Upload
trantuyen
View
213
Download
0
Embed Size (px)
Citation preview
This journal is c the Owner Societies 2010 Phys. Chem. Chem. Phys.
Kinetic studies of the heterogeneous oxidation of maleic and fumaric acid
aerosols by ozone under conditions of high relative humidityw
Juan J. Najera,a Carl J. Percivala and Andrew B. Horn*b
Received 24th November 2009, Accepted 2nd July 2010
DOI: 10.1039/b924775k
In this paper, a kinetic study of the oxidation of maleic and fumaric acid organic particles by
gas-phase ozone at relative humidities ranging from 90 to 93% is reported. A flow of single
component aqueous particles with average size diameters in the range 2.6–2.9 mm were exposed to
a known concentration of ozone for a controlled period of time in an aerosol flow tube in which
products were monitored by infrared spectroscopy. The results obtained are consistent with a
Langmuir–Hinshelwood type mechanism for the heterogeneous oxidation of maleic/fumaric
acid aerosol particles by gas-phase ozone, for which the following parameters were found:
for the reaction of maleic acid aerosols, KO3= (9 � 4) � 10�15 cm3 molecule�1 and
kImax = (0.21 � 0.01) s�1; for the reaction of fumaric acid aerosols, KO3= (5 � 2) �
10�15 cm3 molecule�1 and kImax = (0.19 � 0.01) s�1. From the pseudo-first-order coefficients,
apparent uptake coefficient values were calculated for which a decreasing trend with increasing
ozone concentrations was observed. Comparison with previous measurements of the same system
under dry conditions reveals a direct effect of the presence of water on the mechanism of these
reactions, in which the water is seen to increase the formation of CO2 and formic acid (HCO2H)
through increased levels of hydroxyacetyl hydroperoxide intermediate.
Introduction
Organic material present at the aerosol surface is susceptible to
atmospheric oxidation by a variety of oxidants.1 The chemical
processing of these organic species can alter the surface and
bulk composition of the aerosols, leading to the formation of
increasingly polar compounds which is believed to impact on
physicochemical properties such as particle hygroscopicity,
cloud condensation nuclei (CCN) activity, and light
extinction.2–4 Gas-phase oxidation initiated by reaction with
ozone is an important pathway for the degradation and the
transformation of unsaturated organic compounds in the
atmosphere,5 either by releasing volatile organic compounds
(VOCs) to the gas-phase,6 or by producing secondary organic
aerosols (SOA) which can partition to the condensed phase.7,8
The rates and mechanisms of these reactions, whilst not
completely understood, are nevertheless reasonably well
characterised. The same cannot be said of heterogeneous
ozonolysis reactions, which are known to be strongly
influenced by the chemical composition of the aerosol in which
they are located4 and for which a wide range of conflicting
observations are present in the literature. Consequently, the
potential atmospheric impact of heterogeneous oxidation
reactions is poorly characterized and remains one of the
largest uncertainties in modelling.
Low molecular weight dicarboxylic acids (LMW-DCA)
represent a significant fraction of the organic material found
on collected atmospheric aerosol particles from continental
and marine atmosphere.9–12 LMW-DCA largely remains in
the particle phase due to generally rather low vapour pressures
and high solubility, and may therefore play a role in chemical
reactions in both condensed and aqueous aerosol phase.13
It is likely that the rates and mechanisms of any oxidation
reactions of ozone with dicarboxylic acid aerosols may be
dramatically different in solid and aqueous droplets14 parti-
cularly as a result of the effect of the particle surface on the
partitioning of ozone.
In a previous study of the rates and mechanism of the
ozonolysis of solid maleic and fumaric acid aerosol particles
under dry conditions, the formation of formic acid (HCO2H)
and CO2 as major products was reported.15 Present predomi-
nantly in the gas-phase due to its high vapour pressure,
HCO2H is one of the most abundant mono-carboxylic acids
reported in the atmosphere.16–18 Whilst HCO2H is reported to
be mainly produced via photochemical oxidation of VOCs7,19
and is also emitted directly from several biogenic and anthro-
pogenic sources16–19, any potential new heterogeneous source
may be significant. Lower concentration of formic acid in
the aerosol phase (0.16–0.49 mg m�3) compared to the corres-
ponding gas-phase (0.24–1.07 mg m�3) concentrations were
reported in field studies.7,19 Particle-phase effects of dissolved
HCO2H are also known: a significant amount is present in
the aqueous phase and HCO2H is known to influence
pH-dependent chemical reactions in cloud droplets. It has
a School of Earth, Atmospheric and Environmental Sciences, Facultyof Engineering and Physical Sciences, The University of Manchester,M13 9PL Manchester, UK. Fax: +44 (0)161 3069361;Tel: +44 (0)161 3063945
b School of Chemistry, Faculty of Engineering and Physical Sciences,The University of Manchester, M13 9PL Manchester, UK.E-mail: [email protected];Fax: +44 (0)161 2754598; Tel: +44 (0)161 2754618
w Electronic supplementary information (ESI) available: Comparisonof reaction kinetics obtained from the evolution of HCO2H and ofCO2 (not shown in this paper) with time. See DOI: 10.1039/b924775k
PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics
Dow
nloa
ded
by U
NIV
ER
SIT
Y O
F M
AN
CH
EST
ER
on
26 A
ugus
t 201
0Pu
blis
hed
on 1
3 A
ugus
t 201
0 on
http
://pu
bs.r
sc.o
rg |
doi:1
0.10
39/B
9247
75K
View Online
Phys. Chem. Chem. Phys. This journal is c the Owner Societies 2010
also been identified as a major sink for OH in solution.17 The
partitioning of HCO2H between gas and aerosol phase is
also known to depend on the vapour pressure and relative
humidity (RH%).20
In this paper, measurements of the rate and mechanism of
the oxidation of maleic and fumaric acid aerosols by ozone are
reported under nominally wet conditions (RH% 4 90) using
an aerosol flow tube (AFT) apparatus equipped with a
multi-pass Fourier-transform infrared (FTIR) spectroscopic
detection system. The atmospheric implications of the
measured rates and observed products are discussed.
Experimental
The AFT apparatus employed in this work has been
previously described.15,21 A temperature-stabilized laboratory-
made ultrasonic nebuliser was used to generate micro-droplet
aerosols of maleic and fumaric acid from 0.65 M and 0.090 M
bulk aqueous solutions, respectively. The nebuliser was
immersed in an isothermal bath at 15 1C for maleic acid and
40 1C for fumaric acid (HUBER Ministat 125). Aqueous
solutions of appropriate concentrations of maleic acid
(Aldrich, 99%) and fumaric acid (Aldrich, 99+%) were made
up in deionised water without further purification. Aerosol
droplets from the nebuliser output were entrained in a
humidified nitrogen (BOC Gases) carrier gas flowing at a rate
of 150–200 SCCM (standard cubic centimetres per minute)
before being mixed with a second humidified flow of 250–200
SCCM, controlled using calibrated mass flow controllers
(Type 1179A MKS Instruments). Humidified nitrogen flows
were generated by passing the nitrogen stream through water
bubbler vessels. Post-humidification, particle size distributions
were measured using a commercial process aerosol monitor
(PAM 510, TOPAS GmbH) before the aerosols flow was
admitted into the top of the vertically oriented AFT. Aerosol
particle dimensions were described by lognormal distributions
with standard deviation s o 1.15, where their average particle
diameter (Dp) and number of particles (N0) were typically
between Dp = (2.6 � 2.9) mm and N0 = (1.5 � 3.4) �106 particles cm�3 (Table 1). As described earlier,21 a
correction factor for the particle size measurements is applied
to account for the difference in the refractive index values
between the organic aqueous droplets and the calibrated
TOPAS instrument. Based on the aqueous droplet concentra-
tions and using solution density values of 1.03 g cm�3 for
maleic acid and 1.005 g cm�3 for fumaric acid at 25 1C,22
refractive indexes (nD) of 1.350 and 1.336 for maleic and
fumaric acid, respectively, were used.23 The estimated uncer-
tainties are 3.7% (maleic acid) and 3.5% (fumaric acid).
