# Greenhouse Gas and Ozone

## Greenhouse effect

Greenhouse gases occur naturally in our atmosphere. When solar radiation passes through the atmosphere, most of it is absorbed by the surface, heating the surface, while some is reflected.

When this happens, greenhouse gases absorb the infrared radiation and reemit it towards the surface. The more greenhouse gases present, the more of the IR is re-emitted, and the more the surface of the earth is heated.

Some greenhouse gases include:

• Water vapour: This is the most abundant greenhouse gas and is from evaporation of lakes and oceans, among others.
• Carbon dioxide: This is produced in many natural processes such as respiration and volcanic eruptions but also in industrial processes such as combustion.
• Methane: This is produced in the production of coal, natural gas and oil. It is also emitted through natural processes such as a by-product of digestion.

### Absorption of IR

Infrared radiation can be absorbed by:

• $C=O$ bonds in $CO_{2}$
• $O-H$ bonds in $H_{2}O$
• $C-H$ bonds in $CH_{4}$

When the IR is absorbed, the bonds vibrate which emits energy transferring kinetic energy to other atoms, increasing the temperature.

### Global Warming Potential

The ability of a gas to cause global warming is called its global warming potential. This depends on:

• The lifetime of the gas in the atmosphere
• The ability of the bonds to absorb infrared radiation
• The amount of the gas present in the atmosphere

## Reducing Greenhouse Emissions

In order to slow climate change, the amount of greenhouse gases released into the atmosphere must be controlled. An important milestone was getting countries to agree to the Kyoto Protocol which committed countries to reduce their emissions of greenhouse gases by 5% before 2012. Similar proposals were set up by the EU to control the greenhouse gases produced.

### Carbon capture and storage

Carbon capture and storage (CCS) captures carbon dioxide from power stations and stores it safely instead of releasing it into the atmosphere. Underground porous rocks can act as a sponge in order to store carbon dioxide gas and keep it from leaking.

### Decarbonised fuels

A power station often burns methane gas as a fuel producing carbon dioxide gas.

$$CH_{4(g)} + 2O_{2(g)} \rightarrow CO_{2(g)} + 2H_{2}O_{(l)}$$

A decarbonised fuel is produced by reforming natural gas into a mixture of $H_{2}$ and $CO_{2}$.

$$CH_{4(g)} + 2H_{2}O{(l)} \rightarrow CO_{2(g)} + 4H_{2(g)}$$

The $CO_{2}$ can then be safely extracted and stored so that it does not contribute to the greenhouse effect. A common method of storage is in old oil fields where injecting carbon dioxide can extract more of the oil out. The hydrogen gas can then be burnt as a clean fuel producing no harmful waste.

#### Storage as carbonates

Another possible storage technique is to store it in carbonate rock. This is a naturally occurring process which is very slow so is rarely used in industry.

## Ozone layer

Ozone, $O_{3}$, is an important compound which when present in the upper atmosphere, the stratosphere, is able to prevent harmful UV radiation from reaching the Earth's surface. In the lower atmosphere, the troposphere, it is an air pollutant with harmful effects to the respiratory system.

### Ozone-oxygen cycle

Ozone is being continually formed and broken down in the stratosphere by UV radiation which maintains an equilibrium of ozone.
This is first done by the UV splitting $O_{2}$ into two oxygen radicals.
$$O_{2} + UV \rightarrow 2O{\cdot}$$
These radicals then react with $O_{2}$ forming $O_{3}$
$$O_{2} + O\cdot \rightarrow O_{3}$$
Ozone can then be removed by an oxygen radical reacting with $O_{3}$, forming an oxygen diatomic molecule.
$$O_{3} + O\cdot \rightarrow 2O_{2}$$

This gives an overall reaction of:
$$O_{2} + O\cdot \leftrightharpoons O_{3} + \text{heat}$$

## Ozone Depletion

### Ozone Depletion Potential

The ozone depletion potential is a measure of how much a substance breaks down ozone relative to $CFCl_{3}$.

It has been recognised that $Cl$ radicals in the stratosphere are able to catalyse the breakdown of ozone and are largely a result of human activity. The use of CFCs has resulted in more chlorine radicals being present in the atmosphere therefore breaking down more ozone. This process starts with initiation:

$$CFCl_{3} \rightarrow Cl{\cdot} +\,CFCl_{2}{\cdot}$$

Once the radicals have been formed, propagation can start:

$$Cl{\cdot} +\,O_{3} \rightarrow ClO{\cdot} +\,O_{2} \\ ClO{\cdot} + O \rightarrow Cl\cdot +\,O_{2}$$

Overall this reaction is:
$$O_{3} + O \rightarrow 2O_{2}$$

As the chlorine radical is regenerated it can breakdown more ozone molecules. A single CFC molecule is able to destroy 100000 ozone molecules.

