FT-IR product study on the photo-oxidation of dimethyl sulphide in the presence of NO x...

Atmospheric Environment 35 (2001) 3769–3780

FT-IR product study on the photo-oxidation of dimethylsulphide in the presence of NOx}temperature dependence

Cecilia Arsenea,b, Ian Barnesa,*, Karl H. Beckera, Raluca Mocanub

aBergische Universit .at Gesamthochschule Wuppertal, Physikalische Chemie FB 9, Gauss Strasse 20, D-42097 Wuppertal, GermanybFaculty of Chemistry, Al.I. Cuza University Iasi, Carol I, 11, 6600 Iasi, Romania

Received 28 November 2000; received in revised form 21 February 2001; accepted 7 March 2001

Abstract

The products of the OH radical-initiated oxidation of dimethyl sulphide (DMS) have been investigated as a function oftemperature (284, 295, and 306K) and different initial NOx (NO+NO2) concentrations: initial NO was varied between434 and 2821 ppb and NO2 between 135 and 739ppb. The experiments were performed at 1000mbar total pressure insynthetic air using the photolysis of H2O2 as the OH-radical source and FT-IR spectroscopy to monitor reactants and

products. The major sulphur-containing products identified were SO2, dimethyl sulphoxide (DMSO), dimethyl sulphone(DMSO2), methane sulphonic acid (MSA), methane sulphonyl peroxynitrate (MSPN) and OCS. The variation of theproduct yields with temperature and NOx concentration are consistent with the occurrence of both addition and

abstraction channels in OH radical-initiated oxidation of DMS. Distinct trends in the yields of the various products havebeen observed as a function of temperature, initial NOx conditions and also reaction time as NO is consumed in thesystem. Increasing the initial NO concentration was found to depress the DMSO, SO2 and OCS formation yields and

enhance those of DMSO2, MSA and MSPN. The yield–time behaviour of DMSO2 is supportive of a formationmechanism involving addition of O2 to a (CH3)2S–OH adduct, formed via the addition channel, followed by sequentialreactions with NO and O2. The mechanisms controlling the concentration–time profiles of the individual products underthe present experimental conditions are discussed in detail and consideration is given to possible implications for the

photo-oxidation of DMS under ambient conditions. # 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Organic sulphur; OH radical; Sulphur dioxide; Dimethyl sulphoxide; Troposphere

1. Introduction

The main natural source of reactive sulphur in theatmosphere is dimethyl sulphide (DMS: CH3SCH3)

emitted from marine phytoplankton (Andreae, 1990;Spiro et al., 1992; Bates et al., 1992; Berresheim et al.,1995). In the absence of anthropogenic sources, oxida-

tion of DMS is considered to be a dominant source ofsubmicron particles in marine air (Andreae and Crutzen,1997; Andreae et al., 1995; Bates et al., 1987; Nguyen

et al., 1983). During the day DMS is oxidised mainly by

OH radicals and during the night by NO3 radicals.Based on field study measurements of reactive halogenspecies (X2, XO) and also the behaviour of NMHC it

has been speculated that Cl atoms and BrO radicals, inparticular, might also play a role in the chemistry of themarine boundary layer and consequently in the chem-

istry of DMS oxidation (Andreae and Crutzen, 1997;Berresheim et al., 1995). The present status concerningthe kinetics and chemistry of the various oxidant/DMS

interactions can be found in several reviews (Baroneet al., 1995; Berresheim et al., 1995; Campolongo et al.,1999; Ravishankara et al., 1997).Oxidation of DMS is a complex process, however,

it is well established that the reaction of OH radicals

*Corresponding author. Tel.: +49-202-439-2510; fax: +49-

202-439-2505.

E-mail address: [email protected]

(I. Barnes).

1352-2310/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.

PII: S 1 3 5 2 - 2 3 1 0 ( 0 1 ) 0 0 1 6 8 - 6

with DMS proceeds through two channels: H-atomabstraction by OH from a methyl group and

OH-radical addition to the S atom. The relativeimportance of the addition versus abstraction channelsis temperature dependent (Hynes et al., 1986). Attack of

OH radicals at the CH3 produces water and themethylthiomethyl radical (CH3SCH2), while additionto the S atom produces the dimethylhydroxysulphuranylradical ((CH3)2S–OH), an adduct which can either

dissociate back to reactants or react further to formproducts.

