9. Chemical Reaction of Atmospheric Pollutants

As noted in §1, pollutants may react with each other or with the air to form new chemicals. Some of the more important reactions for pollution studies are now described.

9.1 Nitrogen Oxides and Ozone

The oxides of nitrogen NO (nitric oxide) and NO$_2$ are produced during combustion. There are two sources: (i) From nitrogen in the air in contact with flames at a few thousand K. (ii) From nitrogen present within the fuel.

The dominant reaction producing nitrogen oxides (NO$_{\rm x}$) is

\begin{displaymath} {\rm N}_2+{\rm O}_2\longrightarrow 2{\rm NO}. \end{displaymath}

${\rm NO}$ is a colourless and odourless gas. It reacts with oxygen in the air over a few hours to produce nitrogen dioxide:

\begin{displaymath} 2{\rm NO}+{\rm O}_2\longrightarrow 2{\rm NO}_2. \end{displaymath}

${\rm NO}_2$ is a brown gas which is a respiratory irritant. Some of the ${\rm NO}_2$ reacts with water vapour to form nitric acid:

\begin{displaymath} 3{\rm NO}_2+{\rm H}_2{\rm O}\longrightarrow 2{\rm H}{\rm NO}_3 +{\rm NO}. \end{displaymath}

Another important reaction of ${\rm NO}_2$ is a photochemical reaction producing ozone, ${\rm O}_3$:

\begin{displaymath} {\rm NO}_2 +h\nu \longrightarrow {\rm O}+{\rm NO}, \end{displaymath}


\begin{displaymath} {\rm O}+{\rm O}_2 +{\rm M}\longrightarrow {\rm O}_3 +{\rm M} \end{displaymath}

where $h\nu$ represents a photon of sunlight and ${\rm M}$ is any air molecule (usually ${\rm N}_2$ or ${\rm O}_2$). Ozone is the main contributor to photochemical smog and is a strong respiratory irritant.

9.2 Oxides of Sulphur

All fuels (e.g. oil, coal, gas, wood) contain sulphur. When these are burnt the sulphur is mostly released as sulphur dioxide (${\rm SO}_2$):

\begin{displaymath} {\rm S}+{\rm O}_2\longrightarrow {\rm SO}_2. \end{displaymath}

A sequence of reactions in the atmosphere then produces sulphuric acid:

\begin{displaymath} {\rm SO}_2+{\rm O}{\rm H}+{\rm M}\longrightarrow {\rm H}{\rm SO}_3+{\rm M}, \end{displaymath}


\begin{displaymath} {\rm H}{\rm SO}_3+{\rm O}_2\longrightarrow {\rm SO}_3+{\rm H}{\rm O}_2, \end{displaymath}


\begin{displaymath} {\rm SO}_3+{\rm H}_2{\rm O}\longrightarrow {\rm H}_2{\rm SO}_4. \end{displaymath}

The sulphuric acid condenses onto existing particles or condenses to form new particles. These are often captured by rain drops and fall to the ground as acid rain.

${\rm SO}_2$ can be removed from emissions by reaction with limestone ( ${\rm Ca}{\rm CO}_3$) to form gypsum ( ${\rm Ca}{\rm SO}_4$):

\begin{displaymath} 2{\rm Ca}{\rm CO}_3+2{\rm SO}_2+{\rm O}_2\longrightarrow 2{\rm Ca}{\rm SO}_4+2{\rm CO}_2. \end{displaymath}

9.3 Oxides of Carbon

Carbon dioxide (${\rm CO}_2$) is a naturally occurring gas in the atmosphere. It is absorbed by plants during photosynthesis and is therefore an essential component of the biosphere. The amount of carbon dioxide is increasing in the atmosphere because (i) It is produced during the burning of all fuels (ii) As tropical forests are cut down for timber, there are fewer trees to absorb the ${\rm CO}_2$.

The proportion of ${\rm CO}_2$ in the atmosphere has increased by about 25% since the Industrial Revolution.

The air is quite transparent in incoming solar radiation. This heats the surface of the Earth which in turn emits radiation at much longer wavelengths. This long-wave radiation is strongly absorbed by ${\rm CO}_2$. Thus, increasing the proportion of ${\rm CO}_2$ has the potential to increase the temperature of the atmosphere and oceans (global warming). It is thought that doubling the amount of ${\rm CO}_2$ over the next century would result in a temperature rise of 0.5-5$^\circ$C. There are great uncertainties, particularly connected with the response of the biosphere and absorption of ${\rm CO}_2$ by the oceans. The main consequences of global warming would be (i) changes in climate patterns (with desertification in places) (ii) rising sea levels due to melting of land ice and thermal expansion of the sea.

Burning of fossil fuels also produces carbon monoxide (${\rm CO}$). This is very poisonous. It is produced in equilibrium with ${\rm CO}_2$:

\begin{displaymath} 2{\rm CO}_2\longrightarrow 2{\rm CO}+{\rm O}_2. \end{displaymath}

At high temperatures the equilibrium moves more to the right in the above equation.

9.4 CFCs

CFCs (chloro-fluoro-carbons) are compounds containing chlorine, fluorine and carbon. They are very inert, non-toxic, non-inflammable, invisible and odourless. They have been used as refrigerants and as propellants in aerosol cans. The problem with CFCs is that when they reach the stratosphere (10-50 km) they can release chlorine which reacts to destroy naturally occurring ozone. For example

\begin{displaymath} {\rm CFCl}_3+h\nu\longrightarrow {\rm CFCl}_2+{\rm Cl}, \end{displaymath}


\begin{displaymath} {\rm Cl}+{\rm O}_3\longrightarrow {\rm Cl}{\rm O}+{\rm O}_2, \end{displaymath}


\begin{displaymath} {\rm Cl}{\rm O}+{\rm O}\longrightarrow {\rm Cl}+{\rm O}_2. \end{displaymath}

Stratospheric ozone is responsible for absorbing much of the ultra-violet part of the incident solar radiation. Loss of ozone results in increased UV radiation reaching the surface. This is harmful to humans and animals and possibly to plants. CFCs are rapidly being replaced by other chemicals as a result of international agreements. However, their residence time in the atmosphere is tens to hundreds of years, so the effects of 20$^{\rm th}$ century CFC production will continue for a considerable time.

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