Nitrate ion photochemistry at interfaces: A new mechanism for oxidation of α-pinene

Article abstract of DOI:10.1039/b719495a

The photooxidation of 0.6-0.9 ppm α-pinene in the presence of a deliquesced thin film of NaNO₃, and for comparison increasing concentrations of NO₂, was studied in a 100 L Teflon chamber at relative humidities from 72-88% and temperatures from 296-304 K. The loss of α-pinene and the formation of gaseous products were followed with time using proton transfer mass spectrometry. The yields of gas phase products were smaller in the NaNO₃ experiments than in NO₂ experiments. In addition, pinonic acid, pinic acid, trans-sobrerol and other unidentified products were detected in the extracts of the wall washings only for the NaNO₃ photolysis. These data indicate enhanced loss of α-pinene at the NaNO₃ thin film during photolysis. Supporting the experimental results are molecular dynamics simulations which predict that α-pinene has an affinity for the surface of the deliquesced nitrate thin film, enhancing the opportunity for oxidation of the impinging organic gas during the nitrate photolysis. This new mechanism of oxidation of organics may be partially responsible for the correlation between nitrate and the organic component of particles observed in many field studies, and may also contribute to the missing source of SOA needed to reconcile model predictions and field measurements. In addition, photolysis of nitrate on surfaces in the boundary layer may lead to oxidation of co-adsorbed organics. the Owner Societies.

Full text of DOI:10.1039/b719495a

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PAPER
www.rsc.org/pccp | Physical Chemistry Chemical Physics
Nitrate ion photochemistry at interfaces: a new mechanism for oxidation
of a-pinene
a
a
b
c
b
Yong Yu, Michael J. Ezell, Alla Zelenyuk, Dan Imre, Liz Alexander, John
a
b
a
a
a
Ortega, Jennie L. Thomas, Karun Gogna, Douglas J. Tobias, Barbara D’Anna,
a
a
a
Chris W. Harmon, Stanley N. Johnson and Barbara J. Finlayson-Pitts*
Received 18th December 2007, Accepted 19th February 2008
First published as an Advance Article on the web 18th March 2008
DOI: 10.1039/b719495a
The photooxidation of 0.6–0.9 ppm a-pinene in the presence of a deliquesced thin film of NaNO ,
3
s
2
and for comparison increasing concentrations of NO , was studied in a 100 L Teflon chamber
at relative humidities from 72–88% and temperatures from 296–304 K. The loss of a-pinene and
the formation of gaseous products were followed with time using proton transfer mass
spectrometry. The yields of gas phase products were smaller in the NaNO
NO experiments. In addition, pinonic acid, pinic acid, trans-sobrerol and other unidentified
products were detected in the extracts of the wall washings only for the NaNO photolysis. These
3
experiments than in
2
3
data indicate enhanced loss of a-pinene at the NaNO thin film during photolysis. Supporting the
3
experimental results are molecular dynamics simulations which predict that a-pinene has an
affinity for the surface of the deliquesced nitrate thin film, enhancing the opportunity for
oxidation of the impinging organic gas during the nitrate photolysis. This new mechanism of
oxidation of organics may be partially responsible for the correlation between nitrate and the
organic component of particles observed in many field studies, and may also contribute to the
missing source of SOA needed to reconcile model predictions and field measurements. In
addition, photolysis of nitrate on surfaces in the boundary layer may lead to oxidation of
co-adsorbed organics.
Introduction
the air–particle interface, leading to less recombination of NO
2
ꢀ
ꢀ
3
with O( P), respectively, giving effective
with O and of NO
2
It is now recognized that photochemistry at interfaces can be
unique compared to that in the bulk. This effect can be quite
quantum yields that are larger than those measured in bulk
7,19,20
solutions (f1a = 0.009, f1b = 0.001 at 305 nm).
Nitrate ions and organics in particles have been observed to
6
large; for example, the photolysis of Mo(CO) in 1-decene was
faster by at least three orders of magnitude in aerosols than in
1
bulk liquids. Additional experiments and theoretical calcula-
be correlated in some field studies. For example, in the Mexico
21
City study by Volkamer et al., the formation of SOA and
tions also support the notion of enhanced photolysis at
2
nitrate tracked each other. While this may simply reflect
–6
interfaces.
The enhancement appears largely to be due to
x
simultaneous oxidation of organics to SOA and of NO to
the effects of an incomplete solvent cage at the interface, thus
increasing dissociation at the expense of recombination when
compared to the bulk.
nitrate, it may also represent, at least in part, a closer inter-
relationship through nitrate photochemistry. In short, there
are both fundamental chemical reasons as well as potential
applications to understanding organic particle formation and
growth in the atmosphere that make nitrate ion photochem-
istry in aqueous systems particularly intriguing.
