Meteo 465/565 -- The Middle Atmosphere
Chemistry of the tropical and midlatitude stratosphere.

Important chemical families:

oxygen.

Constitutents: The oxygen chemical family consists of ozone and oxygen atoms.

Source: molecular oxygen. Exchange is through the making and destruction of the O₂ chemical bond

Chemistry: the 4-reaction Chapman mechanism

Interactions: The Chapman mechanism overpredicts the amount of ozone that is observed in the main ozone production regions of the stratosphere. At the same time, ozone is underpredicted at high latitudes. There are two causes. First, transport carries ozone from regions where it is produced to regions where it is destroyed. Second, other chemistries interact with ozone chemistry.

How? The amount of ozone is so much greater than most of the other chemicals in the stratosphere. However, these other chemicals are competitive through catalytic cycles that mimic the O_x termination reaction: O+O₃ ® 2 O₂.

We will go through these chemical families one by one, describing the sources, sinks, and methods for destroying ozone.

The basic mechanism can be described by the figure showing the relationships among the sources, radicals, and reservoir species.

hydrogen.

Source: water vapor, methane.

Water vapor is destroyed in the mesosphere and thermosphere by photolysis at 121.6 nm: H₂O+hn®H+OH.

In the stratosphere, where solar UV below 200 nm does not penetrate, H₂O is destroyed by:

O₃ + hn ® O₂ + O(¹D)

O(¹D) + H₂O ® 2 OH

The amount of water vapor entering the stratosphere is limited by the cold trap at the tropical tropopause. This temperature changes seasonally, but typically it is cold enough that only about 2-3 ppmv water vapor enters the stratosphere.

Methane comes from cows, termites, rice paddies, and many other sources. It has a lifetime of about 10 years in the troposphere, but some of it finds its way to the tropical upper troposphere, where it enters the stratosphere. At present, the methane mixing ratio entering the stratosphere is about 1.74 ppmv. About 8% of the methane is removed from the atmosphere in the stratosphere.

Methane is removed by the methane oxidation sequence:

CH₄ + OH ® CH₃ + H₂O

CH₃ + O₂ + M ® CH₃O₂ + M

CH₃O₂ + NO ® CH₃O + NO₂

CH₃O + O₂ ® CH₂O + HO₂

CH₂O + hn ® HCO + H

® CO + H₂

H + O₂ + M ® HO₂ + M

HCO + O₂ ® HO₂ + CO

CO + OH ® H + CO₂

HO₂ + NO ® OH + NO₂

The net result is that we end up with one CO₂ and two H₂O for each methane molecule that is completely oxidized.

When we add H₂O + 2 CH₄ we get a sum that is roughly constant at about 6 ppmv throughout the stratosphere.

Chemistry: We see that in the process of water vapor and methane oxidation OH and HO₂ are formed. These two radicals are very reactive.

HO_x ozone-destroying catalytic cycles:

middle stratosphere	lower stratosphere	upper stratosphere
OH + O3 ® HO2 + O2 HO2 + O ® OH + O2 net: O + O3 ® 2 O2	OH + O3 ® HO2 + O2 HO2 + O3 ® OH + 2O2 net: O3 + O3 ® 3 O2	OH + O ® H + O2 H + O2 + M ® HO2 + M HO2 + O ® OH + O2 net: O + O ® O2

There can be null cycles that dominate the catalytic cycling of HO_x, but do not destroy ozone:

OH + O₃ ® HO₂ + O₂ ; HO₂ + NO ® OH + NO₂ ; NO₂ + hn ® NO + O ; O + O₂ + M ® O₃ + M ; net: null

The loss of OH and HO₂ is by a few reactions:

OH + HO₂ ® H₂O + O₂. Here, both water vapor and molecular oxygen are reformed.

OH + NO₂ + M ® HONO₂ + M (nitric acid). Nitric acid can be converted back by photolysis, but generally is a sink. Notice that this reaction combines chemicals from two difference chemical families.

Look at the HO_x catalytic cycles. Note that the reaction of HO₂ + NO ® OH + NO₂ is much faster than HO₂ + O ® OH + O₂. We can see this from the figures in the chapter on Stratospheric Chemistry. What is the result of this?

nitrogen.

Reactive forms: NO (nitric oxide) and NO₂ (nitrogen dioxide)

Sources: 65% Nitrous Oxide, N₂O, which comes from anerobic microbial activity. The current tropospheric mixing ratio is about 320 ppbv. Its atmospheric lifetime is 120 years. Its main destruction is in the stratosphere.

