Characterization of Sulfurous Acid, Sulfite, and Bisulfite Aerosol

Article pubs.acs.org/JPCA

Characterization of Sulfurous Acid, Sulfite, and Bisulfite Aerosol Systems Thomas M. Townsend,*,† Arnaud Allanic, Colette Noonan, and John R. Sodeau* Centre for Research into Atmospheric Chemistry, Department of Chemistry, University College Cork, and Environmental Research Institute, Cork, Ireland ABSTRACT: Acidic tropospheric aerosols contain inorganic species such as sulfurous acid (H2SO3). As the main alkaline species, ammonia (NH3) plays an important role in the heterogeneous neutralization of these acidic aerosols. An aerosol flow-tube apparatus was used to obtain simultaneous optical and size distribution measurements using FTIR and SMPS measurements, respectively, as a function of relative humidity and aerosol chemical composition. A novel chemiluminescence apparatus was also used to measure ammonium ion concentration [NH4+]. The interactions between ammonia and hydrated sulfur dioxide (SO2·H2O) were studied at different humidities and concentrations. SO2·H2O is an important species as it represents the first intermediate in the overall atmospheric oxidation process of sulfur dioxide to sulfuric acid (H2SO4). This complex was produced within gaseous, aqueous, and aerosol SO2 systems. The addition of ammonia gave mainly hydrogen sulfite (SHO3−) tautomers and disulfite ions (S2O52−). These species were prevalent at high humidities enhancing the aqueous nature of sulfur(IV) species. Their weak acidity is evident due to the low [NH4+] produced. Size distributions obtained correlated well with the various stages of particulate compositional development.

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H2SO3 in aqueous solution.5,6 Furthermore, previous studies have established that sulfurous acid cannot exist on Earth due to the high temperature and abundance of water here.7

INTRODUCTION “Sulfurous acid” (H2SO3) has never been characterized or isolated on Earth. This finding is related to the unfavorable conditions that exist for the hydration product of SO2 (SO2·H2O) within our atmosphere.1 Hydrated sulfur dioxide is, nonetheless, an important chemical species as it represents the primary intermediate in the atmospheric oxidation of sulfur dioxide to sulfuric acid. A brief description of the related speciation and equilibria of the aqueous S(IV) system is presented below as a foundation to understanding the experimental work performed in the current experimental spectroscopic study on aqueous aerosols because many contradictions exist in the published literature. The availability of sulfur 3d orbitals promotes covalency in the sulfur−oxygen bonds, and many sulfur(IV) species can form in acidic aqueous solutions, despite the expected simplicity of the SO2/H2O binary system. The identification of these species and their inter-relationships has proved difficult to resolve to date. Sulfur dioxide gas has a Henry’s law constant of 1.22 M atm−1 and can dissolve in water to form the hydrated sulfur dioxide complex, eq 1.2 While the formation of a SO2·H2O complex can, in principle, occur in the gas-phase as well, the binding energy is low (∼1.8−3.5 kcal mol−1);34 hence, this complex is not important in the atmosphere. The hydrated sulfur dioxide complex is the most stable associated S(IV) species in water and has been shown to be more thermodynamically stable than sulfurous acid. Indeed, no experimental evidence has been obtained for the existence of © 2012 American Chemical Society

SO2 + H 2O ⇌ SO2 ·H 2O

(1)

Equation 2 shows the dissociation of hydrated sufur dioxide to initially produce the bisulfite ion (HOSO2−). SO2 ·H 2O ⇌ OHSO2− + H+ −2

K a = 1.54 × 10

(2)

−1

L mol ; pK a = 1.81

It is clear from the chemical formula that the hydrogen atom in the bisulfite ion is bonded to one of the oxygen atoms but can quickly tautomerize to form the sulfonate ion (HSO3), where the hydrogen atom is bonded to the sulfur atom (eq 3). HSO3− ↔ HOSO2−

(3)

The equilibrium constant for this tautomerization has been determined by 17O NMR spectroscopy to be Kc = [HOSO2−]/ [HSO3−] = 4.9 ± 0.1 at 298 K at an ionic strength of 1.0 M.8 The sulfonate ion does not form directly from the hydrated sulfur dioxide complex due to the high reaction barrier:9 first, bisulfite is formed and then tautomerism plays a role in the Received: December 15, 2011 Revised: March 5, 2012 Published: April 3, 2012 4035

dx.doi.org/10.1021/jp212120h | J. Phys. Chem. A 2012, 116, 4035−4046

The Journal of Physical Chemistry A

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the chemical state of SO2(aq) was molecular SO2 and not H2SO3. Initial calculations indicated that the hydrogen sulfonate form, HSO3−, is the most stable form of the tautomers.18 However, Hoffmann19 has argued that the chemical reactivity of bisulfite ions in aqueous solutions is consistent with HO−SO2− being the major reactive tautomer, which is in rapid equilibrium with HSO3−. Baird and Taylor suggested that the H−S bonded tautomer is the most energetically favorable.20 In contrast, ab initio calculations by Stromberg21 indicated that the tautomeric form, bisulfite, is lower in energy. In a later study, Connick et al. proved the existence of the two tautomers in solution by examining the S−O stretching region of Raman spectra of aqueous hydrogensulfite solutions.13 Further confusion has arisen regarding the assignment of bands in the vibrational spectra of hydrogenosulfite solutions containing both tautomers. There is a common misconception in the literature that, in the S−O stretching region, the 1052 cm−1 band is assignable to S2O52− and the 1021 cm−1 band to HOSO2−. However, utilizing both UV/vis and Raman spectroscopies, Connick et al. proved that the 1052 cm−1 band is a composite of both S2O52− and one of the hydrogensulfite tautomers; the 1021 cm−1 band is attributable to just one of the tautomers. However, the experiments were unable to distinguish between each tautomer.13 Risberg et al. concluded from an X-ray absorption and vibrational spectroscopic study that the 1052 cm−1 peak was attributable to the SO2 symmetric stretching mode of HOSO2−, while the 1021 cm−1 band was assigned to the SO3 symmetric stretch of HSO3− only.22 Zhang and Ewing have made a very tentative proposition for the existence of another dimer in solution with an S−S bond, whose chemical formula is the same as Golding’s dimer (H2S2O62−)12 but for which the structure is different. Like Golding’s dimer, the newly proposed complex is said to undergo condensation to form S2O52−, whereas in contrast to Golding’s dimer, it is formed from two HOSO2− ions. It was stated that this structure is more plausible than the structure of Golding’s dimer because it can eliminate water from its structure more easily. However, this proposition is based on the assumption that the HSO3− concentration is very low, a statement for which there is little experimental evidence. The predominant forms of the S(IV) species discussed above, which exist when dissolved in solution, depend on the acidity of the solution in which the SO2 dissolves. The individual reactions in the equilibria, 1−7, are relatively fast.23 For example, the rate coefficient for dissociation of hydrated SO2, k, is 3.4 × 106 s−1 so that the half-life for dissociation of the hydrated SO2 is only 0.2 μs. Similarly, the second ionization process occurs on time-scales of less than a millisecond.24 Thus, regardless of which of the main species, SO2·H2O, SHO3−, or SO32−, is the actual reactant in any particular oxidation step, the equilibria will be re-established relatively rapidly under laboratory conditions and likely under atmospheric conditions as well. For example, given a typical cloudwater pH of 4, SO2 will exist in solution mainly as the hydrogenosulfite ions. The importance of S(IV) aqueous phase oxidation in the atmosphere was first evidenced in the fifties, when Junge and Ryan25 called attention to the great potential of cloudwater for the oxidation of dissolved SO2 by heavy metal catalysis and subsequently a number of possible pathways for S(IV) to S(VI) conversion, often pH-dependent, have been identified in the literature.23,26