Nitrate ion is particularly interesting in that it photolyzes
7
3
below 350 nm to generate OH and O( P):
H2O
NO þhn ! NO2 þ Oꢀ ꢀ! NO2 þ OH þ OHꢀ ð1aÞ
ꢀ
3
We report here evidence for the oxidation of gaseous a-
pinene at the air–solution interface of a deliquesced thin film
of NaNO on the walls of a Teflon reaction chamber during
3
2ꢀ
3
NO + O( P)
-
(1b)
irradiation. a-Pinene is a significant biogenic hydrocarbon
2
2–25
This photochemistry is believed to be at least partly respon-
constituent in many regions
and a well-known SOA
2
3,25–45
sible for the photochemical production of NO in snow-
One might expect an incomplete solvent cage at
x
precursor.
generates not only OH and O( P) in the film via reaction (1),
but also forms gaseous NO that photolyzes to give O and
Irradiation of thin liquid films of NaNO
3
8
–18
3
packs.
x
3
a
Department of Chemistry, University of California Irvine, Irvine, CA
OH. These in turn react in the gas phase with a-pinene to
generate both gas phase products and SOA. To differentiate
9
2
2697-2025, USA. E-mail: bjfinlay@uci.edu; Fax: +1 (949) 824-
420; Tel: +1 (949) 824-7670
b
c
x
this gas phase NO photooxidation of a-pinene from that
occurring at the air–solution interface, some comparison
Environmental Molecular Sciences Laboratory, Pacific Northwest
National Laboratory, Richland, WA 99354, USA
Imre Consulting, Richland, WA 99352, USA
studies were carried out in which NOx was added in the
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absence of the thin liquid film at concentrations simulating
those in the NaNO experiments. A companion paper explores
in detail the gaseous products and SOA formation in the NO
blacklamps were then turned on and NO
x
was added con-
3
tinuously to the chamber by metering in a small flow (less than
ꢀ
1
0.1 L min ) of a 4.64 ppm NO /N mixture (Scott-Marrin).
x
2
2
4
6
addition experiment; only the results needed to differentiate
Nitric oxide is present as an impurity at typical concentrations
the interface chemistry on NaNO from that in the gas phase
3
of B3–4% of the NO . Nitric oxide is also formed immedi-
2
are shown here. Molecular dynamics simulations were also
ately from the NO photolysis, and indeed, this is the major
2
carried out to probe the affinity of a-pinene for the surface of a
concentrated aqueous NaNO solution.
source of NO in this system. The flow rate for the NO
mixture was chosen to reproduce the increase in NO observed
during the NaNO photolysis experiments (see below).
2 2
/N
3
2
The experiments show that a-pinene interacts with the
surface of deliquesced nitrate film during irradiation, leading
to oxidation of a-pinene at the interface. The MD simulations
provide support for this interpretation of the experimental
data. Possible relevance to the atmosphere is discussed.
3
a-Pinene and its gaseous reaction products were monitored
using proton transfer reaction mass spectrometry (PTR-MS,
4
0,48–55
Ionicon Analytik).
This method is based on proton
+
transfer from H
3
O
to compounds with larger proton affi-
nities than the ionizing agent, giving [M+1] peaks. While
some fragmentation of the parent molecule may occur, it is less
severe than for electron impact ionization. Low molecular
weight compounds show less fragmentation and have the
largest ion peak at [M+1] while larger species may have a
fragment ion as the largest peak. One common mechanism for
fragmentation of oxygenated species is the loss of a neutral
water molecule following protonation. The largest peak of m/z
151 from pinonaldehyde (MW = 168) results from this
mechanism. The lack of extensive fragmentation also contri-
butes to the sensitivity of the PTR-MS, providing highly
sensitive detection (B100 ppt). Daily calibrations for a-pinene
and a number of the gas-phase products including methanol,
acetaldehyde and acetone were performed using a certified
multi-component gas-phase standard.
Experimental
4
6
The experimental apparatus is described in detail elsewhere.
Briefly, a collapsible Teflon reaction chamber of total volume
4
2
B100 L and surface area of 1.3 ꢂ 10 cm was irradiated using
two banks of blacklamps (Sylvania 350, 20 W, F20T12/350
BL, 300–400 nm) located on opposite sides of the chambers.
