Article pubs.acs.org/JPCA
Influence of Temperature on the Heterogeneous Reaction of Formic Acid on α‑Al2O3 Ling-Yan Wu,†,‡ Sheng-Rui Tong,*,† Si-Qi Hou,† and Mao-Fa Ge*,† †
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 S Supporting Information *
ABSTRACT: Despite increased awareness of the role played by heterogeneous reactions of formic acid on mineral aerosol, the experimental determination of how these atmospheric reaction rates vary with temperature remain a crucially important part of atmosphere science. Here we report the first measurement of heterogeneous uptake of formic acid on α-Al2O3 as a function of temperature (T = 240−298 K) at ambient pressure using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). From the analysis of the spectral features, crystalline HCOOH was identified at low temperature besides common product (formate ions) on the surface. It was also interesting to find that crystalline HCOOH can continue to react with α-Al2O3. The reaction mechanisms at both room and low temperature were discussed. Furthermore, the reactive uptake coefficients were acquired and found to increase with decreasing temperature. Finally, the atmospheric lifetime of formic acid because of heterogeneous loss on mineral aerosol was estimated at temperatures related to the upper troposphere. pogenic, and biogenic emissions.1 But their sinks are still poorly documented for global or even regional scale. It is reported that formic acid is not significantly removed from the atmosphere by gas-phase reactions because its main loss reaction with OH radical is slow.1 A principal sink of formic acid in the troposphere is via dry or wet deposition,9 which accounts for the removal of 95% and 91% of formic and acetic acid, respectively.10 Meanwhile, formic acid in the atmosphere also showed a correlation with mineral aerosol in field studies.11,12 Additional measurements indicated that formic and acetic acids were the most common monocarboxylic acids found in the mineral aerosol samples.13 Therefore, the heterogeneous chemistry of formic acid with the components of mineral aerosol deserves more study since it can provide a useful proxy to investigate the fate and transport of formic acid as well as how organic acids contribute to the overall organic composition in atmospheric aerosol. The loss pathway involving dry deposition of formic acid to mineral aerosol and surfaces has been gradually simulated in laboratory studies. Previous studies probing the uptake of formic acid at 295 K on mineral aerosol such as Fe2O3, CaCO3, and Al2O3 have been reported.14−17 Grosjean et al18 found that OH and silanol groups are always the most reactive formic acid adsorption sites. These laboratory studies have shown that
1. INTRODUCTION Formic acid (HCOOH) is one of the most abundant organic acids in the atmosphere.1,2 In some regions, the mixing ratio of formic acid can exceed those of HNO3 and HCl.3 Reported concentrations of formic acid are 0.05−6 part per billion (ppb)1,3 and can be as high as 20 ppb.4 Because formic acid has low molecular weight and high polarity, it may be involved in potentially important atmospheric transformations.1 It is now well established that atmospheric HCOOH influences pHdependent chemical reactions in clouds and contributes to the acidity of rain.5 It was estimated that formic and acetic acids may contribute between 16% and 35% of the free acidity in rainwater in the U.S.A.5 while sulfuric and nitric acids contributed only about 10−20% of the hydrogen ion concentration.6 Formic acid is also believed to be an important sink for OH radical in cloudwater, and, as such, it influences oxidation of other important atmospheric species such as SO2.7 However, its lifetime in the troposphere is difficult to reconcile in current atmospheric chemistry models. In some atmospheric modeling studies, formic acid was expected to have long residence times of the order of a few weeks in the troposphere,7,8 whereas a number of other studies predicted that the lifetime of formic acid should be of the order of several hours to a few days (at most) based on daily cycling of its vapor-phase concentrations.1 Thus, it is important to understand the sources and sinks of formic acid. The sources of carboxylic acids are now well recognized to be comprised of chemical transformations of precursors, anthro© 2012 American Chemical Society
Received: July 25, 2012 Revised: September 29, 2012 Published: October 1, 2012 10390