formation of hydrogen sulfonate. The tautomerism process is catalyzed by water molecules, which reduce the ring strain upon inclusion, and the reaction barrier decreases from 76.0 kcal mol−1 to 22 kcal mol−1.9 It is noteworthy, in terms of further reaction, that the pyramidal ion, HOSO2−, which is of Cs symmetry, would provide a stereochemically more attractive site for attack by a molecule than the tetrahedral tautomer HSO3− of C3v symmetry. Dimerization also occurs in this aqueous system to form pyrosulfite (disulfite) ions. It is still uncertain whether they are formed by each tautomer10 or by two of the same bisulfite tautomers,1112 but inevitably, disulfite ions are formed. Raman and UV/vis spectroscopies have been combined to measure the equilibrium constant for dimerization; Kc = 0.088 M−1 in 1 M NaClO4 solution at 25 °C.13 2OHSO2− ⇌ S2 O52 − + H 2O

(4)

OHSO2− + HSO3− ⇌ S2 O52 − + H 2O

(5)

Alternatively, the bisulfite ion can dissociate further to yield the sulfite ion (SO32−) as shown below. OHSO2− ⇌ SO32 − + H+

(6)

K a = 1.02 × 10−7 L mol−1; pK a = 6.97

It is also possible to form the sulfite ion directly from the sulfonate ion (HSO3−) as shown by Horner and Connick.14 HSO3− ⇌ SO32 − + H+

(7)

A summary of the speciation and equilbria in the aqueous S(IV) system is presented in Figure 1. Hydrogen sulfite (SHO3−) represents both of the tautomeric forms, bisulfite (HOSO2−) and hydrogen sulfonate (HSO3−).

Figure 1. Structural mechanisms for the interconversion of species in the aqueous S(IV) system.

Although steps 1−7 are now accepted as a foundation for understanding S(IV) speciation chemistry, there is much confusion in the literature regarding the spectral assignments and energetics of the individual ions. Raman, infrared, UV, and NMR spectroscopies, as well as ab initio calculations, have all been utilized in a range of different studies to gain insight, and progress has been made. For example, it has now been proven unequivocally that crystalline hydrogenosulfites are comprised of only sulfonate ions in the solid state,1516 and the extensive study of Simon and Waldman11 revealed the presence of bisulfite and pyrosulfite ions in aqueous solutions of SO2. Jones and McLaren17 studied the infrared absorption of SO 2 dissolved in water and inferred that, in the aqueous phase, 4036



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Figure 2. Schematic of the aerosol/flow-tube/spectroscopic detection apparatus.

pheric aerosols.30 If NH3 is absorbed into aerosols, it raises the pH of those aerosols, which enhances the rate of oxidation of dissolved sulfur dioxide (SO2) by ozone (O3) to form sulfate (SO42−).31 High concentrations of NH3 are also important for ammonium nitrate (NH4NO3) formation,32 although the affinity of sulfuric acid (H2SO4) for NH3 exceeds that of nitric acid (HNO3). The sulfur budget in particular is greatly affected, as both (NH4)HSO4 (acidic) and (NH4)2SO4 (neutral) are stable.33 In contrast, ammonium nitrate tends to be shortlived and can re-evaporate into its original gas phase constituents (i.e., NH3 and HNO3). Virtually all of the above studies on S(IV)/S(VI) aqueous chemistry have been performed in bulk solution conditions. The transformation mechanisms and intermediates involved are now set on firm experimental and computational foundations although contradiction is still clearly apparent in the literature from the above discussion. Their spectral assignments are therefore tentative guidelines regarding the identification of the species detected herein in the aerosol phase. Therefore, the purpose of the current study is to use the ideas proposed in the solution conditions mainly employed for the sulfurous acid system in an aerosol droplet environment to check their validity for a system potentially more relevant to the atmosphere.