For the NaNO —a-pinene system (referred to as the ‘‘NaNO
3
3
experiments’’ throughout), a thin film of deliquesced NaNO3
was generated on the walls of the chamber by nebulizing with
Ultra Zero grade air a solution of B30 wt% (4 M) NaNO
Fisher Certified ACS grade, 99.9% or Sigma Aldrich ACS
3
(
Reagent Grade, Z 99.0%) in nanopure water (Barnstead 18
MO cm). This generated particles with diameters of B800 nm,
The increase in NO
42C chemiluminescence Trace Gas NO
reaction times, the concentrations of gases other than NO and
NO are expected to be sufficiently small that the NO instru-
ment reading can be taken as the NO concentration. At
longer reaction times, there may be some contribution in the
NO channel from organic nitrates. The size distributions and
2
and NO was measured using a Thermo
x
analyzer. At shorter
4
7
which were deposited on the chamber walls. The chamber
was coated three times with the nebulized NaNO solution,
3
2
2
followed by pumping out the remaining aerosol until the
chamber was flat, and then filling with 100 L of humidified
air. The film was clearly visible covering the walls of the
chamber after each pump-out of aerosol, but disappeared
after filling with 100 L of humidified air. It is therefore
assumed that the deliquesced nitrate film evenly coats the
surface so that the area of the thin film is equivalent to that
of the chamber. The appropriate volume of liquid (1R)-(+)-a-
pinene (Aldrich 99+%), was injected, allowed to evaporate
and mix for several minutes, and then the blacklamps were
turned on to initiate the nitrate photochemistry.
2
2
number concentrations of the airborne SOA formed by the
photochemistry were measured using a SMPS consisting of
either a nano or long differential mobility analyzer (DMA, TSI
Model 3080 or 3085) and a condensation particle counter
(CPC, TSI Model 3022A or 3786).
At the completion of the reaction in some of these experi-
ments, the internal walls of the chamber were rinsed with
ethanol in order to extract the soluble organic compounds on
the walls. The wall washings were analyzed either directly or by
conversion to the trimethylsilyl derivative using N,O-bis(tri-
For the comparison experiments in which NO
x
was added
experi-
continuously to the chamber (referred to as ‘‘NO
x
5
6
ments’’ throughout this paper), the chamber was initially filled
with 80 L of Ultra Zero grade air (Oxygen Services Company,
synthetic blend of O and N , THC o 0.01 ppm, H O o 2.0
methylsilyl)trifluoroacetamide (BSTFA)
with trichloro-
methylsilane catalyst (Supelco #33154-U, BSTFA + TMCS,
99:1) in pyridine (EMD 99.0% GR-ACS) as the solvent.
Analysis was performed using GC-MS (Agilent Model 6850
Series II GC and a Model 5975B VL MSD) with a DB5-ms
column of 60-m length, 0.25-mm internal diameter, and a
0.25-mm film thickness (Agilent Technologies, Inc.).
2
2
2
ppm, CO o 0.5 ppm, CO o 0.5 ppm) which had been
2
humidified by passing through a bubbler containing nanopure
water. The chamber was not filled to the full 100 L capacity
initially in these experiments to allow for the subsequent
addition of NO
Na SO (EMD Chemicals Inc. ACS grade, 99.0%) generated
in the same manner as NaNO was placed on the chamber
x
. In some experiments, a wall coating of
Experiments were carried out at 300 ꢃ 5 K and at atmo-
2
4
spheric pressure. The relative humidity during the NaNO
3
3
experiments was in the range of 74–79%. The deliquescence
relative humidity (RH) of NaNO is 75% so that NaNO
particles on the wall were saturated or slightly super-saturated
walls to investigate the effects of a non-photochemically
reactive salt thin film. The relative humidity and temperature
were measured using a Vaisala probe (Model HMP-238).
a-Pinene was added to the chamber as described above. The
3
3
5
7,58
solutions of NaNO3.
During the NOx experiments, the
RH was in the range from 72–88%.
3
064 | Phys. Chem. Chem. Phys., 2008, 10, 3063–3071
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Computational methods
Simulations of a-pinene interacting with the surface of aqu-
eous NaNO
namics of NaNO
a box of 864 water molecules and 86 Na /NO
3
were completed using classical molecular dy-
3
and either one or ten a-pinene molecules in
+
ꢀ
ion pairs with
3
5
9
periodic boundary conditions in three dimensions. In order
to simulate the liquid–vapor interface, a slab geometry was
2
,60,61
employed.
˚
00 A, with the elongated box dimension along the z-axis
The size of the unit cell was set to 30 ꢂ 30 ꢂ
1
normal to the two liquid–vapor interfaces. The simulations of
a-pinene scattering were carried out at constant volume and
energy (NVE ensemble) after equilibration of the nitrate slab
at constant volume and temperature (NVT ensemble). A 1 ns
simulation of 10 a-pinene molecules interacting with the
solution was carried out in the NVT ensemble at 298 K.