OHþ CH3SCH3 ! CH3SCH2þH2O

ðabstraction pathwayÞ;

OHþ CH3SCH3 ! ðCH3Þ2SOH ðaddition pathwayÞ:

Major end-products of the OH-radical initiated DMSoxidation observed both in laboratory and the tropo-sphere include species such as sulphur dioxide (SO2),

dimethyl sulphoxide (DMSO: CH3S(O)CH3), dimethylsulphone (DMSO2: CH3S(O)2CH3), methane sulphonicacid (MSA: CH3S(O)2OH) and methane sulphonylperoxynitrate (MSPN: CH3S(O)2OONO2) (Arsene

et al., 1999; Barnes et al., 1996; Berresheim et al.,1995; Patroescu et al., 1999; Ravishankara et al., 1997;S�rensen et al., 1996).

It is reasonably well established that in the presence ofNO the CH3SCH2 radical reacts to produce CH3Sradicals:

CH3SCH2þO2þM ! CH3SCH2OOþM;

CH3SCH2OOþNO ! CH3SCH2OþNO2;

CH3SCH2OþM ! CH3SþHCHOþM:

Further reactions of CH3S produce SO2, MSA andMSPN. The fate of the (CH3)2S–OH adduct is not sowell established. Possible reactions of the (CH3)2S–OH

adduct with O2 include:

ðCH3Þ2SOHþO2 ! HO2þCH3SðOÞCH3;

! CH3OOþ CH3SOH;

! OHþ CH3SðOÞ2CH3;

! CH3þCH3SðOHÞOO;

! CH3SðOHÞðOOÞCH3:

The addition pathway is known to produce DMSO andthe results from a recent product study from thislaboratory (Arsene et al., 1999) support that the major

fate of the (CH3)2S–OH adduct under NOx-free condi-tions is reaction with O2 to form DMSO. This result wasin contradiction with a pulsed laser photolysis/pulsed

laser-induced fluorescence study on the reaction of OHwith DMS by Turnipseed et al. (1996) who reported abranching ratio of F=0.5� 0.15 for the production of

DMSO based on the conversion of the HO2 produced toOH by reaction with NO. As discussed in Arsene et al.

(1999) the reason for the discrepancy is not entirelyclear, however, the reason may well be due to reaction

conditions employed in the two studies, i.e. with andwithout NO in the reaction system.Although the ambient NOx levels probably do

not have a large effect on the product distributionof the atmospheric photo-oxidation of DMS (Baroneet al., 1995), it is well established from smogchamber experiments where much higher concen-

trations than those found in the atmosphere areemployed, that the NOx levels in these experimentsinfluence quite substantially the product distribution

(Patroescu et al., 1999). In order to extract meaningfulresults from smog chamber studies on DMS, which canbe applied to the photo-oxidation of DMS under

atmospheric conditions, it is necessary to have a fullknowledge of the influences of NOx on the experimentalsystem.

In an extension of the work within our laboratory ondifferent aspects of the photo-oxidation mechanism ofDMS, we report here a detailed product study of the OHradical initiated oxidation of DMS as a function of

temperature and different initial concentrations of NOx

(NO+NO2). The work is aimed at clarifying someaspects of the influence of NOx on the product

distribution from the photo-oxidation of DMS withthe final goal of constructing a chemical oxidationmechanism for DMS relevant for ambient atmospheric

conditions.

2. Experimental

The experiments were performed in a temperatureregulated 1080 l quartz reaction chamber at a total

pressure of 1000mbar using synthetic air as bath gas.The reaction chamber is equipped with 32 super actinicfluorescent lamps (Philips TL05/40W: 3205l5480 nm,

lmax ¼ 360 nm) and 32 low-pressure mercury lamps(Philips TUV 40W: lmax ¼ 254 nm). The experimentalset-up is described in detail elsewhere (Barnes et al.,1994a). Experiments have been performed at 284, 295

and 306� 2K on DMS/H2O2/air mixtures for differentinitial NOx conditions. The photolysis of H2O2 with thelow-pressure mercury lamps was used to generate the

OH radicals:

H2O2 þ hn ðl¼ 254 nmÞ ! OHþOH:

The concentration–time behaviour of the DMS, NOx

and products were monitored in situ using a multi-reflection White-type mirror system (5.6m base length,

498m total path length) mounted in the reactor chamberand coupled to a Fourier transform infrared (FT-IR)spectrometer (Bruker IFS 88) equipped with a KBr

beam splitter and HgCdTe (MCD) detector. Typicalinitial concentration of DMS and H2O2 were kept fairly

C. Arsene et al. / Atmospheric Environment 35 (2001) 3769–37803770

constant during the experiments at 6–7 ppm andapproximately 25–30 ppm, respectively, where

1 ppm=2.46� 1013molecules cm�3 at 298K. The initialNO concentrations were varied between 434 and2821 ppb. The NO was always associated with an initial

NO2 concentration due to oxidation of NO duringtransfer and injection into the reactor. The initial NO2

concentrations varied between 135 and 739 ppb.In typical experiments infrared spectra were recorded

at 1 cm�1 resolution and 10–15 spectra were collectedover irradiation periods of 20–30min. Concentrations ofall species, reactant and products were determined by

computer-aided spectral abstraction using calibratedreference spectra generated in this laboratory.

