Isoprene , sulphoxy radical-anions and acidity

Isoprene, sulphoxy radical-anions and acidity K. J. Rudziński, L. Gmachowski, and I. Kuznietsova Department of Catalysis on Metals, Institute of Physical Chemistry of the Polish Academy of Sciences, 01-224 Warsaw, Poland Institute of Chemistry, Warsaw University of Technology, 09-400 Płock, Poland Received: 24 September 2008 – Accepted: 14 October 2008 – Published: 12 December 2008 Correspondence to: K. J. Rudziński (kjrudz@ichf.edu.pl) Published by Copernicus Publications on behalf of the European Geosciences Union.


Introduction
Isoprene (C 5 H 8 ) and sulphur dioxide (SO 2 ) are important trace components of the atmosphere that are emitted from biogenic and anthropogenic sources (Georgii and Warneck, 1999;Sharkey et al., 2007).Sulphur dioxide is also produced directly in the atmosphere, for instance by oxidation of dimethylsulphide (Barnes et al., 2006).
The recognised fate of SO 2 is conversion to sulphuric acid and sulphates, in the gasphase, heterogeneous or multiphase processes (Seinfeld and Pandis, 2006).On the other hand, isoprene is quickly oxidised in the gas phase by radicals (OH, NO 3 ) and by ozone, to several primary and many secondary products, such as methacrolein and methylvinyl ketone or methylglyoxal and organic nitrates, respecively (LeBras and the LACTOZ Steering Group, 1997; Fan and Zhang, 2004;Paulot et al., 2008;Taraborrelli et al., 2008).
The discovery of tetraols in ambient aerosol samples collected in Amazon rainforest and boreal K-Puszta forest (Claeys et al., 2004a,b) spawned intensive research on formation of secondary organic aerosol (SOA) in heterogeneous and multiphase reactions of isoprene.For review of this research see (Rudzi ński, 2006(Rudzi ński, , 2008)).In a nutshell, numerous experiments in simulation chambers, unseeded or seeded with ammonium sulphate particles, showed isoprene induced formation of aerosol that contained such compounds as methyltetraols, C 5 alkene triols, 2-methylglyceric acid, glyoxal, methylglyoxal, acetals and hemiacetals, oragnosulphates and organonitrates, as well as oligomers of some of these.Yield and composition of aerosol depended on the presence of NO x or SO 2 in the gas phase, or H 2 SO 4 added to acidify the seed particles.
In particular, sulphuric acid and sulphur dioxide increased the yields of aerosol (Jang et al., 2002;Kleindienst et al., 2006;Surrat et al., 2007b).Sulphuric acid alone promoted the heterogeneous and aqueous formation of isoprene derivatives containing sulphate functional groups.Formation of these organosulphates was originally explained by direct reaction of the acid with products of isoprene oxidation such as aldehydes and polyols (Liggio et al., 2005;Surrat et al., 2007a).However, recent experimental study showed direct esterificaton of alcohols was not feasible kinetically (Minerath et al., 2008).On the other hand, isoprene was shown to inhibit the aqueousphase autoxidation of dissolved SO 2 , or rather sulphite ions (Rudzi ński, 2004).The suggested mechanism of inhibition included direct reaction of isoprene with sulphate Introduction

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Interactive Discussion radical-anions, which led to formation of sulphate esters of isoprene derivatives.The aim of this work was to show that aqueous reactions of sulphoxy radical-anions with isoprene can produce sulphur-containing derivatives of isoprene, and to show how these reactions depend on and influence the acidity of aqueous solutions during oxidation of dissolved S IV species (sulphur dioxide, sulphite ions) to S VI species (sulphuric acid, sulphate ions).

