This indicator shows the dynamics of emissions of gases causing the formation of ground-level ozone in the period 1990–2015 and the structure of emissions of particular gases by pollution source.

An ozone precursor is a substance that contributes to the formation of ground-level (tropospheric) ozone. Ozone precursors include nitrogen oxides (NO_x), carbon monoxide (CO), methane (CH₄) and non-methane volatile organic compounds (NMVOC).

Total emissions of substances causing the formation of ground-level ozone are expressed as the sum of emissions of the four pollutants multiplied by corresponding factors of ground-level ozone formation. Emissions are given in NMVOC equivalents.

Emissions of nitrogen oxides, carbon monoxide, methane and non-methane volatile organic compounds contribute to the formation of ground-level ozone. NO_x, CO, CH₄ and NMVOC enter into complex photochemical reactions in the lower layers of the atmosphere. They are called precursors of tropospheric ozone. Different pollutants have a different potential for the formation of ground-level ozone. In 2015, the most important pollutants contributing to the formation of ozone precursors were nitrogen oxides (49%), followed by carbon monoxide (36%). NMVOC contributed 14%, while the contribution of methane was only 1%.

In the period 1990–2015, emissions of ozone precursors decreased by 54%. The greatest reduction was recorded in emissions of carbon monoxide (66%). Emissions of NMVOC decreased by 55%, nitrogen oxides by 48% and methane by 18%. The greatest reduction of emissions was recorded in the transport sector, largely due to the introduction of stricter emissions standards for motor vehicles. The introduction of catalytic converters for petrol-fuelled vehicles contributed to a considerable decrease in emissions of nitrogen oxides and carbon monoxide. The reduced use of solvents contributed to a reduction in NMVOC emissions (lower content of solvents in paints, varnishes, lacquers, etc.), as did the implementation of legislation in the field of storing, decanting and volatility of fuel petrol. Reduced emissions of nitrogen oxides are also a result of measures taken in thermal power plants and combined heat-and-power plants, e.g. modernisation of technologies and installation of flue gas purification systems, replacement of fuels, as well as the improvement of combustion processes in industry. In 2015, emissions of nitrogen oxides were 22% below the target value that was not to have been exceeded from 2010 onwards. Also, emissions of NMVOC were 20% below the prescribed target values.

In 2015, the main source of total emissions of ozone precursors was transport (41%), followed by the use of fuels in households and the services sector (20%). The contribution of other sectors is less significant.

In 2015, the main source of nitrogen oxides was transport (65%). The use of fuels in households and the services sector contributed most to emissions of carbon monoxide (63%). The main source of methane emissions is agriculture (58%). Industrial processes and the use of solvents are the largest source of non-methane volatile organic compounds (31%).

Slovenia is among countries with relatively high emissions of ozone precursors per capita. With 43 kg of emissions of ozone precursors per capita in 2015, Slovenia was ranked 9^th among EU-28 countries. The average in the EU-28 was 36 kg per capita. The highest emissions in 2015 were recorded in Luxembourg (68 kg/capita) and the lowest in Malta (13 kg/capita).

In order to achieve the set goals, measures were taken in the field of energy efficiency improvement, adoption of stricter emission standards for motor vehicles, reduction of the content of solvents in paints, cleaning agents and other products and consistent implementation of environmental legislation, especially in the field of integrated prevention of industrial pollution (the use best available technologies) and limitation of pollution from large combustion plants.

Ozone precursors significantly contribute to the formation of ground-level (tropospheric) ozone. As ozone is a strong oxidant, it adversely affects human health and ecosystems. High concentrations of ground-level ozone may have a negative impact on the human respiratory system (lungs in particular), cause damage to leaves, reduce the resistance of plants to diseases and reduce crop production. Ozone may also have a damaging impact on plastics and rubber.