3. Results and discussion

The sulphur-containing products identified in theirradiation of DMS/H2O2/NOx reaction mixtures insynthetic air included DMSO, DMSO2, SO2, MSA andMSPN. Formation of OCS was also observed during the

experiments when the NOx concentration fell to lowlevels. Fig. 1(a–f ) shows plots of the measured yield–time profiles for all the sulphur-containing products

observed, as a function of the different temperatures andinitial NOx conditions investigated. The yields of theproducts shown in Fig. 1(a–f ) have not been corrected

for secondary loss processes such as further reactionwith other oxidants, e.g. OH, or wall loss. In order tofacilitate the discussion on trends in the product

behaviour, the yield–time profiles of DMSO, DMSO2,SO2, MSA, MSPN and OCS for 295K and 1000mbarsynthetic air as a function of NOx concentration areshown separately in Fig. 2(a–f ).

SO2 and DMSO undergo considerable further oxida-tion in the reaction system. In order to obtain the‘‘exact’’ formation yields for these compounds correc-

tions need to be performed for reaction with OHradicals and also loss to the reactor walls. Correctionshave been made using the mathematical procedure

described in Tuazon et al. (1986) and also outlined ina recent publication from this laboratory on theoxidation of DMS under NOx free conditions (Arsene

et al., 1999). The constants used for the corrections arethe same as those given in Arsene et al. (1999), thereforedetails are not given here.Figs. 3 and 4 show examples of plots of the DMSO

and SO2 concentrations corrected for wall loss andreaction with OH radicals against the consumption ofDMS, the slopes give the ‘‘true’’ formation yields of the

compounds. Except for the study at 284K, the plots forDMSO in Fig. 3 show quite distinct deviations fromlinearity. Two distinct linear periods are often evident,

an initial period followed by a transition to a secondperiod where the DMSO yield increases compared to the

first period. The onset of the increase occurs at the pointwhen the NO concentration approaches values below

the system detection limit (10 ppb). This behaviour isattributed to a change in the reaction mechanism, whichis discussed below. Similar behaviour was not observed

in the SO2 yield plots, however, since SO2 can be formedin both the abstraction and addition channels (Arseneet al., 1999) the effect here is probably not so great andnot observable within the error limits of the measure-

ments. The corrected formation yields of DMSO andSO2 are shown in Tables 1 and 2, respectively, for theinitial reaction period. The yields are compared to

fraction of the reaction estimated to proceed via theaddition channel.

3.1. General trends in products with temperature andinitial NOx conditions

3.1.1. DMSOThe yield–time behaviour of DMSO varied system-

atically with changes in temperature and initial NOx.The uncorrected yield–time profiles for DMSO (Figs. 1a

and 2a) show that at each temperature and initial NOx

concentration the yield initially increases, passesthrough a maximum and then decreases again. The

magnitude of the yield turning point increases withdecreasing temperature for similar initial NOx condi-tions. The DMSO yield is also strongly dependent on the

initial NO concentration, increasing the initial NOconcentration systemically depressed the DMSO yieldmaximum at each of the temperatures investigated. For

the measurements at 306K, the bell-shaped be-haviour observed at 284 and 295K is not so pro-nounced. After attainment of the yield maximum there isonly a further slow decay of DMSO. In the experiments

at 306K the oxidation of DMS also slows downsignificantly near the point of the DMSO maximum.This behaviour in DMS suggests termination of the

propagation of the OH radical production chain atthis point under the conditions of these experiments. Theprobable explanation for the termination is efficient

scavenging of the OH radicals via the reaction OH+NO2+M!HNO3+M at the very high levels of NO2

which are generated under the conditions of these

experiments. Therefore, the flatness of the DMSO curvesat 306K after the maximum is presently attributed totermination of its photo-oxidation under the conditionsof the experiment.

The increase in the DMSO yield with decreasingtemperature and the shape of the curves is analogous tothe behaviour reported recently in our paper on the

oxidation of DMS under NOx free conditions (Arseneet al., 1999). As stated in the Introduction the OHinitiated oxidation of DMS is known to proceed by both

addition and abstraction channels, the addition channelproducing the dimethylhydroxysulphuranyl radical

C. Arsene et al. / Atmospheric Environment 35 (2001) 3769–3780 3771

((CH3)2S–OH). This adduct can thermally de-compose either back to reactants or products or itcan react further with O2 to form products which

include DMSO.