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2.2. Experimental Methods. Diffuse reflectance infrared Fourier transform spectroscopy was used to probe the condensed phase during the heterogeneous reactions. The experimental setup has been described in detail elsewhere,16,25 thus only relevant features will be described here. The sample compartment of the FTIR spectrometer (Thermo Nicolet 6700) houses a low-temperature reaction chamber (Harrick Scientific, CHC−CHA-3) equipped with a DRIFTS optic (Harrick Scientific, DRP). This reaction chamber is designed for operation from −150 to 600 °C under vacuum and the temperature was controlled by a thermocouple mounted directly underneath. The reaction chamber was part of a flow system, where the gases under study were added to a carrier gas stream of synthetic air (20% O2 and 80% N2). The concentration of the reactive gas inside the reaction cell could be calculated with the known mixing ratio and flow rate of the gas/synthetic air mixture. During the exposure of the alumina sample to HCOOH vibrational spectra were recorded in the spectral range from 4000 to 650 cm−1 with a resolution of 4 cm−1. To improve the signal-to-noise ratio 100 scans were co-added for each spectrum resulting in a time resolution of 1 min. Prior to an experiment, the Teflon tube and the chamber were washed at least three times and then evacuated under high vacuum overnight to make sure that the whole gas supply system and the chamber were clean. α-Al2O3 powder (about 60 mg) was gently packed by tamping the powders down once in the stainless steel cup (10 mm diameter, 0.5 mm depth) and leveling it with a plastic blade. The cup was then put into the DRIFTS reaction chamber and in situ pretreated by heating in synthetic air (20% O2 + 80% N2) at 573 K for 3 h. When the temperature of the sample was cooled to the desired one, a spectrum of the α-Al2O3 powder was recorded and used as the background for each subsequent spectrum. Gaseous formic acid was then introduced as a continuous flow over the α-Al2O3 powder simultaneously with the onset of data collection. During the whole experimental procedure, the temperature uncertainty was ±1 K. To prevent the ZnSe windows from frosting, a circulating water jacket was used to maintain the temperature of the outer surface of the chamber and windows at room temperature during low temperature operation. The absolute number of formate ions formed during the reaction was determined by ion chromatography (IC). The reacted alumina particles were sonicated in 1.5 mL of ultrapure water for 20 min. The filtered solution was analyzed with use of a Dionex ICS 900 system, which was equipped with a Dionex AS 14A analytical column and a conductivity detector (DS5).
heterogeneous chemistry involving some minerals representative of mineral aerosol may be a significant sink for atmospheric formic acid. And the calculated lifetimes of formic acid are shorter than that for removal by reaction with OH radical.16 In addition, relative humidity can play an important role in the heterogeneous reaction of formic acid.16,17 However, previous studies have not yet quantified the heterogeneous uptake of organic acids on mineral aerosol under typical seasonal temperature. It is suggested that the experimental determination of rate for important atmospheric reactions and how these rate constants vary with temperature remain a crucially important part of atmospheric science.19 Hatch et al.20 have studied the heterogeneous uptake of the C1 to C4 organic acids on a swelling clay mineral under typical upper tropospheric temperatures. Moreover, the rate change in the reaction between HCOOH and hydroxyl radical at cold temperature results in a dramatic decrease of formic acid in the upper troposphere.21 However, the experiments were done at low pressure and they did not investigate how the uptake efficiency varied with temperature. Therefore, the uptake of formic acid on α-Al2O3 particles was investigated at temperatures from 240 to 298 K, 1 atm synthetic air, using a diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) reactor in the present study. α-Al2O3 is an important reactive constituent of mineral aerosol and widely used as a model oxide for the study of trace gases in heterogeneous reactions.22−24 A series of reactive uptake coefficients for the heterogeneous reaction of formic acid on α-Al2O3 particles at different temperatures was obtained. The reactive uptake coefficients at different temperatures will supply the basic data for atmospheric chemistry modeling studies. Finally, the importance and implications of the above kinetic parameters in the heterogeneous chemistry of formic acid in the atmosphere are discussed. The results are helpful for the further understanding of the seasonal variation of secondary organic aerosols as well as the lifetime of formic acid in the troposphere.
