The indicator shows the number of days per year when the 24-hour limit concentration 50 µg/m³ and the annual limit value 40 µg/m³ were exceeded in the selected Slovenian urban areas that are most affected by pollution.

“Particulate matter” means a combination of finely divided solid or liquid particles suspended in the air, of various physical and chemical compositions.

A portion of airborne particles is formed as a consequence of direct emissions (primary particles), others are formed as products of various processes in the atmosphere (secondary particles).

Particle size distribution is usually described in terms of trimodal size distribution. Particles ranging between 0.01 and 0.1 µm belong in the so-called nucleation area, particles ranging between 0.1 and 1 µm belong in the accumulation area, and particles larger than 1 µm are known as coarse particles.

Measurements of particlulate matter (PM10) concentration levels in several urban areas came into practice with the introduction of a new air quality monitoring network (i.e. PHARE monitoring network) in March 2001. As is clearly evident from Figure ZR8-1, particulate matter poses a serious problem not only in Slovenian urban areas but also throughout Europe. In 2002, 24-hour limit values were exceeded more than 35 times per calendar year at five monitoring sites. Since the latter are mainly placed in heavy traffic locations, emissions from traffic bear a significant influence on particulate matter concentration levels in ambient air.