ARTICLE pubs.acs.org/JPCA
Uptake of Methacrolein into Aqueous Solutions of Sulfuric Acid and Hydrogen Peroxide Ze Liu,†,‡ Ling-Yan Wu,†,‡ Tian-He Wang,† Mao-Fa Ge,*,† and Wei-Gang Wang*,† †
Beijing National Laboratory for Molecular Science (BNLMS), State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People's Republic of China ‡ Beijing National Laboratory for Molecular Science (BNLMS), State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Peking University, Beijing 100871, People's Republic of China ABSTRACT: Multiphase acid-catalyzed oxidation by hydrogen peroxide has been suggested to be a potential route to secondary organic aerosol (SOA) formation from isoprene and its gasphase oxidation products, but the kinetics and chemical mechanism remain largely uncertain. Here we report the first measurement of uptake of methacrolein into aqueous solutions of sulfuric acid and hydrogen peroxide in the temperature range of 253293 K. The steady-state uptake coefficients were acquired and increased quickly with increasing sulfuric acid concentration and decreasing temperature. Propyne, acetone, and 2,3-dihydroxymethacrylic acid were suggested as the products. The chemical mechanism is proposed to be the oxidation of carbonyl group and CdC double bonds by peroxide hydrogen in acidic environment, which could explain the large content of polyhydroxyl compounds in atmospheric fine particles. These results indicate that multiphase acid-catalyzed oxidation of methacrolein by hydrogen peroxide can contribute to SOA mass in the atmosphere, especially in the upper troposphere.
in water.16 Also H2O2 can be produced through aqueous-phase reactions.1719 Thus, this multiphase acid-catalyzed oxidation can possibly occur in the troposphere. Although research demonstrated that the SOA components could be formed through this process, the chemical mechanism has not been well-known yet. Furthermore, to the best of our knowledge, information on kinetics is still missing except for a previous investigation on the uptake of 2-methyl-3-buten-2-ol,20 which is one of the crucial aspects to assess the atmospheric implication of this process. Methacrolein (MAC) is one of the principal first-generation oxidation products of isoprene, with a molar yield of 2028%.21,22 Besides this natural source, MAC is also directly emitted by anthropogenic sources.23 These make MAC emitted into the atmosphere at a rate higher than 100 Tg year1, assuming a global emission of isoprene is 500750 Tg year1.24 Hence, in this work, the uptake of MAC into aqueous solutions of H2SO4 and H2O2 was studied over the temperature range of 253293 K. The steady-state uptake coefficients, γss, were acquired for the first time, and the chemical mechanism was proposed, indicating this process has a potential contribution to SOA formation in the upper troposphere.
1. INTRODUCTION Secondary organic aerosols (SOAs) make up a significant fraction of tropospheric aerosols and have received considerable attention due to their links to visibility reduction, climate change, and adverse health effects.13 SOAs are formed from a wide variety of natural and anthropogenic precursors, but the kinetics and chemical mechanisms driving their formation are only partially understood. Formation of SOAs has generally been explained by atmospheric oxidation of volatile organic compounds (VOCs) followed by gas-to-particle partitioning.4 However, it has become more and more evident that chemical processes in the aerosol phase may also substantially contribute to global SOA loading. Jang et al. found that inorganic acid, such as sulfuric acid (H2SO4), could catalyze carbonyl compounds heterogeneous reactions and consequently lead to potentially multifold increase in SOA mass.5 Subsequently, many studies showed the heterogeneous reactions of VOCs with acidic particles might play an important role in SOA formation. 