After entering the AFT, the aerosol flow was further diluted
into a humidified sheath flow of 4500 SCCM. The main AFT
reactor consists of a 100 cm long, vertically oriented Pyrex
glass flow tube with an internal diameter of 4.0 cm.
A temperature-regulated jacket was used to maintain constant
temperature within the AFT. A moveable stainless steel
injector of internal diameter 0.953 cm was inserted axially
down the centre of the flow tube, through which a 50 SCCM
variable oxygen/nitrogen flow mixture was injected. A total
humidified flow of 5000 SCCM through the AFT results in an
average linear flow velocity of B7 cm s�1, a Knudsen number
of B0.05 and a Reynolds number of B170.
Ozone (O3) was generated by flowing pure oxygen through
commercial ozone generator (BMT802, BMT Messtechnik
GmbH). The ozone concentration entering the AFT was
controlled by combining a variable oxygen flow rate with a
nitrogen flow to make a total flow of 50 SCCM. The ozone
concentration in the AFT was determined from the
integrated area of the n3 infrared absorption band of ozone24
(1000–1043 cm�1, baseline 1000–1043 cm�1) in the measured
spectra following a methodology previously described.15 The
ozone concentration in the AFT varied between 29–371 ppm
(7.2 � 1014–9.1 � 1015 molecules cm�3). The uncertainty in the
determined ozone concentration was estimated to be �3%.
Taking into account the average particle mass of the aqueous
aerosol particles (Table 1, based on size measurements) and
the aqueous droplet concentration, the concentrations of
ozone were kept in excess of the total numbers of maleic
and fumaric acid molecules by factors of 3–30 and 10–100,
respectively. Under these experimental flow conditions at high
relative humidity, it was essential to ensure that ozone is well
mixed with the humidified particle flow upon entering the flow
tube, as well as to keep constant the ratio of ozone to aerosol
concentration. By monitoring the ozone infrared band at n3 asa function of the injector position in a humidified flow without
particles, it was determined that full mixing of the ozone into
the bulk flow by molecular diffusion occurs on the time scale
of B3 s. Consequently, the reaction time was varied by setting
the moveable injector to different positions from 35 to 82 cm
along the length of the flow tube, which is equivalent to a
reaction time of 5–12 s. For each ozone concentration, ozone
Table 1 Summary of particle sizes and key experimental parameters in this study
Maleic acid Dpa/mm 2.6 2.8 2.7 2.6 2.6 2.6
N0a/particle cm�3 1.6 � 106 1.5 � 106 2.1 � 106 2.2 � 106 2.1 � 106 2.1 � 106
Ma/g cm�3 8.8 � 10�8 1.0 � 10�7 1.3 � 10�7 1.2 � 10�7 1.2 � 10�7 1.2 � 10�7
RH% 91 91 93 90 91 91T/1C 21.6 20.7 20.2 22.2 22.0 21.7[O3]/ppm m 29 76 165 239 321 371
[O3]/ppm k 32 78 171 242 317 368
Fumaric acid Dpa/mm 2.8 2.8 2.9 2.8 2.8 2.8
N0a/particle cm�3 2.5 � 106 3.0 � 106 2.6 � 106 2.5 � 106 2.6 � 106 3.4 � 106
Ma/g cm�3 2.4 � 10�8 2.9 � 10�8 2.8 � 10�8 2.4 � 10�8 2.5 � 10�8 3.3 � 10�8
RH% 91 91 91 90 90 91T/1C 21.6 21.7 22.2 22.5 22.1 21.9
a Dp = particle mean diameter, N0 = number of particles, M = average mass particle.
Dow
nloa
ded
by U
NIV
ER
SIT
Y O
F M
AN
CH
EST
ER
on
26 A
ugus
t 201
0Pu
blis
hed
on 1
3 A
ugus
t 201
0 on
http
://pu
bs.r
sc.o
rg |
doi:1
0.10
39/B
9247
75K
View Online
This journal is c the Owner Societies 2010 Phys. Chem. Chem. Phys.
generation was initiated and a series of spectra were recorded
as a function of the position of the injector (i.e. reaction time).
Infrared spectra recorded before and after each experiment
verified that no gas-phase compounds were present in the light
path and that no aerosol material deposited on the cell
windows during the experiment.
The downstream end section of the flow tube is coupled to a
laboratory-made glass cell with the White mirror system. In
these experiments, the number of passes in the White Cell
was set in 16, corresponding to an optical-path length of
(47.0 � 0.9) cm.15,21 The humidified aerosol flow crosses the
infrared detection perpendicular to the multi-pass optics
immediately prior to exiting the flow tube. Infrared spectra
were acquired in situ using a modified commercial FTIR
spectrometer (Nexus, ThermoNicolet) over the wavenumber
range from 4000 to 600 cm�1. Spectra were measured at a
resolution of 4 cm�1 from the co-addition of 256 scans.
Gas-phase water absorption bands were removed by subtraction
of a pure water spectrum obtained in the absence of aerosol.
Ozone absorptions were also removed using a reference
spectrum.
Pressure, temperature and RH% (Table 1) within the flow
tube were continuously monitored using a capacitance
manometer (MKS Instruments 722A), T-type thermocouples
located along the flow tube and a combined T & RH% probe
(Vaisala HMP234). The RH% in the AFT was determined
from the integrated infrared area of one of the lines of the
bending mode of gas-phase water (1872–1864 cm�1, baseline
1875–1862 cm�1) in the measured spectra. This latter
procedure was calibrated against the Vaisala probe in flows
of humidified nitrogen, with the spectroscopic RH% error
estimated as �3%. In this study, experiments were performed
at room temperature (B293 K) and atmospheric pressure
(B1 atm).
Results and discussion
Characterization of aqueous maleic and fumaric acid aerosols
Aerosol droplets were produced by ultrasonic nebulisation of
aqueous organic solutions and combined with humidified
flows. The stabilisation time ts of maleic (or fumaric) acid
aqueous droplet can be estimated from the lifetime of a pure
water particle with the same particle size at the same tempera-
ture and RH%.25 For average values of 91% RH and
Dp E 2.6 mm at 298 K for these experiments, a value of
ts E 0.2 s suggests that droplet processes of condensation and
evaporation are in equilibrium with the surrounding humidity
under these conditions. Therefore at these high relative
humidities, there is insufficient time or potential for evaporation
of the water content and hence the organic concentrations in
the aerosol droplet are assumed to quantitatively reflect the
composition of the nebulised solutions. The room temperature
DRH (Deliquescence Relative Humidity) of maleic acid
aerosols has been reported as 87.5%,26,27 hence the maleic
acid aerosols are assumed to remain liquid prior to reaction.