2. EXPERIMENTAL SECTION 2.1. Sources of Chemicals. α-Al 2 O 3 powder was purchased from Alfa Aesar (with a stated purity of 99.99%). The specific surface area of the α-Al2O3 powder was determined according to the Brunauer−Emmett−Teller (BET) method from nitrogen adsorption−desorption isotherms which were obtained at 77 K. The surface area of the particles is measured to be 9.11 ± 0.13 m2 g−1 (Autosorb-IQ automatic equipment (Quanta Chrome Instrument Co.)). Gaseous formic acid (1.23 ± 0.02 × 1014 molecules cm−3) used in this study was obtained by diluting formic acid (>97%, Alfa Aesar, used as received) by N2 (>99.999%, Beijing Tailong Electronics Co., Ltd.) in a glass bottle and the partial pressures were monitored by absolute pressure transducer (MKS 627B range 0 to 1000 Torr). Mass flow controllers (Beijing Sevenstar electronics Co., LTD) were used to adjust the flux of diluted formic acid and N2 to an expected concentration. O 2 (>99.999%, Orient Center Gas Science & Technology Co., Ltd.) was used to simulate the ambient air. N2 and O2 were dehumidified by silica gel and molecular sieve before flowing into the system, and the RH was less than 1%, which was called dry condition. Ultrapure water with resistivity of 18.2 MΩ·cm was purified by the Thermo Scientific Barnstead Easypure II systems (Model UF).
3. RESULTS AND DISCUSSION To explore the role of temperature in the heterogeneous reaction of HCOOH, the following aspects must be investigated. First, the products formed in the reaction at different temperatures must be identified. Second, how do the kinetics parameters, for example, uptake coefficients, vary with temperature. 3.1. Temperature Effect on the Surface Species. The interaction of HCOOH (1.23 ± 0.02 × 1014 molecules cm−3) with α-Al2O3 particles using the surface sensitive DRIFTS method has been studied as a function of temperature (240 to 298 K) at ambient pressure. In all uptake experiments the DRIFTS spectrum of the unreacted mineral aerosol sample has been used as a background spectrum. Therefore, reaction products formed during the uptake can be observed as positive 10391
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involved the reactions of HCOOH.32−35 From a comparison of experimental results, Chapman et al.33 pointed out that there was, in general, a correlation between a frequency of the free molecule in the gas and a set of crystalline frequencies. It was reported that the gas HCOOH frequency was at 1773 cm−1.33 However, the CO bond length is not the same in the solid and the gas state, the mean frequency of crystalline HCOOH would shift to low frequency, about 1720 cm−1.33 This is close to our value, 1706 cm−1. Moreover, because the coupling between neighboring molecules in crystalline HCOOH is principally through hydrogen bonds,33,34 it was expected that the largest shifts in the frequencies were associated with the O−H hydrogen bonds. Using single-crystal X-ray diffraction techniques, the molecules were found to be arranged in the form of infinite chains in the crystal and each molecule was linked to two neighbors by hydrogen bonds.36,37 With the presence of hydrogen bonds, the physisorbed formic acid molecules were allowed to aggregate into more densely packed crystalline form.38 It is evident that two strong bands at 2921 and 2562 cm−1 were observed at temperatures below room temperature. These results are in excellent agreement with the IR spectrum of crystalline formic acid as well as the previous IR studies of crystalline formic acid physisorbed on metals.33−35,38,39 Thus, it is reasonable to deduce that crystalline HCOOH was formed at temperatures below 277 K. 3.1.3. The Surface Species at Low Temperature. To further analyze the surface species as a function of temperature, a Gaussian curve-fitting procedure was used to deconvolute overlapping bands from 1800 to 1450 cm−1.30,40 The fitting was undertaken until reproducible results were obtained with the coefficient of determination (R2) greater than 0.995. The integrals of the peak at 1706 cm−1 as a function of temperature are plotted in Figure 2. It can be found that the concentration
absorption bands whereas negative bands indicate the loss of the corresponding species. Due to lattice vibrations of the solid samples, the sensitivity was significantly lower in the spectral region about 1100 cm−1. Therefore, the spectral range extending from 1200 to 3900 cm−1 was selected for all the spectra below. 3.1.1. Common Absorbance Peaks. Figure 1 shows the spectra of reaction products after α-Al2O3 exposure to formic
Figure 1. In situ DRIFTS spectra (absorbance units) of surface products after α-Al2O3 exposure to formic acid for 200 min at different temperatures. Spectra have been offset for clarity.
