Environ. Sci. Techno/. 1995, 29, 202-206
Upper limit for the Rate Co&cient for the Reaction HO2 NO2 HONO 02
+
-
+
Atmospheric Chemistry Division, National Center for Atmospheric Research, P.O. Box 3000, Boulder, Colorado 80307
Experiments have been carried out in a large stainless steel reaction vessel t o investigate the rate of homogeneous production of nitrous acid, HONO, from the reaction of HO2 radicals with NO2: H02 NO2 HONO 02. The product distribution was determined using Fourier transform infrared (FTlR) spectrometry. No evidence for the reaction was found, and an upper limit for the rate coefficient of 5 x 10-l6 cm3 molecule-' s-' was derived. This value is small enough to exclude the reaction from being a significant source of nitrous acid in the atmosphere.
+
Nitrous acid, HONO, has been identified in the continental troposphere in regions of elevated NO,. During daytime hours, the photolysis of HONO leads to the production of OH radicals: HONO
GEOFFREY S. TYNDALL,* JOHN J . O R L A N D O , A N D JACK G. CALVERT
-
Introduction
+
+ hv - OH + NO
(1)
This process is quite fast (t x 500 s for overhead sun) and thus limits the daytime concentration of HONO. During nighttime hours, however, HONO has been found to build up to levels exceeding 1 part per billion (ppb) (1-9, and photolysis of this HONO can provide the major source of OH in the early hours after sunrise. The mechanism of production of nitrous acid in the urban and semirural atmosphere has intrigued atmospheric chemists for many years. The only homogeneous reaction which is known to produce HONO in the troposphere is the direct combination of OH and NO: OH
+ NO + M - HONO + M
(2)
This reaction will presumably only be important in the daytime, since both OH and NO are produced photochemically and their concentrations typically become very small at nighttime. However, the observations of ppb amounts of nighttime HONO (1-3) indicate that mechanisms other than reaction 2 are operating to produce HONO. Stockwell and Calvert (4)modeled the nighttime LosAngeles atmosphere and considered several possibilitiesfor HONO formation, both homogeneous and heterogeneous in nature. One of these was a bimolecular reaction of HOz and NO,: HO,
+ NO, - HONO + 0,
(3a)
This reaction was proposed by Simonaitis and Heicklen (6) and by Coxand Derwent (7) to explain product distributions in the HOz-NO-NOz reaction system. The reaction between HO, and NO, is now known to proceed predominantly via an addition mechanism to produce peroxynitric acid (8-10) with reaction kinetics displaying typical falloff behavior (11): HO,
+ NO, + M - HO,NO, + M
(3b)
Analysis of the results of the low pressure study of reaction 3b by Howard (8) allowed a small pressure-independent (bimolecular)channel at low pressure, with a rate coefficient of approximately 3 x cm3 molecule-' s-l. Extrapolation of such results to zero pressure is difficult in larger molecular systems due to broadening of the fall-off curves, so that the intercept of 3 x 10-l5 cm3molecule-' s-l cannot be regarded as unambiguous. However, computer simulations carried out by Stockwell and Calvert (4) and Cantrell et al. (12) showed that if reaction 3a occurs at this rate, it could account for up to 50% of the HONO observed in the moderately polluted troposphere. Previous FTIR studies in which H02N02 was generated i n situ using reaction 3b have noted that no significant amount of HONO was produced (9,13-16'). Graham et al. * Author to whom correspondence should be addressed e-mail address:
[email protected].
202 ENVIRONMENTAL SCIENCE & TECHNOLOGY I VOL. 29, NO. 1, 1995
0013-936X/95/0929-0202$09.00/0
Q 1994 American Chemical Society
TABLE 1
Summaty of Initial Conditions and Products Formed for Experiments To Determine k3r expt no.
1, light dark 2, light dark 3, light dark 4, light 5, light 6,light dark
(N4 cm-3)
Hz (1O'a cm-3)
02 (10'7 cm-3)
NO, (1014 cm-3)
timee
2.1
6.4
6.4
1.4
2.1
6.4
2.1
6.4
2.1 2.1 7.0
6.6 2.6 6.4
540 1980 360 1620 990 2100 780 3000 540 1290
Cl2
+
IOl5
Experimental Section The reaction vessel used was a temperature-regulated stainless steel cylinder with an intemal diameter of 16 cm, length 250 cm, and surfacelvolume 0.25 cm-l (17). The experiments involved the photolysis of C12-H2-02-N02NZmixtures using a xenon arc lamp, the output of which was filtered by a Corning 7-54 glass filter (240 < i< 400 nm). Photolysis of Clz under these conditions leads to the formation of HOz radicals and consequentlyto peroxynitric acid:
+ hv - 2C1
-
(4)
HC1+ H
(5)
+ 0, + M -HO, + M HO, + NO, + M HO,NO, + M H
-
(3b)
The concentration of Ha was kept sufficiently high that combination of C1 atoms with NO2 was less than a 5% channel for C1 atoms: C1+ NO,
+M
-
ClNO,
+M
(7)
At the irradiation wavelengths used, NOz was also subject to photolysis: NO,
+ hv-
NO
+0
(8)
The presence of NO in the system could potentially lead to the formation of HONO and HN03 through the conversion of HOZto OH:
+ NO OH + NO + M HO,
+ NO, HONO + M
OH
5.0 0.7 7.3 0.8 15.1 3.6 4.9
molecule ~ m NO - instead ~ of NOz in initial mix.
(15) estimated k3a to be less than 5 x 10-l6 cm3molecule-' s-l, but no experiments were designed specifically to investigate HONO formation quantitatively, and it is possible that it was being formed and then lost heterogeneously. In order to assess the importance of reaction 3a in producing HONO at room temperature, we have carried out the first experiments designed to detect the reaction products explicitly using FTIR spectrometry. Conditions were chosen to eliminate the production of HONO from other sources and to minimize the extent of its heterogeneous loss. We find no production of HONO and conclude that the process will not be significant in the atmosphere.
C1+ H,
10" x k (crd molecule-' s-1)
2.8
