A few atmospheric species related to air quality
Nitrogen dioxide (NO2), already notorious as one of the main causes of photochemical smog and acid rain, is also responsible for respiratory tract irritation problems (it can enhance asthma, chronic bronchitis, etc. ) and other health ailments.
This gaseous compound is mainly produced by combustion processes in industrial plants and car engines.
Next to nitrogen dioxide, sulphur dioxide also contaminates the lower atmosphere. Although volcanic eruptions sporadically release very large amounts of this gas, the largest source of sulphur compounds is the combustion of fossil fuels (coal and oil with high sulphur content) as well as several industrial processes (e.g. copper smelting).
Once in the atmosphere, this gas is oxidized into sulphuric acid which rapidly combines with water to form fine droplets, i.e. sulfate aerosols.
Air quality on the global scale seems to be deteriorating
Most citizens living in modern societies are aware of hazards associated with polluted living conditions. That's probably what led industrialized countries to change their policies and to more efficiently manage their emissions, by this way maintaining a sustainable air quality.
Yet on the global scale, air quality seems to be slowly deteriorating and the situation is particularly delicate in many cities of the new emerging countries where impacts of pollutants on health are of alarming proportions. Many mega-cities in developing countries suffer from heavy air pollution, to a large extent due to the explosion of trafic.