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The mechanism of SO2 oxidation by CH3O2 radicals. Rate coefficients for the reactions of CH3O2 with SO2 and NO
Volume 65, number 2
CHEMICAL PHYSICS LE-l-l-ERS
IS August 1979
THE MECHANISbi OF SO, OXIDATION BY CH,O, RADICALS. RATE COEFFICIENTS FOR THE REACTIONS OF CH,O, WITH SOz AND NO R. SIMONAITIS and J. HEICKLEN Deparfment of Chenlirtry ad Iottosphere Research Laboratoy. The Pennsyhanlh State Unil*ersit_y. (Inil-ersity Park. Pemzg hanin 1680-7. USA Reeehed 13 April 1979: in fin;tl tbrm 11 5lny 1979 The reacttonsof CH30z nitIt SO2 and NO haw been studied bv steady state photo&s of azomethane in the presence of Ot-SOz-SO mixtures at 196 M and I ;Ltm total pressure. Thr quantum yield of NO otidntion by CHxO= m.dicxG is increased substanthlIy when SO? is added ro the system indicating an SOa induced chain oxidation of X0. The rare isw gbes - CH30 t KOz (1). Combining this ratio with the absolute value ofkr = 8.2 X IO-” cm3 se1 gibes X-2 = 10
l_ Introduction
3_ Experimental
The reactions of peroxy mdicxtk with SO1 may be important in the atmospheric oxidarion of SOz. i\ recent study in our laboratory of the reaction of CH,O, with SO2 using a time resolved flash photolysis-kinet-
hlixtures of azomethane. NO, SO?, O2 and in some runs N, were irradiared at 366 nm in a 10 cm long by 5 cm diameter cylindrical quartz cell. The radiation was from an Illumination Industries high pressure Hg arc lamp, ?OOW(type202-1003) \vhich had passed through a Corning filter (CS 7-37). NO was monitored continuously during irradiation using the chemiluminescent reaction of NO with 0, as described in a previous publication [Z] _ The initial rates of NO removal were corrected for the time constant of the measurement system of 0.42 s-t, leading to maximum correction of =7%_ Absorbed lighr intensities. fa, \\ ere determined by the photolysis of azomeihane-02-NO mixtures. The quantum yietd for x0 removed in this system is known to be 4.0 [3]. NO @latheson) w-as purified by distrllation from --IS6 to -196°C. The white coIor of the solid h0 indicated it to be free of NO, _ Before use, NT was slowly passed through traps makrtained at -196oC. O2 (Matheson Co., Research grade) was used without purification SO, was punfied by conventional trap to trap distillation under vacuum_ Azomethsne was prepared and purified as descrrbed in an earlier pubhcation [4]_
ic spectroscopy technique showed that CH;O, and
SO1 react CH,O? + SO, + products
(1)
with a rate coefficient of 52 X 10-t” cm3 s-l at 296 K and 1 arm total pressure [ I] _ Of course, that study provides no indication of the mechanism or the nature of the products of reaction (I)_ In order to determine the mechanism of SO, oxidation by perosy radicals,
we have studied the competitive reactions of CH,O, with SO, and x0. It was expected that the competitive technique would also provide relative rate coefficients for the reaction of CH,O, with SO, and the atmospherically very important reaction of CH;02 with NO CH301 •t- NO + CH;O •t- NO, _
(3
Since the absolute value of X-t is now known. an absolute vatue of kl could then be computed_
361
Votume 65. number 2
15 August 1979
CtIEMICAL PHYSICS LETTERS
3_ Results When azomethane is photoiyzed at 366 nm and 296 K in the presence of 5\‘0 (5-11 mTorr), Iow [O,] ((320 Ton) and the total pressure brought to 700 Torr with X:1 ~ the ir;O rxmsumption quantum yield, --9 {x0), increases from a vxhxe of?_0 in the absewx of SO, to a near Iirniting vahie of 129 when [SO,]/[KOl =4.67X I03. Wh&~ azomethane is photoiyzed in the presence of NO (~9 mTorr) and high [O,l (*7OOTorr), --cl, (x0) = 10.0. When SO, is added to a mix1 ure of this composition, -cP {SO] declines to a limiting mIue of =I4 for &O-,j/fNO] >5 X 103_The resultroftheseexperinxnts arc presented in iabtc I and in graphical for-m in fig_ I _
implies that NO removal is promoted by a chain process invoking SO?_ Initiation of the chain process occurs by CH3 QCH, + fw -+ 2CI-E, + N?,
rate = 1,
CH, + O2 + hl + CH301 + bl_
(3)
The CH301 radical must carry the chain. since as we shaIf argue below, the CH30 radical cannot do this_-
The high limiting quantum yieid for i\‘O removai Iabk 1
Hcacrion of mcrhytperusy radicak 1% irh SO2 and X0 at 196 B and I Mm Coin1 pressure --_- _ _ ~_ __ ___ _ _ _- -. __-__-_.-__
lo-3{so,I~[~
_
_
_
I.\01
_.