acid (1.23 ± 0.02 × 1014 molecules cm−3) for 200 min at different temperatures under dry conditions (RH < 1%). Many common absorbance peaks, such as 1378, 1393, 2750, 2867, and 3726 cm−1, were observed at all experimental temperatures. These common absorbance peaks are assigned with the aid of previous infrared studies that involved the reactions of formic acid. Assignment of the vibrational frequencies is shown in Table 1S (Supporting Information). The band at 1378 cm−1 and the shoulder at 1393 cm−1 are associated with C−H inplane bend and symmetric stretching mode (νs(OCO)), respectively, in agreement with the assignment on other Al2O3 surfaces.16,26,27 The band at 2867 cm−1 is expected for the C−H strenching vibration, while the peak at 2750 cm−1 is due to the overtone of OCO symmetric stretching (1393 cm−1) and C−H in-plane bend (1378 cm−1).26−29 The broad band at about 1590 cm−1, which is assigned to the OCO antisymmetric stretching mode (νas(OCO)),15,16 indicates that formate anions were formed on the surface at temperatures from 298 to 240 K. Accompanied by the above peaks, a negative band at about 3726 cm−1 is observed. And it was ascribed to the loss of OH surface species in several publications.30,31 The decrease in intensity of the band at 3726 cm−1 and a concomitant growth of a band at lower frequency at 1590 cm−1 suggest that the basic OH group was reactive and removed or exchanged upon formation of formate. Therefore, the hydroxyl groups were consumed and formate ions were formed in the heterogeneous reaction of HCOOH on α-Al2O3 at either room temperature or lower temperature. 3.1.2. The Formation of Crystalline HCOOH. However, it is surprising to find that some new peaks at 1706, 2562, and 2921 cm−1 began to appear when the temperature is below 277 K, as shown in Figure 1. The assignments of these absorbance peaks come from the examination of spectra in the literature that
Figure 2. The integrated absorbance for the crystalline HCOOH stretching region from 1780 to 1685 cm−1 as a function of temperature.
of crystalline HCOOH on the surface of α-Al2O3 increases with decreasing temperature. However, the concentration of crystalline HCOOH increases quickly until the temperature is below 277 K. This may be due to more physisorbed formic acid molecules being adsorbed on the surface of alumina at lower temperature and then crystalline HCOOH was formed more easily. 10392
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With the purpose of probing the details of the heterogeneous reaction of HCOOH ([HCOOH] = 1.23 ± 0.02 × 1014 molecules cm−3) at low temperature, in situ DRIFTS spectra of HCOOH on α-Al2O3 surface at 240 K were collected, as shown in Figure 3. Upon adsorption of HCOOH on the
Figure 4. IR spectra of the reaction products after the HCOOH flow was cut off and the particles were purged by synthetic air at 240 K for 60 min.
Figure 3. Absorption spectra recorded during the reaction of HCOOH ([HCOOH] = 1.23 ± 0.02 × 1014 molecules cm−3) on αAl2O3 particles at 240 K. The inset shows the temporal evolution of the integrated absorbance of the solid formic acid absorption band (1706 cm−1) and the absorption band (2562 cm−1).