All simulations were completed using the Amber 8 suite of
6
2
programs. Long-range electrostatic interactions were calcu-
3,64
6
lated using the particle mesh Ewald method
with a real
˚
space cutoff of 12 A. Water molecules were modeled using the
6
polarizable POL3 water model and the internal degrees of
5
freedom of the water molecules were constrained using the
6
6
SHAKE algorithm. Nitrate ions were modeled using the
6
parameters recommended by Salvador and co-workers and
7
used in two subsequent studies on nitrate in interfacial envir-
6
8,69
onments.
Generalized Amber Force Field. Each simulation of a
pinene/slab collision consisted of a well-equilibrated NaNO
slab and a-pinene molecule(s) using a time step of 1 fs. In order
The a-pinene molecules were modeled using the
70
3
7
1
to avoid the polarization catastrophe due to the large electric
field in solutions with a large number of polarizable molecules,
the induced dipoles were calculated using a method developed
Fig. 1 NO
2
and a-pinene concentrations, and total gas phase product
counts measured by PTR-MS for (a) photolysis of NaNO
(634 ppb) system at 301–304 K, RH varied from 79 to 74%; (b)
photolysis of NO —a-pinene (963 ppb) system at 302–304 K, RH
3
—a-pinene
7
2
previously, with the induced dipole scaling chosen to
preserve the properties of neat water.
x
varied from 86 to 72%.
reacted a-pinene, whereas they represent 30% in the NOx
photooxidation. Most of this difference is due to the smaller
Results and discussion
Fig. 1a shows that upon irradiation of a deliquesced NaNO
3
yield of gas phase pinonaldehyde in the NaNO
New particles are formed and grow in both the NaNO
NO addition experiments. Fig. 2 shows the loss of a-pinene as
a function of the mass of suspended aerosol formed, calculated
3
experiments.
thin film, a-pinene is lost after a brief delay. Gaseous products
represented in Fig. 1 by the total product ion counts are
formed simultaneously. Fig. 1b shows similar data for the
3
and
x
4
6
comparison experiment in which NO
in the absence of the NaNO film at such a rate that its rate of
increase mimicked that in Fig. 1a. In both cases, NO remained
x
was added to a chamber
from the measured volume of the particles using a density of
ꢀ3
1.21 g cm . It is clear that the mass of SOA formed per ppb of
a-pinene reacted is smaller for the NaNO experiments. For
example, for 100 ppb loss of a-pinene, 43 mg m SOA are
3
3
ꢀ
3
at B2–3 ppb throughout the experiments. Given typical RO
2
ꢀ
11
3
cm molecule s , this level
ꢀ1 ꢀ1
+
NO rate constants of B10
of NO is sufficient that in the gas phase, the RO reaction with
2
ꢀ11
3
NO will dominate over that with HO (k B 10
cm
ꢀ1
Table 1 Molar yields of gas phase products measured in NaNO
3
photochemical experiments compared to NOx photooxidation of a-
pinene
2
ꢀ1
ꢀ1
s ) or RO2 (k B 10 –10 cm molecule
9
ꢀ13
ꢀ17
3
molecule
1
ꢀ
ꢀ3
s
) at typical mid-10 radical cm
concentrations of
HO and RO
2
2
.
% Molar yield (ꢃ1s)
NaNO experiment
% Molar yield (ꢃ1s)
x
NO experiment
46
Product
3
Table 1 summarizes the major gas phase products identified
by PTR-MS, which included acetone, pinonaldehyde, acetal-
dehyde, formaldehyde, acetic acid and pinene oxide. These
Pinonaldehyde
Acetone
Acetaldehyde
Acetic acid
Methanol
Formaldehyde
Formic acid
Pinene oxide
8.2 ꢃ 1.8
15.1 ꢃ 2.1
7.1 ꢃ 0.9
4.3 ꢃ 1.8
0
60 .3 ꢃ 1.8
1.6 ꢃ 0.6
22 ꢃ 6
12 ꢃ 3
3.9 ꢃ 1.7
8.6 ꢃ 1.9
0.6 ꢃ 0.3
5 ꢃ 1
x
compounds are also formed in the NO photooxidation, along
4
6
with small amounts of methanol and formic acid. However,
the total yield of gas phase products is significantly smaller
than for the NOx experiments. On a carbon basis, the gas
phase products in the NaNO3 case represent 17% of the
2.5 ꢃ 1.4
0.9 ꢃ 0.1
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addition, the relative importance of (1a) and (1b) may change
at the interface compared to the bulk. For example, Miller
7
et al. have carried out theoretical calculations on nitrate ions
5
in water clusters of increasing size and find that at least up to
3
00 water molecules (corresponding to a 1–2 nm diameter
particle), the nitrate ion prefers to reside at the surface. In
addition, the solvation of the surface nitrate ion is different
from that in the bulk. At the surface, a significant portion
(B43%) of the nitrate ions have only one O atom hydrogen-
bonded to water, and another large portion (B47%) have two
O atoms that are H-bonded. However, in the interior, most
have three O-atoms hydrogen bonded to water. The lower
extent of hydrogen-bonding for the surface nitrate may rea-
sonably be expected to increase the efficiency of NO produc-
2
tion during photolysis, and perhaps increase the importance of
channel (1b) compared to channel (1a). In any event, although
Fig. 2 Measured loss of a-pinene for the NaNO
ments shown in Fig. 1 as a function of mass of suspended aerosol
particles formed (M ).
