Main pollutants
Carbon monoxide (CO) is derived from incomplete combustion processes of carbon-containing compounds. It is a gas which, above all in urban areas, is generated principally by motor vehicle traffic, and above all by petrol-powered vehicles. CO emissions from vehicles are greater during deceleration and in congested traffic. Its concentrations are closely linked to local traffic intensity, and its daily levels mirror the volume of vehicular transit, reaching maximum values at peak traffic times at the start and end of the day, above all on weekdays. During the central hours of the day, its values tend to decrease, in part due to the atmosphere’s greater dispersive capacity. In Lombardy from the early 1990s, the concentrations of CO have been decreasing, principally due to the introduction of vehicle catalytic converters and to improvements in internal combustion engine technology (introduction of Euro 4 vehicles). - Source: ARPA Lombardy -
Nitrogen oxides (NO and NO2) are emitted directly into the atmosphere as a result of all high-temperature combustion processes (heating plants, vehicle engines, industrial combustion, power stations, etc.), the oxidation of atmospheric nitrogen and, to only a small degree, the oxidation of the nitrous compounds contained in the fuels that are utilised. In the case of motor vehicle traffic, the highest concentrations of these pollutants are recorded when the vehicles are moving fast and during acceleration, because the production of NOX is greater when the ratio between air and fuel increases, in other words when there is more oxygen available for combustion. On emission, most nitrogen oxides are in the form of NO, with an NO-NO2 ratio in which the former is by far the greatest. It can be estimated that the NO2 content in emissions constitutes between 5 and 10% of the total for oxides of nitrogen. Nitrogen monoxide is not contemplated in legislation, because, at the concentrations normally found in atmospheric air, it does not cause damage to human health and the environment. However, its levels are measured, because, as a result of its oxidation forming NO2, and the role that it plays in other photochemical processes, it contributes to the production of tropospheric O3. On the other hand, maximum values are defined for nitrogen dioxide, as summarised in table 2. - Source: ARPA Lombardy -
Ozone (O3) is a secondary pollutant, for which there are no major direct sources of emission. It is formed by chemical reactions in the atmosphere involving its precursors (principally nitrogen oxides and volatile organic compounds), reactions that occur in the presence of high temperatures and intense solar radiation and that give rise to the formation of a group of different compounds, which include, in addition to ozone, nitrates and sulphates (constituents of fine particulate matter), peroxyacetylnitrate (PAN), nitric acid and other compounds, which together form the characteristic summer pollution known as photochemical smog. Unlike primary pollutants, whose concentrations depend directly on the quantities of each respective pollutant emitted by sources present in the area, the formation of ozone is therefore more complex. The chemistry of ozone begins with the presence of nitrogen oxides, large amounts of which are emitted in urban areas. As a result of the effect of solar radiation (hereinafter denoted hν), the formation of ozone occurs through the photolysis of nitrogen dioxide:
NO2+hν →NO+O* (1)
Atomic oxygen, O*, reacts rapidly with the molecular oxygen in the air, in the presence of a third molecule that does not take part in the reaction itself but that absorbs excess vibrational energy and therefore stabilises the ozone molecule that is formed:
O* + O2 + M → O3 + M (2)
Once formed, ozone reacts with NO and regenerates NO2:
NO + O3 → NO2 + O2 (3)
These three reactions form a closed cycle which, on its own, would not be enough to cause the high levels of ozone that can be found in conditions favourable to the production of photochemical smog. The presence of other pollutants, such as hydrocarbons, for example, constitutes a different pathway for the oxidation of nitrogen monoxide, causing the production of NO2 without absorbing ozone, therefore altering the balance of the cycle described above and allowing the accumulation of O3. Ozone concentrations reach their highest values during the afternoons of sunny summer days. In addition, as ozone is formed during the movement of air masses containing its precursor compounds, emitted primarily within urban districts, the highest concentrations are found principally in suburban zones downwind of the major built-up areas. In addition, in cities, the presence of NO tends to reduce ozone concentrations, above all near roads with high volumes of traffic. - Source: ARPA Lombardy -
Airborne particulate matter is formed of a mixture of solid and liquid particles, with different chemical and physical properties, and of different sizes. They may be of primary origin, namely emitted directly into the atmosphere as a result of natural or anthropic processes, or of secondary origin, in other words formed in the atmosphere due to chemical reactions and of principally human derivation. The major natural sources are erosion and the overturning of soil, along with fires, pollen, sea spray and volcanic eruptions; anthropic sources can be identified principally in the form of combustion processes (vehicular traffic, the use of fuels, industrial emissions). The aggregate of particles suspended in the atmosphere is called TSP (Total Suspended Particles). To assess the impact of particulate matter on human health, a distinction can be made between a fraction capable of penetrating the upper respiratory tracts (nose, pharynx, larynx) and a fraction that can reach the lower parts of the respiratory system (trachea, bronchi, alveoli in the lungs). The former comprise particles with an aerodynamic diameter below 10 µm (PM10), while the second comprise particles with an aerodynamic diameter below 2.5 µm (PM2.5). Currently, European and national legislation has defined limit values on daily concentrations and annual averages for PM10 only, while for PM2.5, the European community, in cooperation with national organisations, is currently working on the necessary assessments. - Source: ARPA Lombardy -
GREENHOUSE GASES
Carbon dioxide (CO2): the main cause of the enhanced greenhouse effect (due to human activities) is carbon dioxide, which is responsible for this accelerated effect to a proportion of over 60%. In industrialised countries, CO2 accounts for more than 80% of greenhouse gas emissions. - Source: European Commission -
Methane (CH4): the second-most important cause of the enhanced greenhouse effect is methane. Since the start of the industrial revolution, methane concentrations in the atmosphere have doubled, contributing 20% to the acceleration of the greenhouse effect. In industrialised countries, methane is responsible for an average of 15% of emissions. Man-made sources include mining and the use of fossil fuels, livestock farming (animals feed on plants that ferment in their stomachs, emitting methane, which is also contained in manure), rice cultivation (rice paddies produce methane because the organic matter in the soil decomposes in the absence of sufficient oxygen) and from rubbish tips (also in this case, organic matter decomposes in the absence of sufficient oxygen). When it is released into the atmosphere, methane traps heat 23 times more efficiently than CO2, although its cycle is shorter, between 10 and 15 years. - Source: European Commission -
Nitrous oxide (N2O): nitrous oxide is emitted naturally from the oceans, rainforests, and bacteria in the soil. Sources attributable to human activities include nitrate-based fertilisers, the combustion of fossil fuels and the production of industrial chemicals using nitrogen, for example for sewage treatment. In industrialised countries, N2O is responsible for approximately 6% of greenhouse emissions. Like CO2 and methane, nitrous oxide is a gas whose molecules absorb the heat that is escaping into space, and it has a heat absorption capacity 310 times higher than CO2. Since the start of the industrial revolution, nitrous oxide concentrations in the atmosphere have increased by about 16%, contributing 4-6% to the acceleration of the greenhouse effect. - Source: European Commission -