Ammonia is the most abundant alkaline component in the atmosphere27 and is a key player in the partitioning of atmospheric sulfates and nitrates, yet very few studies to date have been dedicated to elucidating its heterogeneous behavior. The relevance of ammonia to the atmospheric environment lies not only in its contribution to N deposition to land surfaces and water bodies, which often leads to euthrophication and/or acidification.28 It also has the capacity to neutralize atmospheric acids. A substantial part of the acid generated in the atmosphere, by the oxidation of sulfur dioxide (SO2) and nitrogen oxides (NOx), is neutralized by ammonia as attested by the high concentration of particulate ammonium in the troposphere.2 Ammonium aerosols have a longer residence time in the atmosphere than gases and can be transported and dispersed over greater areas.29 Gaseous NH3 is very stable as regards oxidation in the atmosphere and is extremely soluble in water. However, its lifetime is limited due to a very effective heterogeneous conversion to particulate matter in the form of the NH4+ ion. Gaseous NH3 is removed from the atmosphere by the following three methods: wet deposition (remote sources), dry deposition (local sources), both of a nonchemical nature and reactions with acid species to form aerosols. Virtually all NH3 and NH4+ (collectively termed NHx) emission occurs in the form of NH3, with NH4+ ion in the atmosphere originating from reactions of NH3. The role of ammonia in neutralizing acidic aerosols has received considerable attention in literature concerning environmental acidification and the health effects of atmos-

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EXPERIMENTAL METHODS Apparatus. The apparatus on which the experiments were conducted was based on an aerosol flow-tube instrument connected to an FTIR spectrometer and adapted for NH34037



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purposes of accuracy. Small volumes were handled with a calibrated 1 cm3 Gilson Pro-pipet. Ammonia was taken from a dilute (100 ppmv) mixture in nitrogen (99.8% purity; manufacturer, BOC Gases). A small flow of this mixture (typically in the range 100−300 sccm) was passed through the injector. The dilution of this flow by the main flow resulted in an average [NH3]0 in the reactor that was typically in the range between 1−3 ppmv. Introduction of the acidic species to the flow-tube, using the two different methods of aerosol generation is discussed below. Experimental flow conditions (typically 1000−3000 sccm) were achieved by using a series of MKS Type 247 mass flow controllers (3000, 500, and 20 sccm) and ensured that the necessary laminar flow conditions were always prevalent in the reactor (Re < 100). Pressure was monitored using an MKS Baratron Type 626A pressure gauge through an inlet port. The flow-tube was operated close to atmospheric pressures at all times. A slight increase in the pressure was noted on the addition of the aerosols generated from the constant output atomizer technique. The relative humidity of the carrier flow (compressed air) through the flow-tube was tuned between 1 and 90% and monitored by a digital hydrometer (Rotronic A2 Hygromer). The humidity was increased by the addition of water vapor to the air flow. It was entrained into the flow by bubbling at a known level of compressed air through a humidifier of deionized water at room temperature. The exit of the humidifier was packed with glass wool to stop any droplets becoming entrained into the flow. This flow was then mixed within the conditioner with the aerosols. The dew point of the flow-tube effluent was monitored by the hydrometer. Aerosol Generation. Aerosol particles for most of the studies reported here were generated by passing ca. 2500 sccm of dry filtered air through a constant output atomiser (COA, TSI Model 3075). Aerosols of a given composition were formed from atomising solutions of liquids or suspensions of solids in liquids of the species of interest dissolved in water. The solution was entrained into a spray region, where droplets beyond a certain size (typically submicrometer) impacted on a collector, which in turn empties back into the reservoir. A range of particle sizes are produced initially but large particles are removed by impaction. Drying the aerosols after their generation is an important factor in the nature of the final aerosol produced since the process may alter both the physical and chemical nature of the particles. In experiments for which a low relative humidity (45% RH) was desired. Once dried or wetted, the particles were entrained into the apparatus for size selection, experimental processing, and detection. Reservoir solutions of 0.1 to 1 wt % concentration were sufficiently concentrated so that the aerosol particle mass was high enough for FTIR observation, yet sufficiently dilute that frequent obstruction of the atomizer did not occur. This type of particle generator produced particles that were well described by a log-normal distribution with a peak diameter typically in the region of 100 nm as shown by the scanning mobilty particle sizer (SMPS) measurements described in more detail below.

monitoring using a novel chemiluminescence approach. The aerosol flow-tube was used to obtain simultaneous optical and size distribution measurements as a function of relative humidity and aerosol composition. The full system was thereby devised to study the potential interactions between acidcontaining aerosols with a variety of trace gases. A constant flow of mainly submicrometer particles (either acidic species or ionic salts) were generated as described later in this section. The components of the flows were chosen to closely resemble those found in the atmosphere. The chemical aerosols were then passed through a vertically aligned laminar flow reactor, with gaseous ammonia introduced via a sliding injector. A variety of detection techniques were employed to monitor changes in the composition of both gas- and condensed-phase species as described below. A schematic diagram of the flow reactor apparatus is shown in Figure 2 below. The apparatus comprised three main sections: {1} aerosol generation, {2} a reaction zone (i.e., a vertical flow-tube with several inlet and outlet ports), and {3} a detection region. The flow-tube was limited to operate at atmospheric pressure because of the aerosol generation system employed in this study. The constant output atomiser (COA) required a high flow rate (>2500 standard cubic centimeter per minute, sccm) to generate particles. Therefore, in order to establish a reasonable interaction time (∼100 s) between the trace gas and the aerosols, the flow-tube itself was designed to be of sufficient length (150 cm) to allow such an interaction time to be achieved. The flow reactor body consisted of four Pyrex glass tubes, each with an internal diameter of 74.5 mm and with a combined length of 1500 mm. Each individual Pyrex glass tube was flanged on open ends, to allow for assembly. Vacuum grease was used to ensure a tight seal between each of the flanges. The first tube is the inlet and has four sample ports that allow the attachment of a trace gas injector, pressure gauge, carrier air flow inlet, and an aerosol flow inlet. It also has a cooling jacket that can be used for temperature dependent studies. The flow profile in this region changes from turbulent to laminar. The main body of the flow reactor is found in the second tube where laminar conditions prevail. The third tube houses the optical windows for spectroscopic detection and is coupled to a Fourier transform infrared spectrometer (BioRad Excalibur FTS 3000 with external MCT detector). The arrangement was designed to ensure that the windows are not exposed to the aerosol flowing in the central core of the flow-tube, and therefore, very little material condenses on them during the experiment. The FTIR path length through the flow-tube is approximately 250 mm. The final tube encases the oxalic acid coated denuder to allow sampling of particulate NH4+ ions. This allowed for the selective removal of gaseous ammonia from the mixed air-flow without effecting particle concentration. It also has two outlets that allow aerosol sampling. Without the use of an annular denuder, one would not be able to distinguish between ammonium, adsorbed ammonia, and residual (unreacted) gas phase ammonia. The exhaust section at the bottom of the system houses a relative humidity (RH) and temperature sensor in the form of a digital hydrometer (Rotronic A2 Hygrometer). Experimental Conditions. Concentrated aqueous acid solutions were prepared using standard volumetric apparatus, and stock solutions were made up to at least 500 cm3 for 4038



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Figure 3. FTIR spectra of sulfur dioxide and water at 10% RH with gas-phase water absorption lines (bottom spectrum, a) and without gas-phase water absorption lines (top spectrum, b) after subtraction. Absorption of gas-phase CO2 can be seen between 2200 and 2400 cm−1.