3 x
and NO experi-
a-pinene is not taken up into the NaNO thin film in the dark,
3
0
3
reaction with OH and O( P) at the surface during irradiation
would provide an additional sink for a-pinene at the surface of
the NaNO thin film.
3
ꢀ
3
formed in the NO
NaNO experiment.
The gaseous NO
x
experiment, but only 27 mg m in the
3
To test this hypothesis, a search for oxidation products on
the chamber walls was carried out using GC-MS analysis of
2
and NO levels were very similar in the two
sets of experiments. It would therefore be expected that if only
gas phase photochemistry was responsible for the oxidation of
a-pinene, the formation of gaseous and particulate products
should also be comparable. However, as discussed above, the
extracts of the wall washings. In order to mimic the NaNO
3
salt film on the chamber walls but without the nitrate photo-
chemistry, comparison NO addition experiments were carried
x
out using a deliquesced coating of Na SO on the chamber
2
4
SOA yield is smaller for the NaNO experiment. In addition,
3
walls. While quantitative recovery of small amounts of organic
products in the presence of large quantities of salt was difficult,
qualitative identification of products was achieved. For both
data such as those in Fig. 1 show that, at long reaction times,
the gaseous product counts per ppb of a-pinene reacted is 40.2
ꢃ 1.4 (2s) for the NO
for the NaNO experiment. The difference corresponds to an
additional B14% a-pinene loss (13% due to gas phase
products and 1% due to particulate products) in the NaNO
x
experiments, but only 30.6 ꢃ 0.8 (2s)
the reactive NaNO
Na SO coated chamber, pinonaldehyde was recovered from
the salt coatings after irradiation. However, for the NaNO
3
coated chamber and the unreactive
3
2
4
3
3
film, pinonic and pinic acids, trans-sobererol and some uni-
case that cannot be accounted for as gas phase products or
SOA. Taking into account the volume and surface area of the
reaction chamber and expressing this difference per unit sur-
dentified organics were also observed. These were not detected
when NO
x
2 4
was irradiated with a-pinene in the Na SO coated
chamber experiments. While trans-sobrerol will react with O3,
9
ꢀ2 ꢀ1
face area, there is an extra loss of 2.9 ꢂ 10 a-pinene cm
s
its lifetime is expected to be 3.4 h at 40 ppb O (a typical mid-
3
in the NaNO system for the experiment shown in Fig. 1a.
3
range concentration in our experiments) if the rate constant is
the same as that for cyclohexene. Our experiments were
typically carried out over 1–2 h, so a large loss of sobrerol
The unique characteristics of the NaNO3 system is the
presence of a photochemically active aqueous thin film in
3
which OH and O( P) are generated during irradiation.
due to reaction with O
in a mixture with other products, the surface-bound sobrerol
may not be as readily available for reaction with O
Increased uptake of oxygenated organics from the gas phase
on NaNO compared to Na SO was ruled out by measure-
3
is not expected. In addition, since it is
a-Pinene taken into or onto this thin film could then be
oxidized by the OH and O( P). If some portion of the products
3
3
.
remains on the surface, lower product yields for the gases and
suspended particles would result, as is observed. Hydro-
3
2
4
7
3
carbons such as a-pinene are not very soluble in water,
ment of the concentration of cyclohexanone, chosen as an
easily detectable oxygenated organic, in chambers that were
uncoated or coated with NaNO or Na SO . There was no
7
4
and even less so in salt solutions. This is supported by the
PTR-MS data; addition of equal volumes of a-pinene in the
dark to Teflon chambers either uncoated or, alternatively,
3
2
4
difference in the gas phase concentrations measured after
addition of equal amounts of cyclohexanone, nor did these
concentrations change with time which would be observed if
there was continuing uptake on the walls. Enhanced uptake of
SOA is also unlikely because the wall loss of SOA was not
coated with a thin deliquesced film of NaNO , gave similar gas
3
phase reactant signals. This rules out significant uptake of
a-pinene into the salt solution.