Method
The experimental method we used to study the transformation of isoprene coupled with autoxidation of S IV catalysed by manganous sulphate was described in detail by Rudzi ński (2004).We used a well stirred glass reactor of 0.785 dm −3 volume, closed with a Teflon cover and thermostatted within a water jacket.The reactor was operated homogenously (no gas phase was present), and in a batch manner.Each experimental run was prepared by filling the reactor with aqueous solution of manganese catalyst and oxygen, and adding an aliquot of aqueous solution of isoprene.Then, the reactor was sealed and the run was started by injecting an aliquot of aqueous solution of sodium sulphite.This solution was prepared from Na 2 SO 3 and Na 2 S 2 O 5 used in different proportions in order to obtain desired initial acidities of the reacting mixtures (Eqs.R1-R3).Only for the lowest initial acidity (pH=2.9),we additionally acidified the mixture with small amount of dilute sulphuric acid.
The variables recorded in each experimental run included pH, concentration of dissolved oxygen, temperature and high resolution UV spectra of reacting solutions.The Introduction

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Full pH and concentration of oxygen were measured continuously, using a Ross Ultra glass combination pH electrode and a 525A+ pH-meter from Thermo Electron, and a 9708-99 oxygen probe (Clark type) with 920 pH-meter from Orion (now also Thermo Electron).These data were recorded using a computer system equipped with a M6281 data acquisition card and a LabView application, both from National Instruments.The UV spectra were recorded periodically with a Jasco V570 spectrophotometer (0.2 nm bandwidth, 0.5 nm data pitch, 1 cm light path), using a closed sampling loop constructed from a T valve, a syringe and a Hellma Suprasil cell.Temperature in the reactor was measured with a mercury thermometer.Post-reaction solutions were analysed off-line, using an API 365 triple quadrupole mass spectrometer with electrospray ionization from Applied Biosystems.Concentrations of isoprene, sulphite ions and bisulphite ions in the reacting solutions were obtained from the recorded UV spectra by subtraction of reference spectra of these species.The accuracy of determination of subtraction coefficients was better than 12%.The reference spectra and the analysis of subtraction errors were given in (Rudzi ński, 2004).

Chemicals
Most of chemicals were purchased from Merck and used without further purification: C 5 H 8 for synthesis grade, stabilised with 100 ppm of 4-tert-butylpyrocatechol, Na 2 SO 3 , Na 2 S 2 O 5 and MnSO 4 • H 2 O ACS pro analysi grade.Sulphuric acid, pro analysis grade, was obtained from CHEMAN.Buffer standards used for daily calibration of pH electrodes were from Thermo Electron.
All solutions were prepared using the Milli-Qplus Millipore water.Oxygen was dissolved in water by equilibration with atmospheric air.Liquid isoprene was dissolved in water with the aid of an ultrasonic bath.
3.1 Influence of isoprene on autoxidation of S IV Autooxidation of S IV catalysed by transition metals and undisturbed by addition of foreign substances is characterised by a fixed ratio of S IV conversion to oxygen conversion, which reflects the 2:1 stoichiometry of the overall reaction (Eq.R4).In the presence of isoprene, the conversion ratio dropped to lower values, due to formation of oxygenated derivatives of isoprene (Table 1, see also mechanism in Sect.4, Figs.7-9).
Our previous work showed that isoprene slowed down the autoxidation of S IV in solutions of pH≥8 (Rudzi ński, 2004).Here, we confirmed this observation for higher concentrations of isoprene, and extended the experiment to neutral and acidic solutions (pH=3-8).
We found that isoprene slowed down the autoxidation of S IV in acidic and basic solu- isoprene could stop it almost completely.On the contrary, Figure 4 shows that in all experiments in neutral solutions, isoprene accelerated the autoxidation.The diverse influence of isoprene could result from subtle changes in a quantitative balance of three superimposed processes whose rates depended in different manner on the acidity of reacting solutions -the scavenging of sulphoxy radicals by isoprene, the formation of sulphoxy radicals during further transformation of isoprene, and the autoxidation of S IV itself.The chemical mechanism utilising this concept was discussed in Sect. 4.
Figures 2-4 show that in all experiments, the addition of isoprene reduced the acidification of reacting solutions.During uninhibited autoxidation of S IV , the solutions got acidified due to changes in the acid-base dissociation balance that took place when sulphite and bisulphite ions were converted into sulphate ions.Isoprene always reduced this acidification markedly, or even reversed it in experiments starting at pH>8.2.This influence was a natural consequence of slower autoxidation of S IV , but also reflected the formation of isoprene derivatives capable of associating with free protons in solutions.