Detection. FTIR Spectroscopy. FTIR spectroscopy was used to monitor both gas- and aerosol-phase compositions. The FTIR spectrometer (BioRad Excalibur) was coupled to an external mercury cadmium telluride (MCT) detector. In this setup, collimated radiation from the FTIR spectrometer was directed through the probing path length, a distance measuring 250 mm. Barium fluoride windows mounted at both ends permitted a single passage of the IR beam. The transmitted radiation was focused by a zinc selenide lens onto the active area of the liquid nitrogen cooled MCT detector. The entire path length of the beam was purged with dry filtered air to reduce background signals and light was excluded to prevent degradation of the BaF2 windows. A background spectrum was taken while the flow-tube was fully purged with dry filtered air. Each spectrum represented the average of 512 scans recorded over the range 4000−750 cm−1 at 2 cm−1 resolution. The output flow from the aerosol generation system was then introduced, and sample spectra were taken depending on the conditions of each particular experiment. As a short path length was imposed by the flowtube geometry, it was not possible to rely, with confidence, on spectroscopic detection for quantitative measurements. Therefore, the primary use of the FTIR spectrometer for this current study was in the area of product characterization, with the chemiluminescence technique being applied for quantitative analysis, as described below. The software supplied with the spectrometer was Win-IR Pro, and it was used, in conjunction with Thermo Galactic’s GRAMS/AI, to perform necessary manipulations on the spectra. These include baseline correction, spectral subtraction, integration, and peak fitting. SMPS. The aerosol suspensions were characterized using a scanning mobility particle sizer (SMPS, TSI 3081) instrument, which determines key parameters such as mode (typically 100 nm for the atomizer technique), number distribution, surface area, and particulate mass by electrostatic means. The TSI SMPS consists of an electrostatic classifier (EC, TSI 3080), a differential mobility analyzer (DMA, TSI 3011), and a condensation particle counter (CPC, TSI 3010).

Chemiluminescence NOx Monitor: Ammonia and Ammonium Ions. Because of the path-length constraints, FTIR spectroscopic detection of ammonia and ammonium ions was limited to the ppmv range and found to be too insensitive for use at the low concentrations used in this study. Therefore, a method for detecting ammonia and ammonium ions was developed, using an Ostwald-type reaction coupled with a commercial chemiluminescence NOx monitor. This approach allowed the monitoring of NH3/NH4+ species at low concentrations (down to ppbv levels) with accuracy (within 5%) and appropriate time response (within 3 min). In summary, the oxidation step utilized a lanthanide oxide system doped with copper oxide operated at an optimum temperature of 1023 K. At this point, ammonium salts decompose, releasing ammonia, which, in turn, is converted to NO by the rare-earth catalyst and detected by the chemiluminescence monitoring instrument (EC9841 NO/ NO2/NOX analyzer). Gas-phase ammonia was admitted into the flow-tube via a 6 mm diameter movable glass injector. From the 100 ppmv standard (NH3/N2 brand), the ammonia concentration was diluted down to the ppmv range using a range of mass flowmeters. The concentration of ammonia in the flow-tube was determined and subsequently compared to the NOx levels. A calibration curve was constructed, and a conversion factor was calculated (1.2 ± 0.03). Ammonia was admitted into the flow-tube via the sliding injector, and the flow rate was varied from 100 to 200 to 300 sccm, resulting in a concentration range of 4.3 to 8.7 to 13 ppmv, respectively. The interaction between ammonia and acidic aerosols results in the formation of ammonium ions. The concentration of these ammonium ions was monitored using the NO x chemiluminescence monitor as discussed above; all ammonium ion concentrations quoted in the results have been converted from the NOx signal, using the conversion factor stated previously. Therefore, these ammonium ion concentration values represent the NOx signal converted to ammonia concentration with units of ppmv. The changes in ammonium ion concentration (by varying experimental conditions: 4039



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increasing humidity and can be correlated to the FTIR spectrum shown in Figure 3. The water subtraction is not clean due to the aqueous nature of the SO2·H2O species resulting in uncertainties on the peak-area values generated by the increasing interference of water lines as RH increases. SMPS. The size distribution in terms of particle number is approximately log-normal, i.e., the plot of dN/d ln Dp vs Dp (N = number of particles x, cm−3, Dp = particle diameter, nm). In all cases, sulfurous acid aerosols generated from aqueous solutions of sulfur dioxide by the atomizer technique gave particle distributions that were monomodal and had an approximately Gaussian size distribution. Figure 5 illustrates

humidity, concentrations, and interaction times) are of central use to this characterization study, while the absolute values are of less importance.

