air quality: pollution from cars, industry, space heating, biomass burning;
acid rain and smog: affects vegetation, human health, weathering;
radioactive isotopes released accidentally from power stations or other
installations, or during weapons tests;
ozone destruction due to the release of chlorine compounds and other
industrial products;
urban heat islands reflecting the composition and energy balance of
urban areas;
carbon cycle: fossil fuel burning, deforestation: global warming.
Atmospheric pollution raises a very complex set of issues, rarely with
obvious or easy solutions. This course will explore these issues, and examine
the need for a co-ordinated, reasoned response. Public debate on atmospheric
issues is all too often dominated by either compacency (i.e. denying the
problem) or scaremongering (i.e. exaggerating the problem). Both distort
the truth, and both divert attention away from the real issues. There is
a clear need for informed opinion, and providing the basis for that is
one of the main aims of these lectures.
air pollutants are substances which, when present
in the atmosphere, may become injurious to life or property or interfere
with their enjoyment or use.
This definition does not specify human sources, so it can include natural sources such as volcanoes. The definition also emphasises the importance of receptors: that is, the effects that substances have on living things or property.
We can identify three important stages in the lifetime of a pollutant:
(1) emissions
(2) atmospheric transport and transformation
(3) sinks: receptor response
Emissions vary according to the rate and type of emission, and the configuration
of the source. Sources can be points (e.g. a factory or power station),
lines (e.g. roads) or areas (e.g. a city), and may be mobile or stationary.
Atmospheric transport and transformation is very important in understanding
pollutants. The state of the atmosphere determines whether pollutants are
concentrated or dispersed, and whether they are likely to remain in areas
and concentrations that put life or health at risk. Atmospheric processes
may also transform primary pollutants into secondary pollutants (e.g. the
formation of photochemical smogs), or allow the settling and removal of
pollutants. Receptor response covers a wide range of effects, including
health effects, vegetation damage, soiling of surfaces, or corrosion of
metals or other materials.
We will begin with a review of the main substances contribributing
to human impact on the atmosphere today. These can be classified as AEROSOLS
and GASES.
Aerosols may act as pollutants in several ways:
Aerosols may cover a wide range of sizes: from less than 0.1 microns
to greater than 100 microns (fine sand).
Composition:
Solids: carbon (soot: smoky fires, biomass burning, diesel engines),
silica (sand or silt particles: windblown material, volcanic ejecta, mining
waste), lead, asbestos, salts, and many more.
Liquids: cloud droplets are a natural liquid aerosol. Most important
pollutant aerosols are acidic droplets (sulphur and nitrogen compounds).
Sulphur compounds: Natural sources: bacteria, sea-spray, volcanoes.The behaviour and effects of aerosols mainly dependent on size:
Human sources: burning of sulphur-bearing materials, such as coal and oil. Also oil refineries and ore smelters.
Anthropogenic sulphur compounds are mainly in the form of Sulphur dioxide (SO2), but other compounds, such as Hydrogen sulphide (H2S) may also be important. Sulphur dioxide is transformed on contact with atmospheric water into acidic solutions.
Transformed in atmosphere to sulphurous and sulphuric acids (H2SO3 and H2SO4). Component of certain types of smogs and acid rain.
Nitrogen: Oxides of nitrogen (particularly NO) produced by combustion transformed in atmosphere into Nitric acid (H+ and NO3).
The amount of particulate pollution reduced in recent years in developed
nations: mainly due to a reduction in domestic coal burning and the introduction
of tall stacks/ scrubbing procedures. They remain a major problem in the
developing world and Eastern Europe, and a serious occupational hazard
in some industries.
Nitrogen compounds
These may be produced naturally as the result of organic decomposition
in soil or oceans. Human sources include the combustion of fuels under
pressure at high temperatures: e.g. in internal combustion engines. They
are also produced during the manufacture of explosives and fertilizers.
