Worldwide, it ranks among the cities with the highest levels of air pollution – and it’s located in the heart of Europe: Sarajevo, the capital of Bosnia and Herzegovina. Previously, the spatial distribution of air pollutants here was largely unknown, as were their sources. Now the Paul Scherrer Institute PSI, using its mobile laboratory, has provided the first reliable data – and found the causes of the high level of pollution.

Sarajevo, the 1984 Olympic city, is always good for record-setting – even, unfortunately, when it comes to air pollution. In winter, the skies above the capital of Bosnia and Herzegovina in southeastern Europe are even more heavily polluted with particulate matter than the air in the Chinese capital Beijing. This was shown by the research project SAAERO (Sarajevo Aerosol Experiment), in which researchers from eight countries took part. Besides the Laboratory for Atmospheric Chemistry in the PSI Center for Energy and Environmental Sciences, the University of Nova Gorica, Slovenia, and the Federal Hydrometeorological Institute of Bosnia and Herzegovina played leading roles. At the beginning of 2023, André Prévôt’s team drove through Sarajevo, completing 39 measurement runs within three weeks – through densely populated residential areas on the slopes of the narrow valley, along main traffic arteries, and into the city centre. For this they used PSI’s smog-mobile: a van containing a complete mobile laboratory with state-of-the-art instruments for determining air quality. The results have now been published in the journal Environment International.

“With these mobile measurements we have, for the first time, made it possible to see where particularly high levels of pollution occur,” says Prévôt, who heads the Laboratory for Atmospheric Chemistry at PSI. “In some cases, there are major differences between neighbouring streets.” The researchers were also able to identify the sources: “Heating with solid fuels such as wood and coal in residential areas is the main factor driving up particulate matter concentrations in the evenings,” says Prévôt. Around two-thirds of all measurements exceeded the WHO’s recommended daily limit for particulate matter (PM2.5) of fifteen micrograms per cubic metre. Brief peaks as high as several hundred micrograms per cubic metre were recorded.

Uneven distribution of pollution in the evening

During the day, particulate matter pollution is distributed fairly evenly within the city. In the evenings, concentrations rise sharply in certain districts – especially in residential areas outside the city centre. There, up to 60 percent of the organic particulate matter originates from wood-burning stoves. The PSI team also found high concentrations of polycyclic aromatic hydrocarbons, which are considered carcinogenic.

The air is also polluted in the old town district of Baščaršija in the east of the city. These emissions, however, do not originate from wood heating, but rather from the kitchens of the many restaurants. “Here you always have the smell of grilled meat in your nose,” says PSI team member and study co-author Katja Džepina. There is one advantage for the residents: At night, fresh air flows into the valley from the east, and the levels of pollutants drop faster than in the west.

Sulphur dioxide from old power plants

Another air pollutant is sulphur dioxide. Eighty-one percent of all European emissions of this gas originate in the western Balkans – primarily from old coal-fired power plants dating back to the Soviet era. When the PSI researchers set off from Zurich with their mobile laboratory, sulphur dioxide levels were barely measurable. But as soon as they reached Bosnia and Herzegovina in the Balkans, the levels skyrocketed and remained high, especially in the valleys in and around Sarajevo.

To improve the air quality over Sarajevo, as many buildings as possible would need to be insulated and connected to the gas network. This is not easy everywhere, especially on the hillsides, and therefore a quick solution is not in sight. Along with gas, cleaner pellet heating systems would also be a viable solution.

Thousands of deaths due to air pollutants

This has consequences for health. In an international study published last year in Nature, in which André Prévôt also participated, researchers investigated the toxicity of specific air pollutants. It’s not just the amount of particulate matter that is important; the critical factor is oxidative stress in the lungs, which can trigger respiratory and cardiovascular diseases and lead to premature deaths. Already in this study, Sarajevo was the unenviable leader. Researchers estimate that reducing air pollutants by 50 percent could save 5,000 lives per year in Bosnia and Herzegovina.

André Prévôt recommends making air quality measurements in the western Balkans more consistent by establishing so-called supersites. These are permanent monitoring stations that continuously record air quality data over several years, thus making it possible to compare the measurements. “The region is still inadequately studied,” says Prévôt. In the coming months his team aims to close this gap, at least partially, by analysing additional data from the measurement campaign conducted in early 2023. The researchers want to investigate, among other things, how the concentration of sulphur dioxide changes the chemistry in the atmosphere.