Atkinson, R.

Harris, G.W.

Pitts Jr., J.N.

English

YorkSpace

TEMPERATURE DEPENDENCE OF THE REACTION OH + S02 + M -+ HS03 + M FOR M :;; Ar AND SF6C.W. HARRIS. R. ATKINSON'" and J.N. PITTS Jr.Absolule ratc constants kif' ha~c becn det.:nmncd. uSing a 113sh-photob slS-rcsonance-fluorescence technique. for the:ltmo«phcnc:JlI} Imparl3nt reaction of OH radlc:Jl. 'I.uth 502 for M = Ar and SF6 over the temperature range 298-424 K attOI:JI prcsslI[cs from 98-653 Torr At ==650 Torbtotal pressure. the Arrhemus e'\prCSSlOns k~I(M = Ar) = 1.16 X l(1t4X e",p(2370:!: 300)/RTI cm J molcculc- t 5- 1 • kt/(M = SF6) = I 27 X 1(11J e",p[(1495 ± 300)/RTI cm J molecule-Is-I,""cre obtamed \\here the (nc~atl"c) aCtlV:lIIOn energlcs are In caI/mole The ratc const:lnts at 298 K and ==650 Torr arck ~1(l\1 = Ar} = (6 49 = 0 8~) x 1(113 cm 3 molcculc-1 s-/ and k~l(M =sr6) = (l 61 ± O.22) X url2 cm J molecule-1 s-l A~aluc of k\J1(l\1 = N2 ) = ~ 0 X 1(114 c'\p( 1900/Rn cm3 molecule-Is- I at appro\.unatcly atmosphenc pressure was estimatedtrom thesc rcsults '1.'1. ItIt k~l(M = N 2 ) = I 0 X Hrl2 cm3 molecule-I 5- 1 at ==650 Ton total pressure and 298 K. in g;ood ag.ree-ment '1.\ Itlt J. reccnt c\aluatlOn1. IntroductionTIle reaction of the hydro\.yl radIcal with S02'OH + S02 + M -0. HS03 + l\I (1)is an Important depletion process for SO., m the pollutedtroposphere and m plumes from fossll-fu~ledpowerplants [1.2].Rate constants k 1 for thIS reactIon. which IS m thefall-off regIon between second- and thud-order kmetlcsover the pressure range accesSible to laboratory studIes.have been obtamed. mamly at room temperature, byboth absolute [3-6] and relative rate (7-11] techni-ques. Absolute rate constants k 1 have been determmedover sigmflcant pressure ranges only for M = Ar (20-500 Torr (4) and 25-648 Torr [6) total pressure) andM =He (50-500 Torr [4]). whIle values of k 1 havebeen obtamed. relative to the rate constant for the re-actIon of OH radicals with CO. at 760 TOff for M =N?+ O2 by Co>.. [8.9] and over the range 20-1000 Torr -total pressure for M = N? by Castleman and co·workers[IO.I 1] _The sole tempe"'i-ature-dependence stud)' IS thatof Castleman and Tang [I I] , Cdrned out In the low-pressure third-order kinetiCS region.There IS hence a need for the modellmg of the tro-posphere and of power-plant plumes to determine k 1as a function of temperature at ambient pressure ofM:;; N., and 0., However. Since these dtluent gases can-not be-used at-high pressure with the flash-photol} SIS-resonance techmque. because of quenching of theOH(A 2~+) state. rate constantsk 1 have been deter-mmed m thIS work for M = Ar and SF6 up to :::::650 Torrtotal pressure over the temperature range 298-424 K.From these results and analogous data for the reachonof OH radicals With N02 [12]. the values of k 1 at at-mospheric pressure for M =N2 can be eshmated2. ExperimentalThe apparatus and techmques used have been de-scnbed previously [6,13] . Hydro,,"yl radicals were pro-duced by the pulsed vacuum ultraviolet photolysis ofH20 at wavelengths longer than the CaF2 cutoff~ 1250 A). OH radical concentrations were monitoredas a function of ttme after the flash by resonance fluo-rescence, using a cooled EMl9659QA photomultlpherfitted WIth an interference filter transmitting the 3064 Aband of OH(A 2~+ • v' =0 - X 2n. v" :;; 0) The mter-section of the detection system aperture and the reso-nance radiatIon beam dermed a fluorescence viewingzone at the center of the reaction vessel whose crosssectIon was ==2 cm m diameter. This region was wellseparated from the reaction vessel walls, mimmizingwall losses of the OH radicals. The reaction cell wasenclosed m a furnace which could be held constant tobetter than ±1 K over the temperature range 295-475K. the gas temperature being measured by a chromel/alumel thermocouple mounted inside the reaction vessel.The flash lamp was typically operated at dischargeenergies of 20-80 J per flash at repetition rates of oneflash in four seconds Signals were obtamed by photoncounting in conjunction with multichannel scalingDecay curves of OH radicals were