J. Phys. Chem. 1993,97, 11699-11705
11699
HOBr Kinetics: Reactions of Halogen Atoms, Oxygen Atoms, Nitrogen Atoms, and Nitric Oxide with HOBr P. S. Monks,'*t F. L. Nesbitt,l M. Scanlog* and L. J. Stief' Laboratory for Extraterrestrial Physics, NASAIGoddard Space Flight Center, Greenbelt, Maryland 20771 Received: June 7 , 1993; In Final Form: September 8, 1993'
Relatively little is known about the role of HOBr in stratospheric chemistry. Potentially, HOBr could act as a temporary reservoir for stratospheric BrO, compounds. Recently, the first direct measurement of the rate coefficient for the reaction BrO HO2 HOBr 0 2 , which is thought to be the main production channel for HOBr in the stratosphere, was found to be 6 times larger than previously suggested at (3.3 f 0.5) X 10-11 cm3 molecule-' s-l. In this work we have investigated the reactions of HOBr with F(,P), Cl(,P), O(3P), N(4S), Br(,P), and NO(,lI) using the discharge-flow kinetic technique coupled to mass spectrometric detection. The HOBr was formed in situ by the reaction sequence F H2O O H + HF, O H Bra HOBr Br. Using an excess of the atomic species or NO, the rate coefficients at T = 298 K for the HOBr reactions were determined to be k(F) = (2.0 f 0.7) X 10-l0, k(C1) = (1.1 f 0.3) X 10-l0, k ( 0 ) = (2.5 f 0.4) X k(N) < 2 X and k(N0) < 5 X l&14 (all units cm3 molecule-' s-l). No reaction between Br HOBr could be observed, but it was not possible to quantify this because of the large amount of interference from background Br2 on the HOBr signal. This study represents the first determination of the rate constants for thesereactions. Attention is paid to possible interferences from reaction of HOBr precursors. The reactions of HOBr with C1 and 0 are markedly faster than the analogous reactions of HOC1. The atmospheric lifetime of HOBr with respect to reaction with 0 is ca. 0.03 h and with respect to reaction with C1 is ca. 9.6 h at z = 35 km (calculated maxima in [HOBr]).
-
+
+
+
-
-
+
+
+
htroduction
-
ozone destruction, since the rate of reaction 4 was too slow
+ 0, BrO + HO, Br
Bromine compounds are known to interact with ozone,lS2 in both the perturbed and natural stratosphere. The discovery of the massive seasonal depletion of ozone3in the Antarctic spring centered attention on the role of biogenic and anthropogenic chlorinated speciesin polar regions. Analogousto the interactions of the chlorine species implicated in ozone depletion are the stratospheric bromine species.13 In 1986,McElroy et ale4pointed out the importance of bromine oxide chemistry in the antarctic destruction of ozone via the synergistic C10 cycle
-
Br
+ 0,
BrO
+ 0,
(1)
Cl
+ 0,
ClO
+ 0,
(2)
+ C1+ 0,
(3)
BrO
+ C10net:
Br
203-30,
Present knowledgeof reaction 3 indicates that themajor products are not atomic bromine and chl~rine.~ Bromine oxide concentrations in the Antarctic vortex are in the region 4-7 p p t ~ .The ~,~ bromine cycle (1, 4-6) was thought1P8to play w active part in NAS/NRC Research Amxiate.
* JOVEScholar. Present addresr: FairmontStateCollege,Fairmont, WV
26554. Present address: Coppin State College, Baltimore, MD 21216. Abstract published in Advance ACS Abstracts, October 15, 1993.
BrO
+ 0,
HOBr
+ 0,
HOBr+hv-OH+Br OH
+ 0,-,HO, + 0,
net:
(1) (4)
(5)
(6)
203-30,
Recent kinetic measurements8q9have shown the room temperature rate for reaction 4 to be 6 times larger than previously thought, leading to the possibility of the increased importance of this catalytic cycle in the depletion of ozone.8 An analogous C1driven catalytic cycle to (1, 4-6) was suggested by Solomon et al.1° to account for the C1 atom and HO, coupling in the polar spring, One of the intermediate species in the catalytic cycle (1,4-6) is HOBr, the brominated analogue of HOCl. Hypobromous acid (HOBr) could act as a temporary reservoir for BrO, in the stratosphere.' The main stratospheric loss process for HOBr is thought to be photolysis,8 but as yet there are no data to confirm this hypothesis. In the troposphere, HOBr transport is facilitated by cloud-pumping mechanisms.ll There is little chemical information on HOBr. There exists infrared (IR) sptctra12J3 and a matrix IR spectrum.14 The kinetics of HOBr have been studied in solution by spectrophotometry.l5 In combustion chemistry HOBr has been detected as a product from the addition of halon 1301 (CF3Br) to cool flames.16 The halon fire suppressantsact as an OH "radical interceptor" in the early stages of high-temperature combustion processes. There has only been limited kinetic and spectrorpcopicwork on the analogous Cl species HOCl.17-24 In addition, the kinetic experimentsare difficult to perform, and the rate constant values have somewhat larger than usual uncertainties. HOCl is
0022-365419312097-11699$04.00/0 Q 1993 American Chemical Society
Monks et al.
11700 The Journul of Physical Chemistry, Vol. 97, No. 45, 1993
potentially a temporary reservoir for stratospheric chlorine. More recently, it has been suggested2sthat the reactions of HOCl with HCl on polar stratospheric clouds may lead to more rapid ozone destruction by releasing greater amounts of C10,. In this paper we describe the first measurements of the gasphasekineticsofthereactionof HOBr withF(2P),C1(2P),Br(2P), O('P), N(4S), and NO(2II). The measured rate coefficients are compared to equivalent measurements, when available, for reaction with HOCl. The implications of this work for the atmospheric lifetime of HOBr, with respect to the reactions of Cl and 0,are discussed briefly.
+ CH4 titration reaction F
+ CH4 -,CH, + H F
(9)
k,(T=298 K) = 8.0 X lo-'' cm3 molecule-' s-' (ref 31) Chlorine atoms were formed at the tip of the sliding injector by the rapid reaction
F + C1,
-
C1+ FCl
(10)
k,,(T=298 K) = 1.2 X lo-'' cm3 molecule-' s-'
Experimental Seetion
(refs 34,35)
D h h g e Flow Reactor. All experiments were performed in a Pyrex flow tube, -60 cm long and 28 mm in diameter, the inner surface of the tube being lined with Teflon. The flow tube was coupled Diu a two-stage stainless steel collision-freesampling system to a quadrupole mass spectrometer (Extranuclear Laboratories Inc.) that was operated at low electron energies (typically