tmTorr i
--_-.-
ISO, I lo21 48 (Torr) (Torr) (em” s-1) _ _ - _ --__ __~ _ - _ _ __ - -___ ___-__.--_
-‘P {NO)
low fO2 j a) CL23 0.3s 1.01 t -03 1.60 I.Sl
ZOi
1.25 466
trw lo, I bJ -
1.1-r 2.39 J_W S-19
t3.t-t 102 516 8_-85 9.37
11.9 209 40-9 76.7 773
700 700 700 700 700 600
7.1-t 6.3s 6-99
20.0 16-7 Ia.8
6.46
16.9
6.68
d.63 Y96 6.84 _- __-_. __- __ - __ - -_--- _ _ _ _ -_-- _____-__ ------ __--- -__- ---..-_-
ZJ xDtat pr-urc LCCL nu~~~~imxt JI 700 Torr =irh “) jCtisS$X1~ 1 = 1_67-1.71Tcxr.
added vL ~,[CI13S~CII~ 1 = O-SO--1_9STorr_
11.7 14.6 --__
Volume 65. number 2
CkIE.\!ICAL PIIYSICS LETTERS
The propagation steps we propose are CH,O1 + SO2 + CH,O,SO,,
(I)
CH302S02 + O2 + CH302S0202,
(4)
CH302S0201 f NO --f CH,O,SO, + NO1_
(3
CH;O,SO, + CH;02 -I- SO; _
(6)
One termination step is surely CH30, +NO-+CHjO+NO1_
(2)
However this step is not the only termination step beQuse it would not lead to an upper limiting value for -@ {KO} at high [SOz]/[NO] ratios_ Other tenninating steps Lzin be envisioned with the CH301S02 radical CH,O+O, 4 CH;O -I- s o ; ,
(7)
CH30,S0, + A+ products,
(8)
where A is NO (most likely), another radirzl, or the wall. If reactions (7) and (S) are important, then at high [S02]/[i\O] ratios, the upper limiting value for -+ {NO} should depend on the O2 pressure, contrary to our results. Thus we can conclude that these reactions are not important in our system. This contrasts with the prediction of Benson [5] that CHj02S02
will be unstable and decompose via reaction (7). This
leaves us with the conclusion that the other terminaring reaction is CH302S03 + A - products.