accumulated from100-4250 flashes, dependmg on the signal strengths.OH half-hves ranged from 1.52-92.4 ms and radIcalconcentrations were mOnitored over at least three half-lives. In all cases the flash duration (~llJ.s)was negli-gible in comparison to the OH radical half-hves en-countered.In order to a"oid the accumulation of photolysis orreaction products, all experiments were carried outunder flow conditions so that the gas mixture in thereaction vessel was replemshed every few flashes. Thepartial pressure of H?O in the reaction cell ranged from0.05-0.16 Torr. Thegases used had the followmg puritYlevels accordmg to the manufacturer' Ar ~99.998%;SF6 ~9.99%; S02 ~9 98% (taken from a cylinderora I 06:t 0 02% miJ\ture in argon). All flows weremonitored by cahbrated flowmeters and the gases werepremi>..ed before entering the reaction vessel.3. Results and discussionThe reaction of OH radicciIs with S02 was studiedover the temperature range 298-424 Kat =:::650 Tontotal pressure of argon, and over the range 98-653 Torrtotal pressure ofSF6 . Under the experImental condi-tions used. the pseudo-f"llst-order decays of the OHradical concentrations. [OHJ • are given by the integratedrate expression[OHJO/[OH], =So/S,=exp{(k +k~I[S02])(t- to)}' (I)where [OH] 0 and [OH], represent the concentrationsof OH at times to and t. respectively, So and Stare thecorrespondmg resonance-fluorescence intensities, k isthe first-order rate for removal ofOH in the absenceof added reactant (primarily attnouted to diffusion outof the viewing zone and to reaction with impUrities),and ktI is the bimolecular rate constant for the reactionof OH radicals with S02'For M =Ar at 298 K. and for M =SF6 at both thelowest temperatures and total pressures. Some nonexpo-nentiality in the OH radical decays was observed. asnoted in the previous room-temperature study of thisreaction in this laboratory (6)- This nonexponentialitywas probably caused either by a long-lived chemilumi-nescence or by enhanced scattering of the resonancelamp radiation by aerosol particles formed subsequentto the imtial reaction. Decay rates were obtained underconditions where this effect was negligible for at leasttwo half-lives.Fig. 1 shows a plot of the OH radical decay rateagamst S02 concentration for the temperatures andtotal pressures studied for M == Ar. while fig_ 2 showsa plot of ktJ(M = SF6) against the total SF6 pressurefor the temperatures 298 and 424 K, and table I givesthe rate constants ktl for 1\01 = Ar and SF6 obtained byleast-squares analysis of plots such as those shown infig. l. -Fig 3 shows the data at ==650 Torr total pressurefor M =Ar and SF6. and at~ lOO Torr total pressurefor M =SF6 , plotted m Arrhenius form_ Least-squaresanalysis of these results yields the Arrhemus expres-sionsk~J(l\f=Ar, ==650 Torr)= 1.16 X 10-14 exp[(2370 ± 300)/RT]cm3 molecule-1 s-I,ktl(M = SF6 • >=::::100 Torr)=3.72 X 10-14 exp[(1610 ± 300)/RT)cm3 molecule-1 s-1.krl(M =SF6' ==650 Torr)= 1.27 X 10-13 exp [(1495 ± 300)/RT]cm3 moJecule-1 s-I,where the mdicated errors in the (negative) activationenergy (in caI/mole) are the estimated overall error limits.400 Torr424 K3001- (6501129orr_ / 8K~ 1 /""6i~4To~ri I /-'~~o r ,2 4 ' !6 ' I10 ' I12"1014[502] molecule cm-3rig. 1 Plots of OH radical d.:cay Tate against 502 concentration ofr M = AT at the temperatures and total pressures studiedThe present room-temperature rate constant forM = AI at 650 Torr total pressure, (6.49 ± 0.84) X IQ-13cm3 molecule-1 5-1, In IS excellent agreement WIth thevalue of k~l(M = Ar, 648 Tau) = (6.55 ± 0.66) X IQ-13cm3 molecule-1 s-1 determined in the previous studyTemperature Total pressure 1013 X k~J(K) (Tou) (cm3 molccule-1 5-1 )M==Ar 298 ± 1 650±4 649: 0.84354 ± 1 650 ± 2 322: 036424 ± 1 404± 1 162 = 0 28653 ± 2 198:!: 020M= 5F6 298 ± 1 98:1: 2 S70:!: 1 18203:!: 2 7.83 :!: 0 79400= 2 12.3 ± 16643:!: 2 16.1:22354:!: 1 lOO:!: 1 361 ± 058652: 2 103=16424 ± 1 lOO:!: 1 254±031201 :t 2 3.61 ± 058408:!: 2 600 ± 064650± 2 7.61 :t 0.80~16"':"u 12:ll...:;u..."0E 8""eu25-.:s:,..,..