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RESULTS The studies presented in this article center on the use of FTIR spectroscopy and SMPS particle sizing to investigate the behavior of aerosols formed from the solvation of gaseous sulfur dioxide in water at various humidities relevant to the atmosphere. Ammonia, the most abundant alkaline species in the troposphere, was subsequently added and detected using a NOx analyzer. In aqueous solution, sulfurous acid, SO2·H2O dissociates into hydrogensulfite, sulfite, and disulfite ions depending on temperature and equilibrium conditions. Discussion of the FTIR spectrum of sulfurous acid must be carried out in the context of contributions from the presence of these ions. Since ammonia is the only soluble base found in the atmosphere in significant quantities, it plays a principal role in neutralizing these acidic aerosols, converting them to new nonvolatile aerosol particles; (NH4)2SO3, (NH4)2S2O5, and NH4HSO3. In addition, various particle parameters such as size, number, concentration, and surface area were measured using a scanning mobility particle sizer (SMPS). For comparison and correlation, the FTIR spectra were measured in tandem with the SMPS. Results obtained for various sulfurous acid dispersions under a variety of experimental conditions were examined. Ammonia was subsequently allowed to interact with these aerosols and the amounts quantified using a NOx technique. Sulfur Dioxide and Water. FTIR Spectroscopy. SO2 (dispersed in the three phases) show IR absorptions in the region centered on 1350 cm−1: aqueous- (1347 cm−1), liquid(1356 cm−1), and gas-phase (1373 cm−1). They were assigned to asymmetric sulfur dioxide fundamentals with reference to Zhang and Ewing (Figure 3).12 A weak absorption can also be seen at 1155 cm−1, which represents the symmetric stretching mode of the molecule. Spectra were more defined using higher concentrations (1−4 wt %; an aerosol/particle distribution generated from a wt % SO2·H2O aqueous solution). However, the similarity with gas-phase spectra indicates that, in aqueous solution, SO2 is molecular, nonlinear, and of C2v molecular symmetry. Pure sulfur dioxide spectra could not be recorded given the experimental instrumentation. The IR absorptions due to sulfur dioxide were not as clear when increasing the humidity to 50% RH due to the dilution of the system and solvation of the molecular sulfur dioxide. Calculation of the peak areas due to the absorption at ∼1350 cm−1 (using 2 wt % at various humidities) as a function of RH is shown in Figure 4. It shows that the peak area decreases with

Figure 5. Size distributions of 1 wt % sulfurous acid aerosols at varying RH values.

SMPS results using a sheath flow of 2 L min−1 and a sample flow of 0.2 L min−1. A mode (maximum in the size distribution) at approximately 30 nm, indicating a hydrated cluster form of SO2, was noted for these experiments, the mode slightly decreases with increasing humidity. The particle number density increases from ∼1 × 106 to 1.5 × 106 particles cm−3 when increasing the humidity from ∼10% RH to 50% RH as seen in the graph. This behavior illustrates the affinity of water to the aerosol and the formation of the aqueous phase of the sulfur(IV) ions. The increased stabilization of initial particle nuclei in the low nano range, therefore invisible to SMPS measurement, would justify increasing numbers. Sulfur Dioxide with Water and Ammonia. FTIR Spectroscopy. The spectrum obtained for this experiment is shown in Figure 6. The subtraction is cleaner due to removal of initial aqueous sulfur dioxide spectra rather than water spectra. Hydrogen sulfite isomers (1200−1030 cm−1, attributed to their S−O stretching vibration bands) and disulfite ions (the intense absorption feature centered at approximately 928 cm −1 indicating S−O bond symmetric stretching) are observed when NH3 is introduced. The asymmetric stretches attributable to sulfur dioxide in the 1350 cm−1 region are inverted indicating loss of this species in this system. Absorption bands are also seen at 1193 cm−1 associated with the hydrogen sulfite ion, 1120 cm−1 representing the S−H absorption and also at 1048 cm−1 attributed to the bisulfite tautomer. The latter feature is inverted indicating loss; there is also an indication of the bisulfite tautomer at 1080 cm−1 and evidence for the hydrogen sulfonate tautomer at 1030 cm−1. No correlation between the absorption peak at 1030 cm−1 (S−O stretching vibration band) representing a hydrogen sulfonate ion and the symmetric stretching vibration near 2550 cm−1, indicating an S−H bond, could be determined12 as the S−H stretching mode of HSO3− was not observed presumably

Figure 4. Peak areas for the ∼1350 cm−1 peak at various RH. 4040



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Figure 6. FTIR spectrum of 1 wt % SO2·H2O aerosol and 300 sccm ammonia at 60% RH.

Figure 7. FTIR spectra of 1 wt % SO2·H2O and 100 (a), 200 (b), and 300 (c) sccm ammonia at 60% RH.

which solely represents water from the SO2·H2O complex. Differences may be due to a combination of overlapping bands in this region, which makes it difficult to measure the precise intensity values and peak areas of absorptions. The absorbances of the spectra are arbritary and the main focus here is product characterization rather than quantitative reproducibility when comparing similar conditions for spectra of different experiments. Also present are absorptions at 1195, 1035, and 941 cm−1 representing a mixture of hydrogen sulfite ions, the hydrogen sulfonate tautomer and the disulfite ion, respectively. With increasing ammonia concentrations, gaseous ammonia peaks at 966 and 954 cm−1 also emerge indicating that neutralization has occurred within the system. These ammonia absorptions were assigned previously when recording pure NH3 gas spectra. Table 1 illustrates the peak area values for the abovementioned absorptions as a function of ammonia concentration. The 1195 and 941 cm−1 absorptions increase in

due to its low absorption cross-section and low oscillator strength.5 It also has been noted that the symmetric stretch of SO32− at 966 cm−1 is not IR active in solution.34 The sulfite anion may be sensitive to perturbation in terms of contact ion pairing or solvent separated ion pairing.35 The C3v symmetry for sulfite affects the occurrence of overtone or combination modes, and the water molecules are hydrogen bonded to the anion, which results in a lowering of the symmetry. The ammonium deformation peak (1434 cm−1) is also present, which has a maximum value of 0.007 ± 0.001. In summary, the main evidence points toward the formation of a disulfite species either coordinated or dissociated with ammonium ions. Hydrogenosulfite tautomers are also present, though in lesser abundance. The assignment of an −OH stretching vibration at 3247 cm−1 representing the bisulfite ion can be made in reference to assignments made for bands at 314522 and 3622 cm−112 in Figure 7. This absorption is not as pronounced in Figure 3, 4041



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1030 cm−1 has previously been assigned to the hydrogen sulfonate tautomer. As alluded to in the introduction, the speciation associated with aqueous S(IV) chemistry can be extremely complicated to interpret in full detail. Nonetheless, from the FTIR spectra obtained of the aerosols, it is clear the main species present is mainly the disulfite ion along with a small contribution from hydrogenosulfite tautomers based on comparative cumulated area for each ion. Ammonia appeared in the gaseous form at low RH and high concentrations, but RH > 60% resulted in the observation of ammonium ions. It should be noted that a recognized contaminant related to the oxalate ion are also present in the IR spectra due to its use in previous experiments. Flushing and cleaning experiments were unsuccessful in removing all traces of the material, but its IR features are readily recognized as shown in Figure 8. Table 2 lists all of the observed FTIR bands