(91
The diperosy radical, CH302S0102. formed by reac:ion (a)_ must be Introduced ds the NO o_Gdidng species. because it has the requisite peroxy radical structure for 0 atom transfer_ Reaction (5) is the direct analog of the well known reactions of peroay r3dicals with NO RO1fKO-RO+NO?,
(10)
where R E alkyl or H. The k!OS070, radical. which is similx to the CH,02S020, radical, 11~s been shown to be stable by Benson [5] from thermodynamic arguments_ The methylperosy sulfate radical, CH; 02SO;, formed in reaction (5) is proposed to be unstable and decomposes via reaction (6) to regenerate CH302. The methylperoxy sulfate radical is structurally similar to the methylperoxynitrates which owing to the weak
15 August 1979
0-N bond readily decompose to the peroxy radical and NO2 [X61
ROli\;Oq f bl --f RO> + NO, + Bl_
ill)
The proposed methylperoxy sulfate radical is the rddi-
cai derived from the methylester of the known perox?_sulfuric acid (Cares’ acid)_ From the flash photolysis experiments and the present experiments a lifetime of CH302S03 of 0.01 --I s is indicated, which is significantly shorter than that of the corresponding nitrates [Z6]. The fate of the CH30 radical must also be considered. The reactions which must be considered are: CHjO -r NO - CHjONO,
(12)
CH,O -I- SO? - CH,OSOI_
(13)
Reactions (2) and (12) are well known reactions which lead to a quantum yield of 1.0 for iK0 removal in the absence of SO, [3] and contrlbute to chain termination in the p&ewe of SO, _ Reaction (13) could be follower by a series of reactions analogous to those for CH,0,S02 except that a chain cannot be propagated bemuse CH3 OSO; has a strong O-S bond [S] and \koould not decompose_ Kate that CH,0S03 is the sulfate radical derived from the known and stable dimethj Isulfate. Reaction (13), if it occurs, is hkely to ultimately lead to termination by a step involving removal of X0 Thus reactions (12) and (13) ~111 probably have very nearly the same effect on --CD {NO}. OH radicals_ if they were produced by some means, could not propagate a chain because the reaction OH T SO2 (+ hl) + HOSOl (+ XI)
(141
is fast with kil = I X lo-” cm3 s-l under our conditions [7 S] , and the radical HOSO-, is chain terminating because the sequence
HOSO + 07 - HOSOIO1,
(15)
HOS0,02 -I- x0 - ho-, T HOSO,
(16)
to HOSO, N hi& hhr CH, 050; is stabie [3]. The general kinetic expression for -9 {NO] over the entire [S07]j[NO] range is cumbersome, but it has simple litniting forms. At sniall values of [SO,] / [NO] it becomes
kids
363
Volume 65. number 2
CHEMICAL PHYSICS LElTERS
Eq_ (I) predicts that the initial slope of the graph of -a {x0) versus [SO, ] / [NO] shown in fig_ 1 is Z/1-,/ k2_ From thegraph iris estimated that x’& =(2-S +-O-S) X 10-3_ Combining this value with the value of k, = 82 X 1O-15 cm3 s-~ obtained directly by flash pbotolysis [I] gives k2 = 10-rt-5 * *-? cxn3 s-l_ It is interesting that the rate coemcient for the reaction of the hydroperoxy radical with NO is about a factor of 3-5 greater_ It is important to note that this method for obtaining k2 is not dependent on the details of the mechanism_ All that is required is that chain termination be by reaction (Z),which is certainly the case for [SO? j / [NO] -+ O_ Our value of k2 = IO- M-5 * O2 cm3 s-1 is in e_sact agreement with the value found recently by Adachi and Basco -[9] in a flash photolysis experiment, bur is considerabiy smaller rhan the value of (S.0 + 20) X IO- 12 cm3 s-l found by Plumb et al_ [IO] in a flow reactor sampled by a mass spectrometer.
x1 HI-git f 021
1.5 August I979