-2 4400 600 800TOTAL PRESSURE (Torr)Fig. 2. Plots ofk~i~1 =SF6) agamst the total pressure of SF6at 298 and 424 K.5"'Or2Ir--'i--~.r--'---Y---T-----I i I i I i1.,0..131 , I « , J , I24 28 32 36IOOO/T (K)Fig 3. Arrhemus plot of log k~1 agamst 1000/T(I() for M = Arand SFc; at ::::650 Ton total pressure. and for M == SF6 at =100Ton total pressurem this laboratory [6]. As anticipated. for example byanalogy WIth the reaction of OH radIcals with NOZ [12].SF6 is a more effiCient third body than Ar. However.perhaps surprismgly. the rate constants for M =SF6 • asshown in table 1 and fig. 2. have not attained the second-order lunltmg high-pressure value at =::650 Torr totalpressure. From Lindemann plots. the second-orderlimiting high-pressure rate constants were estimated [0be ~3.0X 10-12 cm3 molecule-1 s-1 at 298 K and~I 5 X 10-12 cm3 molecule-1 s-1 at 424 K.As seen from fig. 3 the bimolecular rate constantskrl have negative temperature dependencies (equivalentto T-34 for M = AI at 650 Torr total pressure. andr-2.3 and r-2 1 for M = SF6 at lOO and 650 Ton to·tal pressure). These temperature dependencies are ofa similar sIZe and magmtude to that reported by Anastasland Smith [I2] for the linllting lugh-pressure bimolec-ular rate constant for the reachon of OH radicals withNOz and provide a further opportunity to test the pre.dIctlonsof RRKM theory against experiment. How-ever. as expected from unirnolecu!ar theory. these tem-perature dependencies are less negatIve than that deter-mined by Castleman and Tang [11] (E == -2.8 kcalmole-I) m the low-pressure third-order region. and at[11 J G. Calvcrt, F. Suo J.W. Bottenheun and O.P. Strausz,Atmos Environ 12 (1978) 197.121 A EJ. Egg)eton and R.A. Cox.. Atmos. Environ. 12 (1918)227.[31 G W. Hams and R.P. Wayne, I. Cbem. Soc. Faraday Trans.I 71 (1975) 613.[41 D D. Davis. A.R. Ravishankara and S. Fiscller. Geophys.Res Letters 6 (1979) 113.[51 S Gordon and W A ~Iulac.lntem. S. Chel11. KineticsSI (19'75) 289.(6) R. AtkinsoD. RA. Perry and J.N. PlttsJr•• J. Chem. Phys.65 (1976) 306.650 Torr total pressure. the temperature dependencefor M == SF6 IS less negative than that for M == M.For tropospheric modelling purposes. the importantresults are the values ofkri for M ~ N2 and O2 at at-mospheric pressure [1.2]. From the relative efficien-Cles of Ar. N 2 and SF6 in the analogous reactions ofthe OH radical with NOz [12]. with NI being inbetween Ac and SF6 in third-body efficiency. values ofk¥I(M::: N2) of 1.0 X 10-12 cmJ molecule-1 s-1 at298 K. 5.8 X 10-13 cm3 molecule-1 s-1 at 354 K and3.9 X 10-13 cm3 molecule-1 s-1 at 424 K can be es-timated at ~SOTorr from the present rate-eonstantresults. Hence for tropospheric purposes (since N2 andO2 are of comparable third-body efficiency [12] >, theArrhemus expression. applicable to approximately at~mospheric pressure.k~lo.t == N2):; 4.0 X 10-14 exp(l900jRT)cm3 molecule-1 5-1is recommended (the activation energy is in callmole).This value for k~l(M = N 2) at 298 K and atmosphericpressure. which is derived by interpolation betweendirectly measured rate constants Which differ in mag-mtude by a relatively small amount. is essentially iden-tical with the value recommended recently by Calvedet a1. [Il (k¥I(P.1 =air) == (1.1 ± 03) X 10-12 cm] mot-ecule-1 s-l) from an evaluation of the existing rate con-stant data.References650Torr SFS2010')..>l:2,,1(f'O!..~ ''''0,2'QJ:;~E SolO-"....Eu(7] W A. Paync. L J. Sttefand D D Davls. J. Am. Chcm Soc.95 (1973) 7614.18) RA Co'.J Photochem 3 (1974) 291(9J RA Co'\.. Intern J alCm KmetlcsSl (1975) 379.[IOJ A.W CasttemanJr.RE Davls.H R \lunkeh\1tz.INTan," and W P. Wood. Intern J Chcm. Kmetlcs SI (1975)629(Ill A W. Castleman Jr. and I N Tang, J. Photochem 6 (1977)349(121 C. Anasl:lSI and I W.M. Smith. J. Chem Soc FaradayTrans 11 72 (1976) 1459(131 R Atkmson. D A Hanscn and J N Pills Jr. J. Chem Phys62 (1975) 3284;63 (1975) 1703.

Temperature Dependence of the Reaction OH + SO2 + M = HSO3  + M for M = AR and SF6

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Temperature Dependence of the Reaction OH + SO2 + M = HSO3 + M for M = AR and SF6

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