610 Recently, multiphase acid-catalyzed oxidation by hydrogen peroxide (H2O2) was suggested to be a new route to SOA formation from isoprene and its gas-phase oxidation products, which could account for the polyhydroxy compounds observed in atmospheric fine particles.11,12 In the troposphere the acidity of aerosols changes drastically with altitude or temperature, ranging from 60 to 80 wt % in the upper troposphere to less than 1 wt % in the lower troposphere.1315 H2O2 in ambient air can enter the aqueous aerosols and exist with a certain concentration according to its high gas-phase concentration and high solubility r 2011 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Uptake Experiments. Uptake measurements were carried out by using a rotated wetted wall reactor (RWWR) coupled Received: October 20, 2011 Revised: December 1, 2011 Published: December 03, 2011 437
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Table 1. Summarization of the Experimental Conditions and Steady-State Uptake Coefficients [H2SO4] (wt %)
a
[H2O2] (wt %)
T (K)
flow rate (cm3(STP) min1)
p (Torr)
γssa (104)
60
1
293
155.5 N2
55.2
0.162 ( 0.006
60
1
283
155.5 N2
55
0.166 ( 0.004
60
1
273
155.5 N2
50.7
0.178 ( 0.006
60
1
263
155.5 N2
50.7
0.187 ( 0.007
60
1
253
155.5 N2
50.7
0.193 ( 0.002
70
1
293
155.5 N2
16.2
1.39 ( 0.04
70
1
283
155.5 N2
15.8
1.74 ( 0.07
70 70
1 1
273 263
155.5 N2 155.5 N2
15.6 15.6
2.34 ( 0.02 3.44 ( 0.03
70
1
253
155.5 N2
15.5
80
1
293
220 He
13
4.95 ( 0.05 23.4 ( 0.5
80
1
283
220 He
13
27.1 ( 0.9
80
1
273
220 He
13
33.1 ( 1.5
80
1
263
220 He
13
38.3 ( 2.5
80
1
253
220 He
13
43.6 ( 3.7
Each value is the average of at least three measurements, and the error corresponds to one standard deviation (σ).
to a single-photon ionization time-of-flight mass spectrometer (SPI-TOFMS). This equipment was built and used in our previous works,20,25 so here is a brief description. The RWWR consisted of a horizontal glass flow reactor and a rotating inner cylinder (length L = 30 cm, inner radius R = 1.25 cm). Solution (∼3.5 mL, corresponding to a film thickness of ∼0.15 mm) was placed in the inner cylinder, which was rotated at 1015 rpm to maintain an even liquid film on the wall. A glass stirring bar rested on the bottom of the inner cylinder to make sure that the solution was mixed and spread sufficiently. The mixture of nitrogen (or helium) and water vapor in equilibrium with the solution was used as carrier gas so that the change of solution composition can be avoided over one experimental period of time. As reactant gas, MAC diluted in nitrogen (or helium) was introduced into the main flow at a small flow rate (10% of the carrier gas) through a movable injector. The movable injector centered in the RWWR allowed for the variation of the contact distance between reactant gas and solution. The total flow rates have been listed in Table 1. In the range of 253293 K, the temperature was controlled by flowing cold ethanol through the outer jacket of RWWR and was measured by a thermocouple. Typically, the total pressure was 1355.2 Torr, and the MAC concentration in the reactor was (1.98.1) 1014 molecules cm3. In this situation, the measurements were operated under the approximate laminar flow conditions. The variation of MAC concentration during uptake was monitored by SPI-TOFMS. MAC was ionized by 118 nm vacuum ultraviolet (vacuum-UV) laser, which was one of the soft ionization techniques with single-photon energy of 10.5 eV. The vacuum-UV laser was generated by focusing the third harmonic (355 nm, ∼30 mJ/pulse) of a Nd:YAG laser in a tripling cell that contained about a 250 Torr argon/xenon (10/1) gas mixture. To separate the vacuum-UV laser beam from the 355 nm fundamental beam, a magnesium fluoride prism (apex angle = 6°) was inserted into the laser beams. In this case, one is quite sure that the mass signal is by ionization purely through the vacuum-UV laser radiation with gentle power (∼1 μJ/pulse, pulse duration ≈ 5 ns). For MAC, only the molecular ion peak was observed at m/z = 70. The intensity of the molecular ion peak was recorded to investigate the uptake behaviors and calculate the steady-state uptake coefficients.