The most common gaseous nitrogen compound produced by combustion is
NO (nitric oxide): a relatively harmless gas. But it tranforms readily
into NO2 (Nitrogen dioxide), a yellow-brown, irritant gas, which is a major
component of urban smogs. NO also transforms into nitric acid (see above)
and nitrous oxide (N2O, a greenhouse gas). Oxides of nitrogen are collectively
termed NOx, where x is a variable number. NOx compounds include NO, NO2,
NO3, N2O5.
Hydrocarbons
Hydrocarbons are compounds of hydrogen and carbon, naturally produced
by decomposition of vegetation. Anthropogenic hydrocarbons are produced
by incomplete combustion or evaporation of fuels (e.g. petrol, lighter
fuels, solvents). One of the most important hydrocarbons is Methane (CH4)
produced in biomass burning and agricultural activities. This is a major
greenhouse gas, with 21 times more effect on atmospheric energy budgets,
per molecule, than CO2. Methane is created by decomposition in rice paddies,
and in the guts of grazing animals.
Ozone
Ozone is the triatomic form of oxygen (O3) [Most oxygen in atmosphere
is in form of O2] Ozone in upper atmosphere is essential to life on earth,
but in the lower atmosphere it is a serious pollutant, formed by the transformation
of hydrocarbons and nitrogen compounds in presence of sunlight, and is
a major component of urban smogs. Ozone causes irritation of the lungs,
and can lead to asthma attacks.
Chlorofluorocarbons
(CFCs) are entirely man-made compounds, created by modifying hydrocarbon
molecules. Until recently, they were produced in quantity as refridgerants,
propellants, sovents and fillers. They were initially thought to be inert
and harmless, but are now identified as major greenhouse gases and play
a part in crucial reactions in the stratosphere leading to destruction
of stratospheric ozone.
Carbon monoxide (CO)
This is an odourless, colourless gas, lethal in high concentrations.
Sources include the incomplete combustion of carbon compounds, and as a
product of engines, metal processing, oil refineries, and cigarettes.
Cigarettes are single most important source of CO to which humans are
exposed. When inhaled, CO fixes onto blood haemoglobin, reducing its ability
to fix oxygen and reducing efficiency of respiration.
CO2 (Carbon dioxide)
Carbon dioxide is not poisonous, but it can have harmful effects. It
is best known as a greenhouse gas, and is produced naturally as by-product
of metabolism and by volcanoes. It is produced by human activities by:
• complete combustion of carbon compounds: burning fossil fuels. 5
billion tonnes produced annually by fossil fuel burning, 90% from developed
world.
• deforestation: burning and indirectly by reducing CO2 fixing by plants.
5-20% of man's contribution. Deforestation of rainforests is currently
of major concern, but deforestation in temperate regions was important
in the past, providing a larger source of atmospheric CO2 than fossil fuels
between 1850 and 1950 (c. 120 billion tonnes was released due to deforestation,
double the contribution from fossil fuel use over that time).
In high concentrations CO2 can displace oxygen, causing asphyxiation.
Eg. Lake Nyos, Cameroon, August 1986: Cloud of CO2 released from volcanic
lake, released from solution by heavy rainfall and dilution of surface
water. Cloud sank into valleys, displacing lighter oxygen and asphyxiating
1700 people.
Other gases
Range of compounds released in small amounts, but may be of local importance.
Eg. Bhopal, India: one of world's worst air pollution disasters.