However, the photochemical production of OH and O( P)
3
also occurs at the surface of the NaNO
3
film, and indeed, may
3
significantly larger in the NaNO coated chamber compared to
be greatly enhanced compared to the bulk. While this area is
largely unexplored, one would expect an incomplete solvent
cage at the air–particle interface, leading to less recombination
an uncoated chamber. In addition, the measured ratio of the
products was different in the wall washings compared to that
in the suspended SOA collected on filters. For example, the
ratio of pinic to pinonic acids was B25 in the SOA from
various runs, while in the wall washings, this ratio was B2.
ꢀ
ꢀ
3
of NO
2
with O and of NO
2
with O( P), respectively, giving
higher effective quantum yields for reactions (1a) and (1b). In
3
066 | Phys. Chem. Chem. Phys., 2008, 10, 3063–3071
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4
6
Finally, as described elsewhere, the addition of excess NO to
the chamber in the NO photooxidation quenched the forma-
x
tion of O and the associated SOA formed by the ozonolysis of
3
a-pinene. When 2 ppm NO was present initially in the NaNO3
experiments, the gas phase ozonolysis was similarly quenched.
However, various products including pinonaldehyde, pinonic
acid, pinic acid and trans-sobrerol were still observed in the
wall washings. In short, the experimental evidence points to
3
a-pinene being oxidized at the surface of the NaNO thin film
during irradiation.
Molecular dynamics simulations
In order for a-pinene to be taken up and oxidized on the
surface of deliquesced NaNO3 film, the hydrocarbon must
have some finite residence time on the aqueous salt surface.
This is not intuitively likely, given the low solubility of
hydrocarbons in water, and even lower solubility in salt
Fig. 4 Twenty five independent simulations of one a-pinene scatter-
ing event. The evolution of the a-pinene center of mass z-coordinate
vs. time is shown with the Gibbs dividing surface, which denotes the
location of the air–solution interface, set to z = 0.
7
4
solutions. In order to explore this possibility, molecular
dynamics (MD) simulations of this system were carried out
to develop a molecular scale picture of a-pinene interacting
with the surface of an aqueous sodium nitrate solution (B6.8
M). An example initial configuration for a-pinene interacting
vast majority of cases, the a-pinene adsorbs to the interface
and stays in the interfacial region during the entire simulation
period.
with the interface of aqueous NaNO
3
is shown in Fig. 3a. A
Fig. 5 shows the density profiles from a 1 ns simulation of 10
snapshot of a-pinene impinging on the aqueous NaNO
3
inter-
a-pinene molecules interacting with concentrated aqueous
face is shown in Fig. 3b. Two computational approaches were
used to probe this interaction. First, 75 individual 100 ps
simulations of a-pinene scattering either off or sticking onto
the liquid–vapor interface of aqueous sodium nitrate have
been carried out. A selection of 25 scattering events, with
different initial a-pinene orientations and/or initial velocities,
is shown in Fig. 4. Similar results were observed for the
additional 50 trajectories (not shown). The a-pinene sticking
probability was found to be independent of the initial condi-
tions for both the nitrate slab and the gas phase a-pinene. In
all cases, the a-pinene resides at the interface for some time
and, in rare events, it desorbs during the simulation. In the
3
NaNO . The density profiles have been normalized using
zmax
R
rðzÞdz ¼ 1 and then averaged about the center of the slab
i.e. averaged over the two interfaces) and they are shown
zmin
(
relative to the water Gibbs dividing surface (set to z = 0). The
density profile shows that nitrate resides primarily below the
water interface, consistent with recent work on aqueous nitrate
6
8,69,76
solutions,
and that a-pinene resides near the interface,
as expected for a hydrocarbon on salt solutions. Fig. 5 illus-
trates that the nitrate and a-pinene density profiles do overlap
to some extent near the interface, suggesting that contact
between a-pinene and nitrate photolysis products is likely in
the interfacial region, and thus oxidation of a-pinene at the
Fig. 3 (a) Typical initial snapshot for one a-pinene interacting with
the solution–air interface of aqueous sodium nitrate. (b) Snapshot of
Fig. 5 Density profiles of a-pinene, water, nitrate and sodium ions
for a 1 ns simulation of 10 a-pinene molecules interacting with the
a-pinene interacting with the surface later in the simulation. Owater
red, Hwater = white, Nnitrate = blue, Onitrate = purple, Na = green,
a-pinene = teal, Ha-pinene = white.
=
3
surface of aqueous NaNO . The density profile of each component
was normalized such that its integral over the half slab is unity. The
range of overlap will not depend on the normalization.
C
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interface by aqueous NaNO
experiments is reasonable.