2.2. Analysis of Products. Profiting from the multichannel real-time detection of the SPI-TOFMS, we observed the products in gas phase during the uptake measurements and acquired the molecular weights of the products. Furthermore, the gasphase species were also collected in a U-shape collector located in a liquid nitrogen bath and then analyzed offline by gas chromatography (GC6820, Agilent Technologies; 60 m DB-1 capillary column, 0.53 mm i.d.; 1.0 μm phases) and FTIR spectrometer (Nicolet 6700, Thermo Scientific). Infrared spectra were recorded at a resolution of 2 cm1, and 32 scans were usually averaged for each spectrum. 2.3. Chemicals. MAC (95%, Alfa Aesar), H2SO4 (>96 wt %, Beijing Chemical Reagents Co.), and H2O2 (30 wt % aqueous solution, Beijing Chemical) were used as purchased. The reactant gas was prepared by injecting MAC into an evacuated 15 L glass flask to 7.6 Torr and pressurizing with pure nitrogen (or helium) to 2 atm. The aqueous solutions of H2SO4 and H2O2 were prepared by mixing ultrapure water (with resistivity of 18 MΩ 3 cm) with H2SO4 and H2O2. The solution was replaced after each uptake experiment.
3. RESULTS AND DISCUSSION 3.1. Uptake Behaviors. Uptake measurements were performed by exposing the aqueous solutions of H2SO4 and H2O2 to MAC, while monitoring the MAC signal at m/z = 70 by the SPI-TOFMS. Figure 1 depicts the temporal profiles of the MAC signal during the uptake measurements at 293 K. As the H2SO4 concentration of the solution increased, uptake behavior changed from reversible to irreversible. The reversible uptake was observed for all of the solutions with H2SO4 concentration below 50 wt %. Figure 1a depicts the uptake profile of MAC into 40 wt % H2SO4 + 1 wt % H2O2 solution. The signal dropped instantly upon exposure and returned to its original level as the solution was saturated. Pushing the injector back to its initial position produced an opposite change in signal, corresponding to desorption of MAC. The similarity in shape and total area of the adsorption and desorption curves meant that MAC partitioned reversibly into solution. For 60 wt % H2SO4 + 1 wt % H2O2 solution, two types of behavior were both observed in 438
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Figure 2. MAC signal loss as a function of exposed distance for aqueous solutions of 60 (9), 70 (b), and 80 wt % (2) H2SO4 with 1 wt % H2O2 at 293 K.
uptake coefficients (γss), defined as the probability that the gasphase molecule will be taken up irreversibly by the liquid,27 were calculated for 60, 70, and 80 wt % H2SO4 + 1 wt % H2O2 solutions. The calculation method was described as follows: As an uptake experiment just began, the movable injector was placed at its maximum position downstream. In that situation, the solution was unexposed and the unperturbed concentration of MAC could be obtained as the original signal, S0. Then, the injector was moved upstream to expose the solution to MAC. The steady-state uptake was indicated by a constant offset between the original signal and the steady-state uptake signal with time, S. The observed first-order rate constant for removal of MAC from the gas phase, kobs (s1), was calculated from S ð1Þ ln ¼ kobs L=vav S0 where L (cm) is the contact distance of MAC and the solution and vav (cm s1) is the average gas flow velocity of MAC. kobs was determined more accurately by placing the injector at various positions in the reactor to change the contact distance. The rate constant for removal of MAC, k (s1), can be determined by correcting kobs for diffusion2830
Figure 1. Uptake profiles of MAC into aqueous solutions of H2SO4 and H2O2 at 293 K: (a) reversible uptake into 40 wt % H2SO4 + 1 wt % H2O2 solution; (b) combined reversible and irreversible uptake into 60 wt % H2SO4 + 1 wt % H2O2 solution; (c) irreversible uptake into 80 wt % H2SO4 + 1 wt % H2O2 solution.
