Environ. Sci. Technol. 1993,27,982-983
Evidence for the Heterogeneous Formation of Nitrous Acid from Peroxynitric Acid in Environmental Chambers T. Zhu, 0. Yarwood,? J. Chen, and H. Nlki' Centre for Atmospheric Chemistry and Department of Chemistry, York University, 4700 Keele Street, North York, Ontario, Canada M3J 1P3
Introduction Our recent FTIR study (I) has shown that the visible (X I 400 nm) irradiation of Brz-HCHO-air mixtures can provide a clean photolytic source of HO2 radicals (i.e., reactions 1-3) for laboratory studies of the relevant atmospheric reactions, particularly those involving reactants or products which can be readily photodissociated by UV (300 IX I400 nm) irradiation, e.g., NOz, NO3, HONO, etc. Br,
+ hv (A > 400 nm)
Br + HCHO
HCO + 0,
-
-
Br
+ HBr HO, + CO HCO
HO,
+ NO, + M
-
HO,NO,
+M
Experimental Section The FTIR facility and the general experimental procedures have been described in detail previously (I). Briefly, a 180-m pathlength IR cell (30 cm i.d., 2 m long 140-LPyrex cylinder with multipassed internal gold-coated White cell mirrors) surrounded by 26 visible fluorescent lamps GE F40CW; X > 400 nm) was used as a photochemical reactor. IR spectra were recorded in the range of 50O-37OO cm-1 using a Ge-coatedbeam splitter and liquid helium cooled Cu-Ge detector. The interferometer was operatd at a resolution of 1/16 cm-1 in 90 s (16 scans). In order to time resolve the HOzNO2 decay and HONO formation, some spectra were recorded in 30 s (4scans). Concentrations of all the reactants and products were determined using the calibrated absorbance spectra recorded previously in this laboratory (e.g., ref 3). Results and Discussion Illustrated in Figure 1 are the concentration-time profiles of HONO, HN03, and HO2NOz observed before, + Systems Applications International, 101 Lucas Valley Rd., San Rafael, CA 94903. Environ. Sci. Technol., Voi. 27, No. 5, 1993
I
30 s i r r a d i a t i o n 1.5
a n
-
Y
c
0 .3
4
m
L
1.0
-
l
a
(2)
(4) Notably, in modeling studies of smog chamber data, the photolysis of HONO formed from heterogeneous reactions involving NO, is commonly postulated as a dominant background source of the HO radical (e.g., ref 2). The results described below suggest that HOzNO, formed from the HO2 + NO1 reaction may also produce HONO heterogeneously, thereby contributing to the excess formation of HO radicals frequently observed in smog chamber experiments.
882
-E
(1)
(3) Presented in this paper is new evidence for the rapid heterogeneous decomposition of peroxynitric acid (HOzN02) to HONO and HN03 on the gas reactor walls. This reaction was encountered while attempting to produce the HOzNO2 in known amounts by adding NO2 to the above photochemical system for quantitative in situ IR calibration of HOzNO2 via reaction 4: -+
2.0
20
30
40
Time ( m i n ) Flgure 1. Concentration-time profiles of H02N02,HONO, and HN03 before, during, and after 30-svisible irradiation of a mixture containing Br2 (20 pprn), HCHO (3.9pprn), and NO2(6.6 ppm) in 700 Torr of air.
during, and after 30-9 visible irradiation of a mixture containing Br2 (20 ppm), HCHO (3.9 ppm), and NO2 (6.6 ppm) in 700 Torr of air. When the mixture was kept in the dark for 20 min prior to the irradiation, a small concentration (0.02 ppm) of HONO was detected, while "03 remained virtually unchanged from the initial level. These observations are consistent with the slow heterogeneous reaction of NO2 with HzO on the reactor wall to form HONO ( 4 ) . In contrast, 1.53 ppm of HOzNO2 and 0.16 ppm of HONO were formed during the short irradiation (30 s). Subsequently, the HO2NO2 concentration decayed while the HONO (and to some extent, HN03)concentration continued to increase rapidly during the first 3 min in the dark following the irradiation, eventually reaching a plateau. Uselman et al. (5) have found that the HO2NOz decay consists of both first- and second-order components, but that the heterogeneous reaction of HO2NOz alone is the source of the first-order decay term. Under our experimental condition, the decay curve of HOzNO2 is dominated by the first-order component, indicating that HO2N02 decayed predominantly on the reactor wall producing HONO and "03. The only known homogeneous reaction forming HONO in HOz-NO2-HO2N02 system is reaction 5:
HO, + NO,
-
HONO + 0,
(5)
Although laboratory studies (6-8) have shown that the reaction of HOz and NO2 proceeds mainly via formation of HOzNOz (reaction 41, Stockwell and Calvert have pointed out that even a small branching ratio k d k 4 could account for the observed level of HONO in the atmosphere (9). To verify the contribution of reaction 5 to HONO in our system, we carried out numerical simulation of the experimental results. Using k4/k5 = lo3,which is the lower limit given by Graham et al. (8), simulation results indicate that reaction 5 alone could account for only 5% of the HONO observed during the first 30-s dark decay of 0013-936X/93/0927-0982$04.00/0
0 1993 American Chemical Society
Table I. Summary of Yields of HONO and date 08-17-89 08-17-89 08-21-89 08-21-89 09-06-89 09-21-89 04-12-90 04-12-90 04-18-90 04-18-90 10-01-91 10-01-91
7.2 9.0
4.5 5.6 5.4 4.3 4.2 4.0 3.9 3.8 3.9 4.5
20 20 20 20 20 20 20 5 5 5 20 20
Formed in Dark Decay of HOzNOz
"03
initial concn (ppm) HCHO Brz
[HOzNOzloa
A [HONO]/ At*
NOz
(PPd
(ppm min-I)
5.2 5.2 2.6 1.9 5.3 5.1 1.5 2.2 2.2 2.0 6.6 6.8
2.1 2.7 1.0 1.5 0.7 1.9 0.8 1.2 1.0 1.2 1.5 1.7
0.020 0.028 0.014 0.006 0.028 0.038 0.008 0.008 0.006 0.010 0.036 0.034
yield ( % ) c HONO "03 15 17 18 4 58 41 16 8 20 8 52 52
32 54 40 33 38 37 84 36 48 33 60 52
Initial HOzNOz concentration after the UV irradiation. Average HONO formation rate during the first 5 min HOzNOz dark decay. formed during the first 5 min HOzNOz dark decay.