10% NO made by solar proton events and galactic cosmic rays;

25% NO from lightning in the tropical upper troposphere, carried into the stratosphere.

The main destruction of N₂O in the stratosphere is:

N₂O + hn ® N₂ + O.

More than 92% of the N₂O is destroyed this way. However, the more interesting destruction pathway is:

N₂O + O(¹D) ® 2 NO

NO and NO₂ can catalytically destroy ozone by the catalytic cycle:

NO + O₃ ® NO₂ + O₂

NO₂ + O ® NO + O₂

net: O + O₃ ® 2 O₂

An important point is that although this is the main ozone destroying catalytic cycle, it is not the main catalytic cycle involving NO and NO₂. That cycle is:

NO + O₃ ® NO₂ + O₂

NO₂ + hn® NO + O

O + O₂ + M ® O₃ + M

net: null cycle.

So the lesson is that the most important catalytic cycles may well not be the most important ozone destroying catalytic cycles.

Other reactions happen to NO and NO₂. These are usually with reactive species from other chemical families to form reservoir species. Two prominent examples are:

NO₂ + OH + M ® HONO₂ + M

NO₂ + ClO + M ® ClONO₂ + M

In the troposphere, HONO₂ is rapidly taken up on surfaces. In the stratosphere, where there are few surfaces but some UV, HONO₂ can be photolyzed to Oh and NO₂. In addition , the reaction HNO₃ + OH ® NO₃ + H₂O also reverses HNO₃.

Faster photolysis occurs with ClONO₂. Its lifetime throughout much of the stratosphere is hours.

With the nitrogen family, there are also some important reaction sequences that involve aerosols:

NO₂ + O₃ ® NO₃ + O₂ (NO₃ is photolyzed in seconds and thus exists only in the dark.)

NO₃ + NO₂ ® N₂O₅

N₂O₅(gas) + H₂O(surface) ® 2 HNO₃(surface)

chlorine.

Reactive forms: Cl (atomic chlorine) and ClO (chlorine monoxide)

Sources: Total chlorine sources: 3.8 ppbv in 1998.

chemical	formula	percent in 1997	a or n	lifetime (years)
CFC-11	CCl3F	23	a	50
CFC-12	CCl2F2	28	a	120
carbon tet	CCl4	12	a	42
CFC-113	CClF2CClF2	16	a	85
methyl chloroform	CH3CCl3	10	a	4.8
HCFC-22	CHClF2	3%	a	12
methyl chloride	CH3Cl	15	n	1.3
hydrogen chloride	HCl	3	n

The annual emissions of the anthropogenic compounds dropped precipitously after the Montreal Protocol was signed in 1987. As a result, the tropospheric amount of total anthropogenic chlorine peaked at about 3 ppbv in 1992-1994 and is now in rapid decline. Of course, the only way that these chlorofluorcarbons are being destroyed is by cycling them through the stratosphere.

The CFCs are being replaced by HCFC, which contain a hydrogen. The lifetimes of these compounds is controlled by tropospheric OH. The lifetimes are typically days to months. We need to worry about the degradation products, such as COCl₂, phosgene, which is toxic.

Those compounds that contain an H are primarily destroyed in the troposphere by reaction with OH. The CFCs are so good because they aren’t destroyed in the troposphere, but only the stratosphere.

So, look at the CFCs, taking CFC-11 as an example:

CCl₃F + hn ® CCl₂F + Cl

Cl + O₃ ® ClO + O₂

Now, just as for other chemicals, ClO has several reaction pathways:

ClO + NO ® Cl + NO₂

or

ClO + O ® Cl + O₂

Which cycle results in ozone loss and which one is a null cycle?

Right, the ClO + O ® leads to ozone destruction.

This is basically the work that won Sherry Rowland and Mario Molina the Nobel Prize in Chemistry in 1994. What they didn’t know was:

ClO + NO₂ + M ® ClONO₂ + M

They knew about

Cl + CH₄ ® HCl + CH₃

So we see how the chlorine family connects to the reservoir species.

Figure on distributions in the stratosphere.

Two distributions:

Total organic chlorine, CCl_y

Total inorganic chlorine, Cl_y

The sum should be constant in the atmosphere by conservation of mass.

bromine.