Table 1. Peak Areas for the Major Peaks Detected in 1 wt % SO2·H2O Aerosol Following Ammonia Addition at 60% RH 1 wt % SO2·H2O

1195 cm−1

1035 cm−1

941 cm−1

+100 sccm NH3 +200 sccm NH3 +300 sccm NH3

0.00585 0.00865 0.02230

0.00031 0.00440 0.00367

0.00963 0.02379 0.04596

intensity with increasing ammonia; however, the absorption at 1035 cm−1 decreases after 300 sccm NH3 addition suggesting complete deprotonation and disufite ion formation. Closer inspection of several of the IR bands observed indicated that they are asymmetrical. In order to extract quantitative data from the spectra, these features can be resolved into components using computational modeling. Such a process was performed using the spectroscopic software package, Thermo Galactic’s GRAMS/AI, which includes baseline correction, spectral subtraction, integration, and, most importantly, peak fitting. Figure 8 shows the spectral deconvolution obtained for SO2·H2O mixtures with ammonia at high RH (50%). Deconvolution of the absorption at 1200 cm−1 reveals three bands at 1220, 1200, and 1180 cm−1 representing both of the hydrogen sulfite isomers and also a disulfite ion.12 To help confirm this prediction, more IR bands were included in the analysis. For example, the bending mode of the S−H bond associated with hydrogen sulfonate is assigned to the absorption at 1150 cm−1; the 1090 cm−1 band is a further feature connected with the bisulfite isomer (the S− OH stretch). The peak at 1070 cm−1 can be assigned to the S− O stretching vibration of S2O52−, which is supported by the deconvolution study published by Zhang and Ewing.12 Here, the 1050 cm−1 band representing the bisulfite tautomer was well-fitted by two Gaussian features at 1063 and 1047 cm−1. The band at 1050 cm−1 is the position of the S−O stretching mode associated with the bisulfite tautomer, and the band at

Table 2. Observed FTIR Bands (cm−1) and Their Assignments wavenumber

vibrationa

species

reference

1350 1150 3247 1200 1090 1050 1180 1120 1030 1220 1070 930 1442

νa(SO2) νs(SO2) νO−H νS−O ν(S−OH) νs(SO2) νas(SO3) δS−H νs(SO3) νS−O νs(SO2) νsS−O δN−H

SO2(aq) SO2(aq) HOSO2− HOSO2− HOSO2− HOSO2− HSO3− HSO3− HSO3− S2O52− S2O52− S2O52− NH4

12,22 12,22 12,22 GRAMS/AI 22,35 GRAMS/AI GRAMS/AI22,35 GRAMS/AI35 22,35 12,35 GRAMS/AI35 12,35 earlier experiments

a Notation of vibrational modes: ν, stretching; δ, bending; s, symmetric; a, asymmetric.

Figure 8. Deconvolution of the FTIR spectrum of 4 wt % SO2·H2O and 300 sccm NH3 at 50% RH. The red curve is the source spectrum, and the green and blue curves are the deconvolution peaks. 4042



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Figure 9. FTIR spectra of 1 wt % SO2·H2O + 300 sccm ammonia at (a) 20% RH and (b) 60% RH.

Figure 10. Sizes of distributions of 4 wt % SO2·H2O aerosols upon the addition of ammonia at 12% RH.

Figure 11. Sizes distributions of 1 wt % SO2·H2O upon the addition of ammonia at 12% RH.

for a relatively high humidity for the formation of hydrogenosulfite/disulfite species, which are necessary to promote reaction with ammonia to form ammonium ions. It also further indicates the affinity and prevalence of these sulfur(IV) ions in aqueous (aerosol) conditions. Finally, of note in the spectra is that the symmetric stretches of sulfur dioxide are observed to be inverted indicating that this parent species is removed from the system at both humidities.

related to the ammonia experiments and their assignment to specific vibrational modes. Figure 9 illustrates the presence of gaseous ammonia (centered in the ∼950 cm−1 region of the spectrum) for 1 wt % SO2·H2O at 20% RH and 300 sccm NH3. These absorptions appear after the introduction (injection and exposure) of ammonia to the nebulized droplets of SO2·H2O at low humidity in the flow-tube. This finding illustrates the need 4043



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Figure 12. Ammonium ion concentration vs ammonia flow rate at 15% RH, for each of the aqueous sulfur dioxide concentrations.

acid studies as opposed to studies of stronger acids previously such as sulfuric acid mainly due to the unknown concentration of the species used and the weaker acidity of the aqueous solution of sulfur dioxide. An increase of approximately 0.1 ppmv [NH4]+ occurred to 1 wt % SO2·H2O with the addition of NH3. Although it cannot be detected by FTIR, it is known that the sulfite ion favors alkaline conditions36 due to its high pKa as opposed to the HSO3−/ OHSO2− isomers (hydrogen sulfonate and bisulfite). Complete ionization and eventual disulfite formation would also favor ammonium ion production. The presence of acidic H+ ions from the hydrogen sulfite tautomers would also contribute to the formation of ammonium. An overall decrease in ammonium ion concentration was observed when the humidity was increased after the addition of ammonia to the sulfur dioxide containing aqueous solutions as shown in Figure 13 during this NH3 uptake experiment.