ðCH3Þ2S2OHþD! CH3SCH3þOH;

ðCH3Þ2S2OHþO2! products including DMSO:

Based on the above mechanism the importance of the

addition channel and hence DMSO formation willincrease with decreasing temperature. This is in broad

agreement with the experimental observations reportedhere. The bell-shape of the yield–time curves is causedby a period of rapid production from OH+DMS

followed by a period of fast removal via OH+DMSOas this removal reaction gradually becomes morecompetitive with the production process.

However, the simple mechanism above cannot explainthe significant dependence of the DMSO yield on theinitial NO concentration. This is an effect that has notbeen previously reported. The simplest explanation for

this phenomenon is that the DMS–OH also undergoes a

Fig. 1. Plots of the yields of DMSO (a), DMSO2 (b), SO2 (c), MSA (d), MSPN (e), and OCS (f) as a function of temperature (284, 295

and 306K) and initial NOx concentration against reaction time.


reversible addition of O2.

ðCH3Þ2S2OHþO2þM Ð CH3SðOHÞðO2ÞCH3þM:

Further reaction of this peroxy radical with NO formsan alkoxy radical which reacts further with O2 to formdimethyl sulphone (DMSO2).

CH3SðOHÞðO2ÞCH3þNO ! CH3SðOHÞðOÞCH3þNO2;

CH3SðOHÞðOÞCH3þO2! CH3SðOÞ2CH3þHO2:

Since formation of DMSO2 is also observed in NOx-freeproduct studies of DMS photo-oxidation (Arsene et al.,

1999), there are obviously other routes forming DMSO2.Heterogeneous reactions of DMS with H2O2 at thereactor walls can lead to formation of DMSO2, however,

in the present work this was insignificant at thetemperatures of the experiments. Self reaction of theCH3S(OH)(O2)CH3 radical or reaction with otherperoxy radicals could also lead to the formation of the

CH3S(OH)(O)CH3 alkoxy radical and subsequently

Fig. 2. Plots of the yields of DMSO (a), DMSO2 (b), SO2 (c), MSA (d), MSPN (e), and OCS (f) for 295K and 1000mbar synthetic air

as a function of different initial NOx concentrations against reaction time.


Fig. 3. Plots of the DMSO concentration (corrected for wall

loss and reaction with OH radicals) against the consumption of

DMS at 284, 295 and 306K for different initial NOx

concentrations.

Fig. 4. Plots of the SO2 concentration (corrected for wall loss

and reaction with OH radicals) against the consumption of

DMS at 284, 295 and 306K for different initial NOx

concentrations.


DMSO2, e.g.

CH3SðOHÞðO2ÞCH3þCH3SðOHÞðO2ÞCH3

! 2CH3SðOHÞðOÞCH3þO2;

CH3SðOHÞðO2ÞCH3þCH3O2

! CH3SðOHÞðOÞCH3þCH3OþO2:

It is quite probable that such routes are operative

under NOx-free conditions in smog chambers. Thereaction of OH with DMSO is also known to produceDMSO2 (Arsene et al., 1999; S�rensen et al., 1996)possibly via

OHþ CH3SðOÞCH3! CH3SðOÞðOHÞCH3;

CH3SðOÞðOHÞCH3þO2! CH3SðOÞ2CH3þHO2:

However, this formation route cannot explain theobserved behaviour of DMSO with NOx. The yieldmaximum of DMSO2 would be expected to be shifted tolonger reaction times compared to that of DMSO and

no effect on variation of NO would be expected.Reaction of NO2 with the OH–DMS adduct or evenan OH–DMSO adduct are also potential routes to

DMSO2 formation:

CH3SðOHÞCH3þNO2! CH3SðOHÞðOÞCH3þNO;

CH3SðOHÞðOÞCH3þO2ðNO2Þ

! CH3SðOÞ2CH3þHO2ðHONOÞ;

CH3SðOÞðOHÞCH3þNO2! CH3SðOÞ2CH3þHONO:

Table 1

Corrected yields for the formation of DMSO in the reaction of OH with DMS in 1000mbar of synthetic air as a function of

temperature for the initial reaction period (see text for details). The yields are compared to the fractions of the reaction occurring via

the addition pathway calculated using the results of Hynes et al. (1986) for the specific reaction conditions

Temperature (K) NO (ppb) NO2 (ppb) Corrected DMSO yields

(M% yield)

Contribution of addition

pathway (%)

284 434 135 34 52

937 356 26

1200 515 22

295 685 251 27 33

886 368 19

1070 505 17

306 1579 141 15 17

2123 689 11

2821 739 9

Table 2

Corrected yields for the formation of SO2 in the reaction of OH with DMS in 1000mbar of synthetic air as a function of temperature

for the initial reaction period (see text for details). The yields are compared to the fractions of the reaction occurring via the addition

pathway and abstraction pathways calculated using the results of Hynes et al. (1986) for the specific reaction conditions

