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: Andrew.b.horn@manchester.ac.uk;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

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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.

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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

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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.

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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

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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

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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.

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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).

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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.

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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.

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