:o give the radical HO,SO, which then propagates a chain analogous to the CH302 chain. The problem with this mechanism is that the reported value for kl, = 9 X lb-I6 cm3 s-l [ 121 is too iow for reaction (20) to compete with reaction (IS). We suspect that the reported value for kzO is foe low, and is probably closer to the vaIue for the reaction of CH,O, radicals with SO, which is S3 X 10-u cm5 s-l, since there is no reason why reaction (20) shou!d be so much slower than reactiotl (I)_ If this is the case then the high values of--G {NO) are due to simultaneous operation of the HO, and CH302 chains in the competitive regime_ As [SO,] / [NO] increases the CH301 chain becomes domix& and --9 {NO} approaches the same limiting value at high [02] as at low [02]_ If this explanation is correct then it follows that at 12 Torr of SO,, reaction (I 3) cannot completely dominate reaction (17), or there wouId be no HO, radicals_ Since k17 = 5-S X 10si6 cm3/s at 298 K [13], then R,3 < 4 X IO-l4 cm’fs, a value very much lower than that for the analogous reaction (14) with HO radicals_
At high O1 pressures tile reaction of CH30 radicals with 0, CIl>O + 0, + CH,O + HO, _
(17)
will compete witlt reacrion (12) since the relative rate coefficient k&k il -4-77 10-S [II]_Thehighvalue of -*P ix01 = 30 in the absence of SO2 is no doubt due to a chain propagated by the HO, and OH radicals involving azometbant via the reactions t IO, i- ir;o + NO2 + OH*
(18)
OH + CII,K=KCH, + radical products_
(19)
Upon the addition of 11 Torr of SO, and at [SO? ]I [XOJ = I _I7 X 103, CH301 radicals will react competitively with SO2 and KO (= 1: 1). That portion \\hich rexis with SO2 will react as described before to propzste the Clf30T chain_ How-ever the OH wili be complerely scakengez by SO,, so that the chain inducxd by reaction (f9) is complerely surpresscd- Nevertheless --9 {x:0> is considerably greater than at iow [Oz] for the same SO1 and ir;O pressures. Thus there must be mother clnin sequence_ One possible explanation for the high quantum yield is that HO2 radicals produced by reaction (17) react with SO, IlO_,- f so, + bl + lto+o~ + hl. 364
(20)
Acknowledgement This work was supported by the Atmospheric Sciences Section of the National Science Foundation through Grants No_ ATM 76-83378 and ATM 7S16833 and by the National _eronautics and Space Administration through Contract No. NAS7-IOOwith the Jet Propulsion J._aboratory for which we are grateful_
References
111 E. Sanhuen. R. Simonaitis and J. Heichlen. Intern J.
Chem. Kineria (1979). to be published. 121 R_ Simon&is nnd I_ H&&n, Intern. J. Chem Kinetic3 10 (1978) 67. 131 R_ Simonaitis and J_ Hcickten. J_ Phyr Chem_ 78 (197-t) 2517. 141 H-4_ Webe and J_ Heickkn, J_ Am_ Chem_ Sot. 95 (1973) 1. [S J SW. Benson, Chem. Rev. 75 (1978123. [6 j R- Simon&k and J. Heidilen, Chem. Phys. Letters 62 (1979) 473; _
171
R-4. Graham. AN. Winer and J-N_ Piits. Chem. Phyr Lerrea51 (1977) 215. D-D_ Davis. A.R. Rabishhanhn and S. Ksher. Geophyr Res_ Letters6 (1979) 1 I3_
Vohune 6.5, number 2
CHEiWCAL PHYSICS LETTERS
[SJ RF_ Hampson Jr. and D_ Gawk. Reaction Rate and Photochemiul Data for Atmospheric Chemistry - 1977, NBS Special Pubiication No. 513 (1977). [91 H. Adachi and N. Basco. Chem. Phls. Letters 63 (1979) 490. [IO] LC. Plumb, K-R. Ryan. J.R. Steven and M.F.R. blulcahy, private communiution (1979).
i5 August 1979
[ 111 H.A_ Wiebe, A. Villa, TX. Hellman and J. Heicklen. J. Am. Chem. Sot. 95 (1973) 7. [ 121 W.A. Payne, L.T. Stief and D.D. Davis, J. Am. Chem. Sot. 95 (1973) 7614. [ 131 G D_ Mendenhall, D_M_ Golden and SW_ Benson, Intern. J. Chem. Kinetics 7 (1975) 725.