To the authors’ knowledge, the DRH for fumaric acid has not
been experimentally determined. Since fumaric acid is less
soluble than maleic acid and based upon reports of DRHs in
the literature for other less soluble organics,27–29 it is likely to
be above 90%. There is some evidence from the infrared
spectra that the aerosols are deliquesced, although this is far
from certain. However, given the timescales and the high
humidity in both the carrier stream and the sheath flow, it is
assumed that they remain liquid prior to reaction.
Representative extinction infrared spectra obtained for
aqueous maleic and fumaric acid aerosols at RH above 90%
are shown in Fig. 1a after subtraction of gas-phase water
absorption lines. Reference spectra15 of dry aerosols of maleic
acid (1.3% RH) and fumaric acid (2.9% RH) aerosols are also
shown for comparison. The sloping baselines are the result
of Mie scattering. Since extensive spectral manipulation
(i.e. subtraction of water vapour and ozone spectral lines) is
required in order to obtain an absorption spectrum, uncer-
tainties larger than those obtained for dry aerosols can be
expected in the quantitative analysis. The extinction infrared
spectrum of aqueous maleic acid aerosols under wet
conditions agrees well with that reported previously in the
literature.26 For both maleic and fumaric acid aerosols at high
RH% (shown as an expanded view in Fig. 1b and c respec-
tively), the characteristic condensed-phase water features at
3650–2500 cm�1, 1640 cm�1 and 685 cm�1 correspond to the
n1–n3 (OH stretching), n2 (HOH bending) and L2 (H bonding
libration) modes, respectively.30 The fumaric acid aerosols
show a poorer signal-to-noise (S/N) ratio compared to those
of maleic acid as a direct result of their lower concentration in
aqueous droplet (shown as M in Table 1). Consequently, the
spectral subtraction of gas-phase water in fumaric acid is
poorly resolved and sharp features centred at 3830, 3730 and
1520 cm�1 in the wet fumaric acid spectrum are associated to
artefacts from the imperfect subtraction of water vapour.31
Furthermore, the presence of the condensed phase water
features in fumaric acid is less discernible.
A more detailed comparison of distinctive infrared absorp-
tion features for dry and wet maleic acid particles is shown in
Fig. 1b. In dry aerosols, the features at 1460 cm�1 and
1332 cm�1 have been assigned to vibrational modes associated
with the H bonded d(CO–H) and a unique surface trans
structure respectively (see Najera et al., 200915 for a full
description of band assignments). These features disappear
entirely in the wet aerosols and are taken to be indicative of
changes in internal hydrogen bonding upon solvation. The
intramolecular H-bonded d(CC–H)0 mode at 1436 cm�1 and
the d(CC–H) mode at 1175 cm�1 are still visible, although the
latter shows an apparent increase in relative intensity. Whilst
some subtle changes are noticeable for the stretching modes of
n(CO)/n(CO)0 at 1262 cm�1 and n(CC) at 949 cm�1, no
significant changes can be observed at B1720 cm�1 (n(CQO)
and n(CQO)0) and at 1630 cm�1 (n(CQC)) (although it should
be noted that the latter is overlapped to the n2 water mode). In
contrast, the spectra of dry and wet fumaric acid aerosols are
identical (as shown in Fig. 1c), since intramolecular bonding is
not possible in the trans CQC conformation.15
Ozonolysis mechanism of aqueous maleic and fumaric acid
aerosols
Reaction of these aerosols with ozone is shown in Fig. 2 and 3,
in which only spectral features due to gas-phase CO2 and
Dow
nloa
ded
by U
NIV
ER
SIT
Y O
F M
AN
CH
EST
ER
on
26 A
ugus
t 201
0Pu
blis
hed
on 1
3 A
ugus
t 201
0 on
http
://pu
bs.r
sc.o
rg |
doi:1
0.10
39/B
9247
75K
View Online
Phys. Chem. Chem. Phys. This journal is c the Owner Societies 2010
HCO2H are seen. There are no obvious changes to the features
in the aerosol phase, suggesting that wet maleic and fumaric
acid aerosols appear superficially to show the same basic
reaction mechanism as their dry counterparts.15 Similarly,
neither glyoxylic nor oxalic acid are seen in either phase,
although these have previously been identified in bulk solution
experiments.32 Interestingly, a substantial increase in product
yield can be inferred from a semi-quantitative analysis by
comparison of the infrared integrated areas of these gas-phase
products obtained at different RH% regimes (discussed later),
especially considering the lower organic acid concentration in
the aqueous aerosols compared to their dry counterparts. This
increased yield may simply be the result of increased ozone
partitioning to the particle surface, but it is also possible that a
subtly modified reaction mechanism occurs. Scheme 1 shows a
basic mechanism for the reaction of maleic and fumaric acid
aerosols with gas-phase ozone, summarising the previously
observed dry aerosol chemistry and including the possible
effect of water vapour. As usual in the ozonolysis of
unsaturated organic species, an unstable primary ozonide is
formed which rearranges and decomposes to form an excited
state Criegee intermediate (ECI) and glyoxylic acid. In the dry
reaction, ECI either stabilises to SCI (stabilised Criegee inter-
mediates) or undergoes a series of reactions yielding gas-phase
products such as CO2 and HCO2H and oxalic acid in the
condensed phase. Additionally, the SCI may also react with
glyoxylic acid to form a secondary ozonide. Under conditions
of excess of water, the addition of a water molecule at SCI
allows for an additional pathway involving the formation
of hydroxyacetyl hydroperoxide (HAHP, also called
2-hydroperoxy-2-hydroxyacetic acid),33–38 as illustrated in
Scheme 1. Yamamoto et al.33–38 have reported that this species
readily dehydrates to HCO2H and CO2. Although there is
another decomposition channel for HAHP in which H2O2 and
H2CO are formed,35,36 neither of these species was observed in
the infrared spectra in either the gas- or condensed-phase.
Neither is there any evidence for the formation of OH radicals
from the surface aqueous reaction,5 in agreement with the
observations from gas-phase experiments in which only very
low OH yields from alkene–ozone reactions were observed.39
Furthermore, if a significant amount of OH radicals are
produced, HCO2H would readily decompose to H2O2, oxalic
Fig. 1 (a) Representative extinction infrared spectra for aqueous (grey lines) and dry (black lines) maleic and fumaric acid aerosols. The increases
in the slope of the baseline at higher wavenumber are a result of Mie scattering of the infrared beam by the particles. The dotted lines in the spectra
indicate condensed-phase water bands at 3500–2500 cm�1, 1700 cm�1 and 680 cm�1. (b and c) Expanded view of the IR spectra for aqueous (grey)
and dry (black) maleic and fumaric acid aerosols. In all cases, gas-phase water lines have been subtracted for clarity and the spectra have been
vertically scaled and offset.
Dow
nloa
ded
by U
NIV
ER
SIT
Y O
F M
AN
CH
EST
ER
on
26 A
ugus
t 201
0Pu
blis
hed
on 1
3 A
ugus
t 201
0 on
http
://pu
bs.r
sc.o
rg |
doi:1
0.10
39/B
9247
75K
View Online
This journal is c the Owner Societies 2010 Phys. Chem. Chem. Phys.
acid and CO2.20,40,41 The quantitative agreement between the
CO2 and HCO2H kinetic results suggests that this channel is
therefore not favoured. On the basis of these observations, and
by comparison to the dry studies, it is proposed that both the
HAHP channel and the SCI isomerisation channel operate in
parallel in wet aerosols, resulting in faster reaction and
increased yield.
Kinetics of the ozonolysis reaction with aqueous organic aerosols
In a previous study of dry aerosol particles, the main reaction
products of the ozonolysis of dry maleic and fumaric acids
were observed to be gas-phase HCO2H and CO2.15 These are
clearly identifiable by their characteristic features at 2347 cm�1
and 667 cm�1 for CO2 (not shown), and at 1790, 1118 and
640 cm�1 for HCO2H (Fig. 2 and 3).24 The quantification of
evolved products using CO2 is problematical because of inter-
ference from atmospheric contributions in the light path of the
spectrometer.15 However, since the formation of HCO2H can
only be the result of the heterogeneous reaction between ozone
and organic aerosols, reaction progress can be monitored
effectively by following the intensity of the relevant absorption
bands. A series of experiments were conducted in which the
formation of gas-phase HCO2H as a function of the reaction
time (t) was monitored over a range of gas-phase ozone
concentrations ([O3]gas) through the appearance of the
characteristic gas-phase infrared absorption band at 1790 cm�1.
Fig. 2 Expanded view (1900–700 cm�1) of extinction infrared spectra
obtained during the oxidation of aqueous maleic acid aerosols with
321 ppm ozone. Spectral changes can be seen as exposure time to
ozone was varied between 0 to 12 seconds. Note the increase in the
intensity of HCO2H absorption bands at 1790 and 1118 cm�1.
Gas-phase water and ozone lines have been subtracted for clarity.
Fig. 3 Expanded view (1900–700 cm�1) of extinction infrared spectra
obtained during the oxidation of aqueous fumaric acid aerosols with
317 ppm ozone. Spectral changes can be seen as exposure time to
ozone was varied between 0 to 12 seconds. Note the increase in the
intensity of HCO2H absorption bands at 1790 and 1118 cm�1.
Gas-phase water and ozone lines have been subtracted for clarity.
Scheme 1
Dow
nloa
ded
by U
NIV
ER
SIT
Y O
F M
AN
CH
EST
ER
on
26 A
ugus
t 201
0Pu
blis
hed
on 1
3 A
ugus
t 201
0 on
http
://pu
bs.r
sc.o
rg |
doi:1
0.10
39/B
9247
75K
View Online
Phys. Chem. Chem. Phys. This journal is c the Owner Societies 2010
Representative spectra are shown in Fig. 2 and 3 for
wet maleic and fumaric acid aerosols respectively. The
concentration of HCO2H was obtained from integration of
the peak area between 1800 and 1780 cm�1 (background
1805.0–1777.6 cm�1). As a check, the concentration of
HCO2H was also obtained by integrating the peak area
between 1100 and 1080 cm�1 (background 1100–1080 cm�1).
Quantitative agreement was observed by both methods,
although the 1800–1780 cm�1 region is preferred as the
signal-to-noise ratio is higher. To correct for fluctuations in
aerosol size and number density between experiments, the
integrated areas were normalised to the input aerosol
measurements from the process aerosol monitor. Typical
profiles of the normalised integrated peak area representative
of HCO2H formation from maleic acid reaction are shown
in Fig. 4 as a function of the reaction time, along with
comparable values previously reported for dry maleic acid
aerosols.15 From this, it is clear that the rate of formation
of HCO2H (from comparable ozone concentrations) is
substantially higher at increased RH%. It should also be
noted that the organic concentration in the aqueous aerosols
is at least 3 times smaller than that in the dry aerosols.
For each ozone concentration, the HCO2H concentration at
the end of the reaction, [Cinf], was calculated by fitting the
integrated HCO2H peak area versus time plot with an
exponential Box–Lucas function y(x) = a(1 � exp(bx)). The
fitting statistics for values derived [Cinf] values for each ozone
concentration are 0.89o R2 o 0.94 and 0.93o R2 o 0.96 for
maleic acid and fumaric acid aerosols respectively. Plots of
ln(1 � [C]/[Cinf]) versus reaction time are shown in Fig. 5 and 6
for maleic and fumaric acid respectively. Given the linearity of
these plots, the reaction between these organic particles and
ozone appears to be fairly well-described by pseudo first-order
kinetics, hence kI values were obtained from least squares
linear regression on this data. The uncertainty on kI was
determined as the standard error of the slope at the 95%
confidence interval.
Reactive uptake of ozone by reaction of maleic or fumaric
acid in aqueous droplets involves convolution of several
simultaneous processes.42–46 Principally, these are (i) gas-phase
diffusion of ozone to the droplet surface, (ii) accommodation
of ozone at the surface of the aqueous droplet, (iii) diffusion
of ozone within the droplet, and (iv) subsequent chemical
reaction with an organic molecule either in the surface layer or
dissolved in the droplet. The characteristic times47 associated
with these chemical processes can be evaluated and compared
in order to determine those most likely to be rate-limiting, as
summarised in Table 2. Gas-phase diffusion of ozone to
the particle surface occurs on a timescale given by tg =
Dp2/(4p2Dg(O3)), where Dg(O3) is the gas-phase diffusion
coefficient of ozone (B0.2 cm2 s�1)48 and Dp is the
average particle diameter. For Dp = 2.7 mm (typical of these
Fig. 4 Integrated absorption band of HCO2H formed as a function
of the reaction time for the oxidation of aqueous maleic acid aerosols
with [O3] = 76 ppm (black squares), 165 ppm (open circles) and
371 ppm (grey diamonds). Results are compared with those values
obtained from a previous study15 of the ozonolysis of dry maleic acid
aerosols with [O3] = 150 ppm (open squares), 450 ppm (up triangles)
and 750 ppm (open diamonds).
Fig. 5 Oxidation of aqueous maleic acid aerosols with [O3] = 29 ppm
(open squares), 76 ppm (black squares), 165 ppm (open circles),
239 ppm (grey up triangles), 321 ppm (open down triangles) and
371 ppm (grey diamonds). Decay plots based on the formation of
HCO2H as a function of the reaction time. The pseudo-first-order rate
coefficients were determined from the slope of linear least-squares fits
to the data (solid lines).15
Fig. 6 Oxidation of aqueous fumaric acid aerosols with [O3] =
32 ppm (open squares), 78 ppm (black squares), 171 ppm
(open circles), 242 ppm (grey up triangles), 317 ppm (open down
triangles) and 368 ppm (grey diamond). Decay plots based on the
formation of HCO2H as a function of the reaction time. The pseudo-
first-order rate coefficients were determined from the slope of linear
least-squares fits to the data (solid lines).15
Dow
nloa
ded
by U
NIV
ER
SIT
Y O
F M
AN
CH
EST
ER
on
26 A
ugus
t 201
0Pu
blis
hed
on 1
3 A
ugus
t 201
0 on
http
://pu
bs.r
sc.o
rg |
doi:1
0.10
39/B
9247
75K
View Online
This journal is c the Owner Societies 2010 Phys. Chem. Chem. Phys.
experiments), tg E 9 � 10�9 s. The mass accommodation time
is given by tH = Dl(O3)[4HRT/(acO3)]2, where Dl(O3) is the
aqueous-phase diffusion coefficient of ozone (B1.8 �10�5 cm2 s�1),47 H is the Henry law constant for ozone in
solution (1.15 � 10�2 M atm�1),20,41 R is the universal gas
constant (0.082 atm K�1 M�1), T is an average experimental
temperature (22 1C), a is the mass accommodation coefficient
for ozone in organic aqueous solution, and cO3is the mean
speed of ozone molecules (3.61 � 104 cm s�1). Although a has
not been reported for comparable organic acid solutions, tHcan be estimated asB2� 10�10 s using a= 1� 10�2, a typical
value for aqueous droplets.41,48 The time constant for diffusion
in a spherical aqueous droplet (taq) of 1 � 10�4 s is estimated
using taq = Dp2/(4p2Dl(O3)).
