The Key Air Pollutants
Air pollutants arise from a wide variety of sources, although they are mainly a result of the combustion process. The largest sources include motor vehicles and industry. The main air pollutants are reviewed according to: 1) description, 2) occurrence in air, and 3) major sources. The following units of measurement are used throughout: ppb = parts per billion; ppm = parts per million; µgm-3 = micrograms per cubic metre; mgm-3 = milligrams per cubic metre; µm = micron or micrometre (one millionth of a metre).
Oxides of Nitrogen (NOx)
NOx is a collective term used to refer to two species of oxides of nitrogen: nitric oxide (NO) and nitrogen dioxide (NO2).
Annual mean concentrations of NO2 in urban areas are generally in the range 10-45 ppb (20-90 µgm-3). Levels vary significantly throughout the day, with peaks generally occurring twice daily as a consequence of "rush hour" traffic. Maximum daily and one hourly means can be as high as 200 ppb (400 µgm-3) and 600 ppb (1200 µgm-3) respectively.
Globally, quantities of nitrogen oxides produced naturally (by bacterial and volcanic action and lightning) far outweigh anthropogenic (man-made) emissions. Anthropogenic emissions are mainly due to fossil fuel combustion from both stationary sources, i.e. power generation (21%), and mobile sources, i.e. transport (44%). Other atmospheric contributions come from non-combustion processes, for example nitric acid manufacture, welding processes and the use of explosives.
Sulphur Dioxide (SO2)
SO2 is a colourless gas. It reacts on the surface of a variety of airborne solid particles, is soluble in water and can be oxidised within airborne water droplets.
Annual mean concentrations in most major UK cities are now well below 35 ppb (100 µgm-3) with typical mean values in the range of 5-20 ppb (15-50 µgm-3). Hourly peak values can be 400-750 ppb (1000-2000 µgm-3) on infrequent occasions. Natural background levels are about 2 ppb (5 µgm-3).
The most important sources of SO2 are fossil fuel combustion, smelting, manufacture of sulphuric acid, conversion of wood pulp to paper, incineration of refuse and production of elemental sulphur. Coal burning is the single largest man-made source of SO2 accounting for about 50% of annual global emissions, with oil burning accounting for a further 25-30%.
Carbon Monoxide (CO)
Carbon Monoxide is a colourless, odourless, tasteless gas that is slightly lighter than air.
Natural background levels of CO fall in the range of 10-200 ppb. Levels in urban areas are highly variable, depending upon weather conditions and traffic density. 8-hour mean values are generally less than 10 ppm (12 mgm-3) but have been known to be as high as 500 ppm (600 mgm-3).
CO is an intermediate product through which all carbon species must pass when combusted in oxygen (O2). In the presence of an adequate supply of O2 most CO produced during combustion is immediately oxidised to carbon dioxide (CO2). However, this is not the case in spark ignition engines, especially under idling and deceleration conditions. Thus, the major source of atmospheric CO is the spark ignition combustion engine. Smaller contributions come from processes involving the combustion of organic matter, for example in power stations and waste incineration.
O3 is the tri-atomic form of molecular oxygen. It is a strong oxidising agent, and hence highly reactive.
Background levels of O3 in Europe are usually less than 15 ppb but can be as 100 ppb during summer time photochemical smog episodes. In the UK ozone occurs in higher concentrations during summer than winter, in the south rather than the north and in rural rather than urban areas.
Most O3 in the troposphere (lower atmosphere) is formed indirectly by the action of sunlight on nitrogen dioxide - there are no direct emissions of O3 to the atmosphere. About 10 - 15% of tropospheric O3 is transported from the stratosphere where it is formed by the action of ultraviolet (UV) radiation on O2. In addition to O3, photochemical reactions involving sunlight produce a number of oxidants including peroxyacetyl nitrate (PAN), nitric acid and hydrogen peroxide, as well as secondary aldehydes, formic acid, fine particulates and an array of short lived radicals. As a result of the various reactions that take place, O3 tends to build up downwind of urban centres where most of NOx is emitted from vehicles.
Particulate matter is a complex mixture of organic and inorganic substances, present in the atmosphere as both liquids and solids. Coarse particulates can be regarded as those with an aerodynamic diameter greater than 2.5 µm (micrometres), and fine particles less than 2.5 µm. Coarse particles usually contain earth crustal materials and fugitive dust from roads and industries. Fine particles contain the secondarily formed aerosols, combustion particles and re-condensed organic and metallic vapours. The acid component of particulate matter generally occurs as fine particles. A further distinction that can be made is to classify particulates as either primary or secondary, according to their origin. Primary particulates are those emitted directly to the atmosphere while secondary particulates are those formed by reactions involving other pollutants. In the urban context, most secondary particulate matter occurs as sulphates and nitrates formed in reactions involving SO2 and NOx.
2. Reported concentrations vary according to the sampling technique. In urban areas typical annual mean values are 10 - 40 µgm-3 (gravimetric sampling) although short-lived pollution episodes such as Bonfire night can cause particulate concentrations to rise to several hundred µgm-3. Background levels in rural areas range form 0-10 µgm-3.
3. Particulate matter is emitted from a wide range of sources, the most significant primary sources being road transport (20%), homes (20%), construction, mining and quarrying (13%), industrial combustion plants and processes (10%) and public power generation (10%). Natural sources are less important; these include volcanoes and dust storms. Particulate matter can also be formed by the transformation of gaseous emissions such as oxides of sulphur and nitrogen and VOCs.
Volatile Organic Compounds (VOCs)
VOCs comprise a very wide range of individual substances, including hydrocarbons, halocarbons and oxygenates. All are organic compounds and of sufficient volatility to exist as vapour in the atmosphere. Methane is an important component of VOCs, its environmental impact principally related to its contribution to global warming and to the production of ozone in the troposphere. Regional effects derive from non-methane VOCs (NMVOCs), such as benzene and toluene.
Most measurements of total VOCs are in terms of their carbon content, without analysis as individual compounds. The major contributor to VOCs is normally methane with a local background concentration of 1.6 ppm. Whilst most other individual compounds (e.g. benzene) are present in urban air at concentrations of a few ppb, or less, total NMVOCs will amount to several hundred ppb concentrations.
Hydrocarbons are emitted from petrol evaporation and incomplete combustion, and from leakage of natural gas from distribution systems. Oxygenates arise in vehicle exhausts and via atmospheric chemical reactions. Evaporation of solvents, used in paints or industrial degreasing processes, cause a release of hydrocarbons, oxygenates and halocarbons to the atmosphere.
C6H6 is a colourless, clear liquid. It is fairly stable but highly volatile, i.e. it readily evaporates.
Ambient concentrations of benzene are typically between 1 - 50 ppb. Levels close to major emission sources can be as high as several hundred ppb. The urban background mean concentration of benzene is 1 to 2 ppb (3 to 6 µgm-3); rural areas average 0.5 to 1 ppb (1.5 to 3 µgm-3). Mean annual concentration can be 5 ppb (15 µgm-3) on urban roadsides.
About 80% of man-made emissions come from petrol-fuelled vehicles. This results from both the benzene content of the fuel and partial combustion of the petrol. A further 5% comes from the handling, distribution and storage of petrol and approximately 1% comes from oil refining. Emissions also come from benzene-producing and handling industries, the burning of wood and other organic material, and the use of benzene as a laboratory reagent.