1. Bromine Chemistry in the Troposphere Events of rapid O3 depletion are observed in arctic surface air in spring, with concentrations dropping from 40 ppbv (normal) to less than 5 ppbv in just a few days. These O3 depletion events are associated with elevated bromine, which appears to originate from the volatilization of sea salt bromide deposited on the ice pack. In this problem we examine the mechanism for Br-catalyzed O3 loss thought to operate in arctic surface air. Consider a surface air parcel in the arctic at the onset of an O3 depletion event. The air parcel contains 40 ppbv O3, 50 pptv Bry (sum of Br, BrO, HOBr, and HBr), 10 pptv CH2O, 3 ï‚´ 107 molecules cm-3 HO2, and 1 ï‚´ 105 molecules cm-3 OH. The air density in the parcel is 3 ï‚´ 1019 molecules cm-3. Bromine chemistry is described by the reactions: Br + O3 ï‚® BrO + O2 k1 = 6 ï‚´ 10-13 cm3 molecule-1 s-1
Br + O3 gives BrO + O2 k1 = 6 ï‚´ 10-13 cm3 molecule-1 s-1 (1)
BrO + light + O2 gives Br + O3 k2 = 1ï‚´ 10-2 s-1 (2)
BrO + BrO gives 2Br + O2 k3 = 3 ï‚´ 10-12 cm3 molecule-1 s-1 (3)
Br + CH2O gives HBr + CHO k4 = 6 ï‚´ 10-13 cm3 molecule-1 s-1 (4)
BrO + HO2 gives HOBr + O2 k5 = 5 ï‚´ 10-12 cm3 molecule-1 s-1 (5)
HBr + OH gives Br + H2O k6 = 1.1 ï‚´ 10-11 cm3 molecule-1 s-1 (6)
HOBr + light gives OH + Br k7 = 1 ï‚´ 10-4 s-1 (7)
a. Draw a diagram of the Bry cycle. Identify a catalytic cycle for O3 loss consisting of only two reactions, and highlight this cycle in your diagram.
b. Show that reaction (2) is the principal sink for BrO. What is the rate-limiting reaction for O3 loss in the catalytic mechanism you described in question a? Briefly explain.
c. Write an equation for the O3 loss rate (-d[O3]/dt) in the catalytic mechanism as a function of [BrO]. What would the O3 loss rate be if BrO were the main contributor to total bromine (that is, if [BrO] ï‚» 50 ppt)? Would you predict near-total ozone depletion in a few days? d. Ozone loss can in fact be slowed down by formation of HBr or HOBr.
a. Explain briefly why.
b. Assuming steady state for all bromine species, calculate the concentrations of HOBr, HBr, BrO, and Br in the air parcel. How does the resulting O3 loss rate compare to the value you computed in question c? Would you still predict near-total O3 depletion in a few days?
c. It has been proposed that O3 depletion could be ehnhanced by reaction of HOBr with HBr in the arctic aerosol followed by photolysis of Br2: HBr + HOBr ï‚¾aerosol ï‚¾ ï‚®ï‚¾ Br2 + H2O Br2 + hï® ï‚® 2Br. How would these two reactions help to explain the observed O3 depletion? Draw a parallel to similar reactions occurring in the stratosphere.
[To know more: Haussman, M., and U. Platt. Spectroscopic measurement of bromine oxide and ozone in the high Arctic during Polar Sunrise Experiment 1992, J. Geophys. Res. 99:25399-25413, 1994].