The FTIR spectrum shown in Figure 9 also illustrates the disappearance of the gaseous ammonia absorptions with increasing humidity and corresponding emergence of the disulfite absorption in the 950 cm−1 region. SMPS. The profiles shown in Figure 10 shows the measured size distributions obtained for the aerosols with increasing humidity and the addition of ammonia. In fact, there was no change in size distribution, particle diameter, or particle concentration noted as conditions were changed. As observed in Figure 10, particle diameters are in the region of 60 nm implying a cluster composition of SO2·H2O, ammonia, and water, and a shift of the particle size distribution is not noted with the introduction of ammonia. However, the particle numbers were shown to decrease slightly with its addition during the course of the experiment suggesting enhanced incorporation of ammonia by the particles with time. Comparing to Figure 5, it is observed that the SO2 wt % fraction has a positive influence on the distribution mode; a shift from 30 nm to 60 nm. The particle distributions, with mode ranging from 40 to 50 nm, shown in Figure 11 differ dramatically during each stage of this experiment when ammonia is introduced followed by increasing humidity. This aerosol is generated from 1 wt % SO2·H2O mixtures (i.e., a lower concentration than used in the experiments displayed in Figure 10). The concentration of particles increases from 4.3 × 105 to 1.9 × 106 cm−3 with the addition of 200 sccm NH3. However, a dramatic decrease of particle concentration is observed with the addition of 300 sccm, which is accompanied by a broader particle distribution. This suggests that saturation of the aerosol by ammonia occurs alongside a shrinking of the particles during the course of the experiment. The presence of gaseous ammonia observed in the FTIR spectra under these conditions enhances the likelihood of such an interpretation. By further increasing humidity, both the particle numbers and mode increase. NOx Analysis. The NH4+ ion concentrations for interaction with sulfurous acid, at various ammonia flow rates, were calculated from the chemiluminescence NOx data using the calibration factor of 1.2 as discussed previously. Inspection of this NH4+ ion concentration data leads to the conclusion that increasing the concentration of sulfurous acid aerosols has a direct, positive effect on the amount of ammonium ions formed, as illustrated in Figure 12. As expected, increasing the NH3 concentration increases the ammonium ion concentration. The amount of ammonium ion formation was low for sulfurous

Figure 13. Effect of humidity on ammonium formation in the 1 wt % SO2·H2O and 300 sccm NH3 system.

Initially, there is an increase of the NH4+ concentrations due to increased proton stabilization in the presence of sulfur(IV) ions. However, with increasing humidity, there is gradually a disappearance of [NH4+] over the course of the experiment. In contrast, introduction of ammonia to SO2·H2O at initial high humidities led to an increase of ammonium ion as previously shown by FTIR in Figure 6, possibly due to the establishment of more condensed aerosol and increased favorable conditions for ammonia uptake by the sulfur(IV) ions, which are predominantly in the aqueous phase. This phenomenon would lead to more uptake of ammonia. 4044



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The weak acidic nature is further enhanced regarding [NH4+] production. A concentration of 0.150 ppmv was noted at 20% RH, and this increased to 0.250 ppmv with increasing humidity due to the further interaction of ammonia with the acidic species. It has also been reported that increasing the alkalinity of sulfur(IV) results in formation of sulfite ions36 due to complete deprotonation. While sulfite ion could not be detected by FTIR, this hypothesis would also favor the abundance of disulfite ion. When increasing the humidity to this binary system of SO2·H2O and NH3, the ammonium ion concentration decreased indicating the hygroscopicity of these aerosols, the uptake of water of the aerosol causing dilution. Possible reactions occurring are shown in eqs 8 and 9.

DISCUSSION

The SO2·H2O complex is a major component of tropospheric aerosols as well as being present throughout the lower and middle stratosphere. The studies presented here center on the use of FTIR spectroscopy and SMPS particle sizing to investigate the behavior of aerosols formed from the solvation of gaseous sulfur dioxide in water at various humidities relevant to the atmosphere. Ammonia, the most abundant alkaline species in the troposphere was subsequently added and its transformation to ammonium detected using a chemiluminescence NOx analyzer. In aqueous solutions, sulfurous acid, SO2·H2O, dissociates into hydrogensulfite, sulfite, and disulfite ions depending on temperature and equilibrium conditions. Discussion of the FTIR spectrum of sulfurous acid must be carried out in the context of contributions from the presence of these ions. Since ammonia is the only soluble base found in the atmosphere in significant quantities, it plays a principal role in neutralizing these acidic aerosols, converting them to new nonvolatile aerosol particles; (NH4)2SO3, (NH4)2S2O5, and NH4HSO3. In addition, various particle parameters such as size, number, concentration, and surface area were measured using a scanning mobility particle sizer (SMPS). For comparison and correlation, the FTIR spectra were measured in tandem with the SMPS. Results obtained for various sulfurous acid dispersions under a variety of experimental conditions were examined. Ammonia was subsequently allowed to interact with these aerosols and the amounts quantified using a NOx technique. Dissolved suspensions of sulfur dioxide in deionized water produced sulfur dioxide absorptions in FTIR spectra at 1350 cm−1 and 1150 cm−1. Three sharp peaks were observed indicating gaseous, aqueous, and aerosol SO2 absorptions, which were clearer at initial high humidities; however, when humidity was increased during experiments, after the establishment of the sulfur dioxide water complex at a low humidity, the 1350 cm−1 absorption band decreased in area indicating dilution of the system. There was no change in particle size distribution observed with a mode and concentration of approximately 30 nm and 1 × 106 cm−3, respectively. The mode slightly decreased and the particle concentration marginally increased with increasing humidity due to the formation of a more aqueous system of SO2·H2O. The main species produced for SO2·H2O interactions with ammonia were hydrogen sulfite tautomers (hydrogen sulfonate and bisulfite) and pyrosulfite (disulfite). Sulfite (SO32−) is not IR active in solution.37 The absorptions increased in peak area with increasing ammonia concentration at mainly high humidities enhancing the aqueous nature of sulfur(IV) species. At high flow concentrations of ammonia (300 sccm), however, and at low humidities (20% RH), gaseous residual gaseous ammonia was detected suggesting the weak acidity of these anions. In relation to the SMPS data, the particle size and concentration increase with increasing humidity and ammonia addition with diameters and particle number in the range of 60 nm and 1 × 106 cm3 for concentrations between 2 and 4 wt %. However, at 1 wt %, further addition of ammonia results in saturation of the particles as the concentration drops with 300 sccm NH3 introduced. Particle distributions are seen to vary also indicating alternating combinations of sulfur dioxide, ammonia, and water.