365
CHEMICAL PHYSICS LE-l-l-ERS
IS August 1979
THE MECHANISbi OF SO, OXIDATION BY CH,O, RADICALS. RATE COEFFICIENTS FOR THE REACTIONS OF CH,O, WITH SOz AND NO R. SIMONAITIS and J. HEICKLEN Deparfment of Chenlirtry ad Iottosphere Research Laboratoy. The Pennsyhanlh State Unil*ersit_y. (Inil-ersity Park. Pemzg hanin 1680-7. USA Reeehed 13 April 1979: in fin;tl tbrm 11 5lny 1979 The reacttonsof CH30z nitIt SO2 and NO haw been studied bv steady state photo&s of azomethane in the presence of Ot-SOz-SO mixtures at 196 M and I ;Ltm total pressure. Thr quantum yield of NO otidntion by CHxO= m.dicxG is increased substanthlIy when SO? is added ro the system indicating an SOa induced chain oxidation of X0. The rare isw gbes - CH30 t KOz (1). Combining this ratio with the absolute value ofkr = 8.2 X IO-” cm3 se1 gibes X-2 = 10
l_ Introduction
3_ Experimental
The reactions of peroxy mdicxtk with SO1 may be important in the atmospheric oxidarion of SOz. i\ recent study in our laboratory of the reaction of CH,O, with SO2 using a time resolved flash photolysis-kinet-
hlixtures of azomethane. NO, SO?, O2 and in some runs N, were irradiared at 366 nm in a 10 cm long by 5 cm diameter cylindrical quartz cell. The radiation was from an Illumination Industries high pressure Hg arc lamp, ?OOW(type202-1003) \vhich had passed through a Corning filter (CS 7-37). NO was monitored continuously during irradiation using the chemiluminescent reaction of NO with 0, as described in a previous publication [Z] _ The initial rates of NO removal were corrected for the time constant of the measurement system of 0.42 s-t, leading to maximum correction of =7%_ Absorbed lighr intensities. fa, \\ ere determined by the photolysis of azomeihane-02-NO mixtures. The quantum yietd for x0 removed in this system is known to be 4.0 [3]. NO @latheson) w-as purified by distrllation from --IS6 to -196°C. The white coIor of the solid h0 indicated it to be free of NO, _ Before use, NT was slowly passed through traps makrtained at -196oC. O2 (Matheson Co., Research grade) was used without purification SO, was punfied by conventional trap to trap distillation under vacuum_ Azomethsne was prepared and purified as descrrbed in an earlier pubhcation [4]_
ic spectroscopy technique showed that CH;O, and
SO1 react CH,O? + SO, + products
(1)
with a rate coefficient of 52 X 10-t” cm3 s-l at 296 K and 1 arm total pressure [ I] _ Of course, that study provides no indication of the mechanism or the nature of the products of reaction (I)_ In order to determine the mechanism of SO, oxidation by perosy radicals,
we have studied the competitive reactions of CH,O, with SO, and x0. It was expected that the competitive technique would also provide relative rate coefficients for the reaction of CH,O, with SO, and the atmospherically very important reaction of CH;02 with NO CH301 •t- NO + CH;O •t- NO, _
(3
Since the absolute value of X-t is now known. an absolute vatue of kl could then be computed_
361
Votume 65. number 2
15 August 1979
CtIEMICAL PHYSICS LETTERS
3_ Results When azomethane is photoiyzed at 366 nm and 296 K in the presence of 5\‘0 (5-11 mTorr), Iow [O,] ((320 Ton) and the total pressure brought to 700 Torr with X:1 ~ the ir;O rxmsumption quantum yield, --9 {x0), increases from a vxhxe of?_0 in the absewx of SO, to a near Iirniting vahie of 129 when [SO,]/[KOl =4.67X I03. Wh&~ azomethane is photoiyzed in the presence of NO (~9 mTorr) and high [O,l (*7OOTorr), --cl, (x0) = 10.0. When SO, is added to a mix1 ure of this composition, -cP {SO] declines to a limiting mIue of =I4 for &O-,j/fNO] >5 X 103_The resultroftheseexperinxnts arc presented in iabtc I and in graphical for-m in fig_ I _
implies that NO removal is promoted by a chain process invoking SO?_ Initiation of the chain process occurs by CH3 QCH, + fw -+ 2CI-E, + N?,
rate = 1,
CH, + O2 + hl + CH301 + bl_
(3)
The CH301 radical must carry the chain. since as we shaIf argue below, the CH30 radical cannot do this_-
The high limiting quantum yieid for i\‘O removai Iabk 1
Hcacrion of mcrhytperusy radicak 1% irh SO2 and X0 at 196 B and I Mm Coin1 pressure --_- _ _ ~_ __ ___ _ _ _- -. __-__-_.-__
lo-3{so,I~[~
_
_
_
I.\01
_.
