,
the nitrooxyhydroperoxide signal drops
∼
6 ppb, while the
signal for the dinitrooxyepoxide (at
m/z
293) rises
∼
2.3 ppb,
indicating that the epoxide (and OH) yield from the NO
3
ox-
idation of the nitrooxyhydroperoxide is
∼
35 %, compared
Atmos. Chem. Phys., 12, 7499–
7515
, 2012
www.atmos-chem-phys.net/12/7499/2012/