Emissions from household heating and road vehicles are the dominant sources of primary PM. Photo: J. Aaron Farr/flickr.com/ CC BY
Particle pollution is not a purely local problem. Transboundary pollution transport is a major contributor to urban PM2.5 levels, and reductions in both primary PM and secondary precursor emissions are needed to bring down PM2.5 to safe levels.
In 2009, the annual average urban roadside PM2.5 concentrations exceeded the World Health Organization’s recommended level in cities in most member states. It is estimated that 400,000 people die prematurely every year across the EU because of air pollution, which means that poor air quality kills ten times as many people as road traffic accidents. At the same time, many cities are struggling to meet the EU air quality standards. There is a strong need for emission reductions, both to achieve legal compliance and to protect public health.
A new report presents an assessment of source contributions to PM2.5 levels at air quality monitoring stations in cities in 21 EU member states. The present situation (based on data for 2009) is compared with the envisaged result in 2030 of the Commission proposal for a revised National Emissions Ceilings (NEC) Directive. The focus is on the attribution of PM2.5 concentrations at urban roadside stations to the different source sectors and also to show the spatial contributions, i.e. if the sources are of local, national, international or natural origin.
The source allocation shows that while source contributions vary widely between individual countries, all spatial domains contribute. In particular, it becomes clear that PM pollution cannot be considered a purely local problem. For several member states, such as Belgium, Czech Republic, Netherlands, Hungary, and Austria, transboundary transport of PM and PM precursor pollutants is a major contributor to urban PM2.5 levels. It is therefore very difficult for these countries to decrease their urban PM2.5 to safe levels without coordinated international action. On the other hand, several regions show very high local increments, pointing to the need for local measures to reduce ambient PM2.5.
Transboundary transport is dominated by secondary pollution, while primary PM plays a role mostly for local sources. Hence, reductions in both primary PM and secondary precursor emissions will be needed to bring down PM2.5 to safe levels.
Emissions from household heating and road vehicles are the dominant sources of primary PM. In many member states, especially those with very high PM concentrations (e.g. Poland, Czech Republic, Slovakia, Romania, and Bulgaria), solid-fuel combustion for domestic heating is the largest single source sector.
The introduction of particle filters for diesel vehicles is expected to reduce primary PM from road transport by almost two thirds up to 2030, so the remaining PM emissions from this sector will be mainly from non‐exhaust sources (road abrasion, brake and tyre wear). Unless new action is taken, household heating will remain an important source of PM emissions, particularly in areas where coal or inefficient biomass burning is used. More stringent product standards for domestic heating appliances (stoves, boilers, etc.) and accelerated substitution of inefficient solid-fuel burning by cleaner alternatives such as district heating, heat pumps, natural gas or efficient biomass combustion could achieve additional emission reductions that are not considered in the Commission’s proposal.
Overall, the Commission’s proposal would cut primary PM emissions by half by 2030. As a consequence, secondary aerosols are expected to become the dominant contributor to the remaining PM2.5 concentrations.
The formation of secondary aerosols involves several pollutants from different source sectors, and the various chemical processes make it difficult to uniquely trace them back to a single source. However, the formation of ammonium sulphate and ammonium nitrate is critically steered by the availability of ammonia (NH3). Ammonia emissions come primarily from agricultural sources, and form, together with sulphur dioxide (SO2) and nitrogen oxides (NOx) from power generation, industry and transport, secondary inorganic particles.
It is concluded that future trends in secondary PM will very much depend on measures to reduce agricultural NH3 emissions. The Commission’s NEC Directive proposal for 2030 aims to cut NH3 emissions by only 27 per cent relative to 2005. At the same time, NOx should be reduced by 69 per cent, and SO2 by 81 per cent.
Altogether, implementation of the Commission’s proposal is expected to reduce ambient PM2.5 levels by 50 per cent or more in most member states by 2030. This would, according to the report, result in attainment of the WHO guideline value for average, urban roadside PM2.5 levels in seven member states, although concentrations at some peak locations could still be higher, which would require additional local measures.
The report: Urban PM2.5 levels under the EU Clean Air Policy Package (October 2014). TSAP Report 12,Version 1.0. Report to the European Commission by the International Institute for Applied Systems Analysis (IIASA).
Figure: Source contributions to ambient PM2.5 at urban traffic stations in the Czech Republic and the Netherlands, in the base year 2009 and for 2030 assuming adoption of the Clean Air Policy Package proposed by the Commission. Source: IIASA GAINS