3
photochemistry as seen in the
removed in the form of organic nitrates in the gas phase
4
6
y
products and SOA, and the NO analyzer does not measure
8
6
The photochemical oxidation of a-pinene could potentially
occur either at the interface between air and deliquesced
organic nitrates with 100% efficiency. It is assumed that OH
diffuses to find an a-pinene, that reaction occurs on every
encounter, which is reasonable based on the known rapid gas
NaNO , or in the bulk. Reaction at the interface seems most
3
8
6
likely based on the MD simulations and the known low
7
solubility of a-pinene in a concentrated salt solution. Sup-
porting our conclusion that a-pinene is being oxidized on the
phase kinetics, and that the surface a-pinene is rapidly
regenerated as it reacts, which is also justified given the high
collision rate compared to the surface concentration. The rate
of oxidation of a-pinene at the interface should then be equal
4
7
7
3
NaNO aqueous film are experiments by Borghesi et al., who
9
ꢀ2 ꢀ1
s .
reported oxidation of gaseous benzene during irradiation of
solid nitrate salts, with the yield increasing with relative
humidity up to 83% RH. The effect of increased water vapor
likely reflects increased mobility and availability of nitrate ions
on the salt surface as the water vapor concentration increases,
to the rate of OH generation, i.e., 8.4 ꢂ 10 cm
However, the experimental data show that the additional loss
of a-pinene in the NaNO
3
experiments compared to the NO
ꢀ2 ꢀ1
2
9
experiments is B2.9 ꢂ 10 a-pinene cm
s . Thus, about
30% of the OH generated in the thin film by the nitrate ion
photochemistry oxidizes a-pinene.
5
7,58
followed by deliquescence at 75% RH.
surface mobility is well-known for salts such as NaCl
and NaCl on which a surface layer of nitrate has been formed
Water-induced
7
8,79
Given that pinonaldehyde is a major product of the OH-a-
2
9,87–92
pinene reaction in the gas phase,
it is reasonable to
8
0–85
3
by reaction with gaseous HNO .
assume this will also be the case for reaction at the surface.
9
3
The MD simulations show that the residence time of
However, given the low vapor pressure of pinonaldehyde, it
is likely to remain on the surface. This would result in loss of
a-pinene from the gas phase without generation of gas phase
pinonaldehyde, consistent with the lower yields of pinonalde-
a-pinene on the surface is at least a nanosecond, i.e., the rate
9
ꢀ1
. From gas-
constant for desorption, k
d
, must be r10
s
kinetic molecular theory, the number of collisions of 1 ppm
7
1
ꢀ2 ꢀ1
s .
a-pinene with the surface is 1.4 ꢂ 10 collisions cm
3
hyde observed for the NaNO experiments (Table 1). This
Using the upper limit for k , a lower limit to the steady-state
d
smaller yield also suggests that the OH that is generated in the
ꢀ
8
ꢀ2
surface coverage of a-pinene is 1.4 ꢂ 10 cm . An estimate of
NO3 photolysis is not directly injected into the gas phase
the total rate of OH production can be obtained from the
before oxidizing a-pinene.
gaseous NO that is generated during an experiment (Fig. 1).
2
Potential mechanisms of formation of pinonaldehyde, pi-
nonic and pinic acids and trans-sobrerol are shown in Schemes
1 and 2. Given that channel (1a) in the nitrate ion photolysis
predominates at least in the bulk, it seems likely that this
interface chemistry will be driven primarily by OH, with some
Since one OH is formed for each NO that is generated in the
2
gas phase (reaction 1a), the equivalent rate of production of
9
ꢀ3 ꢀ1
OH is 1.1 ꢂ 10 cm
s , or expressed as number of OH per
9
2
cm of thin film per second, the production rate is 8.4 ꢂ 10
ꢀ
2
ꢀ1
3
cm
s
. This will be a lower limit, since some of the NO
2
is
contribution from O( P). Scheme 1 shows one mechanism of
Scheme 1 Possible mechanisms of formation of products observed in the NaNO
3
thin film from OH oxidation of the a-pinene.
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3
3
Scheme 2 Possible mechanisms of formation of products observed in the NaNO thin film from O( P) or O3,surface oxidation of the a-pinene.
formation of pinonaldehyde, pinonic acid, pinic acid and
trans-sobrerol from OH chemistry. Pinonic acid is formed
from the oxidation of pinonaldehyde, while pinic acid is
formed from the oxidation of pinonic acid or from the initial
addition of OH to the more substituted carbon of the double
order of magnitude smaller than actually measured in Mexico
2
1
100
City. While recent studies suggest that oxidation of semi-
volatile emissions from diesel exhaust may contribute signifi-
cantly to resolving such discrepancies, this would not explain
the even larger discrepancies between free tropospheric mea-
9
1
101
bond of pinene. The latter requires that O
2
subsequently
surements and models of SOA in the ACE-Asia studies; nor
adds to an a-hydroxy radical and then forms the alkoxy
radical, rather than abstracting a hydrogen to form a ketone
as commonly happens in the gas phase. The proposed forma-
tion of trans-sobrerol involves either OH addition to an alkyl
radical generated after initial addition of OH to the double
bond and opening of the 4-membered ring, or the reaction of
would it explain increased organics in particles above clouds
measured during the MASE campaign off the coast of north-
1
02,103
ern California.
sistent with the large fraction of contemporary carbon
In addition, this hypothesis is not con-
1
04
typically found in carbonaceous aerosol particles.
