This indicator shows the number of days when the daily limit value for PM₁₀ particles was exceeded and the trends in mean annual levels of PM₁₀ and PM_2.5 particles at monitoring sites in Slovenia in the period 2005–2016.

Atmospheric particles, i.e. aerosols, are tiny solid and liquid particles evenly distributed (suspended) in the air. Determination of aerosol size is of key importance in measuring and modelling aerosol dynamics. The diameter of particles is most often described by the term ‘aerodynamic diameter.’ Particles of similar shape and size, but with a different density, have a different aerodynamic diameter. Based on their diameter, we distinguish between PM₁₀ particles (with an aerodynamic diameter below 10 µm), PM_2.5 particles (with an aerodynamic diameter below 2.5 µm) and PM_1.0 particles (with an aerodynamic diameter below 1 µm). The size of particles formed during the interactions between gases and particles in a gaseous or liquid state is usually under 1 µm. These are called fine particles. Particles over 1 µm in size are called coarse particles.

In terms of origin, we distinguish between primary and secondary particles. Primary particles originate from sources on land, while secondary particles result from various transformations in a polluted atmosphere. Particles can be of natural (e.g. pollen, dust, sea salt, smoke from wildfires, meteoric dust, volcano ash, etc.) or anthropogenic origin (emissions from energy facilities, industry, transport, agriculture, small heating units). Depending on their source, particles have various chemical compositions, shapes and physical states.

PM₁₀ coarse particles are a result of soil erosion, resuspension from roads and emissions from industrial facilities. The main components of PM₁₀ particles are secondary anorganic ions (sulphate, nitrate and ammonium) and organic matter. Their share depends on emission sources and meteorological conditions. An exception to this is when the particles are found in locations with a traffic volume that results in a large share of mineral dust in the air. Sources of coarse particles (above 2.5 µm) include transport (but not direct traffic emissions), natural sources, resuspension of particulate matter in the atmosphere and long-distance transport. PM_2.5 particles are small and light – they usually remain suspended in the air for a longer period of time and travel over longer distances than larger particles.

As they enter the respiratory system, particles may cause health problems, such as irritated eyes, asthma, bronchitis, lung damage, development of cancer, etc. In addition to negative impacts on human health, particulate matter is attributed to have certain negative impacts on the environment, such as reduced visibility due to air pollution with PM_2.5. The particles also affect ecosystems (acidification of rivers and lakes, eutrophication of the sea, damage to forests and crops as well as reduced biodiversity), largely due to their long-range transport because of their small size. They also may cause damage to materials and cultural monuments.

In 2016, PM₁₀ levels were slightly lower than a year earlier. The permitted number of exceedances of the daily PM₁₀ limit value (35) was surpassed at ten monitoring sites in urban environments. In the continental part of Slovenia, the permitted number of exceedances was surpassed at the urban monitoring sites of Kranj, Hrastnik, Velenje and Maribor-Vrbanski plato. As usual, the permitted number of daily limit value exceedances was not surpassed at monitoring sites in the Primorska region. The largest number (66) of exceedances in 2016 was recorded at the Ljubljana Center monitoring site, where traffic volume is very high. The annual PM₁₀ limit value was not exceeded at any of the monitoring sites in 2016, although the average annual PM₁₀ concentration of the Ljubljana Center monitoring site was very close to the limit value.

In the last period, elevated levels of particulate matter seem to largely be a result of combustion of biomass in small heating units. Burning of firewood in obsolete old furnaces and boilers represents the highest share in particulate matter emissions. In addition, extremely unfavourable meteorological conditions can be characteristic of the winter season, when polluted air is trapped in basins and valleys for longer periods of time due to frequent and distinct temperature inversions.

Pollution trends in the period 2002–2006 indicate that higher levels were measured in 2016 than in 2014. Nevertheless, a decreasing trend of particulate matter levels has been recorded from 2002 onwards, especially at urban locations. We estimate that this is largely a consequence of reduced emissions from industry. In recent winters, favourable weather conditions have contributed to reduced concentrations. In the rural environment, the trend of decreasing levels of particulate matter is less distinct. In rural areas, the use of wood biomass for heating is on the rise, which contributes to increased emissions. Consequently, the impact of favourable weather conditions in recent winters has not been so strong. A similar trend has been observed in the number of days with an exceeded daily limit value. For PM_2.5 particles, the only limit value that has been defined is annual, which is favourable as regards specific conditions in Slovenia. In Slovenia, elevated values of particulate matter are characteristic of winter months, while lower concentrations in the warm period of the year contribute to the reduced annual average. Since the beginning of measurements, as well as in 2016, the PM_2.5 limit value has not been exceeded at any of the four monitoring sites. Annual trends of PM_2.5 levels indicate that air pollution with PM_2.5 particles remain more or less at the same level.

In 2016, the indicator of average exposure to PM_2.5 was 21 µg/m³ (LJ Biotehniška) and 18 µg/m³ (MB Vrbanski) in the unexposed urban environment. The permitted exposure level (20 µg/m³) for 2016 was exceeded at the Ljubljana Biotehniška fakulteta monitoring site.