atmospheric chemistry

CHEM 415, Atmospheric Chemistry Problem Set #3 (25 points possible) Reference Material: Finlayson-Pitts and Pitts Chapter 5 DUE DATE: May 27, 2014 at 1:30PM.

1. Some reactions that appear to be simple bimolecular processes exhibit pressure-dependent rate constants that are not consistent with a direct concerted reaction. A classic example of great atmospheric importance is the reaction:

OH + CO  H + CO2 rate = k

bi [OH][CO]

It is now believed that this reaction is not a simple bimolecular process but rather a combination of several elementary steps in which a bound adduct, HOCO*, is formed as an intermediate ( the * indicates an energetically excited molecule).

Rate constant OH + CO  HOCO* ka (association) HOCO*  OH + CO kd (dissociation) HOCO* + M  HOCO + M kq (quenching) HOCO*  H + CO2 kr (reaction)

a. Assume that the excited adduct, HOCO*, is in steady state, and solve for kbi in terms of the rate constants for the four elementary reactions above.

b. Sketch out how this rate constant should vary with pressure, from the low pressure limit to the high pressure limit.

c. For atmospheric applications, the rate constant for this reaction is often parameterized as kbi = Aexp (-Ea/RT)

where A = 1.5  10-13 (1 + 0.6 Patm) cm3 molec-1 s-1 (Patm is the pressure in atmospheres) and Ea/R = 0 kcal mol-1.

Assume an OH concentration of 8  105 molec cm-3, calculate the lifetime of CO with respect to the reaction with OH at 298K.

2. 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  10

7 molecules cm-3 HO2, and 1  10 5 molecules cm-3 OH. The air density

in the parcel is 3  1019 molecules cm-3. Bromine chemistry is described by the reactions:

Br + O3  BrO + O2 k1 = 6  10 -13 cm3 molecule-1 s-1 (1)

BrO + h  2O Br + O3 k2 = 1 10-2 s-1 (2)

BrO + BrO  2Br + O2 k3 = 3  10 -12 cm3 molecule-1 s-1 (3)

Br + CH2O  HBr + CHO k4 = 6  10 -13 cm3 molecule-1 s-1 (4)

BrO + HO2  HOBr + O2 k5 = 5  10 -12 cm3 molecule-1 s-1 (5)

HBr + OH  Br + H2O k6 = 1.1  10 -11 cm3 molecule-1 s-1 (6)

HOBr + h  OH + Br k7 = 1  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]  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  aerosol Br2 + H2O

Br2 + h  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].