Clearly
there are a number of other processes that are also contribut-
ing to formation and growth of SOA, and these are not
included or properly represented in current models.
2
an alkoxy radical with HO . Whether these are feasible on the
surface remains to be explored.
Scheme 2 shows potential routes for oxidation of a-pinene
at the interface by O( P). Addition of O( P) to form pinene
The combination of experiments and theory presented here
shows that organic compounds such as a-pinene can be
oxidized at the surface of deliquesced nitrate salts during
3
3
9
4
oxide in the gas phase is well known, and B3% of the ozone
reaction also generates pinene oxide in the gas phase.
3
2,94,95
3
irradiation due to the production of oxidants such as O( P)
Formation of pinene oxide at the interface by direct addition
3
of O( P) and perhaps from the ozone reaction is therefore
and OH by nitrate ion photochemistry. Given the surface
75
propensity of nitrate ions in small clusters/particles, this
likely. Hydrolysis of pinene oxide is known to give trans-
chemistry is likely to be especially important during new
particle formation. This process is consistent with the observa-
9
6,97
sobrerol,
not surprising. In separate experiments, an authentic sample of
gaseous pinene oxide was added to a NaNO coated chamber
so that observation of this in the wall washings is
1
05
tion of nitrate and organics in the smallest particles (6–20
nm) that can currently be measured using particle mass
spectrometry, and it may contribute to the ‘‘missing SOA’’
3
and trans-sobrerol was measured in the wall washings. In the
2
1
gas phase, pinonic acid is a first generation ozonolysis product
reported, for example, by Volkamer et al. While a-pinene
was used in these experiments as ‘‘proof of principle’’, this
mechanism is likely to apply to a broad spectrum of organics
such as the terpenes and aromatic hydrocarbons. Experiments
are underway to probe this chemistry and to quantify it for
inclusion in models of SOA formation in the atmosphere.
Finally, both nitrate and organics are deposited on surfaces
9
formed from both Criegee intermediates, and pinic acid can
8
be formed from the Criegee intermediate that has the diradical
8,99
on the secondary carbon.
9
Atmospheric implications
The formation of new particles in air and their growth is not
well understood. For example, predicted SOA is about an
8
6,106–108
in the boundary layer.
The present work suggests that
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during the day, photolysis of nitrate ions on these surfaces will
lead to oxidation of co-adsorbed organics. This is consistent
18 H. W. Jacobi, T. Annor and E. Quansah, J. Photochem. Photo-
biol., A, 2006, 179, 330–338.
19 P. Warneck and C. Wurzinger, J. Phys. Chem., 1988, 92,
1
09
with the recent work of Donaldson and coworkers
who
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278–6283.
observed the loss of nitric acid in mixed films of HNO and
3
20 H. Herrmann, Phys. Chem. Chem. Phys., 2007, 9, 3935–3964.
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2
006GL026899.
Acknowledgements
2
2 J. D. Fuentes, M. Lerdau, R. Atkinson, D. Balcocchi, J. W.
Bottenheim, P. Ciccioli, B. Lamb, C. Geron, L. Gu, A. Guenther,
T. D. Sharkey and W. Stockwell, Bull. Am. Meteorol. Soc., 2000,
We are grateful to the US Department of Energy (Grant #
DE-FG02-05ER64000) for support of this work. This research
was in part performed in the Environmental Molecular
Sciences Laboratory, a national scientific user facility spon-
sored by the US Department of Energy’s Office of Biological
and Environmental Research at Pacific Northwest National
Laboratory (PNNL) and supported by the US Department of
Energy Office of Basic Energy Sciences, Chemical Sciences
Division. PNNL is operated by the US Department of Energy
by Battelle Memorial Institute under contract # DE-AC06-
8
1, 1537–1575.
23 A. Calogirou, B. R. Larsen and D. Kotzias, Atmos. Environ.,
999, 33, 1428–1439.
1
2
2
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9 R. Winterhalter, R. Van Dingenen, B. R. Larsen, N. R. Jensen
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7
6RL0 1830. Additional support was provided by the AirUCI
2
Environmental Molecular Science Institute (Grant
#
CHE-0431512) funded by the National Science Foundation.
We also thank A. Campbell and R. Gephart for facilitating the
collaborative studies, C. M. Berkowitz, J. Hubbe and
W. Wang for technical assistance, and T. E. Kleindienst for
helpful discussions.
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