Transformation of isoprene
The analysis of UV spectra of reacting solutions showed that in all experiments isoprene decayed proportionally to the decay of sulphite.The ratio of these conversions ranged from 0.21 to 0.40, and to 1.21, the latter value being rather inaccurate as calculated from conversions lower than 1% (Table 1).In neutral and slightly acidic solutions, we saw weak light absorption by non-sulphate reaction products with broad peak at 240 nm (Fig. 5).

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Full 2-methyl-but-2-enyl) ester (m/z=180.9).The discovered compounds have double bonds that can react further, for instance with radical species.Saturated organosulphates produced in such reactions would be similar to sulphate esters of methylglyceric acid and 2-methyltetrols that had been detected in field samples of atmospheric aerosols (G ómez Gonz ález et al., 2007;Surrat et al., 2007a).

Autoxidation of S
IV catalysed by manganese was studied extensively in the context of environmental protection and atmospheric chemistry (Pasiuk-Bronikowska et al., 1992;Berglund and Elding, 1995;Brandt and van Eldik, 1995;Fronaeus et al., 1998;Grgić and Berčič, 2001;Ermakov and Purmal, 2002;Kuo et al., 2006).It is a chain reaction initiated by Mn III and propagated by sulphoxy radical-anions which also regenerate Mn III .In the absence of inhibitors, the chain terminates via several radical-radical reactions.
The chemical mechanism constructed to explain our experiments with transformation of isoprene coupled with autoxidation of S IV consisted of three groups of reactions -autoxidation of Mn II to Mn III in presence of sulphite or bisulphite ions (Fig. 7), autoxidation of sulphite and bisulphite ions (S IV ) catalysed by MnSO 4 (Fig. 8), and transformation of isoprene initiated by sulphate radical-anions (Fig. 9) or by sulphite radical-anions (not shown, since analogous to the scheme for sulphate radical-anions).The individual reactions in the schemes were identified by symbols of rate constants rather than by numbers.Values of rate constants used in simulations were collected in Table 2, along with available references.Introduction

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Full 4.1 Group 1 -autoxidation of Mn II in presence of sulphite or bisulphite ions (Fig. 7) Chemical-kinetic modeling of our experiments required some mechanism for the initial oxidation of Mn II to Mn III in the presence of sulphite or bisulphite ions.Common reaction pathways suggested in literature included oxidation of Mn II by such impurities as other transition metal ions (Grgić and Berčič, 2001), and several oxidation schemes based on formation of manganese-sulphite complexes and oxygen adducts (Bassett and Parker, 1951;Berglund and Elding, 1995).Here, we used a mechanism first introduced by Rudzi ński and Pasiuk-Bronikowska (2000), which utilised the idea of formation of µ-peroxo-dimer of metal complexes with organic ligands (Wilkins, 1971;Mimoun, 1980), followed by intramolecular one-electron oxidation of Mn II and decomposition to Mn III , H 2 O 2 and hydroxyl ions (Gubelmann and Williams, 1983).The scheme comprised reversible formation of Mn II -sulphite complexes (k kf a /k kba and k kf b /k kbb ), reversible addition of oxygen molecule to this complex followed by intramolecular oneelectron oxiation of Mn II (k af a /k aba and k af b /k abb ), then formation of dimer from the The extent of Mn III autoxidation was very minor, and its purpose was to start the autoxidation of S IV by supplying the very initial amount of Mn 3+ in the absence of any other initiator.When the autoxidation chain grew longer, Mn II was oxidised back to Mn III in several reactions with sulphoxy radicals and ions.This is exactly why manganese is also called a catalyst of autoxidation.).The chain initiator, Mn 3+ , was regenerated in reactions of Mn 2+ cations with sulphite radical-anions (k ox11 ), sulphate radical-anions (k ox2 ) and peroxymonosulphate ions (k ox12 ).
4.3 Group 3 -transformation of isoprene initiated by sulphoxy radical-anions (Fig. 9) In this mechanism, isoprene could react with sulphite and sulphate radical-anions.Each radical could add to one of two double bonds of isoprene at any of four locations (1, 2, 3 or 4).We assumed there were no kinetic differences between these additions, so they could be represented by a single reaction for each type of sulphoxy radicals.The final products of isoprene transformation included in the mechanism were the compounds indicated by mass spectra of post-reaction solutions (Fig. 6).
Figure 9 shows the mechanism of isoprene transformation initiated by addition of sulphate radicals (k s1 ).The alkyl radicals were formed that accepted oxygen molecules Introduction