Figure 1b: MAC was found to be taken up and released at a later time, but, in addition, a constant offset in the signal was observed. This may be due to the occurrence of a slow reaction, making some of the dissolved MAC molecules transform to products irreversibly. As the H2SO4 concentration of the solution increased to 70 wt % or higher, the reaction in solution became so fast that the uptake displayed a totally irreversible feature and exhibited no saturation in Figure 1c. Uptake measurements of MAC into H2SO4 solutions were performed before, and only the reversible uptake was observed over the content of 080 wt %.26 Thus, the reaction contributing to irreversible uptake behavior was attributed to the acid-catalyzed oxidation by H2O2. Over the temperature range of 253293 K, the same uptake behavior was observed for the solution with the same composition. 3.2. Uptake Kinetics Parameter: Steady-State Uptake Coefficient. To estimate the uptake kinetics, the steady-state
1 1 1 ¼ k kobs kdiff
ð2Þ
3:66Di r2
ð3Þ
kdiff ¼
where r (cm) is the inner radius of the rotating cylinder, Di (cm2 s1) is the diffusion coefficient which can be calculated from the FullerSchettlerGidding method,31 and kdiff is the diffusion-limited rate (s1). Finally, γss can be acquired from γss ¼
4kV ωA
ð4Þ
where ω (m s1) is the mean molecular speed of MAC, V (cm3) is the volume of the reaction zone, and A (cm2) is the geometric area of the exposed solution. Figure 2 shows the loss of MAC as a function of injector position at 293 K. The variation of ln(S/S0) was found to decrease linearly versus the contact distance of MAC and the 439
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dissolution of MAC is fast in the RWWR, so the temperature dependence can be explained by discussing the adsorption/ desorption and reaction. For 60 wt % H2SO4 + 1 wt % H2O2 solution, the steady-state uptake is limited mainly by the slow reaction. Over the temperature range of 253293 K, γss increased slightly with decreasing temperature, which may be due to the temperature not being an important factor to the reaction. In concentrated solution, for instance in 70 or 80 wt % H2SO4 + 1 wt % H2O2 solution, the reaction became so fast that the steadystate uptake is limited basically by the adsorption. The MAC molecules stick to the surface of solutions more efficiently at lower temperature, leading to a bigger value of γss.32 3.3. Products and Mechanism. The products were observed in the gas phase by SPI-TOFMS during the uptake measurements into 6080 wt % H2SO4 + 1 wt % H2O2 solutions. Figure 3a shows the real-time mass spectrum after exposure of MAC to 70 wt % H2SO4 + 1 wt % H2O2 solution at 293 K, and Figure 3b is a plot of the temporal profiles of all ion peaks and background. As the exposure began, the intensity of MAC signal decreased while two new peaks appeared at m/z = 40 and 58. These two peaks dropped off after the exposure was terminated. This did not occur during the uptake of MAC into 6080 wt % H2SO4 solutions, so the new peaks should be attributed to the acid-catalyzed oxidation of MAC by H2O2. To investigate the products further, the gas-phase mixture was collected and analyzed by gas chromatography and FTIR spectrometer. Besides MAC, three peaks were observed in the gas chromatography experiment, which meant that three products were formed through this acid-catalyzed oxidation. The retention time of the first two products was the same as that of the reference propyne and acetone, respectively. Referring to the new peaks at m/z = 40 and 58 in Figure 3a, we believe that two of the products should be propyne and acetone. Figure 4 exhibits the IR spectra of gas-phase species before (a) and after (b) exposure to 70 wt % H2SO4 + 1 wt % H2O2 solution at 293 K. Obvious changes appear between 1600 and 1800 cm1 in Figure 4b, which should be caused by a CdO stretch from the products. Exhibited bands at 3345 and 3323 cm1 are attributed to an OH stretch vibration. Also evident are the bands at 1206 and 1160 cm1, which are believed to belong to COH stretch vibration. Thus, it is reasonable to speculate that 2,3-dihydroxymethacrylic acid was one of the products, which has been suggested by Claeys et al. in their previous study.11 The bands between 2383 and 2306 cm1 are a distinct feature of CO2. On the basis of the information supplied by SPI-TOFMS, gas chromatography, and FTIR spectrometer, the three products are suggested to be propyne, acetone, and 2,3-dihydroxymethacrylic acid. The last compound was not observed by SPI-TOFMS, which might be due to 2,3-dihydroxymethacrylic acid existing in aqueous phase more readily and having a very low concentration in the gas phase according to its high polarity and low volatility. However, 2,3-dihydroxymethacrylic acid was enriched during the products collection, so it can be observed in the FTIR experiments. The proposed chemical mechanism is shown in Figure 5. Both carbonyl and CdC double bond can be oxidized by H2O2 in acidic environment, transforming to carboxyl and epoxy groups. The epoxy group is protonated and then turns to either primary or ternary carbonium ions. The scission of the CC of the primary carbonium ion leads to the formation of either acetone through enol isomerization or propyne through dehydration, with CO2 and H2O as the byproduct. The primary carbonium
Figure 3. Real-time monitoring for uptake of MAC into 70 wt % H2SO4 + 1 wt % H2O2 solution at 293 K: (a) mass spectrum recorded after exposure; (b) temporal profiles of all ion peaks and background in a.