47 In a recent study of the
reaction of ozone with fumarate containing droplets,47 the
liquid-phase bimolecular rate coefficient k2 was calculated as
(2.7 � 2) � 105 M�1 s�1. Using this result as an average value
for the rate constant in the bulk with concentrations of 0.09 M
and 0.65 M for fumaric and maleic acid respectively, char-
acteristic aqueous droplet reaction times tr of ca. 4 � 10�5 s
(6 � 10�6 s) are obtained. Finally, also shown in Table 2, the
diffuso-reactive length is estimated to be ranging between
0.10–0.27 mm using lc ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiDlðO3Þ=k2½Org�
p. This parameter
represents the characteristic distance that an ozone molecule
diffuses before reaction. Given that the value obtained
(albeit using an approximated rate constant) is much smaller
than the particle radius used in these experiments and that the
associated time constant for the aqueous oxidation reaction is
somewhat faster than the characteristic diffusion time
constant, it seems reasonable to conclude that the reaction is
mainly confined to the surface region of the particles. In
studying the reactive uptake of ozone by pure oleic acid
aerosols, Smith et al.49 have interpreted the reactive uptake
as primarily occurring within a thin layer to the particle
surface, for which an analytical expression derived from flux
calculations reveals an exponential dependence of consump-
tion of the particle-based reactant (oleic acid) with time. This
is one of two possible scenarios for reactions which do not
occur throughout the whole of the particle, the second of
which involves reaction in a diffusion-limited region close to
the particle surface. In this second case, a square-root
dependence of oleic acid consumption with time would be
expected. However, they point out that it is in practice
extremely difficult to separate these scenarios with noisy
experimental data. Based upon both previous work on dry
maleic and fumaric acid ozonolysis and the reasonably good fit
of the experimental data obtained here to an exponential
process (as shown in Fig. 5 and 6) the dominance of a
predominantly surface-located reaction is evident.
For a consistent description of competitive co-adsorption
and surface saturation effects in these apparently surface-
mediated reactions, a Langmuir–Hinshelwood approach in
which the overall uptake process is controlled by both
adsorption of ozone at the surface and by reaction within a
thin surface layer has been adopted to treat these data, as
previously reported for dry maleic and fumaric acid aerosols.
The nature of the reactive double-layer at the surface in such a
scenario is similar to that given by Poschl et al.45 in which
chemical reactions occur within the surface double layer and
involve only adsorbed species or components of the quasi-
static surface layer. The kI values obtained from the data in
Fig. 5 and 6 are plotted as a function of ozone concentrations
for maleic (grey squares) and fumaric (black diamonds) acid
aqueous aerosols in Fig. 7. The error bars correspond to the
standard errors of the pseudo-first-order coefficients. In the
Langmuir–Hinshelwood mechanism, the reaction rate is
proportional to the product of the ozone and organic reactant
concentrations at the aerosol surface at low gas-phase ozone
concentrations. At higher ozone concentrations, the surface
coverage of ozone approaches saturation because a limited
number of surface sites are available for the ozone to
adsorb and consequently, the rate of the reaction becomes
independent of the ozone concentration. This saturation effect
is not clearly defined in these data given the high ozone
Table 2 Comparison between the characteristic time associated with each rate determining process for the heterogeneous reaction betweengas-phase ozone and aqueous maleic/fumaric acid aerosols
Process Equation Associated time/s
Gas-phase diffusion tg = Dp2/(4p2Dg(O3)) B9 � 10�9
Mass accommodation diffusion tH = Dl(O3) [(4HRT/(ac)]2 B2 � 10�10
Aqueous phase diffusion taq = Dp2/(4p2Dl(O3)) B1 � 10�4
Aqueous droplet reaction tr = 1/k0*[Org] B(0.6–4) � 10�5
Diffuso-reactive length lc2 = Dl(O3)/(k
0*[Org]) B(0.1–0.3) mm
Fig. 7 Pseudo-first-order reaction rate constants for aqueous maleic
acid (grey squares) and fumaric acid (black diamonds) aerosol
particles as a function of gas-phase ozone concentration. The grey
(maleic acid) and black (fumaric acid) lines show a fit of the data to the
Langmuir–Hinshelwood mechanism using non-linear least-squares
curve fits of eqn (1) based on the rectangular hyperbola function
(see text). The error bars correspond to the standard errors of the
pseudo-first-order coefficients.
Dow
nloa
ded
by U
NIV
ER
SIT
Y O
F M
AN
CH
EST
ER
on
26 A
ugus
t 201
0Pu
blis
hed
on 1
3 A
ugus
t 201
0 on
http
://pu
bs.r
sc.o
rg |
doi:1
0.10
39/B
9247
75K
View Online
Phys. Chem. Chem. Phys. This journal is c the Owner Societies 2010
concentrations, although there is some evidence that the kIvalues reach a plateau at [O3] above 1 � 1015 molecules cm�3.
The applicability of the LH mechanism to these data at high
RH% is therefore far from clear, since it is difficult to
eliminate the possibility that some aspects of the increased
rate of reaction may be due to increased rates of diffusion of
products and reactants via some physical effect (i.e. essentially
increasing the reacto-diffusive length and thereby involving a
greater volume of the particle in the reaction). However,
application of the LH treatment to these data and direct
comparison with dry maleic and fumaric acid aerosol reactivity15
produces a number of chemically reasonable scenarios.
In the Langmuir–Hinshelwood-type mechanism, the
relationship between kI and gas-phase ozone concentration
can be modelled using:42,45,50
kI ¼kImaxKO3
½O3�gas1þ KO3
½O3�gasð1Þ
where KO3is the ozone gas-to-surface equilibrium constant
and kImax is the maximum pseudo-first-order rate coefficient
observed at high ozone concentrations. For both maleic and
fumaric acid aerosol particles, the kI data in Fig. 7 were fitted
using a non-linear least-squares fit of eqn (1) based on a
rectangular hyperbola function, [y(x) = abx/(1 + bx)]. The
error bars correspond to the standard errors of the slopes of
the corresponding decay plots. The fitted curves show a
saturation trend at these experimental ozone concentrations.
In Table 3, the parameters KO3and kImax for aqueous maleic
and fumaric acid aerosol particles obtained from this fitting
procedure are listed and compared with values reported in the
literature under high relative humidity conditions. The calcu-
lated KO3for maleic acid (9 � 4 � 10�15 cm3 molecule�1) and
fumaric acid (5 � 2 � 10�15 cm3 molecule�1) aqueous aerosols
in this study are similar in magnitude to those previously
reported on aqueous salt51 and azelaic acid52 surfaces, as well
as on Pyrex53 and inert substrates,54 although three orders of
magnitude lower relative to soot surfaces.53 [NB: it should
be noted however that, as any surface reaction proceeds
(i.e. as either the concentration or the length of the reaction
time for ozone exposure is increased), an accumulation of
other condensed phase products may occur on the particle
surface and consequently, the presence of more oxidised
species may change the nature of the surface by making it
more hydrophilic. Consequently, at longer times and higher
exposures, KO3may describe the partitioning of ozone to
oxidatively modified aqueous maleic/fumaric acid aerosol
surfaces. This possible effect is not encapsulated in the simple
LH treatment presented here.] The kImax values obtained for
maleic acid (0.21� 0.01 s�1) and fumaric acid (0.19� 0.01 s�1)
aqueous aerosol particles ozonolysis are higher to those previously
reported rates in the oxidation of benzo[a]pyrene,52 sodium
oleate,51 PAHs55 and cypermethrin,54 suggesting a faster
reaction on aqueous aerosol surfaces.
