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. Na´jera, a Carl J. Percival a 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, K O 3 = (9 4) 10 15 cm 3 molecule 1 and k I max = (0.21 0.01) s 1 ; for the reaction of fumaric acid aerosols, K O 3 = (5 2) 10 15 cm 3 molecule 1 and k I max = (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 CO 2 and formic acid (HCO 2 H) 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 located 4 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 droplets 14 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 (HCO 2 H) and CO 2 as major products was reported. 15 Present predomi- nantly in the gas-phase due to its high vapour pressure, HCO 2 H is one of the most abundant mono-carboxylic acids reported in the atmosphere. 16–18 Whilst HCO 2 H is reported to be mainly produced via photochemical oxidation of VOCs 7,19 and is also emitted directly from several biogenic and anthro- pogenic sources 16–19 , any potential new heterogeneous source may be significant. Lower concentration of formic acid in the aerosol phase (0.16–0.49 mgm 3 ) compared to the corres- ponding gas-phase (0.24–1.07 mgm 3 ) concentrations were reported in field studies. 7,19 Particle-phase effects of dissolved HCO 2 H are also known: a significant amount is present in the aqueous phase and HCO 2 H is known to influence pH-dependent chemical reactions in cloud droplets. It has a School of Earth, Atmospheric and Environmental Sciences, Faculty of 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: Comparison of reaction kinetics obtained from the evolution of HCO 2 H and of CO 2 (not shown in this paper) with time. See DOI: 10.1039/b924775k PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics Downloaded by UNIVERSITY OF MANCHESTER on 26 August 2010 Published on 13 August 2010 on http://pubs.rsc.org | doi:10.1039/B924775K View Online
11
Embed
Kinetic studies of the heterogeneous oxidation of maleic ... · and fumaric acid molecules by factors of 3–30 and 10–100, respectively. Under these experimental flow conditions
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
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
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
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
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.