Air quality index (ICQA)

Diòxid de nitrogen (NO2)

Brownish coloured gas with an irritating odour. Toxic in high concentrations. Intervenes in the formation of photochemical smog.

Sulphur dioxide (SO2)

One of the main sulphur compound contaminants, resulting from the combustion process. Together with flue gases, it is considered a basic indicator of air pollution. It is measured using the thorine method.

Air quality index (AQI)

Synthetic indicator, created on the basis of data for the emission of the four main primary contaminants for which current legislation has established maximum levels (PST, SO2, NO2 and CO). This parameter can take values between 100 and -400; the higher the value, the lower the level of contamination.

Monòxid de carboni (CO)

A colourless, odourless and highly toxic gaseous chemical compound that is produced as a consequence of deficient combustion in conditions with a lack of oxygen. It is used as an indicator of traffic intensity.

Nitrogen oxide (NOx)

Compound pollutants containing nitrogen and oxygen. The most characteristic are nitrogen monoxide (NO) and nitrogen dioxide (NO2). They are emitted by driving vehicles and by the stationary combustion of fuels. These oxides combined with the action of sunlight lead to photochemical processes that produce other more toxic pollutants, such as ozone.

Ozone (O3)

A molecule formed by three oxygen atoms (O3) that are found naturally in the atmosphere. Its maximum concentration is at an altitude of about 20 km, in the stratosphere, called the ozone layer, which protects living beings from ultraviolet (UV) rays from the sun, and is therefore of benefit to us. Ozone is also found in the layer of the atmosphere that is closest to the Earth's surface, what is known as the troposphere, which reaches an approximate altitude of between 8 km over the poles and 20 km over the equator. This ozone is called tropospheric ozone. At concentrations above the usual levels, it can be considered an atmospheric pollutant.

PM10

Solid and/or liquid particles that enter the atmosphere from natural and anthropogenic (manmade) sources. PM10 means particulate matter which passes through a size-selective inlet with a 50 % efficiency cut-off at 10 µm aerodynamic diameter.

Of the processes involved in the atmospheric pollution the emission of pollutants has to be pointed out in first term. In the process of emission determinate amounts of one or several pollutants are launched at the atmosphere. Once issued at the atmosphere the pollutants remain there during a certain time, known as time of residence. This time of permanence at the atmosphere is related with their chemical reactivity, with the wash of the atmosphere made by the rain and the capacity of the environment to disperse them.

The last phase of the atmospheric pollution has to include the effects of the pollutants on the living beings, the structures (constructions, monuments...) and about the intrinsic properties of the atmosphere (reduction in visibility, alteration of the balance of heat of the earth|land-atmosphere system, alterations about the climate...).

Cycle of the atmospheric pollution

The pollution of the air is a process that starts with the emissions in the air on the part of the different polluting emitting focuses at the atmosphere. Once these substances are at the atmosphere they suffer different effects of transport and/or transformation. As a result of these processes, in a determinate point a determinate concentration of each pollutant, which is known as a level of immission, is given. The levels of immission or of quality of the air are those that determine the effect of a pollutant on health or the environment.

Therefore, to minimize the atmospheric pollution it is necessary, on the one hand, the control of the atmospheric emissions (emission levels) and, on the other one, the control and the surveillance of the presence of the pollutants in the air in different receptive points (immission levels).

On treating the problem of the atmospheric pollution it is necessary to have present that, even though there is a certain relationship between emission and immission, these parameters are not necessarily equivalents because between both there is a process of transport and dispersion through the atmosphere, that it can disperse or concentrate the pollutants or even to modify its nature.

The pollutant sources can be classified into three groups:

• Of natural origin (the emissions of a volcano).
• Of natural origin speeded by the human activity (for example, a fire).
• Of anthropogenic origin: it is the emission introduced into the atmosphere by the activity of the man (emissions of exhaust pipe of the cars, emissions of industrial activities, etc.).

The emissions are the amount of pollutant that ends up at the atmosphere from a source, as for example the nitrogen oxides issued by the punctual and mobile focus that represents a car, the sulphur dioxide that is issued through a chimney or the issued particles by a spread source, as the wind when it blows on a dry surface. However, the immissions are the concentration of the pollutant (or level) in each point of the territory, that is, what a person would breathe in that point.

The relationship between emission and immission is not direct. This means that for a same emission we can have an immission in a very different determinate point, since once the pollutant it has been issued at the atmosphere, it suffers physical and chemical transformations (especially transport and dispersion, but also chemical reactions, deposition, aggregation, etc.) depending on the state of the atmosphere and changing with time.

The air quality

The immissions are related with effects on health and the environment. As a result of evaluating these effects the air quality degree is established, that is inversely proportionate to the pollution or immission levels (to more pollution less quality).

Atmospheric pollution is generally measured in micrograms or milligrams per cubic metre of air. The main pollutants are PM 10, lead and benzene. Other compounds are also considered: hydrogen sulphide, sulphur dioxide, ozone, carbon monoxide and nitrogen dioxide, which are measured by the automatic network. The main indicator is the air quality index (AQI).

Unavailable information is represented using the symbol ":". When the value is lower than that of the minimum unit to be able to estimate the statistical operation or if it effects statistical confidentiality, the symbol used is "..".