Comparison of these results to those obtained from the
heterogeneous ozonolysis of dry maleic and fumaric acid
aerosols reveals a significant feature. Whilst maleic acid
aerosols at low (1.3–2.6) and high (90–93) RH% have
comparable values of KO3(3.3 � 0.5 � 10�16 cm3 molecule�1
and 9 � 4 � 10�15 cm3 molecule�1), the difference between the
kImax values (0.038 � 0.004 s�1 and 0.21 � 0.01 s�1) suggests
that the increased overall rate at high RH% is primarily driven
by an increase in the affinity of ozone for the aqueous aerosol
surface rather than by the surface rate constant between ozone
and maleic acid; that is, with increasing RH% for maleic acid
aerosols, KO3increases by 27.3 and kImax increases by only 5.53.
A similar trend is apparent for fumaric acid aerosols, where
KO3and kImax increase by 31.2 and 3.96, respectively. This
implies that the partitioning of ozone to the surface is the most
important factor in determining the rate limiting step in the
kinetics of HCO2H product formation at both low and
high RH%.
Determination of apparent uptake coefficients for aqueous
maleic and fumaric acid aerosol particles
As shown previously,15 these results can also be expressed in
terms of reactive uptake coefficient g, defined as the ratio of the
number of collisions that result in a reaction to total number
of collisions between the surface gas-phase ozone and
maleic/fumaric acid aqueous aerosols at the particle surface.
For a simple bimolecular reaction mechanism, g can be
expressed as:45,50,56
g ¼ 4kI
sorgcO3½O3�
ð2Þ
Table 3 Comparison of reaction rate parameters obtained from heterogeneous reaction of ozone and organic aerosol proxies at high relativehumidity
References Aerosol substrate RH% kImax/s�1 KO3
/cm3 molecule�1 g
King et al.37 Aqueous fumarate 80 1.1 � 0.7 � 10�5
Kwamena et al.52 BaPa on azelaic acid 72 0.060 � 0.018 2.8 � 1.4 � 10�15 1.1–0.6 � 10�6
Kwamena et al.53 PAHsab on Pyrex tubes 48, 57 0.0064 � 0.0018 2.8 � 0.9 � 10�15
McNeill et al.51 NaOla on aqueous NaCl 62–67 0.05 � 0.01 4 � 3 � 10�14 4–1 � 10�5
Najera et al.15 Maleic acid 1.3–2.6 0.038 � 0.004 3.3 � 0.5 � 10�16 1.7–0.5 � 10�7
Najera et al.15 Fumaric acid 2.9–4.4 0.048 � 0.007 1.6 � 0.5 � 10�16 1.4–0.6 � 10�7
Poschl et al.56 BaP on soot 25 0.016 � 0.001 2.8 � 0.2 � 10�13 6–2 � 10�6
Segal-Rosenheimer andDubowski54
Cypermethrinc on inertsubstrate (ZnSe)
80–90 0.0007 � 0.0001 4.7 � 1.7 � 10�16
This work Maleic acid 89.8–93.4 0.21 � 0.01 9 � 4 � 10�15 7.3–0.7 � 10�6
This work Fumaric acid 90–91.5 0.19 � 0.01 5 � 2 � 10�15 5.5–0.6 � 10�6
a BaP = benzo[a]pyrene, PAHs = polycyclic aromatic hydrocarbons, NaOl = sodium oleate. b Coated-wall flow tube experiments. c Attenuated
total internal reflectance and long-path IR cell (ATR-FTIR).
Dow
nloa
ded
by U
NIV
ER
SIT
Y O
F M
AN
CH
EST
ER
on
26 A
ugus
t 201
0Pu
blis
hed
on 1
3 A
ugus
t 201
0 on
http
://pu
bs.r
sc.o
rg |
doi:1
0.10
39/B
9247
75K
View Online
This journal is c the Owner Societies 2010 Phys. Chem. Chem. Phys.
where sorg is the maleic/fumaric molecular cross section
(3.8 � 10�15/3.7 � 10�15 cm2 molecule�1).15
The data in Fig. 7 reveal that the heterogeneous reaction
between ozone and maleic/fumaric acid aerosols is not
independent of the ozone concentration as it would be for a
genuine elementary bimolecular process. To model the effect
of the Langmuir–Hinshelwood surface mechanism, eqn (2)
must be modified as follows in order to extract the apparent
reactive uptake g:
g ¼ 4kImaxKO3
sorgcO3ð1þ KO3
½O3�Þð3Þ
Transformation of the kI values in Fig. 7 into apparent uptake
coefficients using eqn (2) is plotted versus ozone concentration
for maleic acid (grey squares) and fumaric acid (black diamonds)
aerosols in Fig. 8. The error bars of the uptake coefficients
were calculated by considering the uncertainties of other
sources of error (e.g. ozone flow concentration15). As the
ozone concentration is increased, values of g decreased from
7.3 � 10�6 to 0.7 � 10�6 (maleic acid) and from 5.5 � 10�6 to
0.6 � 10�6 (fumaric acid), as listed in Table 3. Non-linear fits
with a two-parameter exponential function were applied to the
uptake data using eqn (3), represented by solid lines for maleic
(grey) and fumaric (black) acid aerosol particles in Fig. 8. At
zero ozone concentration, the limiting values of the apparent
uptake coefficient are 5.6� 10�5 on maleic acid and 3.0� 10�5
on fumaric acid aerosol particles. These are essentially the
values which will prevail under atmospheric conditions of
50 ppb O3.
The apparent reactive uptake coefficients for ozone
determined in this study for maleic and fumaric acid aqueous
aerosols are B5–10 times higher than those reported for
aerosol particles under dry conditions, and are closer to the
values obtained for benzo[a]pyrene on azelaic acid52 and
sodium oleate on salt aqueous aerosols.51 This behaviour
reflects the fact that the wet particle surface becomes less polar
relative to the dry surface. In a recent experimental study,
uptake coefficients for gaseous oxidants (ozone) on 10 mm size
droplet composed of aqueous fumarate anions obtained using
Raman spectroscopy in a laser tweezers experiment have been
reported37 in which an uptake coefficient of 1.1 � 0.7 � 10�5
was determined (shown as a black hexagon in Fig. 8). Aqueous
aerosols therefore clearly show increased ozone partitioning to
the surface, increasing the effective rate of the surface reaction
compared to dry materials.
