5.2. Air pollution#
Sources, sinks and trends
Aims:
To survey the main constituents of air pollution around the world and over time.
To outline the main sources and sinks of different air pollutants, focusing on
and particulate matter. To review historic trends of air pollutants and how they are measured.
To apply the Beer-Lambert law to determine the abundance of air pollutants using remote measurement techniques.
Key Points:
Air pollution is not a new phenomenon and work to legislate air pollution can be traced back to the middle ages.
The main air pollutants are oxidising gases (
, ) and particulate matter.
is only produced naturally from lightning, it’s main source is the internal combustion engine.
has a weak chemical-bond which allows low energy photons from the sun to break it apart. Since the industrial revolution the amount of
has increased dramatically and disproportionately. COVID lockdowns showed that
can be controlled very quickly.
What is air pollution#
Air pollution is a phrase used to describe the presence of contaminants in the air that are harmful for life, to buildings or to climate.
There are lots of different air pollutants. Most are present at trace levels (c.f. the majority of the atmosphere is made up of

Fig. 5.7 A list many important air pollutants.#
Sources of #
The oxides of nitrogen (
Sources |
|
---|---|
Fossil fuel combustion |
|
Release from soils |
|
Biomass burning |
|
Lightning discharges |
|
Ocean surface |
|
Aviation |
|
Injection from the stratosphere |
|
Total sources |
Sinks |
|
---|---|
Wet decomposition of nitrate (land) |
|
Wet deposition of nitrate (ocean) |
|
Dry deposition of |
|
Total sinks |
undergoes photolysis (photodissociation) rapidly. can be observed easily from space.
Making use of point 2 above, the figure below shows maps of the distribution of

Fig. 5.8 Measurements from the Copernicus Sentinel-5P satellite mission between April and September 2018. Credit ESA and KNMI.#
Photodissociation#
The photo-reaction:
Has an associated rate constant (or photolysis frequency)
The rates of change of the molecules are:
Calculation of photolysis frequency #
The photolysis frequency
where
To calculate the photon intensity with the atmosphere at altitude
where
where the summation is over all
To an excellent approximation we need only to consider
In each case
where
Calculation of
The major factors determining fluxes of radiation in the visible and UV wavelengths in the atmosphere are absorptions by
Figure 5.2 shows the regions of the spectrum where the bands of
A detailed knowledge of the absorption properties of the atmosphere is required for a full understanding or for accurate modelling of atmospheric photochemistry.
However, there are a number of wavelengths which are of particular significance for understanding some of the basic principles.
Firstly,
Secondly
Also,
Changes in over time.#

Fig. 5.9 Modelled changes in

Fig. 5.10 A map of cambridge with monitoring stations shown.#

Fig. 5.11 The impacts of the COVID lockdowns on