METO 637

LESSON 7

Catalytic Cycles

• Bates and Nicolet suggested the following set of reactions:

OH + O3 → HO2 + O2

HO2 + O → OH + O2

net reaction O + O3 → O2 + O2

• This is called a catalytic cycle. In this case the OH radical is the catalyst, in that it destroys odd oxygen but is not consumed itself. This cycle can be generalized to be

X + O3 → XO + O2

XO + O → X + O2

net reaction O + O3 → O2 + O2

• Since the overall reaction consists of two steps, each one must be exothermic for the reactions to be efficient

Catalytic Cycles

• The rate coefficient for the first step of the catalytic cycle are usually much faster than the reaction O+O3→O2+O2 and the catalytic cycle is favored.

• There are many species that fill the role of X. The most important are H, OH, NO, Cl, Br, and possibility I.

• The cycles are then said to involve HOx, NOx, ClOx species.

Catalytic Cycles• Other catalytic cycles which do not fit into the O+XO mold have

been identified

• OH + O →O2 + H

H + O2 + M →HO2 + M

HO2 + O → OH + O2

Net O + O + M → O2 + M

• OH + O3 → HO2 + O2

HO2 + O3 → OH + O2

Net O3 + O3 → 3O2

• Cycle does not need atomic oxygen, can be effective at low altitudes where the concentration of atomic oxygen is low.

The leaky bucket model

Catalytic Cycles

Fraction of the odd-oxygen loss rate

Summary of Homogeneous Chemistry

Reservoir Species• So far we have treated the catalytic cycles as independent of one

another. We refer to the species within a cycle as a family, e.g. the nitrogen family.

• However, the species in one family can also interact with those of another family, e.g.

ClO + HO2 → HOCl + O2 (Hypochlorous acid)

HO2 + NO2 + M → HO2NO2 + M (pernitric acid)

ClO + NO2 + M → ClONO2 + M (chlorine nitrate)

OH + NO2 + M → HNO3 + M (nitric acid)

NO3 + NO2 + M → N2O5 + M (nitrogen pentoxide)

• Although these compounds can be dissociated back to their parent molecules, stratospheric circulation moves them to the poles, where the solar radiation is weak, and dissociation unlikely. They are called reservoir species.

Reaction between cycles

• Consider the following reactions:HO2 + NO → OH + NO2

ClO + NO → Cl + NO2

• Both of these reactions short circuit the catalytic cycles, and hence reduce their efficiency.

• The full reaction cycle for the second reaction isCl + O3 = ClO + O2

ClO + NO → Cl + NO2

NO2 + hν → NO + O Net O3 + hν → O2 + O• Known as a null cycle

Natural Sources and Sinks

• The catalytic families HOx, NOx, ClOx, and BrOx, appear to be present in the natural ‘unpolluted’ atmosphere. In today’s atmosphere the levels of ClOx and BrOx have been increased by anthropogenic sources.

• Most of the stratospheric NOx originates from tropospheric N2O, which is of biogenic origin (e.g. soils). This reacts with the O(1D) to start the NOx chemistry

O(1D) + N2O → NO + NO• The main sources of the OH radical are

O(1D) + H2O → OH + OH

O(1D) + CH4 → OH + CH3

• The CH3 radical reacts to produce other hydrogen species including water vapor. Most stratospheric water vapor comes from methane ‘oxidation’.

Natural Sources and Sinks

• The most abundant natural source of ClO is methyl chloride.

• The major contributors are the oceans. Much comes from the decay of organic matter. In wet conditions on land we get methane (CH4), in the sea we get CH3Cl.

• The chlorine is released by reactions with the OH radical, and by photodissociation above 30 km.

• Natural bromine enters the stratosphere principally as methyl bromide, CH3Br, which is produced by algae in the oceans.

Summary of Homogeneous Chemistry

Summary of Homogeneous Chemistry

Heterogeneous Chemistry

• Heterogeneous reactions play an important part in stratospheric chemistry.

• The interest in these reactions was first sparked by the so called ‘ozone hole’ over the antarctic, where they play a crucial role through the surfaces of ice crystals.

• However, it is also recognized that reactions on sulfate particles ( the product of volcanic eruptions) can be important away from the poles.

• Heterogeneous processing is particular important in relation to the reservoir species.

Heterogeneous Chemistry

• Such reactions include

N2O5 + H2O → 2HNO3

ClONO2 + H2O → HOCL + HNO3

• The following reaction is critical to the ‘ozone hole’ theory

ClONO2 + HCl → Cl2 + HNO3

• Other surface reactions include

HOCl + HCl → H2O + Cl2

HOBr + HCl → H2O + BrCl

N2O5 + HCl → ClNO2 + HNO3

Ratio of atomic oxygen to ozone

Comparison of measurements to theory for OH and HO2

Mxing ratio profiles for NOy

Chlorine containing species