By Jens C. Pedersen

The contribution to air pollution from residential wood combustion has now been mapped in unprecedented detail by researchers from the National Environmental Research Institute (NERI), Aarhus University. A comprehensive monitoring campaign in the Danish town Slagslunde indicates that residential wood combustion is a major contributor to particle pollution. When compared to the amount of heat produced, pollution from wood stoves far exceeds that from other methods of heating. However, the results also indicate a possible overestimation of the total emission from residential wood combustion in Denmark.

NERI researchers specialising in air pollution have focused on the Danish village Slagslunde in North Sjælland. Not because this is an area with a particularly high level of air pollution, but because the researchers needed an area with a high proportion of wood burning stoves, where users of wood burning stoves could also provide detailed information about their wood burning habits. The successful project gave the NERI researchers an unprecedented insight into the habits of users of wood burning stoves and the resulting impact on air pollution. Project leader Helge Rørdam Olsen, NERI, describes the results:

 ”We have achieved a unique insight into the wood burning habits of the Danes. At the same time our measurements have provided us with a "fingerprint" of the pollution caused by residential wood combustion. This is a useful tool when distinguishing between contributions from wood smoke and contributions from other sources.”

 Helge Rørdam Olesen draws attention to a fact, which may cause confusion in the public debate about pollution from residential wood combustion:

“When discussing particle pollution it is important to make a distinction between the emission of particles and the concentration of particles in the outdoor air. There is a difference, because only a fraction of the particles in the outdoor air were “born” as particles – these are the so-called primary particles. A large proportion of the outdoor particles were not born as particles, but as gases. Consequently, although residential wood combustion is responsible for more than half of the total Danish emission of primary particles, the contribution to the particle concentration in outdoor air is far below 50%. Other major sources are shipping, agriculture, traffic, and transboundary contributions.”

The investigation indicates that in an area with a high degree of wood combustion activity – such as Slagslunde – the contribution from residential wood combustion to the concentration of fine particles (PM_2.5) will amount to approx. 2 micrograms per m³ on a yearly basis in a year with normal weather conditions. The corresponding figure for another previously monitored area, the village of Vindinge, which has less wood burning activity is ca. 0.5 micrograms per m³. These figures can be compared to the level of PM_2.5 concentration in background air, which amounts to 10-15 micrograms per m³. Thus, for an area with a high level of wood combustion activity the contribution from residential wood combustion constitutes 10-20% of the particle pollution. This applies to the yearly average, while during cold spells the contribution will be considerably higher.

When the scientists compare their measurements of pollution in the air with results of model calculations based on wood burning habits they are able to perform backward calculations of the so-called emission factors, i.e. the amount of grams emitted particles per kilo burnt wood. It is difficult to estimate emission factors of general validity, because besides depending on the wood stove, the emission factor is very sensitive to the habits of the “stove keeper”.

The results indicate that the emission factors in Slagslunde are somewhat lower than the factors usually applied in Danish emission inventories.

 Helge Olesen comments: “It is difficult to draw definitive conclusions as there are many uncertainties involved. If, for example, the closest neighbours to the monitoring station belong to the group of very careful stove keepers – possibly encouraged by the publicity surrounding the monitoring campaign – the results cannot be considered generally representative for Denmark.”

NERI researchers have previously demonstrated that the firing habits of wood stove users are extremely important to emissions. Emission levels 10-100 times above normal can occur as the result of old stoves, wrong wood burning habits or burning of wet wood and waste.

Part of large cooperation

The investigations are part of the research project WOODUSE, an inter-disciplinary project covering many aspects of wood burning. The project comprises investigations of emissions, pollution levels in indoor and outdoor air, impacts on health, and social aspects. Four institutions have taken part in the project (NERI; the National Research Centre for the Working Environment; Department of Public Health, University of Copenhagen; Department of Civil Engineering, Technical University of Denmark). The project is funded by the Danish Council for Strategic Research.

Contact: Senior Adviser Helge Rørdam Olesen, tel. +45 4630 1151, hro@dmu.dk

Brændefyrings bidrag til luftforurening. Nogle resultater fra projektet WOODUSE. Olesen, H.R., Wåhlin, P. & Illerup, J.B. 2010: . Danmarks Miljøundersøgelser, Aarhus Universitet. 71 s. - Faglig rapport fra DMU nr. 779. Summary | Sammenfatning | Hele rapporten i PDF-format (1,54 MB)

Air pollution from residential wood combustion in a Danish village. Measuring campaign and analysis of results. Wåhlin, P., Olesen, H.R., Bossi, R. & Stubkjær, J., 2010. National Environmental Research Institute, Aarhus University. 49 pp. – NERI Technical Report No. 777. Summary | Sammenfatning | Hele rapporten i PDF-format (2,21 MB)  

Brug af brændeovne i Slagslunde. Resultater fra en spørgeskemaundersøgelse inden for projektet WOODUSE. Helge Rørdam Olesen, Steen Solvang Jensen & Jesper Stubkjær 2010:  Danmarks Miljøundersøgelser, Aarhus Universitet. 36 s. - Arbejdsrapport fra DMU nr. 260. Sammenfatning | Summary| Hele rapporten i pdf-format (3,79 MB).

The contribution to air pollution from residential wood combustion has now been mapped in unprecedented detail by researchers from the National Environmental Research Institute (NERI), Aarhus University. A comprehensive monitoring campaign in the Danish town Slagslunde indicates that residential wood combustion is a major contributor to particle pollution. When compared to the amount of heat produced, pollution from wood stoves far exceeds that from other methods of heating. However, the results also indicate a possible overestimation of the total emission from residential wood combustion in Denmark.