solution. Because all decays followed first-order kinetics, we obtained the decay rates kobs from the slopes using a linear regression method. The steady-state uptake coefficients calculated have been summarized in Table 1. The uptake of gas by solution is a complex interaction that can be divided into a series of processes, including gas-phase diffusion, adsorption/desorption at the surface, reaction at the surface, dissolution, liquid-phase diffusion, and reaction in the bulk liquid. The steady-state uptake coefficient was governed by some or all of these processes.32 In this research, the liquid-phase diffusion is not considered because the solution in the RWWR was mixed sufficiently and continually. Also the reaction at the surface seems to be equivalent to the reaction in the bulk liquid. In addition, the gas-phase diffusion is neglected due to the values of γss being corrected for that. So, in sum, the uptake of MAC is influenced mainly by three processes: adsorption/desorption, dissolution, and reaction. The values of γss in Table 1 were found to increase with increasing H2SO4 concentration in solution, which could be explained as follows. As the H2SO4 concentration increased, the adsorption was enhanced because of the increasing stickiness of the solution. Meanwhile, the reaction became much faster. Thus, the steady-state uptake coefficient increased drastically although the dissolution of MAC was proved to change slightly with the increasing H2SO4 concentration.26 The values of γss were also found to increase with decreasing temperature. This negative temperature dependence is pronounced when the H2SO4 concentration is 70 or 80 wt %. The 440
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Figure 4. IR spectra of gas-phase species before (a) and after (b) exposing to 70 wt % H2SO4 - 1 wt % H2O2 solution at 293 K.
Figure 5. Proposed chemical mechanism for the multiphase acid-catalyzed oxidation of MAC by H2O2.
ion can also transform to 2,3-dihydroxymethacrylic acid through hydration, which may be the only pathway for the ternary carbonium ion. The products were not quantified, but the 2,3-dihydroxymethacrylic acid was considered to be the main product because of the higher stability of the ternary carbonium ion than the primary one. This mechanism can provide an explanation for the high concentration of polyhydroxyl compounds in atmospheric fine particles. Meanwhile, the propyne and acetone produced can be emitted into ambient air and influence the atmosphere oxidation capacity through reactions with atmospheric oxidants, although the primary carbonium pathway seems to be the minor one. In the previous work,20 it was found that the multiphase acid-catalyzed oxidation of 2-methyl-3-buten-2-ol by H2O2 led to the formation of less stable primary and secondary carbonium, which makes the chemical mechanism and then the atmospheric implication more complicated. Thus, further studies with other VOCs will
be necessary to understand this multiphase acid-catalyzed oxidation process.
