What is Particulate Matter
Particulate Matter (PM) is a pollutant made of a mixture of complex organic and inorganic substances at solid or liquid state and in highly variable ratios. Particulates are able to adsorb on their surface many different toxic substances such as sulfates, nitrates, metals and volatile organic compounds.
From a dimensional point of view, PM is classified in many different size:
- Gross PM with a diameter bigger than 10 µm (1 µm = 1/1000 mm);
- PM10 – particulates with a diameter smaller than 10 µm;
- PM10-2.5 – Coarse PM (with a diameter between 2.5 and 10 µm);
- PM2.5 – Fine PM (with a diameter smaller than 2.5 µm);
- Ultra-Fine PM (UFP, with a diameter smaller than 0.1 µm).
The size of the particulates determines the residence time in atmosphere: sedimentation and precipitation processes wash-out Gross and Coarse PM from the air in few hours after their emission. PM2.5 may remain suspended in air up to days or weeks after the emission and, in particular, the Ultra-Fine fraction may be transported for very long distances.
Concentration levels of PM10 in urban areas increase in autumn-winter time, that is when emissions from residential heating, especially from fireplaces burning wooden biomasses, are added to traffic ones. Moreover, weather conditions during these periods may increase the ground levels of PM10.
Atmospheric phenomena such as thermal inversion, cause the accumulation of particulate matters to the ground, preventing their dispersion.