' Yields of HONO and "03
HO2NO2, and 20% during the first 5 min. Even by increasing k5, we could not simulate the shape of the formation curve of HONO. Thus, it was concluded that were in the present experiment the HONO and "03 formed via the heterogeneous decomposition of H02N02. The heterogeneous decomposition of H02N02 is likely to depend upon the condition of the reactor walls. Table I lists representative results obtained in the irradiation of HCHO-N02-Br2-air mixtures over a period of 2 years. Over this period of time, the reactor was used for studies of numerous reaction systems, and thus its wall condition had presumably changed to some extent. HONO and HN03 formation was always observed in the irradiation of HCHO-NO2-Br2-air mixtures during the 2-year period, with the HONO yield ranging from 8 % to 58% of the H02N02consumed, and the HN03 yield from 32% to 84%. Thus, the product ratio in the heterogeneous decomposition of H02N02 is dependent on wall conditions, although both HONO and "03 were always observed. In numerous previous studies of hydrocarbon/NO, smog chamber data, the background radical sources have been assumed to be an initial concentration of nitrous acid, [HONOIo and a constant flux of HO radicals, kH~.2,4J0-12 However,the origin of initial HONO and kHo has not been elucidated. It has been suggested that [HONOIo is the concentration of HONO formed in the chamber during the injection of the NO,, when the local NO, concentration is high, or that [HONOlo is formed prior to injection on the vacuum line or in the syringe (10). The constant flux of HO radicals might be due to photocatalyzed or photoenhanced reactions occurring on the surface of the smog chamber (10, 13). In our experiments, the HONO formation rate during the first 5-min dark decay of HOzNO2 ranged from 0.006 to 0.038 ppm min-1 (Table I) and is more than 20 times greater than the reported values Of kHo ranging from 0.014 to 0.29 ppb min-1 (2,4,10-12). Presumably, the formation rate of HONO depends on both the initial H02N02 concentration and the reactor wall
conditions. In the smog chamber studies, HO2 formed from the irradiation of hydrocarbons and NO, in air could react with NO2 to form H02N02. Our results conducted in a glass reactor suggest that HO2N02 formed in smog chamber experiments may also produce HONO heterogeneously, thereby contributing significantly to the constant flux of HO radicals kHO. Acknowledgments We thank NSERC and British Gas/Consumers Gas for financial support. H.N. is the holder of the British Gas/ Consumers Gas/NSERC/AES Industrial Research Chair in Atmospheric Chemistry at York University. Literature Cited (1) Yarwood, G.; Niki, H.; Maker, P. D. J. Phys. Chem. 1991, 95,4113-4777. (2) Leone, J. A.; Flagan, R. C.; Grosjean, D.; Seinfeld, J. H. Int. J. Chem. Kinet. 1985, 17, 177-216. (3) Niki, H.; Maker, P. D. Adv. Photochem. 1990,15,69-137. (4) Sakamaki, F.; Hatakeyama, S.; Akimoto, H. Int. J. Chem. Kinet. 1983, 15, 1013-1029. (5) Uselman, W. M.; Levine, S. Z.; Chan, W. H.; Calvert, J. G.; Shaw, J. H. Chem. Phys. Lett. 1978,58,431. (6) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P. Chem. Phys. Lett. 1977, 45, 564-566. (7) Howard, C. J. J. Chem. Phys. 1977, 67, 5258-5263. (8) Graham, R. A.; Winer, A. M.; Pitts, J. N., Jr. Chem. Phys. Lett. 1977, 51, 215-220. (9) Stockwell, W. R.; Calvert, J. G. J. Geophys. Res. 1983,88 (C11). 6673-6682. (10) Glasson, W. A.; Dunker, W. A. Environ. Sci. Technol. 1989, 23. 910-918.
(11) Carter, W. P. L.; Atkinson, R.; Winer, A. M.; Pitts, J. N.,
Jr. Int. J. Chem. Kinet. 1982,14, 1071-1103. (12) Carter, W. P. L.; Atkinson, R. Environ. Sci. Technol. 1987, 21, 610-679. (13) Akimoto, H.; Tagaki, H.; Sakamaki, F. Int. J . Chem.Kinet. 1987, 19, 539-551. Received for review October 2, 1992. Revised manuscript received January 4, 1993. Accepted January 8, 1993.
Envlron. Sci. Technol., Vol. 27, No. 5, 1993 g83