4. ATMOSPHERIC IMPLICATIONS MAC can be depleted irreversibly through the multiphase acid-catalyzed oxidation by H2O2, but in order to illustrate the atmospheric importance, the lifetime of this process needs to be estimated and compared with that of homogeneous oxidation by OH/O3/NO3. The aerosol is often neutralized by NH3 in the troposphere, but the higher aerosol acidity approach that is used in this paper has also been reported in the upper troposphere.1315 Hung and Wang reported that the value of the H2O2 concentration in the aerosol phase was up to 63 ng m3.33 Given the typical aerosol mass loadings at that site are in the range of 10200 μg m3 and the water generally comprises less than 50% of the aerosol mass loading, the concentration of H2O2 is a little lower than that 441
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used in this study. The steady-state uptake coefficient at 70 wt % acidity and 253 K is used to calculate the atmospheric lifetime by the following equation: τ¼
4 γss ωσ
(6) Jang, M. S.; Carroll, B.; Chandramouli, B.; Kamens, R. M. Environ. Sci. Technol. 2003, 37, 3828. (7) Limbeck, A.; Kulmala, M.; Puxbaum, H. Geophys. Res. Lett. 2003, 30, 1996. (8) Zhao, J.; Levitt, N. P.; Zhang, R. Y. Geophys. Res. Lett. 2005, 32, L09802. (9) Surratt, J. D.; Lewandowski, M.; Offenberg, J. H.; Jaoui, M.; Kleindienst, T. E.; Edney, E. O.; Seinfeld, J. H. Environ. Sci. Technol. 2007, 41, 5363. (10) Liggio, J.; Li, S. M. Atmos. Chem. Phys. 2008, 8, 2039. (11) Claeys, M.; Wang, W.; Ion, A. C.; Kourtchev, I.; Gelencser, A.; Maenhaut, W. Atmos. Environ. 2004, 38, 4093. (12) B€ oge, O.; Miao, Y.; Plewka, A.; Herrmann, H. Atmos. Environ. 2006, 40, 2501. (13) Cobourn, W. G.; Djukichusar, J.; Husar, R. B. J. Geophys. Res. 1980, 85, 4487. (14) Ferek, R. J.; Lazrus, A. L.; Haagenson, P. L.; Winchester, J. W. Environ. Sci. Technol. 1983, 17, 315. (15) Curtius, J.; Sierau, B.; Arnold, F.; de Reus, M.; Strom, J.; Scheeren, H. A.; Lelieveld, J. J. Geophys. Res. 2001, 106, 31975. (16) Hasson, A. S.; Paulson, S. E. J. Aerosol Sci. 2003, 34, 459. (17) Anastasio, C.; Faust, B. C.; Allen, J. M. J. Geophys. Res. 1994, 99, 8231. (18) Valverde-Canossa, J.; Wieprecht, W.; Acker, K.; Moortgat, G. K. Atmos. Environ. 2005, 39, 4279. (19) Chen, Z. M.; Wang, H. L.; Zhu, L. H.; Wang, C. X.; Jie, C. Y.; Hua, W. Atmos. Chem. Phys. 2008, 8, 2255. (20) Liu, Z.; Ge, M. F.; Wang, W. G.; Yin, S.; Tong, S. R. Phys. Chem. Chem. Phys. 2011, 13, 2069. (21) Zimmermann, J.; Poppe, D. Atmos. Environ. 1996, 30, 1255. (22) Lee, W.; Baasandorj, M.; Stevens, P. S.; Hites, R. A. Environ. Sci. Technol. 2005, 39, 1030. (23) Biesenthal, T. A.; Shepson, P. B. Geophys. Res. Lett. 1997, 24, 1375. (24) Guenther, A.; Karl, T.; Harley, P.; Wiedinmyer, C.; Palmer, P. I.; Geron, C. Atmos. Chem. Phys. 2006, 6, 3181. (25) Liu, Z.; Ge, M. F.; Yin, S.; Wang, W. G. Chem. Phys. Lett. 2010, 491, 146. (26) Noziere, B.; Voisin, D.; Longfellow, C. A.; Friedli, H.; Henry, B. E.; Hanson, D. R. J. Phys. Chem. A 2006, 110, 2387. (27) Howard, C. J. J. Phys. Chem. 1979, 83, 3. (28) Murphy, D. M.; Fahey, D. W. Anal. Chem. 1987, 59, 2753. (29) Hanson, D. R.; Burkholder, J. B.; Howard, C. J.; Ravishankara, A. R. J. Phys. Chem. 1992, 96, 4979. (30) Gershenzon, Y. M.; Grigorieva, V. M; Ivanov, A. V.; Remorov, R. G. Faraday Discuss. 1995, 100, 83. (31) Fuller, E. N.; Schettler, P. D.; Giddings, J. C. Ind. Eng. Chem. 1966, 58, 19. (32) Davidovits, P.; Kolb, C. E.; Williams, L. R.; Jayne, J. T.; Worsnop, D. R. Chem. Rev. 2006, 106, 1323. (33) Hung, H. F.; Wang, C. S. J. Aerosol Sci. 2001, 32, 1201. (34) Godin, S.; Poole, L. R. Scientific Assessment of Ozone Depletion. Global Ozone Research and Monitoring Project, Report No. 44; World Meteorological Organization: Geneva, Switzerland, 1998. (35) Kwok, R. S. C.; Aschmann, S. M.; Arey, J.; Atkinson, R. Int. J. Chem. Kinet. 1996, 28, 925. (36) Canosa-Mas, C. E.; Cotter, E. S. N.; Duffy, J.; Thompson, K. C.; Wayne, R. P. Phys. Chem. Chem. Phys. 2001, 3, 3075.