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Full to form peroxyalkyl radicals (k s2 ).The peroxyalkyl radicals could oxidise sulphite (k s3b ) or bisulphite ions (k s3a ), while turning into alkoxy radicals.The alkoxy radicals reacted with sulphite (k s4b ) or bisulphite ions (k s4a ) to form a hydroxy derivative of isoprene and to regenerate sulphite radical-anions.In a parallel reaction with oxygen, the alkoxy radicals turn into an oxy derivative of isoprene and produce hydroperoxy radicals (k s5 ).Hydroperoxy radicals reacted with sulphate radical-anions, producing sulphate ions (k s6 ).An analogous scheme (not shown here) was constructed for transformation of isoprene starting with the addition of a sulphite radical-anion to a double bond.The rate constants for respective reaction were distinguished by adding "2" in the subscript (k 2s1 , k 2s2 , k 2s3a , k 2s3b , k 2s4a , k 2s4b , k 2s5 ).The isoprene derivatives were similar to those obtained in the transformation initiated by sulphate radical-anions, but contained −OSO 2 substituent groups instead of −OSO 3 .

Simulation of experiments
Basing on the chemical mechanism described above, a corresponding set of ordinary differential equations was constructed and solved using a Mathematica 2.3 package.
Reversible reactions were treated kinetically, each one as a pair of independent reactions.Rate constants used for simulations were collected in Table 2. Constants, for which no experimental estimates were available, were adjusted using trial and error guesses.
The icals in solution to influence the overall inhibiting or accelerating action.In the present mechanism, 'all the work' had to be done by isoprene, which appeared to decay faster.
The ratio of simulated conversions of sulphite and oxygen was 2.0 in the absence of isoprene and 1.64-1.66 in the presence of isoprene, while the ratio of simulated conversions of isoprene and sulphite ranged from 0.28 to 0.31 (Table 3).The calculated ratios agreed well with those determined experimentally (Table 1).In the absence of isoprene all sulphite and bisulphite ions were converted into sulphate ions.Simulation showed that isoprene reduced this conversion by about 30%, as part of S IV species became the substituent groups in produced esters.
The acidity of reacting mixtures was reproduced quite well, with the exception of experiments that started at pH o =8.3, in which the experimental acidity decreased, while the simulated acidity slightly increased.This disagreement could be explained by ability of functional groups in isoprene derivatives to bind free protons from solutions.Generally, the fairly good agreement of simulated and experimental data shows that the proposed mechanism of isoprene degradation during autoxidation of S IV is a reasonable approximation of the real-life chemistry.However, the rate constants used for simulation should be considered a working-set only, and not a set of determined values.