SHO3− + H+ + NH3 ⇌ NH4SHO3

(8)

SO32 − /S2 O52 − + 2H+ + 2NH3 ⇌ (NH4)2 SO3 /(NH4)2 S2 O5

(9)

With regards to the bulk phase, it is found that ammonium sulfite is formed when ammonia and sulfur dioxide are reacted together with excess water present for solid state reactions of a low temperature matrix.38 However, with substoichiometric amounts of H2O, ammonium disulfite is the main species. (NH4)2S2O5 reacts further with water to form ammonium bisulfite. This would suggest the presence of SO32− in this system for the aerosol phase. Other studies of the gas-phase in a flow system by Scargill39 show that, when sulfur dioxide is in excess, ammonium disulfite is formed, but when ammonia is in excess, the speciation changes to ammonium sulfite. Initial spectra of ammonia introduction to SO2·H2O would also support this observation with a decrease in disulfite concentrations over time.

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CONCLUSIONS AND ATMOSPHERIC IMPLICATIONS In the current study, regarding the behavior of sulfurous acid, sulfur dioxide in three different phases (gaseous, aqueous, and aerosol) was produced. As ammonia is introduced, the speciation of the SO2·H2O aerosols alters significantly. In considering a system containing H2SO3, NH3, and water only and given the temperature, relative humidity, and the levels of available ammonia, the following question arises: what is the aerosol composition? The main species produced were hydrogen sulfite tautomers (hydrogen sulfonate and bisulfite) and predominantly pyrosulfite (disulfite). There are many physical and chemical steps in aqueous phase oxidation of sulfur(IV) to sulfate, the oxidation itself is only one portion of a sequence of processes. There is increasing evidence that there are some species, and perhaps some chemistry, that are unique to the interface.4 For example, in the case of SO2, it was observed that the uptake of this gas into the water droplets was faster than expected based on the known kinetics in bulk solution.40 It has also been suggested that a surface complex was formed between sulfur dioxide and water at the interface.41 This fourth phase is an area that remains to be explored. Under extreme conditions of large droplets (∼10 μm) and very high oxidant concentrations, the chemical reaction times may approach those of diffusion, particularly in the aqueous-phase. In this case, mass transport may become limiting. However, it is believed that, under most conditions typical of the troposphere, this will not be the case and that the 4045



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(22) Damian Risberg, E.; Eriksson, L.; Mink, J.; Petterson, L. G. M.; Skripkin, M. Y.; Sandstroem, M. Inorg. Chem. 2007, 46, 8332−8348. (23) Martin, L. R. In Kinetic studies of sulfite oxidation in aqueous solution in SO2, NO and NO2 Oxidation Mechanisms: Atmospheric Considerations; Calvert, J. G., Ed.; Butterworth-Heinemann: Boston, MA, 1984; pp 63−100. (24) Schwartz, S. E.; Freiberg, J. E. Atmos. Environ. 1981, 15, 1129− 1144. (25) Junge, C. E.; Ryan, T. G. Meteorol. Soc. 1958, 84, 46−55. (26) Moller, D. Atmos. Environ. 1980, 14, 1067−1076. (27) Asman, W. A. H.; Sutton, M. A.; Schjorring, J. K. New Phytol. 1998, 139, 27−48. (28) Fowler, D.; Sutton, M. A.; Smith, R. I.; Pitcairn, C. E. R.; Coyle, M.; Campbell, G.; Stedman, J. Environ. Pollut. 1998, 102, 337−342. (29) Aneja, V. P.; Roelle, P. A.; Murray, G. C.; Southerland, J.; Erisman, J. W.; Fowler, D.; Asman, W. A. H.; Patni, N. Atmos. Environ. 2001, 35, 1903−1911. (30) Suh, H. H.; Spengler, J. D.; Koutrakis, P. Environ. Sci. Technol. 1992, 26, 2507−2517. (31) Asman, W. A. H.; Janssen, A. J. Atmos. Environ. 1987, 21, 2099− 2119. (32) Russell, L. M.; Pandis, S. N.; Seinfeld, J. H. J. Geophys. Res., [Atmos.] 1994, 99, 20989−21003. (33) Brost, R. A.; Delany, A. C.; Huebert, B. J. J. Geophys. Res., [Atmos.] 1988, 93, 7137−7152. (34) Brown, J. D.; Straughan, B. P. J. Chem. Soc., Dalton Trans. 1972, 16, 1750−1751. (35) Herlinger, A. W.; Long, T. V. Inorg. Chem. 1969, 8 (12), 2661− 2665. (36) Pichler, A.; Fleissner, G.; Hallbrucker, A.; Mayer, E. J. Mol. Struct. 1997, 408/409, 521−525. (37) Brown, J. D.; Straughan, B. P. J. Chem. Soc., Dalton Trans. 1972, 16, 1750−1751. (38) Hisatune, I. C.; Heicklen, J. Can. J. Chem. 1975, 53, 2646. (39) Scargill, D. J. Chem. Soc. Assoc. 1971, 2461. (40) Chang, S. G.; Toossi, R.; Novakov, T. Atmos. Environ. 1981, 15, 1297−1292. (41) Schwartz, S. E.; White, W. H. Adv. Environ. Sci. Technol. 1983, 12, 1−116.

chemical reaction rate will be rate determining in the S(IV) aqueous-phase oxidation. Quantifying the role of sulfurous acid aerosols in atmospheric processes requires, among other things, assessments of particle number density and composition. The generation techniques employed here are not new nor are they unique to this study. The use of FTIR spectroscopy in conjunction with a laminar flow-tube and a SMPS to establish the nature of these acidic aerosols was particular to this study and yielded some very novel and interesting results concerning the growth of aerosols under a variety of atmospherically relevant conditions. The results of this study may have significant implications for tropospheric heterogeneous chemistry. In particular, they may provide greater insight in the understanding of speciation within sulfur(IV) containing aerosols.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (T.M.T.); j.sodeau@ ucc.ie (J.R.S.). Present Address †

Department of Physical Chemistry, University Castilla-La Mancha, Ciudad Real, Spain. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Irish Higher Education Authority, Science Foundation Ireland, and the PRTLI-3 Programme for Research.

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REFERENCES

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