tmTorr i
--_-.-
ISO, I lo21 48 (Torr) (Torr) (em” s-1) _ _ - _ --__ __~ _ - _ _ __ - -___ ___-__.--_
-‘P {NO)
low fO2 j a) CL23 0.3s 1.01 t -03 1.60 I.Sl
ZOi
1.25 466
trw lo, I bJ -
1.1-r 2.39 J_W S-19
t3.t-t 102 516 8_-85 9.37
11.9 209 40-9 76.7 773
700 700 700 700 700 600
7.1-t 6.3s 6-99
20.0 16-7 Ia.8
6.46
16.9
6.68
d.63 Y96 6.84 _- __-_. __- __ - __ - -_--- _ _ _ _ -_-- _____-__ ------ __--- -__- ---..-_-
ZJ xDtat pr-urc LCCL nu~~~~imxt JI 700 Torr =irh “) jCtisS$X1~ 1 = 1_67-1.71Tcxr.
added vL ~,[CI13S~CII~ 1 = O-SO--1_9STorr_
11.7 14.6 --__
Volume 65. number 2
CkIE.\!ICAL PIIYSICS LETTERS
The propagation steps we propose are CH,O1 + SO2 + CH,O,SO,,
(I)
CH302S02 + O2 + CH302S0202,
(4)
CH302S0201 f NO --f CH,O,SO, + NO1_
(3
CH;O,SO, + CH;02 -I- SO; _
(6)
One termination step is surely CH30, +NO-+CHjO+NO1_
(2)
However this step is not the only termination step beQuse it would not lead to an upper limiting value for -@ {KO} at high [SOz]/[NO] ratios_ Other tenninating steps Lzin be envisioned with the CH301S02 radical CH,O+O, 4 CH;O -I- s o ; ,
(7)
CH30,S0, + A+ products,
(8)
where A is NO (most likely), another radirzl, or the wall. If reactions (7) and (S) are important, then at high [S02]/[i\O] ratios, the upper limiting value for -+ {NO} should depend on the O2 pressure, contrary to our results. Thus we can conclude that these reactions are not important in our system. This contrasts with the prediction of Benson [5] that CHj02S02
will be unstable and decompose via reaction (7). This
leaves us with the conclusion that the other terminaring reaction is CH302S03 + A - products.
(91
The diperosy radical, CH302S0102. formed by reac:ion (a)_ must be Introduced ds the NO o_Gdidng species. because it has the requisite peroxy radical structure for 0 atom transfer_ Reaction (5) is the direct analog of the well known reactions of peroay r3dicals with NO RO1fKO-RO+NO?,
(10)
where R E alkyl or H. The k!OS070, radical. which is similx to the CH,02S020, radical, 11~s been shown to be stable by Benson [5] from thermodynamic arguments_ The methylperosy sulfate radical, CH; 02SO;, formed in reaction (5) is proposed to be unstable and decomposes via reaction (6) to regenerate CH302. The methylperoxy sulfate radical is structurally similar to the methylperoxynitrates which owing to the weak
15 August 1979
0-N bond readily decompose to the peroxy radical and NO2 [X61
ROli\;Oq f bl --f RO> + NO, + Bl_
ill)
The proposed methylperoxy sulfate radical is the rddi-
cai derived from the methylester of the known perox?_sulfuric acid (Cares’ acid)_ From the flash photolysis experiments and the present experiments a lifetime of CH302S03 of 0.01 --I s is indicated, which is significantly shorter than that of the corresponding nitrates [Z6]. The fate of the CH30 radical must also be considered. The reactions which must be considered are: CHjO -r NO - CHjONO,
(12)
CH,O -I- SO? - CH,OSOI_
(13)
Reactions (2) and (12) are well known reactions which lead to a quantum yield of 1.0 for iK0 removal in the absence of SO, [3] and contrlbute to chain termination in the p&ewe of SO, _ Reaction (13) could be follower by a series of reactions analogous to those for CH,0,S02 except that a chain cannot be propagated bemuse CH3 OSO; has a strong O-S bond [S] and \koould not decompose_ Kate that CH,0S03 is the sulfate radical derived from the known and stable dimethj Isulfate. Reaction (13), if it occurs, is hkely to ultimately lead to termination by a step involving removal of X0 Thus reactions (12) and (13) ~111 probably have very nearly the same effect on --CD {NO}. OH radicals_ if they were produced by some means, could not propagate a chain because the reaction OH T SO2 (+ hl) + HOSOl (+ XI)
(141
is fast with kil = I X lo-” cm3 s-l under our conditions [7 S] , and the radical HOSO-, is chain terminating because the sequence
HOSO + 07 - HOSOIO1,
(15)
HOS0,02 -I- x0 - ho-, T HOSO,
(16)
to HOSO, N hi& hhr CH, 050; is stabie [3]. The general kinetic expression for -9 {NO] over the entire [S07]j[NO] range is cumbersome, but it has simple litniting forms. At sniall values of [SO,] / [NO] it becomes
kids
363
Volume 65. number 2
CHEMICAL PHYSICS LElTERS
Eq_ (I) predicts that the initial slope of the graph of -a {x0) versus [SO, ] / [NO] shown in fig_ 1 is Z/1-,/ k2_ From thegraph iris estimated that x’& =(2-S +-O-S) X 10-3_ Combining this value with the value of k, = 82 X 1O-15 cm3 s-~ obtained directly by flash pbotolysis [I] gives k2 = 10-rt-5 * *-? cxn3 s-l_ It is interesting that the rate coemcient for the reaction of the hydroperoxy radical with NO is about a factor of 3-5 greater_ It is important to note that this method for obtaining k2 is not dependent on the details of the mechanism_ All that is required is that chain termination be by reaction (Z),which is certainly the case for [SO? j / [NO] -+ O_ Our value of k2 = IO- M-5 * O2 cm3 s-1 is in e_sact agreement with the value found recently by Adachi and Basco -[9] in a flash photolysis experiment, bur is considerabiy smaller rhan the value of (S.0 + 20) X IO- 12 cm3 s-l found by Plumb et al_ [IO] in a flow reactor sampled by a mass spectrometer.
x1 HI-git f 021
1.5 August I979
:o give the radical HO,SO, which then propagates a chain analogous to the CH302 chain. The problem with this mechanism is that the reported value for kl, = 9 X lb-I6 cm3 s-l [ 121 is too iow for reaction (20) to compete with reaction (IS). We suspect that the reported value for kzO is foe low, and is probably closer to the vaIue for the reaction of CH,O, radicals with SO, which is S3 X 10-u cm5 s-l, since there is no reason why reaction (20) shou!d be so much slower than reactiotl (I)_ If this is the case then the high values of--G {NO) are due to simultaneous operation of the HO, and CH302 chains in the competitive regime_ As [SO,] / [NO] increases the CH301 chain becomes domix& and --9 {NO} approaches the same limiting value at high [02] as at low [02]_ If this explanation is correct then it follows that at 12 Torr of SO,, reaction (I 3) cannot completely dominate reaction (17), or there wouId be no HO, radicals_ Since k17 = 5-S X 10si6 cm3/s at 298 K [13], then R,3 < 4 X IO-l4 cm’fs, a value very much lower than that for the analogous reaction (14) with HO radicals_
At high O1 pressures tile reaction of CH30 radicals with 0, CIl>O + 0, + CH,O + HO, _
(17)
will compete witlt reacrion (12) since the relative rate coefficient k&k il -4-77 10-S [II]_Thehighvalue of -*P ix01 = 30 in the absence of SO2 is no doubt due to a chain propagated by the HO, and OH radicals involving azometbant via the reactions t IO, i- ir;o + NO2 + OH*
(18)
OH + CII,K=KCH, + radical products_
(19)
Upon the addition of 11 Torr of SO, and at [SO? ]I [XOJ = I _I7 X 103, CH301 radicals will react competitively with SO2 and KO (= 1: 1). That portion \\hich rexis with SO2 will react as described before to propzste the Clf30T chain_ How-ever the OH wili be complerely scakengez by SO,, so that the chain inducxd by reaction (f9) is complerely surpresscd- Nevertheless --9 {x:0> is considerably greater than at iow [Oz] for the same SO1 and ir;O pressures. Thus there must be mother clnin sequence_ One possible explanation for the high quantum yield is that HO2 radicals produced by reaction (17) react with SO, IlO_,- f so, + bl + lto+o~ + hl. 364
(20)
Acknowledgement This work was supported by the Atmospheric Sciences Section of the National Science Foundation through Grants No_ ATM 76-83378 and ATM 7S16833 and by the National _eronautics and Space Administration through Contract No. NAS7-IOOwith the Jet Propulsion J._aboratory for which we are grateful_
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