A tank of Methyl isocyanite ruptured at a Union Carbide fertilizer
plant, releasing deadly gas. Over 1000 people were killed, and tens of
thousands permanently disabled, mainly with serious eye injuries.
| POLLUTANT | RECOMMENDED MAXIMUM | UNITS | AVERAGING TIME |
| Sulphur dioxide | 500 | mg per cubic metre | 10 minutes |
| 350 | 1 hour | ||
| 100 - 150 | 24 hours | ||
| 40 - 60 | 1 year | ||
| Carbon monoxide | 30 | mg per cubic metre | 1 hour |
| 10 | 8 hours | ||
| Nitrogen dioxide | 400 | mg per cubic metre | 1 hour |
| 150 | 24 hours | ||
| Ozone | 150 - 200 | 1 hour | |
| 100 - 120 | 8 hours | ||
| Particulates | |||
| Black smoke | 100 - 150 | 24 hours | |
| 40 - 60 | 1 year | ||
| Total suspended particulates | 150 - 230 | 24 hours | |
| 60 - 90 | 1 year | ||
| Thoracic particles | |||
| (PM 10) | 70 | 24 hours | |
| Lead | 0.5 - 1 | 1 year |
Because of the different thresholds for different substances, a Pollutant
Standard Index (PSI) has been developed to allow easy comparison of air
quality conditions for a range of common pollutants.
| PSI | PM10 (mg/m3)
24 hr |
SO2(mg/m3)
24 hr |
CO (mg/m3)
8 hr |
O3(mg/m3)
1 hr |
NO2(mg/m3)
1 hr |
Descriptor | Health Effects | Warning |
| >400 | >500 | >2100 | >46 | >1000 | >3000 | Hazardous | Premature death of ill and elderly. Healthy people will feel adverse symptoms affecting normal activity. | All persons should remain indoors, keeping windows and doors closed. All persons should minimise physical exertion and avoid traffic. |
| 300 - 399 | 420 - 500 | 1600 - 2099 | 34 - 45.9 | 800 - 1000 | 2260 - 2999 | Hazardous | Premature onset of certain diseases in addition to significant aggravation of symptoms and decreased exercise tolerance in healthy persons. | Elderly and persosns with existing diseases should stay indoors and avoid physical exertion. General population should avoid outdoor activity. |
| 200 - 299 | 380 - 420 | 800 - 1599 | 17 - 33.9 | 400 - 800 | 1130 - 2259 | Very Unhealthy | Significant aggravation of symptoms and decreased exercise tolerance in persons with heart or lung disease, with widespread symptoms in the healthy population. | Elderly and persons with existing heart or lung disease should stay indoors and reduce physical activity. |
| 100 - 199 | 150 - 380 | 365 - 799 | 10 - 16.9 | 235 - 400 | NR | Unhealthy | Mild aggravation of symptoms in susceptible persons, with irritation symptoms in the healthy population. | Persons with existing heart or respiratory ailments should reduce physical activity and outdoor exertion. |
| 50 - 99 | 50 - 150 | 80 - 364 | 5 - 9.9 | 120 - 235 | NR | Moderate | ||
| 0 - 49 | 0 - 50 | 0 - 79 | 0 - 4.9 | 0 - 199 | NR | Good | ||
Pollution levels at a site are not constant, but vary on several timescales,
due to
(1) variations in output from sources
(2) the state of the atmosphere
Some outputs are more or less constant (e.g. power stations, refineries) while others vary on daily, weekly or seasonal basis (domestic fires, car use). E.g. a typical daily cycle of carbon monoxide exists in cities due to traffic: peak concentrations occur during rush-hours.
Additionally, pollution concentrations will change due to changing weather and the ability of atmosphere to disperse, concentrate, transform, and deposit pollutants.
State of the Atmosphere
The state of the atmosphere is of prime importance in determining the
dispersal and concentration of pollutants, and their consequences.
The stability of air masses is a function of the relative values of the environmental and adiabatic lapse rates.
Such conditions commonly occur when high pressure anticyclones develop, associated with cold, clear weather in winter, and warm, dry weather in summer. Convection suppressed by the general sinking of air in an anticyclone and may be helped by chilling of lower air at night, lowering or inverting the environmental lapse rate. In such conditions, pollutants released into atmosphere at low levels will tend to remain there, rather than being dispersed and diluted.
For a more detailed discussion of air stability and its causes, see: