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Wood heating is polluting European mountain valleys

More than 30 million people in Europe live in mountain valleys, many of them in small villages. Dwellings in such places are more likely to be heated using wood as a fuel, especially with the recent rises in the cost of living in general, and in the cost of fossil fuels in particular. Since wood combustion is the largest source of small particulate matter (PM 2.5), it is possible that people living in mountain valleys, and even other rural areas, are exposed to significant levels of air pollution that is dangerous for their health. 

Air quality studies have so far mostly focused on cities. However, in the EU, the UK and the four EFTA countries Iceland, Liechtenstein, Norway and Switzerland, over a quarter of the population lives in rural areas. 

A team of researchers led by the Leibniz Institute for Tropospheric Research (TROPOS) in Germany set out to investigate the pollution levels in shallow mountain valleys, also called hollows. The study was focused on a mountain valley around the village of Retje in Slovenia. 

This shallow valley, in the municipality of Loški Potok, is typical of many mountainous and hilly rural areas in Central and South-Eastern Europe where people use wood for residential heating. The study area is home to 243 households and has a topography that favors the formation of temperature inversions.

The researchers used an instrumented backpack, specially developed by TROPOS, to take mobile readings of air quality along a fixed, six-kilometer route. The team walked the backpack along the chosen route through the valley three times a day – once each in the morning, at midday and in the evening – in December 2017 and January 2018. In total they walked the route 107 times and 642 kilometers on foot, measuring air quality all the way.

Along the route, the readings on the mobile instruments were compared to measurements made by two fixed measuring stations, one at the bottom of the valley in the village, and one on an adjacent hill. This helped to ensure the quality of the data from the mobile platform.  

The researchers measured particulate matter (PM) mass concentrations as well as equivalent black carbon (eBC). Black carbon, commonly known as soot, is a component of particulate matter that derives from incompletely combusted wood. It is known that carcinogenic substances may adhere to the tiny particles of soot, making this component a particular concern in terms of public health. 

The results of the air pollution measurements are published in a paper in the journal Atmospheric Chemistry and Physics. The researchers found that aerosol pollution events in the shallow valley were associated with high local emission intensities originating almost entirely from residential wood burning and shallow temperature inversions. During such temperature inversions the measured mean eBC and PM2.5 mass concentrations in the whole hollow were as high as 4.5 and 48.0 micrograms per cubic meter respectively, which is comparable to levels recorded in larger European urban centers where there is heavy traffic.

Temperature inversions occurred on 70 percent of all the winter nights and mornings during the study period. In the morning, the sun warms the upper parts of the hollow more rapidly than the parts close to the ground, with the morning fog preventing warming near the ground, while the hills on either side may catch the sunlight and warm up. With no wind to mix the two layers, the temperature inversion acts like a lid on a pot, preventing the escape or dispersal of exhaust gases and particles from the combustion of wood. These cannot move upwards and so concentrate in the hollow. As measured by the EU Air Quality Index for fine particulate matter (PM10 and PM2.5), air quality was very poor during such temperature inversions.

“During temperature inversions, pollutant levels in the hollow were highest in the early evening, reaching up to 22 micrograms per cubic meter for black carbon and 560 micrograms per cubic meter for particulate matter. This is the result of domestic wood burning, which increases when people come home after work, and the stable air layer at the bottom of the hollow,” explained Dr. Kristina Glojec, who studied for her PhD at the University of Ljubljana. 

“However, with some wind, both black carbon and particulate matter levels in the basin dropped to less than 1 and 12 micrograms per cubic meter, respectively, which is about four times lower than during a temperature inversion and in line with European regional background levels,” she added.  

During morning and afternoon temperature inversions, in the village of Retje, people living on the lower part of the south-facing slopes were most exposed to the high concentrations of particulate matter, while in the early evening hours, when the inversion is limited to the bottom of the hollow, people there breathe in the highest levels of pollutants.

“These very stable conditions prevent effective mixing of the air in the relief depression, which leads to increased pollutant levels. Therefore, during temperature inversions, particulate matter concentrations in the sink rise to levels comparable to those in larger European city centers and above the EU daily limit value (PM10 = 50 micrograms per cubic meter) as well as above the annual limit value and the WHO daily guideline values (PM2.5 = 20 and 15 micrograms per cubic metre, respectively),” explained Professor Mira Pöhlker from TROPOS.

The researchers state that air pollution in the shallow mountain valley they studied is not limited to this region alone: “The pollutant concentrations measured during the temperature inversions in the rather sparsely populated small relief hollow are worrying, as similar conditions can be expected in numerous hilly and mountainous regions throughout Europe, where about 20 percent of the total population live, 30 percent of whom live in rural relief hollows comparable to the Retje site,” said Professor Griša Močnik.

The study highlights the importance of high-resolution measurements of air quality in rural areas in order to monitor the effects and, ultimately, to reduce the incidence of residential wood-burning, especially in mountainous areas with limited atmospheric self-purification capacity. The team made the following specific suggestions to protect human health in the future research:  

  1. Look at pilot sites on a smaller spatial scale, which could help decision makers in taking effective measures at a local scale.
  2. Raise awareness and knowledge about the air pollution problem of wood burning among the people, including capacity building about the negative health effects, energy efficiency, economical costs of ineffective combustion, optimal use, and regular maintenance of heating appliances, and use of quality fuel (e.g. dry wood).
  3. Inform the inhabitants when atmospheric conditions impede effective dispersion and wood burning is not recommended.
  4. Identify local super emitters, since they could be the main cause of deteriorated local air quality.
  5. Upgrading existing stoves, enhancing energy-related building renovations, and changing the fuel if a better alternative exists are seen as possible options to reduce wood burning pollution.

The study authors conclude by saying that it is important to involve the local population actively in the measures to reduce pollution emissions. They add that everyone should be aware that there is not one universal solution to this complex problem; rather, measures are needed at several levels, taking into account geographical and cultural specificities. 

By Alison Bosman, Staff Writer 

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