Another radical that breaks down ozone is nitrogen oxide $NO{\cdot}$, which is formed in aircraft engines. The propagation steps are shown below:

$$NO\cdot + O_{3} \rightarrow NO_{2}\cdot + O_{2}$$

#### Montreal Protocol

The Montreal Protocol is a global environmental agreement which aims to reduce the usage of ozone depleting compounds such as CFCs.

## Controlling air pollution

Vehicle emissions are one of the major threats to clean air. The internal combustion engine in a modern car emits various atmospheric pollutants, mainly carbon monoxide, oxides of nitrogen and unburnt hydrocarbons.

#### Carbon Monoxide $CO$

This is a poisonous gas emitted through incomplete combustion. Carbon monoxide binds to haemoglobin in blood, reducing the amount of oxygen supplied to organs and tissues. Generally, it remains in the atmosphere for around a month before been oxidised to carbon dioxide.

#### Oxides of nitrogen $NO_{x}$

These are produced in the internal combustion engine, during a high temperature process where some of the nitrogen from air is oxidised by oxygen. Nitrogen oxides are respiratory irritants which can be harmful to humans. Nitrogen oxides in the atmosphere usually get converted to nitric acid.

#### Unburnt Hydrocarbons

These are released in vehicle exhausts from unburnt fuels. Once in the atmosphere, these react with nitrogen dioxides to form low-level ozone. Low-level ozone is harmful as it is a serious pollutant leading to breathing difficulties.

## Catalytic Converters

A typical converter is made from platinum, rhodium and palladium supported by a honeycomb mesh. The exhaust gases pass over the catalyst surface converting harmful gases into less harmful products. There are two main types of catalyst.

### Oxidation catalysts

These are used in diesel engines to decrease the amount of emissions of carbon dioxide and unburnt hydrocarbons. They also remove particulate matter and nitrogen oxides. The following reactions take place:
$$2CO_{(g)} + O_{2(g)} \rightarrow 2CO_{2(g)} \\ C_{12}H_{26(l)} + 18{{}^1 / {}_2}O_{2(g)} \rightarrow 12CO_{2(g)} + 13H_{2}O_{(l)}$$

### Three way catalyst

These are fitted to petrol engines. In this system, nitrogen monoxide reacts with carbon monoxide to form non toxic gases of nitrogen and carbon dioxide.
$$2NO_{(g)} + 2CO_{(g)} \rightarrow N_{2(g)} + 2CO_{2(g)}$$

### How catalysts work

The $CO$ and $NO$ gas molecules diffuse over the catalytic surface of a metal. Some of these are held on the surface by adsorption. Temporary bonds form between the gas molecules and the catalyst which weakens the bonds in the gas molecule resulting in a reaction. After the reaction, the $CO_{2(g)}$ and $N_{2(g)}$ are desorbed from the surface.

## Green Chemistry

Sustainability is the development of processes that prevent the depletion of natural resources. These processes have both environmental and economic advantages for a company:

• Reducing hazardous waste: This is choosing processes that reduce or eliminate waste. This can include ensuring that any waste products are biodegradable or can be recycled.
• Atom economy: Systems with a high atom economy produce few waste products.
• Renewable resources: This reduces the amount of fossil fuels consumed.
• Alternative energy: Using energy sources such as solar energy and wind turbines reduces the amount of fossil fuels consumed.

## Carbon dioxide

While carbon dioxide is often seen as a harmful gas, it has positive uses which are often utilised to reduce the waste from industrial processes:

• Blowing agent in foam: Carbon dioxide can be used as the blowing agent to manufacture expanded polystyrene foam.
• Solvent: While $CO_{2}$ is a gas at room temperature, by altering its room temperature and pressure, it can be converted to a supercritical carbon dioxide or $scCO_{2}$ with a critical temperature of 31°.
• Decaffeinated coffee: $scCO_{2}$ is now widely used for decaffeination of coffee, removing 98% of caffeine.
• Toxic waste: Many organic compounds dissolve in $scCO_{2}$, meaning it can be used to remove toxic materials from waste.
© Andrew Deniszczyc, 2019