13 if reaction 16 is more important than reaction 11 in removing HNO, the mechanism predicts
a{NzO} = kda{k9a4/(kg
+ k9a)k16[02])
ldl
F
(X)
Equation X should apply for low values of Ia/[OZ]*. As this parameter gets large, then a{N,O} should approach 0.055 as a limiting value. Figure 4 is a log-log plot of @ { N,O 1 us. Ia/[O2I2. At low values of the parameter Ia/[0z]2,the plot is well fitted by a straight line of slope 1. The intercept gives klls{kgaq5/(k9 kga)k16)' = 1.0 X lo3 Torr min. Since kga/k9= 0.145 and q5 = 0.76, kll,Ik16? = 6.4 X lo6 Torr sec. As the abscissa becomes larger the deviation from linearity is apparent. The theoretical curve, based on the intercept of 1.0 X lo3 Torr min and the upper limiting value of @ { N 2 0 }= 0.055, is shown in Figure 4. It adequately represents the trend of the data points. There is some scatter in the data. In particular those points corresponding to the lower intensity lie somewhat higher than those corresponding to the higher intensity. Nevertheless the discrepancy is always less than a factor of 2. Since the intensities used differ by a factor of 12, the fit is satisfactory. Now that all the appropriate rate constant ratios have been evaluated, it is of interest to compute the time required for [NO,] to reach its steady-state value. When [O,]/[NO] is very small, eq V applies, and the steady-state ratio [N02]/[NO] is small, the NO, pressure never exceeds a few microns, and this value is reached in the first minute of irradiation. With larger values of [O,]/[NO], eq VI is applicable. For the con-
./
1631
+
t
I
%
I
t''II3I
I
I
I 1 1 1 1 1 1
I
I
,
10-6
IO-'
1 1 1 9 1 1
I
I
I
I888J
lo-4
Io/[Oz]:
T0r;'min-l
in the pbotolysis of Figure 4. Log-log plot of @(N?O]cs. L,/[02]2 C H 3 0 N 0in the presence of NO and O2at 25' and 3660 A.
ditions of the experiments the steady-state value of [NO,]/[NO] never exceeds 1, and only approaches 1 when [NO] < 0.030 Torr. Since eq IV applies, the NO2 pressure never exceeds -0.015 Torr and rarely even approaches this value. For the runs in which it does, I , is sufficiently large so that the steady-state value again is easily achieved in 1 min of irradiation. Acknowledgment. The authors wish to thank Dr. G . R. McMillan for helpful advice on the purification of methyl nitrite. Also the assistance of Chester Spicer is appreciated. This work was supported by the Environmental Protection Agency through the Office of Air Programs under Grant No. AP 01044, for which we are grateful.
Reactions of Methylperoxy Radicals with Nitric Oxide and Nitrogen Dioxide Chester W. Spicer,' Alberto Villa, H. A. Wiebe, and Julian Heicklen"
Contribution f r o m the Department of Chemistry and Center for Air Environment Studies, The Pennsylvania State Unicersity, University Park, Pennsylvania 16802. Receiaed December 8 , 1971 Abstract: The photooxidation of CH3N2CH3was studied at 25' in the presence of NO and NOr. The reaction conditions were [CH3N2CH3] from 1.6 to 29.1 Torr, [O,] from 2.3 to 30.7 Torr, I , from 0.024 to 0.56 p/sec, and [NO] from 19 to 91 p or [NO,] from 31 to 142 p. In most runs about 100 Torr of Nz was also present. Both 'IN and 15Nisotopes were used in the NO and NOr, and the product peaks monitored mass spectrometrically. For some runs gas chromatography was also employed, and in some cases CHJ replaced CHaN,CH3as a source of CHs radicals. With either NO or NOr the major product of the reaction was CH30N02,though it appeared with an
induction period in the NO studies. HCOOH was also produced with an induction period in both studies. Its presence strongly infers the production of CHzOas a primary product. The CH302radicals appear to react with the oxides of nitrogen via CH302 NO -,CH30zN0(loa), CHaOz NO -,CHrO HONO (lob), CH3G NO2 -, CHzO HONOz (12a), CH302 NO, + CH302N02(12c), with kloJklo = 0.6 + 0.1 and kl?c/k12= 0.75 + 0.05, where k l a = kloB kloband k12= k12* k12?. There was no evidence for the reaction between CH3OZand NO producing CH30 NOr, and it occurs < 2 % of the time. The CHsO,NO molecule isomerizes to CHsONO, in a third-order reaction 2CH3OzNO 0, +.2CHsON02 O2(ll), with kll = 0.11 Torr-? sec-I. CH~OZNO? apparently reacts rapidly with both NOz and NO, CH302N02 NO, -,CH30NOz NO3 (14) and CHsOzNOz NO --* CHsONO NO3 (15).
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primary concern in the understanding of photo0 chemical smog formation is the elucidation of the (1) Environmental Protection Agency Air Pollution Trainee.
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mechanism by which nitric oxide is converted to nitrogen dioxide in urban atmospheres. It is well known that the third-order reaction with oxygen ( k = 7 X lo3 M-? sec-I) is much too slow to be important at
Spicer, Villa, Wiebe, Heicklen
i
Reactions of Methylperoxy Radicals with NO and NO,
14
usually thought to arise from the reaction of NOz and 03,our results indicate a possible alternate path for the formation of this important intermediate. Experimental Section
01
o
r
zoo
100
300
500
400
600
700
R E A C T I O N T I M E , aec.
Figure 1. Plot of relative intensities of the mass spectral peaks a t 47 and 43 cs. irradiation time in the photolysis of a CH3NzCH3-0%15NO-Nz mixture: [CH3N2CH3]= 10.4 Torr, [OZ] = 13.2 Torr, [*5NO] = 0.084Torr, [NZ]= 120Torr, I,, = 0.158 p/sec.
atmospheric concentrations of NO. * Several alternative mechanisms have been postulated recently to explain the conversion in urban atmospheres.? One scheme which has been proposed is HO
R
+ R H +HzO
+R
+ Oz(+M) +ROz(+M)
+ N O +RO + NO2 RO + Oz +R'O + HOz HOz + NO +H O + NO2 ROz
(1)
(2) ( 3)
(4) ( 5)
where R H is a hydrocarbon and R'O is an aldehyde or ketone. In this mechanism NO is oxidized by RO, and HO?. Hydroxyl radical, as the chain carrier, is regenerated in reaction 5. Reactions 1 and 2 are well established while reactions 4 and 5 have been examined in our l a b ~ r a t o r y . ~Reaction 3, however, has never been established in the laboratory. We have now studied the photochemical oxidation of azomethane in the presence of both NO and NOz, originally to measure the competition between reaction 3 and reaction 6 2CH30z
+2CH3O
+
0 2
(6)
The results which are reported in this paper show that reaction 3 does not occur as written, at least for methylperoxy radicals, but rather is an addition reaction in which the final product is methyl nitrate. The photolysis of azomethane in the presence of 0, and NO, was also found to yield methyl nitrate as the major product. These results have great significance in the air pollution field since innumerable investigators have postulated reaction 3 as a major step in the mechanism of photochemical smog formation. In addition, NO, has recently been gaining credence as a possible precursor to peroxyacyl nitrate compounds.j Although NO, is (2) J. Heicklen and N . Cohen, Adcan Photochem., 5, 157 (1968). (3; J. Heicklen, I
