METO 621 CHEM Lesson 2

The Stratosphere We will now consider the chemistry of the troposphere and stratosphere. There are two reasons why we can separate these regions (1) The stratosphere absorbs most of the shortwave radiation from the sun, hence the stratosphere has high energy photons to induce photochemistry. The troposphere must make do with lower energy photons. (2) The temperature decreases with altitude in the troposphere, implying a basically unstable atmosphere with considerable vertical mixing. In the stratosphere the temperature increases with altitude. This implies a stable atmosphere with little vertical mixing. Substances injected into the stratosphere take a long time to reach the troposphere, and can build up to significant levels

The Stratosphere

Ozone-only chemistry First approach to the theoretical explanation of the ozone layer was by Chapman, 1930, who proposed a static pure oxygen photochemical stratosphere. The reactions were  odd-oxygen O 2 + h  → O + O O + O 2 + M → O 3 + M 02 O 3 + h → O + O 2 03 O + O 3 → O 2 + O [O + O + M → O 2 + M -25] Reaction 5 can be ignored in the stratosphere. Reactions 1 and 3 give excited atoms, but these are quickly quenched to the ground state. No excited state chemistry is assumed.

Ozone-only chemistry Reactions 2 and 3 interconvert O 3 and O rapidly in the stratosphere. Reaction 2 has a half-life of as little as 100 sec. Ozone has a similar lifetime during the day. Hence we can consider [O + O 3 ] as a species known as odd-oxygen. Hence reactions 2 and 3 ‘do nothing’ as far as odd-oxygen is concerned. Ignoring reaction 5, then reaction 1 is the source of odd- oxygen, while reaction 4 is the ‘sink’. The next figure shows a plot of ozone and atomic oxygen versus altitude.

Ozone-only chemistry Altitude, km

Ratio of atomic oxygen to ozone

Ozone-only chemistry Let the rate of production of odd-oxygen for reaction 1 be P 1, and that for reaction 3 be P 3. In steady state the amount of odd-oxygen produced in reaction 1 must equal the number destroyed in reaction 4. Now consider equations 2 and 3

Ozone-only chemistry

Zonally averaged ozone concentration vs altitude Zonally averaged rate of ozone formation from O 2 photolysis

Ozone-only chemistry The previous figure shows (1) ozone concentrations, (2) the rate of formation of ozone from the photolysis of molecular oxygen, both as a function of altitude and latitude At the equator the ozone layer is centered at 25 km, where the production rate is negligible, while the production rate of atomic oxygen reached a maximum at ~40 km. The lack of a correspondence between ozone concentration and (P 1 ) 1/2 indicates an inadequacy in the Chapman model The first clue as to what was happening was put forward by Bates and Nicolet in 1950 to explain ozone concentrations in the mesosphere.

Catalytic Cycles Bates and Nicolet suggested the following set of reactions: OH + O 3 → HO 2 + O 2 HO 2 + O → OH + O 2 net reaction O + O 3 → O 2 + O 2 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 + O 3 → XO + O 2 XO + O → X + O 2 net reaction O + O 3 → O 2 + O 2 There are many species that fill the role of X. The most important are H, OH, NO, Cl, Br, and possibility I.

Catalytic Cycles The rate coefficient for the first step of the catalytic cycle are usually much faster than the reaction O+O 3 →O 2 +O 2 and the catalytic cycle is favored. The cycles are then said to involve HO x, NO x, ClO x species, and we refer to families.

Catalytic Cycles

Other catalytic cycles which do not fit into the O+XO mold have been identified OH + O →O 2 + H H + O 2 + M →HO 2 + M HO 2 + O → OH + O 2 Net O + O + M → O 2 + M OH + O 3 → HO 2 + O 2 HO 2 + O 3 → OH + O 2 Net O 3 + O 3 → 3O 2 Cycle does not need atomic oxygen, can be effective at low altitudes where the concentration of atomic oxygen is low.

Catalytic Cycles Another cycle of interest is the following: OH + O 3 → HO 2 + O 2 HO 2 + O 3 → OH + O 2 Net O 3 + O 3 → 3O 2 This cycle does not need atomic oxygen, and can be effective at low altitudes where the concentration of atomic oxygen is low.

The leaky bucket model

Fraction of the odd-oxygen loss rate

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 + HO 2 → HOCl + O 2 (Hypochlorous acid) HO 2 + NO 2 + M → HO 2 NO 2 + M (pernitric acid) ClO + NO 2 + M → ClONO 2 + M (chlorine nitrate) OH + NO 2 + M → HNO 3 + M (nitric acid) NO 3 + NO 2 + M → N 2 O 5 + 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: HO 2 + NO → OH + NO 2 ClO + NO → Cl + NO 2 Both of these reactions short circuit the catalytic cycles, and hence reduce their efficiency. The full reaction cycle for the second reaction is Cl + O 3 = ClO + O 2 ClO + NO → Cl + NO 2 NO 2 + hν → NO + O Net O 3 + hν → O 2 + O Known as a null cycle

Natural Sources and Sinks The catalytic families HO x, NO x, ClO x, and BrO x, appear to be present in the natural ‘unpolluted’ atmosphere. In today’s atmosphere the levels of ClO x and BrO x have been increased by anthropogenic sources. Most of the stratospheric NO x originates from tropospheric N 2 O, which is of biogenic origin (e.g. soils). This reacts with the O( 1 D) to start the NO x chemistry O( 1 D) + N 2 O → NO + NO The main sources of the OH radical are O( 1 D) + H 2 O → OH + OH O( 1 D) + CH 4 → OH + CH 3 The CH 3 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 (CH 4 ), in the sea we get CH 3 Cl. 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, CH 3 Br, which is produced by algae in the oceans.

Summary of Homogeneous Chemistry

Mxing ratio profiles for NO y