Reactive forms: Br (atomic bromine) and BrO (bromine monoxide)

Sources: Total chlorine sources: 18 ptv in 1998.

chemical	formula	percent in 1997	a or n	lifetime (years)
methyl bromide	CH3Br	55	a,n	0.7
bromochloromethane	CH2BrCl	0.7	n	120
dibromomethane	CH2Br2	5.7	n	42
Halon-1211	CBrClF2	20	a	20
Halon-1301	CBrF3	14	a	65
Halon_2402	CBrF2CBrF2	5	a

Methyl bromide has about ½ natural sources (biological activity) and ½ anthropogenic sources (fumigation, biomass burning). The ocean serves as a net sink, but is both a source and a sink.

Bromine is still growing in the atmosphere. This will change as the regulation of CH₃Br starts. (Halons are already regulated.)

Those compounds that contain an H are primarily destroyed in the troposphere by reaction with OH, just as for chlorine.

CBrF₃ + hn ® CF₃ + Br

Br + O₃ ® BrO + O₂

Now, just as for other chemicals, ClO has several reaction pathways:

BrO + NO ® Br + NO₂

or

BrO + O ® Br + O₂

Which cycle results in ozone loss and which one is a null cycle?

Right, the BrO + O ® leads to ozone destruction.

BrO + NO₂ + M ® BrONO₂ + M

They knew about

Br + CH₄ ® HBr + CH₃

There is so little Br in the stratosphere that its contribution to ozone loss may seem to be quite small. But it turns out that the bromine chemistry is quite different from the chlorine chemistry, due to the difference between chlorine and bromine bond strengths. This difference is actually much more important than you would think. A much larger fraction of the bromine is in the reactive form than for chlorine. In fact, each bromine atom is able to destroy about 40-100 times as much ozone as each chlorine atom.

In addition, there is an important reaction sequence that connects the two halogen species:

ClO + BrO ® Br + OClO

® Br + ClOO

® BrCl + O₂

Followed by:

OClO + hn ® ClO + O

ClOO ® Cl + O₂

BrCl + hn ® Br + Cl

The pathways are determined by the temperature, but about half of the reaction goes to the last two pathways occur.

So we see how the bromine family connects to the reservoir species.

Figure on distributions in the stratosphere.

Implications of ozone chemistry.

Diurnal variation of stratospheric chemistry.

Reactive stratospheric chemistry occurs in daylight. Note in the figure how the reactive species change as a function of sunlight. Look at the diurnal figure.

The exception to this statement is heterogeneous chemistry, which works as well in the dark as it does in sunlight.

The sum of ozone loss.

1. Because ozone is in much greater abundance than any reactive chemical, the reactive chemicals can destroy significant amounts of ozone only through catalytic cycles.

2. In catalytic cycles, either no or two ozone molecules are destroyed.

3. The rate of destruction is given by the sum of twice the step of the cycle that leads to ozone destruction.

a. d[O₃]/dt = - 2k_O+O3[O][O₃] - 2k_O+NO2[O][NO₂] - 2k_O+ClO[O][ClO] - 2k_O+HO2[O][HO₂] - 2k_HO2+O3[O₃][HO₂] - 2f_Br+Cl k_BrO+ClO[BrO][ClO] - …

4. Let’s look at the reaction rate coefficients to see how much ozone is lost for each molecule of each radical: NO₂, HO₂, ClO, and BrO.

How the interactions among chemical species affect ozone loss.

The abundance of NO_y chemicals is typically ~10-15 ppbv, where NO and NO₂ make up a reasonable fraction. At the same time, the abundance of OH and HO₂ are ~1 pptv and ~20 pptv and the abundance of ClO and BrO are 1-100’s of pptv. Thus, in some sense, the nitrogen family controls the other chemical families. Thus, if the total amount of NO_y or if the partitioning within NO_y is affected, the effects of this change in reactive nitrogen propagates to the other chemical families. We can turn the problem around and say that HO_x and halogens affect NO_x, but in truth, the larger amounts of NO_y give NO_x the advantage.

Why is this important?

A case study: SuperSonic Transports, SST’s, or more recently High Speed Civil Transports, HCSTs. In the early work on this problem in the 70’s, it was concluded that SSTs would destroy large amounts of ozone in the lower stratosphere, reducing the total ozone column by 10’s of a percent in the Northern Hemisphere.

However, a more recent study suggests that if the HSCT’s were to fly below 20 km, the ozone loss would be difficult to distinguish from natural variation. What caused the difference? It was the repartitioning of NO_x.

The main way that HSCTs would destroy ozone is by adding lots of additional NO to the lower stratosphere. Estimates are that NO in the lower stratosphere would roughly double, while water vapor would increase 10-20% and sulfur would increase 10-200%.

However, two effects come into play.