ð5Þ
where ω is the mean molecular speed of MAC and σ is the area density of atmospheric sulfate aerosol (about 2 107 cm2/cm3 34). This calculated value is 16.5 days, which is comparable to the lifetime of homogeneous oxidation by O3 (∼ 15 days), but less than that by OH (∼2 days) and NO3 (∼5 days).35,36 In addition, the temperature in the upper troposphere is lower than 253 K, which may lead to a large steady-state uptake coefficient and a shorter lifetime. Therefore, this process can play a role in the upper troposphere and should be considered in SOA formation.
5. CONCLUSIONS The uptake measurements of MAC into aqueous solutions of H2SO4 and H2O2 were carried out to investigate in detail the kinetics and mechanism of this multiphase acid-catalyzed oxidation, which was pointed out to be a potential route to SOA formation. As H2SO4 concentration increased, the uptake transformed from reversible to irreversible. The steady-state uptake coefficient increased with increasing H2SO4 concentration and decreasing temperature. Propyne, acetone, and 2,3-dihydroxymethacrylic acid were suggested as the products. The chemical mechanism was proposed to be the oxidation of carbonyl and CdC double bond, which could explain the high concentration of polyhydroxyl compounds in atmospheric fine particles. These results demonstrate that the multiphase acid-catalyzed oxidation of MAC by H2O2 can play a role in SOA formation in the upper troposphere. ’ AUTHOR INFORMATION Corresponding Author
*Tel.: 86-10-62554518 (M.-F.G.); 86-10-62558682 (W.-G.W.). Fax: 86-10-62559373 (M.-F.G.); 86-10-62559373 (W.-G.W.). E-mail:
[email protected] (M.-F.G.);
[email protected] (W.-G.W.).
’ ACKNOWLEDGMENT This project was supported by Knowledge Innovation Program (Grant Nos. KJCX2-EW-H01, KJCX2-YW-N24, and KZCX2-YW-Q02-03) of the Chinese Academy of Sciences, the National Basic Research Program of China (973 Program, No. 2011CB403401) of Ministry of Science and Technology of China, and the National Natural Science Foundation of China (Contract Nos. 40925016, 40830101, 21077109, and 41005070). ’ REFERENCES (1) P€oschl, U. Angew. Chem., Int. Ed. 2005, 44, 7520. (2) Rosenfeld, D. Science 2006, 312, 1323. (3) Hallquist, M.; Wenger, J. C.; Baltensperger, U.; Rudich, Y.; Simpson, D.; Claeys, M.; Dommen, J.; Donahue, N. M.; George, C.; Goldstein, A. H.; et al. Atmos. Chem. Phys. 2009, 9, 5155. (4) Seinfeld, J. H.; Pandis, S. N. Atmospheric Chemistry and Physics— From Air Pollution to Climate Change; John Wiley & Sons: Hoboken, NJ, USA, 1997. (5) Jang, M. S.; Czoschke, N. M.; Lee, S.; Kamens, R. M. Science 2002, 298, 814. 442
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