Temperature (K) NO (ppb) NO2 (ppb) Corrected SO2

yields (M% yield)

Contribution of

addition pathway (%)

Contribution of

abstraction

pathway (%)

284 434 135 65 52 48

937 356 43

1200 515 35

295 685 251 41 33 67

886 368 36

1070 505 33

306 1579 141 41 17 83

2123 689 34

2821 739 33


However, if these were major routes to DMSO2

formation their importance would be expected to

increase with increasing reaction time since the NO2

concentration is increasing in the system; this behaviouris not observed.

The formation of DMSO2 via reaction of a DMS–OH–O2 peroxy radical with NO should depress the yieldof DMSO from the addition channel. As can be seen inTable 1, the DMSO formation yield in the initial stages

of the reaction is systemically depressed with increasinginitial NO concentration. Further, in the plots of thecorrected DMSO concentration against the consump-

tion of DMS (Fig. 3) the formation of DMSO increaseswhen the NO concentration falls to low levels. All ofthese facts support that a major mechanism for the

formation of DMSO2 in the reaction system is reactionof a DMS–OH–O2 peroxy radical with NO as describedabove. As discussed below the yield–time behaviour of

DMSO2 appears to be generally in line with thismechanistic conception.It is of interest at this stage to compare the corrected

formation yields of DMSO with the estimated fraction

of the reaction proceeding via the addition pathwaycalculated using the kinetic data of Hynes et al. (1986). Itshould be borne in mind, when comparing the corrected

DMSO yield with the estimation of the OH+DMSreaction proceeding via the addition pathway, that thereare quite substantial uncertainties involved in both the

correction of the DMSO yields, particularly at 306Kwhere the yields are low, and also in the estimates of thefraction of the reaction proceeding via addition asdiscussed in Arsene et al. (1999). Based on the estimates

in Table 1 it is evident that at 284 and 295K thebranching ratios for DMSO formation from CH3S(OH)-CH3+O2 are considerably less than unity under all the

conditions whereas at 306K the differences appear to benot quite so pronounced, particularly at the low NOlevels. The branching ratio decreases with increasing

initial NO and the effectiveness of the depression of theDMSO yield with NO would appear to drop off withincreasing temperature. This general type of behaviour

would be expected for a reaction sequence involvingformation of a thermally unstable DMS–OH–O2 peroxyradical adduct.This finding is in stark contrast to other results from

this laboratory (Arsene et al., 1999), where under NOx-free conditions the DMSO yield was near unity under allthe reaction conditions employed. The branching ratio

values for DMSO formation are, however, quite similarto the branching ratio of F=0.5� 0.15 at 234 and258K for HO2 production from CH3S(OH)-

CH3+O2!HO2+DMSO reported by Turnipseedet al. (1996) in a pulsed laser photolysis/pulsed laser-induced fluorescence study on the reaction of OH with

DMS. Turnipseed et al. measured HO2 in theirexperiments by converting it to OH with NO. In their

analysis of their system Turnipseed et al. may not onlyhave been converting HO2 to OH with NO but they may

also have been disturbing the equilibrium DMS–OH+O2ÐDMS–OH–O2 through the reaction DMS–OH–O2+NO!DMS–OH–O+NO2. Such a reaction if

occurring in their system would have resulted in areduction in the apparent branching ratio for DMSOformation. It is believed that the existence of theaddition of O2 to the DMS–OH adduct with subsequent

reaction with NO offers a plausible explanation for thedifferences in the DMSO yield found between systemsoperating with and without NO.

3.2. Formation of DMSO2

As for DMSO, with the exception of the measure-ments at 306K, the DMSO2 yield goes through adistinct maximum (Figs. 1b and 2b). The highest

DMSO2 yields are obtained at the lowest temperatureand the lowest yields at the highest temperature. In allcases, the DMSO2 yield increases with increasing initialNO concentration. DMSO2 reacts very slowly with OH

radicals (Falbe-Hansen et al., 2000), therefore, apartfrom slow loss to the reactors walls it is not expected toreact further in the system. The bell shape of the

DMSO2 yield curves for the 284 and 295K measure-ments (Figs. 1b and 2b) is what would be expected for aformation mechanism which is active in the initial stages

of the reaction and then ceases when one of the reactantsis exhausted, i.e. in this case NO. At 284 and 295K,therefore, the behaviour of DMSO2 is generally in

qualitative agreement with formation from reaction of aDMS–OH–O2 peroxy radical with NO being animportant formation pathway in the reaction system.For the DMSO2 profiles obtained at 306K the DMSO2

yield increases to a plateau rather than going through amaximum. The behaviour of the DMSO2 yield at 306Kis attributable to the considerable slowing in the DMS

oxidation just after the DMSO maximum as discussedabove. This halts production of DMSO2 and causes itsyield–time plot to approach a plateau. In general, the

overall behaviour of DMSO2 with temperature and NOconditions is not inconsistent with the occurrence of theDMS–OH–O2 peroxy radical mechanism presentedabove.

