Air pollution sources

Air pollutants may be categorised as primary or secondary.

Primary pollutants are directly emitted to the atmosphere, whereas secondary pollutants are formed in the atmosphere from precursor gases through chemical reactions and microphysical processes. Air pollutants may have a natural, anthropogenic or mixed origin, depending on their sources or the sources of their precursors. Key primary air pollutants include particulate matter (PM), black carbon (BC), sulphur oxides (SO₂), nitrogen oxides (NO_X) (including nitrogen monoxide and nitrogen dioxide, NO₂), ammonia (NH₃), carbon monoxide (CO), methane (CH₄), non-methane volatile organic compounds (NMVOCs), including benzene, and certain metals and polycyclic aromatic hydrocarbons, including benzo[a]pyrene (BaP).

Key secondary air pollutants are PM, ozone (O₃), NO₂ and several oxidised volatile organic compounds (VOCs). Key precursor gases for secondary PM are sulphur dioxide (SO₂), NO_X, NH₃ and VOCs. These pollutants and their precursor gases can be of both natural and anthropogenic origin including:

• burning of fossil fuels in electricity generation, transport, industry and households;
• industrial processes and solvent use, for example in the chemical and mining industries;
• agriculture;
• waste treatment;
• natural sources, including volcanic eruptions, windblown dust, sea-salt spray and emissions of volatile organic compounds from plants.

Significant cuts in emissions are essential to improve air quality, as air pollutant emissions are the principal drivers behind air pollution.

At the same time, reductions in emissions do not always automatically result in similar falls in concentrations. There are complex links between air pollutant emissions and air quality. These include emission heights, chemical transformations, reactions to sunlight, additional natural and hemispheric contributions and the impact of weather and topography.

Ground-level (tropospheric) ozone is not directly emitted into the atmosphere. Instead, it forms in the atmosphere from a chain of chemical reactions following emissions of certain precursor gases: NO_X, carbon monoxide (CO) and NMVOCs and methane (CH₄). Ground-level ozone is formed from chemical reactions in the presence of sunlight, following emissions of precursor gases, mainly NO_X, NMVOCs, CO and CH₄.

Nitrogen oxides are emitted during fuel combustion from industrial facilities and the road transport sector. NO_X is a group of gases comprising nitrogen monoxide (NO) and nitrogen dioxide (NO₂). NO makes up the majority of NO_X emissions. NO_X contributes to the formation of ozone and particulate matter.

Particulate matter is a mixture of aerosol particles (solid and liquid) covering a wide range of sizes and chemical compositions. PM is either directly emitted as primary particles or it forms in the atmosphere from emissions of certain precursor pollutants such as SO₂, NO_X, NH₃ and NMVOCs. PM is emitted from many anthropogenic sources, including both combustion and non-combustion sources. Natural emissions of PM also occur, including from sea salt and windblown Saharan dust.

Sulphur dioxide is formed and emitted by combustion of fossil fuels (mainly coal and oil) primarily for electricity generation. High concentrations of SO₂ are associated with multiple health and environmental effects. The highest concentrations of SO₂ have been recorded in the vicinity of large industrial facilities. SO₂ emissions are an important environmental issue because they are a major precursor to ambient PM_2.5 concentrations.

Benzo(a)pyrene is a polycyclic aromatic hydrocarbon (PAH) found in fine PM. Its origin is the incomplete combustion of various fuels. The main sources of BaP in Europe are domestic home-heating, in particular wood- and coal-burning, waste-burning, coke and steel production, and road traffic. Other sources include outdoor fires.

Source: EEA Signals 2013