surface of α-Al2O3, several prominent bands are seen to grow. Bands at 1378, 1590, 1706, 2562, 2750, 2867, and 2921 cm−1 are shown to increase in intensity as time goes on. From what has been discussed above, we have drawn the conclusion that formate ions and crystalline HCOOH were formed at 240 K. The inset in Figure 3 shows the temporal evolution of the integrated absorbance of 1706 and 2562 cm−1 for the crystalline formic acid. For better comparison of these two peaks, the integrated absorbance band at 2562 cm−1 was multiplied by a factor of 7.5. Generally, these two peaks grew at similar rates and showed similar trend changes with reaction time. This behavior indicates that these two peaks were associated with the same product and the process of the deconvolution was reliable. As shown in this figure, the intensity of these two peaks did not grow until the reaction had proceeded for about 20 min. However, the intensity of the peak at 1590 cm−1, which will be discussed in detail in section 3.2, increased as soon as the exposure to HCOOH. Thus there may be some physicalchemical processes that influence the reaction in a different way as a function of temperature. After the heterogeneous reaction for 200 min, the HCOOH flow was cut off and the α-Al2O3 particles were purged by synthetic air at 240 K for 60 min. The intensity of the peak at 1590 cm−1 increases while the ones assigned to the crystalline HCOOH decreased but still existed, as shown in Figure 4. To study whether the crystalline HCOOH would continue to react with α-Al2O3 particles or not, a hermetic reaction was carried out at 240 K. In this reaction, the gas inlet and outlet of the reaction chamber were closed after the heterogeneous reaction of HCOOH with α-Al2O3 for 2 h at 240 K. During the whole process, the sample temperature was kept unchanged at 240 K. It can be seen in Figure 5 that characteristic absorption peaks of crystalline HCOOH are not visible anymore after 174 min of hermetic reaction. Meanwhile, the broad band at about 1590 cm−1, which corresponds to formate, increases as the
Figure 5. IR spectra of the species studied in the hermetic reaction.
closed time grows. Simultaneously, the intensity of the peak at 3726 cm−1 decreases, suggesting a loss of OH on the surface. This suggests that the crystalline HCOOH can continue to react with α-Al2O3 particles to form formate ions. To our knowledge, it is the first time to find a solid−solid phase homogeneous reaction between crystalline HCOOH and αAl2O3 particle rather than only a gas−solid heterogeneous reaction. 3.2. Temperature Effect on the Kinetics Parameters. The effect of temperature (240 to 298 K) on the parameters of kinetics of the heterogeneous reaction of HCOOH (1.23 ± 0.02 × 1014 molecules cm−3) at ambient pressure has been studied. At the beginning of this study, an experiment was carried out to investigate whether the temperature would affect the intensity of the IR signal or not. Therefore, the integrated absorbance of formate with the same concentration (10 mg/g) was investigated as a function of temperature (as shown in Figure 6). The slope, which was close to zero, indicated that the IR signal intensity of formate was approximately independent of temperature in the temperature range used in our study. The following experiments were carried out on the basis of this conclusion. The kinetics of the heterogeneous reaction of HCOOH on the surface of α-Al2O3 particles were followed by using the 10393
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(integrated absorbance) × f = {HCOO−}
(1)
where f is the conversion factor, and the calculated value for formate is 2.46× 1017 ions g−1 ABU−1. Ordinarily, one may expect the reaction rate to decrease with decreasing temperature because the rate constant for the reaction decreases with decreasing temperature. However, it is found in Figure 8 that the rate of formate formation increases
Figure 6. The integrated absorbance of formate (1455−1690 cm−1) with the same concentration (10 mg/g) as a function of temperature. The error bars represent one standard deviation from triplicate experiments.
integrated absorbance−reaction time behavior instead of the Kubelka−Munk method which is known to give rise to unacceptable uncertainty levels in quantitative experiments.41,42 The integrated absorbance of formate at 1590 cm−1 for a series of temperatures as a function of reaction time is shown in Figure 7. As soon as the reactive gas was introduced into the
Figure 8. The relationship between formation rate of formate and temperature. Error bars represent one standard deviation from triplicate experiments.
linearly with decreasing temperature in the range of 240 to 298 K. The correlation coefficient was 0.997. The adsorption of formic acid was found to be promoted on several different substrates such as ice,37,43−45 ammonium nitrate,46 and Pt35 with decreasing temperature. Thus, the lower temperature leads to a higher amount of adsorbed-phase HCOOH, which may make the heterogeneous reaction of HCOOH on α-Al2O3 easier. A parameter often used in atmospheric chemistry to describe the overall kinetics of a heterogeneous reaction is the reaction probability, or reactive uptake coefficient, γ, which is defined as the number of reactive collisions with the surface (d{HCOO−}/dt) divided by the total number of surface collisions Z. γ = (d{HCOO−}/dt )/Z
(2)
The number of reactive collisions can be derived from the reaction rate for the formation of formate. And the total number of collisions can be calculated according to the kinetic gas theory:41
Figure 7. The temporal evolution of the integrated absorbance of the formate absorption band (1590 cm−1) for a series of temperatures.