3.3. Formation of sulphur dioxide

The uncorrected yield–time curves of SO2 show asteep initial rise followed by a much slower increase to aplateau. Since it was difficult to set the initial NOx

conditions to the same level for each of the experimentsat the different temperatures, there is no general patternwith respect to the onset of the approach to the plateau,

which is governed mainly by the level of NO2 in thesystem (see later). However, at all 3 temperatures there is


a general decrease of the yield with increasing initialNOx concentration, the effect being most pronounced at

the lowest temperature. The corrected molar yields ofSO2 for the respective NOx conditions are listed in Table2. The yields of SO2 are much lower than those observed

in NOx-free systems (Arsene et al., 1999), but are similarto those which have been reported previously for DMSoxidation systems containing NOx (Patroescu et al.,1999; S�rensen et al., 1996).

In contrast to DMSO, deviations from linearity weregenerally not observed in the plots of the corrected SO2

concentration against consumption of DMS (Fig. 4).

This is not too surprising since SO2 is not a directproduct and the pathways leading to SO2 formation inthe presence of NOx are much more complex than those

of DMSO and involve the formation of thermallyunstable peroxynitrate intermediates. The complexityof the SO2 formation probably masks any deviations

from linearity. However, the increasing importance ofthe DMSO2 formation pathway with increasing initialNO should be reflected in decreases in the SO2 yield. Asseen in Table 2 the corrected SO2 yield decreases with

increasing initial NO concentration in line with mechan-istic expectations.

3.4. Formation of MSA and MSPN

The yield–time contours of MSA all pass through a

fairly sharp maximum followed by a slow decay causedby loss to the aerosol phase and uptake by the reactorwall. For all of the temperatures the same trend is

observed, i.e. the yield maximum increases with increas-ing initial NO concentration. At 284 and 295K themaximum yield varies between approximately 5 and

12M%. At 306K where the initial NO concentrationsare much higher the MSA yield maximums are muchhigher and vary between approximately 28 and 46M%.This behaviour is in line with NO and NO2 promoting

the formation of MSA as observed in previous studiesvia reactions such as

CH3SOOþNO ! CH3SOþNO2;

CH3SðOÞOOþNO ! CH3SðOÞOþNO2;

CH3SðOÞ2OOþNO ! CH3SðOÞ2OþNO2;

CH3SþNO2! CH3SOþNO;

CH3SOþNO2! CH3SO2þNO;

CH3SO2þNO2! CH3SðOÞ2OþNO

followed by

CH3SðOÞ2OþRH ! CH3SðOÞ2OHþR:

Like MSA, the time-yield contours of MSPN(CH3S(O)2OONO2) also go through a maximum

followed by a slow decay and at all three temperaturesthe yield maximum increases with increasing initial NOx

concentration. However, there are several distinctdifferences compared to the MSA profiles. In contrastto MSA, the molar yield of MSPN increases with

decreasing temperature, the opposite applies for MSA.The position of the MSPN yield maximum also shifts tolonger reaction times with decreasing temperature andbecomes broader. This behaviour is in agreement with

what would be expected for a thermally labile peroxynitrate compound.

CH3SðOÞ2OONO2 þM Ð CH3SðOÞ2OOþNO2þM:

In such an equilibrium, decreases in temperature andincreases in the NO2 concentration obviously favour

formation of the peroxy nitrate. The ratio of NO to NO2

is also important since the equilibrium is in competitionwith

CH3SðOÞ2OOþNO ! CH3SðOÞ2OþNO2:

which favours formation of MSA. The variation of theprofiles of MSA and MSPN in Figs. 1d and e are in

accord with the expected behaviour from the reactionsoutlined above.

3.5. Formation of OCS

The formation of OCS is only observed after an initialdelay in the system. As seen in Figs. 1f and 2f, the onset

of the formation is dependent on the initial NOx

concentrations, the time of the onset increasing withincreasing initial NOx concentration. At the high initial

NOx concentrations of the experiments performed at306K formation of OCS is not observed. Analysis of theconcentration–time profiles shows that the onset of OCS

formation occurs as the NO concentration approacheszero (Fig. 5). This observation is line with other reportsof OCS formation from the oxidation of DMS

previously reported from this laboratory (Barnes et al.,1994b, 1996; Patroescu et al., 1999).Increasing the temperature should have the effect of

increasing the branching ratio for CH3SCH2 formation

in the OH radical-initiated oxidation of DMS, a radicalwhich is considered to be the precursor of the CH3Sradical and possibly methyl thiolformate (CH3SCHO).

