Thousands of wildfires occur all around the globe every year, with significant impacts on air quality and climate. Climate change is expected to cause an increase of the wildfire season length, wildfire frequency and severity. These fires emit particles that are mostly organic and smaller than one micrometer. The current mental model for these particles is that they are transported away from the fire, their concentrations decrease due to dilution, and they are gradually removed either by rain or by deposition to the ground. Our paper shows that this mental model is problematic because the emissions from fires (particles and vapors) react rapidly in the atmosphere especially during the summer, new particulate mass is produced, and the chemical composition and toxicity of the biomass burning organic aerosol changes.

During these atmospheric reactions, biomass burning particles lose their characteristic organic chemical fingerprints and therefore cannot be easily detected as they become similar to the background aerosol. This loss of identity leads to underestimation of their level in previous scientific studies that rely on the presence of the original constituents of the biomass burning particles for their detection. In our work we estimated that secondary biomass burning particulate matter from wildfires (the material formed from the chemical reactions of the primary emissions) was responsible for approximately half of the organic aerosol in Europe during July 2022.

These atmospheric reactions take place rapidly in the atmosphere and convert most of the primary emissions to secondary material in 1-2 days. During that time wildfire air masses travel hundreds of kilometres in the atmosphere. Wildfires from all around Europe affected our sampling site, which was located in a forested background site in Greece.  One of our most surprising results, was that biomass burning emissions from a major fire from Portugal travelled through the United Kingdom, Norway, France, Germany, and many more European countries and finally reached Greece almost 2 weeks after the beginning of the fire.

Our results indicated that the aerosol in this forested background site, which was mostly affected by aged wildfire emissions, had higher potential per unit mass to cause oxidative stress to organisms than that in the centre of Athens, which is the capital of Greece with 4 million inhabitants. Comparisons with other rural, urban and background European sites led the same conclusion.

The chemically aged biomass burning was estimated to be responsible for a significant fraction of the health impacts of fine particles in Europe, causing 10 to 16 thousand deaths every summer. This means that 15-22 out of the 100 deaths from ambient exposure in particulate matter in Europe during summer are associated with biomass burning from wildfires. This is a rather conservative estimate, given that the potential toxicity of the organic aerosol in this forested site was more than that in many other European sites.