Z=
particle layers, the IR bands increased and the integrated absorbance increase approximates linearly with time. In addition, the integrated absorbance increased faster with decreasing temperature. To quantify the rate of formate formation d{HCOO−}/dt, the amount of formate ions formed during the reaction on the particulate sample was determined by ion chromatography. The rate of formate formation was translated from absorption units s−1 to HCOO− s−1 by a conversion factor obtained from a calibration plot. The plot shows that over a large concentration range the DRIFTS signal is proportional to the formate concentration. And the plot gives a conversion factor that enables calculation of the amount of formate formed after the reaction. The formate ion concentration is
1 × A s × [HCOOH] × 4
8RT πMHCOOH
(3)
where As is the effective sample surface, [HCOOH] is the concentration of gas-phase HCOOH, M is the molecular mass of the gas-phase HCOOH, and T is the temperature. The reactive uptake coefficients at temperatures from 240 to 298 K are listed in Table 1. The reactive uptake coefficient at room temperature (2.35 ± 0.03 × 10−3) approximates our previous reported value (2.07 ± 0.26 × 10−3) at 298 K.16 To our knowledge, however, this is the first study reporting the reactive uptake coefficients of formic acid on mineral aerosol at low temperature. The reactive uptake coefficients at these different temperatures can be used directly in atmospheric chemistry modeling studies to predict the formation of 10394
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4. CONCLUSIONS AND ATMOSPHERIC IMPLICATIONS The heterogeneous reaction of HCOOH on α-Al2O3 at various temperatures (240−298 K) was determined experimentally and analyzed by using the DRIFTS technique. Since the kinetics and mechanisms were determined at room temperature previously,16 we focused our attention on the questions of whether the products depend on the temperature and how the kinetics parameters vary with the temperature in the range from 240 to 298 K. Our results show that the temperature plays an important role in this heterogeneous reaction not only by influencing the reactive uptake coefficient but also by changing the reaction mechanism. The heterogeneous reaction of HCOOH with α-Al2O3 at room and low temperature produced common formate ions. However, as temperature decreases, more HCOOH molecules started to condense on the surface and the reaction between crystalline HCOOH and α-Al2O3 began. Therefore, with the increase of physisorbed HCOOH, the reactive uptake coefficient increased with decreasing temperature. The reactive uptake coefficients at various temperatures (240−298 K) were used to estimate the lifetime, τ, for removal of HCOOH by mineral aerosol according to the following simple model:
Table 1. Summary of the Reactive Uptake Coefficients Obtained for the Uptake of Formic Acid on α-Al2O3 Particles at Temperatures from 298 to 240 K, Using DRIFTS Measurements temp (K) 298 285 277 273 263 250 240
γ(HCOOH) BET 1.34 1.52 1.70 1.77 1.96 2.22 2.39
± ± ± ± ± ± ±
0.02 0.04 0.02 0.01 0.02 0.02 0.03
× × × × × × ×
10−6 10−6 10−6 10−6 10−6 10−6 10−6
γ(HCOOH) geometric 2.35 2.65 2.98 3.09 3.43 3.89 4.18
± ± ± ± ± ± ±
0.03 0.07 0.03 0.03 0.04 0.03 0.05
× × × × × × ×
10−3 10−3 10−3 10−3 10−3 10−3 10−3
secondary formate aerosol in the troposphere. As shown in Table 1, the reactive uptake coefficients increase with decreasing temperature from 298 to 240 K. The uptake coefficient at 240 K is about twice as large as the one at room temperature. Thus, the temperature should be considered as an important factor in the heterogeneous reaction of HCOOH. 3.3. Mechanism. It can be concluded from the above results that the temperature has an important influence on the heterogeneous reaction mechanism of HCOOH with α-Al2O3. The reaction mechanism is different at room temperature and low temperature. We discuss the reaction mechanisms for both of these two conditions below. At room temperature, two major steps are proposed for the heterogeneous reaction of formic acid with α-Al2O3. And this mechanism was proved by our previous study.16 The first proposed step is the rapid adsorption of gas-phase HCOOH onto the surface of αAl2O3. The reaction is expressed in reaction R1, where (g) and (ads) symbols denote gas phase and adsorbed phase, respectively. HCOOH(g) ⇔ HCOOH(ads)