Atmospheric implications
The experimental results presented are consistent with a
surface specific, Langmuir–Hinshelwood type mechanism for
the heterogeneous oxidation of aqueous maleic/fumaric acid
aerosol particles by gas-phase ozone. For aqueous maleic acid
and fumaric acid aerosols, measured values of the parameter
KO3(which relates to the ratio of ozone desorption to
adsorption rates) also indicate that the ozone trapping ability
of aerosol surfaces is influenced by the simultaneous presence
of both water and organic acids, leading to an increased rate of
reaction at the droplet surface.57 This result accords well with
recent molecular dynamic calculations,58 where ozone was
found to adsorb at a coated air–water surface more effectively
than that at pure water. It can be inferred therefore that one
aspect of the increased rate and extent of reaction in aqueous
aerosols is a physical one which enhances ozone adsorption via
modification of the surface polarity. A further effect arises
because of the involvement of water molecules in the reaction,
giving rise to an additional, efficient channel (via HAHP). This
reaction would result in less material being transformed into
secondary ozonides. A final effect of water is through
mobilisation: at or above the deliquescence relative humidity,
the mobility of the organic reactants is substantially increased,
which allows both a more rapid replenishment of the organic
reactant at the ozone-rich interface and also the involvement
of a considerably greater proportion of the organic content of
the aerosols in the overall reaction. The atmospheric lifetimes
(t) of these unsaturated dicarboxylic acids at high relative
humidity can be estimated by considering an ozone
concentration of 50 ppb at typical tropospheric conditions.
Using the limiting values of the apparent uptake calculated for
aqueous maleic and fumaric acid aerosols at zero ozone
concentration, the pseudo-first-order rate coefficients are
2.3 � 10�3 and 1.2 � 10�3 s�1 using eqn (2), from which the
calculated lifetimes of pure maleic and fumaric acid aerosols
are 7 and 14 minutes respectively. Lifetimes for dry maleic acid
and fumaric acid are 19 hours and 30 hours, using kI values of
1.5 � 10�5 s�1 and 9.2 � 10�6 s�1 respectively.
For both organic acids under humid conditions, the reaction
of water with the Criegee intermediates might open a reaction
pathway via the formation of hydroxyacetyl hydroperoxide
radicals, which in turn decompose into HCO2H and CO2.
A significant observation of this study is the transformation of
substantial proportion of these short chain acids to HCO2H
and CO2, leading to a significant change in the hygroscopicity
of aerosols. A further consequence of partitioning of the
reaction products to the condensed phase is that they are
Fig. 8 Apparent reactive uptake coefficients as a function of
gas-phase ozone concentration for maleic (grey squares) and fumaric
(black diamonds) acid aerosol particles were calculated using eqn (2).
Black hexagon: literature data from King et al.37 The error bars of the
uptake coefficients were calculated based on experimental uncertainties
(see text for details). Solid fitting lines (non-linear curve fit, see text)
applied to maleic (grey) and fumaric (black) acids data using eqn (3)
are drawn to guide the eye.
Dow
nloa
ded
by U
NIV
ER
SIT
Y O
F M
AN
CH
EST
ER
on
26 A
ugus
t 201
0Pu
blis
hed
on 1
3 A
ugus
t 201
0 on
http
://pu
bs.r
sc.o
rg |
doi:1
0.10
39/B
9247
75K
View Online
Phys. Chem. Chem. Phys. This journal is c the Owner Societies 2010
not available to take part in gas-phase chemistry but may play
a role in particle-associated organic aerosol chemistry. It has
been shown previously that glyoxylic acid and oxalic acid are
the only condensed phase products, but they are not expected
to further react with ozone in the bulk phase due to a small
Henry’s law constant for ozone.
Conclusions
In this study, the kinetics and reaction products of the reaction
between gas-phase ozone and aqueous maleic and fumaric acid
aerosol particles using an aerosol flow tube coupled with a
FTIR spectrometer have been investigated. Single component
organic aerosols were generated from the nebulisation of their
aqueous solution, with sizes ranging from 2.6 to 2.9 mm at RH
90–93%. Continuous spectral monitoring as a function of
contact time over a range of ozone concentrations enabled
the reaction progress to be monitored such that a kinetic
analysis could be carried out from the observation of gas
phase products.
On the basis of the products identified, it is inferred that
both isomers of 2-butenedioic acid aerosol particles exhibit
similar chemistry upon ozonolysis. The formation of CO2 and
HCO2H as the major gas-phase products is consistent with the
Criegee mechanism. It was not possible to quantify the
concentration of other condensed phase products (oxalic or
glyoxylic acid) in this study due to the sensitivity of the
infrared probe to these species within the aerosol phase at
such low concentrations: the gas-phase products are more
easily identified because of their characteristic gas-phase
vibration–rotation envelopes. These species have however
been detected in analogous solution-phase experiments in
which greater amounts of material were produced.32
This work also shows that the heterogeneous reactions of
ozone with maleic and fumaric acid aerosol particles exhibit
pseudo-first-order kinetics for the formation of gas-phase
HCO2H, and the pseudo-first-order rate coefficients display
a Langmuir–Hinshelwood dependence on gas-phase ozone
concentration in both cases. The occurrence of such a kinetic
mechanism is in general agreement with previous observations
of the ozonolysis of unsaturated organic material coated onto
aerosol particles. From the pseudo-first-order coefficients for
2-butenedioic acid isomers, apparent uptake coefficient values
were calculated and a decreasing trend with increasing ozone
concentration was observed, consistent with the previous
published studies.
Acknowledgements
The work reported in this paper was carried out with financial
support of the EPSRC through the award of an Advanced
Research Fellowship to ABH (GR/A00919/02), and by the
Leverhulme Trust through the award of a Research Project
Grant (F/00120/X) which supported JJN.
References
1 Y. Rudich, Chem. Rev., 2003, 103, 5097–5124.2 G. B. Ellison, A. F. Tuck and V. Vaida, J. Geophys. Res., 1999,104, 11633–11641.
3 M. Kanakidou, J. H. Seinfeld, S. N. Pandis, I. Barnes,F. J. Dentener, M. C. Facchini, R. Van Dingenen, B. Ervens,A. Nenes, C. J. Nielsen, E. Swietlicki, J. P. Putaud, Y. Balkanski,S. Fuzzi, J. Horth, G. K. Moortgat, R. Winterhalter, C. E. L.Myhre, K. Tsigaridis, E. Vignati, E. G. Stephanou and J. Wilson,Atmos. Chem. Phys., 2005, 5, 1053–1123.
4 Y. Rudich, N. M. Donahue and T. F. Mentel, Annu. Rev. Phys.Chem., 2007, 58, 321–352.
5 R. Atkinson and J. Arey, Chem. Rev., 2003, 103, 4605–4638.6 A. Bogdan, M. J. Molina, M. Kulmala, A. R. MacKenzie andA. Laaksonen, J. Geophys. Res., 2003, 108, 4302.
7 U. Baltensperger, M. Kalberer, J. Dommen, D. Paulsen,M. R. Alfarra, H. Coe, R. Fisseha, A. Gascho, M. Gysel,S. Nyeki, M. Sax, M. Steinbacher, A. S. H. Prevot, S. Sjogren,E. Weingartner and R. Zenobi, Faraday Discuss., 2005, 130,265–278.
8 R. J. Griffin, K. Nguyen, D. Dabdub and J. H. Seinfeld, J. Atmos.Chem., 2003, 44, 171–190.
9 D. Grosjean, K. V. Cauwenberghe, J. P. Schmid, P. E. Kelley andJ. N. Pitts, Environ. Sci. Technol., 1978, 12, 313–317.
10 K. Kawamura and R. B. Gagosian, J. Chromatogr., A, 1987, 390,371–377.
11 P. Saxena and L. M. Hildemann, J. Atmos. Chem., 1996, 24,57–109.
12 R. Sempere and K. Kawamura, Atmos. Environ., 1994, 28,449–459.
13 P. Saxena, L. M. Hildemann, P. H. McMurray and J. H. Seinfeld,J. Geophys. Res., 1995, 100, 18755–18770.
14 A. R. Ravishankara and C. A. Longfellow, Phys. Chem. Chem.Phys., 1999, 1, 5433–5441.
15 J. J. Najera, C. Percival and A. B. Horn, Phys. Chem. Chem. Phys.,2009, 11, 9093–9103.
16 M. Glasius, C. Boel, N. Bruun, L. M. Easa, P. Hornung,H. S. Klausen, K. C. Klitgaard, C. Lindeskov, C. K. Moller,H. Nissen, A. P. F. Petersen, S. Kleefeld, E. Boaretto,T. S. Hansen, J. Heinemeier and C. Lohse, J. Geophys. Res.,2001, 106, 7415–7426.
