The “photochemical smog” is a typical form of pollution of all the main urban and industrial areas of the world. It occurs in or near areas with a high traffic density, in the presence of specific climatic conditions (no wind or weak winds, high temperatures, etc.), that cause the concentration of polluting gases to increase and prevent them from dispersing. In these areas, the concentrations of some gases (tropospheric ozone, carbon monoxide, particulate, VOC, nitrogen oxides, etc.) very often exceed the threshold values, above which there are risks for human health, farming and natural vegetation.

Atmospheric dust

Atmospheric dust consists of a mixture of solid and liquid particles suspened in the atmosphere varying in composition, source and size. Atmospheric dust particles can be removed out of the atmosphere by dry and wet deposition and fall back on soil, vegetation or watercourses. Atmospheric dust particles can be classified according to their diameter (measured in micrometers or µm. 1000 micrometers equivalent to 1 millimeter) ranging from 0,005 to 100 µm. 

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Benzene

Benzene is a molecule composed of 6 carbon atoms joined in a ring and 6 hydrogen atoms. Benzene is classified as a polycyclic aromatic hydrocarbon (PAH). It’s a liquid substance, but at high temperatures it has a rapid volatilization process passing from a liquid phase into a gas phase. Benzene is either natural and can, for example, be generated by volcanic eruptions or is man-made. In urban centres benzene is almost exclusively generated by human activities as vehicular traffic, oil-refining and fuel distribution. 

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Acid deposition

The term acid deposition refers to the process by which acid particles, gases and precipitation fall from the atmosphere. If this acid deposition takes place in the form of precipitation (rain, snow, fog, dew, etc.) we speak of wet deposition, otherwise the phenomenon is dry deposition. The term 'acid rain' can also be used to describe these phenomena, by which, however, only the phenomenon of wet acid deposition is often referred to.
The substances that give rise to acid deposition are sulphur oxides (SOx) and nitrogen oxides NOx), whose origin in the atmosphere can be either anthropogenic or natural. 

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Ozone

Ozone (O₃) is a gas found in in high levels in the stratosphere, in a region also known as the ozone layer, between 15,000 and 40,000 metres above the surface where it plays an important role screening the sun’s ultraviolet radiations which are harmful for living organisms. In the past years stratospheric ozone levels have declined due to the effect of anthropogenic substances, as chlorofluorocarbons (CFCs), methyl bromide, halon gases and methyl chloroform. Once these gases reach the stratosphere they emit chlorine and bromine, which affect ozone formation.

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We have seen that atmospheric pollution derives from a series of substances that are produced by one or more “sources” (industries, cars and others). Luckily, after a more or less long time spent in the atmosphere, nature can “remove” a certain amount of it. For instance, the carbon dioxide produced by the combustion of fossil fuels and by the breathing of animal and vegetal living organisms is absorbed not only by the vegetation (through the photosynthesis), but is also largely counteracted by seawater, that can fix it through the phytoplankton and stabilise it in carbonated sedimentary rocks. The composition of the atmosphere is therefore in a state of dynamic balance, whose stability depends on the ability of these “self-depuration” processes to counteract or at least reduce the negative effects of man’s activities. The problem arises when the amounts of pollutants emitted in the atmosphere exceed its “self-depurating” ability, increase their concentrations in the air and reach limits that are harmful to man and to nature. In this case, the development model of man and of a country may be no longer “sustainable” in the long term.

Urban pollution

Most Italian cities are affected by urban pollution, which is a major problem. This is demonstrated by data from monitoring stations measuring pollutant concentration. Even in those cities and areas where these stations are lacking we can feel the air is polluted as we experience breathing difficulties or problems. Traffic is currently the main source of pollution in every city. Domestic heating emissions are another major pollution cause during winter.

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Actions to make a difference

What can you do to help reduce air pollution and contrast anthropogenic greenhouse gas concentration? International organisations, national governments and industries can contribute to curb pollutant emissions and “greenhouse gases” enacting specific environmental measures. In many countries excellent results have already been achieved and industries are paying increasing attention to effectively reduce pollutant emissions.

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