τ=
(R1)
(R2)
In contrast, the adsorption of HCOOH would be promoted with decreasing temperature. When the number of surface reactive sites was large compared to the number of formate ions formed, adsorbed-phase HCOOH went on reacting with the surface of α-Al2O3 and more formate ions formed. In this case, more and more surface reactive sites were taken up by formate ions, and the number of surface reactive sites that can provide for gas-phase HCOOH became smaller. Hence, more gas-phase HCOOH molecules adsorbed to the surface with the presence of hydrogen bonds and aggregated into more densely packed crystalline form. The reaction can be expressed in reaction R3, where (s) denotes crystalline. HCOOH(ads) + HCOOH(g) → HCOOH(s)
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ASSOCIATED CONTENT
* Supporting Information S
Assignment of the vibrational frequencies for surface species following exposure of α-Al2O3 particles to formic acid. This material is available free of charge via the Internet at http:// pubs.acs.org.
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(R3)
AUTHOR INFORMATION
Corresponding Author
Finally, the crystalline HCOOH reacted with the hydroxyl groups to form more formate ions. The reaction can be expressed as HCOOH(s) + Al‐OH → HCOO‐Al + H 2O
(4)
where γ is the reactive uptake coefficient, ω is the mean molecular speed, and A is the dust surface area density ranging between 6 × 10−7 and 1.8 × 10−5 cm2 cm−3. Therefore, the atmospheric lifetime of formic acid can be determined as a function of temperature in the range 240−298 K by using the γ values determined in the present work. Two extreme mineral aerosol densities were used to make the estimate. For low A value (4.58 × 10−7 cm2 cm−3), the lifetimes are in the range 17−28 h. However, for high A value (1.37 × 10−5 cm2 cm−3), the lifetimes are in the range 34−56 min. For comparison, the lifetime of HCOOH with respect to the reaction with the OH radical is more than 1 week at room temperature. Clearly, the heterogeneous removal of HCOOH will dominate over gasphase reactions, even at low temperature. At low temperature, the lifetime of HCOOH would be even 2-fold shorter than the one at room temperature. In addition, the temperature in the upper troposphere is lower than 240 K, which may lead to a larger reactive uptake coefficient and a shorter lifetime. Therefore, this process can play a role in the upper troposphere and should be considered in the seasonal variation of secondary organic aerosol formation.
At room temperature, the adsorbed-phase HCOOH would react immediately with hydroxyl groups on the surface of αAl2O3 to form formate ions. The reaction of adsorbed-phase HCOOH with α-Al2O3 can be expressed as HCOOH(ads) + Al‐OH → HCOO‐Al + H 2O
4 γAω
*Tel.: 86-10-62554518 (M.-F.G.); 86-10-62558682 (S.-R.T.). Fax: 86-10-62559373 (M.-F.G.); 86-10-62559373 (S.-R.T.). Email:
[email protected] (M.-F.G.);
[email protected] (S.R.T.).
(R4) 10395
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Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This project was supported by the National Natural Science Foundation of China (Contract Nos. 41105085, 40925016, 40830101, 21077109, and 41005070), Knowledge Innovation Program (Grant No. KJCX2-EW-H01) of the Chinese Academy of Sciences, and the National Basic Research Program of China (973 Program, No. 2011CB933700) of the Ministry of Science and Technology of China.
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