17 C. P. Rinsland, E. Mahieu, R. Zander, A. Goldman, S. Wood andL. Chiou, J. Geophys. Res., 2004, 109, D18308.
18 S. Yu, Atmos. Res., 2000, 53, 185–217.19 R. Fisseha, J. Dommen, K. Gaeggeler, E. Weingartner,
V. Samburova, M. Kalberer and U. Baltensperger, J. Geophys.Res.,2006, 111, D12316.
20 W. L. Chameides, J. Geophys. Res., 1984, 89, 4739–4755.21 J. J. Najera, J. G. Fochessatto, D. J. Last, C. Percival and
A. B. Horn, Rev. Sci. Instrum., 2008, 79, 124102.22 D. Gomez, R. Font and A. Soler, J. Chem. Eng. Data, 1986, 31,
391–392.23 N. A. Lange and M. H. Sinks, J. Am. Chem. Soc., 1930, 52,
2602–2604.24 L. S. Rothman, I. E. Gordon, A. Barbe, D. C. Benner,
P. F. Bernath, M. Birk and V. Boudon, J. Quant. Spectrosc.Radiat. Transfer, 2009, 110, 533–572.
25 G. A. Ferron and S. C. Soderholm, J. Aerosol Sci., 1990, 21,415–429.
26 S. D. Brooks, R. M. Garland, M. E. Wise, A. J. Prenni,M. Cushing, E. Hewitt and M. A. Tolbert, J. Geophys. Res.,2003, 108, 4487.
27 S. D. Brooks, M. E. Wise, M. Cushing and M. A. Tolbert,Geophys. Res. Lett., 2002, 29, 1917.
28 M. T. Parsons, J. Mak, S. R. Lipetz and A. K. Bertram,J. Geophys. Res., 2004, 109, D06212.
29 A. J. Prenni, P. J. DeMott, S. M. Kreidenweis, D. E. Sherman,L. M. Russell and Y. Ming, J. Phys. Chem. A, 2001, 105,11240–11248.
30 C. W. Robertson, B. Cumutte and D. Williams, Mol. Phys., 1973,26, 183–191.
31 D. D.Weis andG. E. Ewing, J. Geophys. Res., 1999, 104, 21275–21285.32 D. J. Last, J. J. Najera, R. Wamsley, G. Hilton, M. McGillen,
C. Percival and A. B. Horn, Phys. Chem. Chem. Phys., 2009, 11,1427–1440.
33 Y. Yamamoto, E. Kiki and Y. Kamiya, J. Org. Chem., 1981, 46,250–254.
34 S. Gab, W. V. Turner, S. Wolff, K. H. Becker, L. Ruppert andK. J. Brockmann, Atmos. Environ., 1995, 29, 2401–2407.
Dow
nloa
ded
by U
NIV
ER
SIT
Y O
F M
AN
CH
EST
ER
on
26 A
ugus
t 201
0Pu
blis
hed
on 1
3 A
ugus
t 201
0 on
http
://pu
bs.r
sc.o
rg |
doi:1
0.10
39/B
9247
75K
View Online
This journal is c the Owner Societies 2010 Phys. Chem. Chem. Phys.
35 A. S. Hasson, G. Orzechowska and S. E. Paulson, J. Geophys. Res.,2001, 106, 34131–34142.
36 P. Neeb, F. Sauer, O. Horie and G. K. Moortgat, Atmos. Environ.,1997, 31, 1417–1423.
37 M. D. King, K. C. Thompson, A. D. Ward, C. Pfrang andB. R. Hughes, Faraday Discuss., 2008, 137, 173–192.
38 A. Leitzke, E. Reisz, R. Flyunt and C. von Sonntag, J. Chem. Soc.,Perkin Trans. 2, 2001, 793–797.
39 E. R. Thomas, G. J. Frost and Y. Rudich, J. Geophys. Res., 2001,106, 3045–3056.
40 N. A. Aristova, N. K. V. Leitner and I. M. Piskarev, High EnergyChem., 2002, 36, 197–202.
41 S. N. Pandis and J. H. Seinfeld, J. Geophys. Res., 1989, 94,1105–1126.
42 M. Ammann, U. Poschl and Y. Rudich, Phys. Chem. Chem. Phys.,2003, 5, 351–356.
43 P. Davidovits, C. E. Kolb, L. R. Williams, J. T. Jayne andD. R. Worsnop, Chem. Rev., 2006, 106, 1323–1354.
44 D. R. Hanson, J. Phys. Chem. B, 1997, 101, 4998–5001.45 U. Poschl, Y. Rudich andM. Ammann, Atmos. Chem. Phys., 2007,
7, 5989–6023.46 D. R. Worsnop, J. W. Morris, Q. Shi, P. Davidovits and
C. E. Kolb, Geophys. Res. Lett., 2002, 29, 1996.
47 S. E. Schwartz and J. E. Freiberg, Atmos. Environ., 1981, 15,1129–1144.
48 B. Muller andM. Heal, Phys. Chem. Chem. Phys., 2002, 4, 3365–3369.49 G. D. Smith, E. Woods, C. L. DeForest, T. Baer and R. E. Miller,
J. Phys. Chem. A, 2002, 106, 8085–8095.50 M. Ammann and U. Poschl, Atmos. Chem. Phys., 2007, 7,
6025–6045.51 V. F. McNeill, G. M. Wolfe and J. A. Thornton, J. Phys. Chem. A,
2007, 111, 1073–1083.52 N. O. A. Kwamena, J. A. Thornton and J. P. D. Abbatt, J. Phys.
Chem. A, 2004, 108, 11626–11634.53 N. O. A. Kwamena, M. E. Earp, C. J. Young and J. P. D. Abbatt,
J. Phys. Chem. A, 2006, 110, 3638–3646.54 M. Segal-Rosenheimer and Y. Dubowski, J. Phys. Chem. C, 2007,
111, 11682–11691.55 N. O. A. Kwamena, M. G. Staikova, D. J. Donaldson, I. J. George
and J. P. D. Abbatt, J. Phys. Chem. A, 2007, 111, 11050–11058.56 U. Poschl, T. Letzel, C. Schauer and R. Niessner, J. Phys. Chem.
A, 2001, 105, 4029–4041.57 B. T. Mmereki, S. R. Chaudhuri and D. J. Donaldson, J. Phys.
Chem. A, 2003, 107, 2264–2269.58 J. Vieceli, O. L. Ma and D. J. Tobias, J. Phys. Chem. A, 2004, 108,
5806–5814.
Dow
nloa
ded
by U
NIV
ER
SIT
Y O
F M
AN
CH
EST
ER
on
26 A
ugus
t 201
0Pu
blis
hed
on 1
3 A
ugus
t 201
0 on
http
://pu
bs.r
sc.o
rg |
doi:1
0.10
39/B
9247
75K
View Online