Conclusions
We showed, by kinetic experiment, product analysis and chemical-kinetic modelling, that isoprene was capable of reacting with radicals in aqueous solutions, and turning itself into reactive radicals.Subtly regulating the balance of sulphoxy radical-anions, isoprene influenced the autoxidation of S IV catalysed by manganese.The autoxidation was significantly slowed down in basic and acidic solutions, and slightly accelerated in neutral solutions.However, production of sulphate ions and build-up of solution acidity were reduced in all cases.Products of isoprene transformation included unsaturated (2-methyl-4-oxo-but-2-enyl) ester, sulphuric acid mono-(4-hydroxy-2-methyl-but-2-enyl) ester.These esters are likely precursors of saturated organosulphur compounds similar to organosulphates detected in aerosol samples from field campaigns and from simulation chamber experiments.Reactions of isoprene with sulphoxy radical-anions have a few potential implications for atmospheric chemistry and atmosphere-biosphere interactions.They are another possible source of new organosulphur components of atmospheric aerosols and waters, which add to heterogeneous sources postulated recently.They can influence distribution of S IV (SO 2 ) and S VI compounds, and reduce formation of atmospheric acidity (sulphuric acid) over isoprene-emitting ecosystems.Probably, reactions of isoprene with sulphoxy radical-anions can also influence distribution of reactive sulphur and oxygen species inside isoprene-emitting organisms (plants, animals and humans) and on their surfaces.Further research is carried out to better identify products of isoprene reactions with sulphoxy radical-anions, and to investigate further reactions of these products with sulphoxy radical-anions and other radicals.Technol ., 41, 5363-5369, 2007b. 20871 Taraborrelli, D., Lawrence, M. G., Butler, T. M., Sander, R., and Drexler et al. (1991).c Huie and Neta (1984).d Huie and Neta (1987).e Averaged in CAPRAM.f 5.5×10 6 Herrmann et al. (1995), ∼ 2.2×10 8 Buxton et al. (1996).g s −1 .h Waygood and McElroy (1992).i 1.3×10 8 Herrmann et al. (1995), 4.8×10 7 Buxton et al. (1996).j McElroy and Waygood (1990).k Rudzi ński (2004).l 1.7×10 9 Buxton et al. (1996).
tions (pH o <7 and pH o >8), but accelerated it in neutral solutions (pH o =7-8).Figures 2 and 3 compare time traces of dimensionless concentration of oxygen (x O2 =[O 2 ]/[O 2 ] o ) obtained from experiments in acidic and basic solutions, with and without isoprene added (red and cyan lines, respectively).In all these cases, isoprene slowed down the adduct and the complex (k d f a /k d ba and k d f b /k d bb ) and intramolecular oxidation of second Mn III , and finally decomposition of the dimer to H 2 O 2 and Mn 3+ (k o2a /k o2b ).Unoxidised sulphite ions were released from the dimer, while oxygen molecule was used to build H 2 O 2 , which could oxidise sulphite ions directly to sulphate ions (k hpa and k hpb ).Two hydroxyl ions were also formed, which resulted in some alkalisation of the reaction mixture.The oxidation of Mn II could take two parallel paths, one startthe two bound by a reversible association-dissociation reaction (k ass /k d ys ).
results of simulation were compared to matching experiments in figures arranged according to initial values of pH of reacting mixtures.Time traces of reactant concentrations were shown in Figs. 10 and 11, while changes of pH of reacting solutions were shown in Fig. 12.Generally, time traces of SIV and oxygen concentrations were reproduced with good accuracy at all acidities.Consumption of isoprene was simulated quite accurately in acidic solutions, but was overestimated in basic and neutral solutions.This observation was explained by a deficiency of the chemical mechanism used, which treated the primary nonradical products of isoprene degradation as nonreactive.Each of these products contained a double bond, which could react with rad- sulphite and sulphate esters -sulphurous acid mono-(2-methyl-4-oxo-but-2-enyl) ester, sulphurous acid mono-(4-hydroxy-2-methyl-but-2-enyl) ester, sulphuric acid mono-

Fig. 1 .Fig. 2 .Fig. 3 .Fig. 4 .Fig. 5 .Fig. 6 .Fig. 7 .Fig. 8 .Fig. 9 .Fig. 10 .Fig. 11 .Fig. 12 .
Fig. 1.Transformation of isoprene coupled with autoxidation of S IV -time traces of reactant concentrations and pH obtained from an experiment in neutral solution (S IV is sum of HSO − 3 was initiated by formation of sulphite radical-anions in reaction of Mn 3+ with sulphite or bisulphite ions (k i b and k i a ), and proceeded via several propagation steps -formation of peroxymonosulphate radical-anions from sulphite radical-anions and oxygen (k p1 ), split reactions of peroxymonosulphate radical-anions with sulphite or bisulphite ions that led to formation of peroxymonosulphate ions and regeneration of sulphite radical-anions (k p21a and k p21b ), or to formation of sulphate radical-anions and sulphate ions (k p22a and k p22b ), and reaction of sulphate radicalanions with sulphite or bisulphite ions, which regenerates sulphite radical-anions (k p3a and k p3b ).
IV cas /k cd y ).Sulphate ions were also produced in reaction of peroxymonosulphate ions with sulphite ions (k f a and k kf b ), which could include formation of an intermediate complex that had not been included in the present mechanism.Chain termination occurred via radical-radical reactions (k t1 , k t21 , k t3 , k t4 ).Reaction of two peroxymonosulphate radical-anions could also regenerate sulphate radical-anions (k t22

Table 2 .
Rate constants used for simulation of chemical mechanism shown in Figs.7-9.

Table 3 .
Model conversion ratios in transformation of isoprene coupled with autoxidation of S IV .