Possible further reaction of CH3S radicals include

CH3Sþ CH3S ! CH3SHþH2C¼¼S;

CH3SþNO2! CH3SOþNO;

H2C¼¼SþOH=hn! HC¼¼SþH2O=H;

HC¼¼SþO2! O¼¼C¼¼SþOH:

The thermal decomposition of CH3SCO radicalsproduced in the photolysis or OH radical-initiated


oxidation of CH3SCHO is a possible source of OCS

formation.

CH3SCHOþOH=hn! CH3SC¼¼OþH2O=H;

CH3SC¼¼OþD! CH3þO¼¼C¼¼S:

The yields of OCS in Figs. 1f and 2f do not represent the

absolute yields. The absolute yields of OCS calculatedfrom the time-point of formation, an on absolute basis,varied from 0.17 to 0.24M% yields (Fig. 6). Acomparison of the molar yields of OCS obtained from

the present study (under 0.2% S) with the molar yieldsof OCS of about 0.7% S obtained from the OH radical-initiated oxidation of DMS under NOx free conditions

(Arsene et al., 1999; Barnes et al., 1994b) shows that thepresence of NOx in the photo-irradiated mixture is veryefficient in suppressing OCS production.

Methyl thioformate was not observed as a product inthe present study which is characterised by very highNOx levels. Formation of MTF has been previouslyobserved in the OH-initiated oxidation of DMS in the

presence of NOx, (Patroescu et al., 1999), however,formation was only observed when the level of NO was530 ppb.

4. Conclusions and atmospheric implications

The OH radical-initiated oxidation of DMS has beeninvestigated as a function of temperature and different

initial NOx concentration. Formation of DMSO,DMSO2, SO2, MSA, MSPN and OCS was observed to

Fig. 5. Plots of the concentration–time profiles for OCS and NO which show that the onset of OCS formation occurs when the NO

concentration approaches zero (examples for 284 and 295K).

Fig. 6. Plots of the OCS concentration against the consump-

tion of DMS at 284 and 295K for different initial NOx

concentrations.


occur in the reaction systems. The level of NO in thereaction system was a critical factor in determining the

yields of DMSO and DMSO2 which were anti-corre-lated.The data support that the major channel for DMSO2

formation is probably reversible addition of O2 to theDMS–OH adduct formed in the addition channel of thereaction of OH with DMS followed by sequentialreactions with NO and O2.

DMS2OHþO2þM Ð OH2DMS2O2þM;

OH2DMS2O2þNO ! OH2DMS2OþNO2;

OH2DMS2OþO2! CH3SðOÞ2CH3þHO2:

This reaction sequence has been proposed previously ina slightly modified form by Yin et al. (1990). In themechanism of Yin et al. the reaction DMS–

OH+O2+M was not reversible and a thermal pathway

OH2DMS2O2! DMSOþHO2

was included. This paper does, however, represent thefirst study where experimental evidence for the occur-rence of such a reaction mechanism has been found.

It is difficult to extrapolate the present results totypical atmospheric concentrations of NO to obtainmeaningful information about the possible significance

of the above reaction sequence in the troposphere.Should the reaction sequence not be significant undertropospheric conditions this would imply a near unit

formation yield of DMSO from the addition channel ofthe reaction of OH with DMS. However, under the lowNOx conditions which often prevail in the remotemarine atmosphere there remains the possibility of

reaction of OH–DMS–O2 adduct with HO2 or otherperoxy radicals which may convert it to DMSO2,particularly at low temperature, for which no informa-

tion presently exists.The present study taken together with the results

published in Arsene et al. (1999) represents a very

detailed body of concentration–time product data on thereaction products of the OH-radical initiated oxidationof DMS as a function of temperature and NOx

conditions. A detailed modelling study of the results is

currently ongoing and should help to elucidate andbetter define the potential atmospheric importance ofmany of the speculative or uncertain aspects of the DMS

oxidation processes.

Acknowledgements

Financial support for this work by the BMBF within

the AFS project and the European Commission withinthe EL CID project is gratefully acknowledged.

References

Andreae, M.O., 1990. Ocean-atmosphere interactions in the

global biogeochemical sulfur cycle. Marine Chemistry 30,

1–29.

Andreae, M.O., Crutzen, P., 1997. Atmospheric aerosols:

biogeochemical sources and role in atmospheric chemistry.

Science 216, 1052–1058.

Andreae, M.O., Elbert, W., de Mora, S.J., 1995. Biogenic sulfur

emissions and aerosols over the tropical South Atlantic. 3.

Atmospheric dimethylsulfide, aerosols and cloud condensa-

tion nuclei. Journal of Geophysical Research 100, 11335–

11356.

Arsene, C., Barnes, I., Becker, K.H., 1999. FT-IR product

study of the photooxidation of dimethyl sulphide: Tempera-

ture and O2 partial pressure dependence. Physical Chemistry

Chemical Physics 1, 5463–5470.

Barnes, I., Becker, K.H., Mihalopoulos, N., 1994a. An FTIR

product study of the photooxidation of dimethyl disulfide.

Journal of Atmospheric Chemistry 18, 267–289.

Barnes, I., Becker, K.H., Patroescu, I.V., 1994b. The

tropospheric oxidation of dimethyl sulfide: a new source

of carbonyl sulfide. Geophysical Research Letter 21,

2389–2392.

Barnes, I., Becker, K.H., Patroescu, I.V., 1996. FTIR product

study of the OH initiated oxidation of dimethyl sulphide:

observation of carbonyl sulphide and dimethyl sulphoxide.

Atmospheric Environment 30, 1805–1814.

Barone, S.B., Turnipseed, A.A., Ravishankara, A.R., 1995.

Role of adducts in the atmospheric oxidation of dimethyl

sulfide. Faraday Discussions 100, 39–54.

Bates, T.S., Cline, J.D., Gammon, R.H., Kelly-Hansen, R.S.,

1987. Regional and seasonal variation of the flux of oceanic

dimethylsulfide to the atmosphere. Journal of Geophysical

Research 92, 2930–2938.

Bates, T.S., Lamb, B.K., Guenther, A., Dignon, J., Stoiber,

R.E., 1992. Sulfur emissions to the atmosphere from natural

sources. Journal of Atmospheric Chemistry 14, 315–337.

Berresheim, H., Wine, P.H., Davis, D.D., 1995. Sulfur in the

atmosphere. In: Singh, H.B. (Ed.), Composition, Chemistry,

and Climate of the Atmosphere. Van Nostrand Reinhold,

New York, pp. 251–307.

Campolongo, F., Saltelli, A., Jensen, N.R., Wilson, J., Hjorth,

J., 1999. The role of multiphase chemistry in the oxidation of

dimethylsulphide (DMS). A latitude dependent analysis.

Journal Atmospheric Chemistry 32, 327–356.

Falbe-Hansen, H., Sorensen, S., Jensen, N.R., Pedersen, T.,

Hjorth, J., 2000. Atmospheric gas phase reactions of

dimethylsulphoxide and dimethylsulphone with OH and

NO3 radicals, Cl atoms and ozone. Atmospheric Environ-

ment 34, 1543–1551.

Hynes, A.J., Wine, P.H., Semmes, D.H., 1986. Kinetics and

mechanism of OH reactions with organic sulfides. Journal of

Physical Chemistry 90, 4148–4156.

Nguyen, B.C., Bonsang, B., Gaudry, A., 1983. The role of the

ocean in the global atmospheric sulfur cycle. Journal of

Geophysical Research 88, 10903–10914.

Patroescu, I.V., Barnes, I., Becker, K.H., Mihalopoulos, N.,

1999. FT-IR product study of the OH-initiated oxidation of

DMS in the presence of NOx. Atmospheric Environment 33,

25–35.


Ravishankara, A.R., Rudich, Y., Talukdar, R., Barone, S.B.,

1997. Oxidation of atmospheric reduced sulfur compounds:

perspective from laboratories. Philosophical Transactions of

the Royal Society of London 332, 171–182.

S�rensen, S., Falbe-Hansen, H., Mangoni, M., Hjorth, J.,

Jensen, N.R., 1996. Observation of DMSO and CH3S(O)OH

from the Gas Phase Reaction between DMS and OH.

Journal of Atmospheric Chemistry 24, 299–315.

Spiro, R.E., Jacob, D.J., Logan, J.A., 1992. Global inventory of

sulfur emissions with 18� 18 resolution. Journal of Geophy-sical Research 97, 6023–6036.

Tuazon, E.C., Mac Leod, H., Atkinson, R., Carter,

W.P.L., 1986. a-Dicarbonyl yields from the NOx}

air photooxidations of a series of aromatic hydro-

carbons in air. Environmental Science and Technology 20,

383–387.

Turnipseed, A.A., Barone, S.B., Ravishankara, A.R., 1996.

Reaction of OH with dimethyl sulfide. 2. Products and

mechanisms. Journal of Physical Chemistry 100, 14703–

14713.


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