Fine dust loves the winter. New findings now provide an explanation.
An international team of scientists has now found out why the concentration is so high, especially in the winter street canyons of large cities. New particles are quickly formed from the exhaust gases, as the researchers describe in the journal "Nature".
The so-called primary particulate matter with a particle diameter of less than 2.5 is mainly formed by combustion processes in engines and heating systems.
Secondary particulate matter is produced when organic substances such as nitric acid, sulphuric acid or ammonia attach themselves to nanoparticles and increase in size like an avalanche. And this is precisely where the international investigation comes in.
Prior to this study, it was unclear how precisely particulate matter can be newly formed in heavily populated urban areas. It is obvious that the tiny particles are more likely to attach themselves to already existing particles and thus become a larger mass than to form completely from scratch.
Since these chemical processes take place at temperatures of less than five degrees Celsius, winter is the preferred period for particulate matter processes such as the one now being demonstrated.
"Under warmer conditions, the particles are too volatile and could therefore not contribute to growth," explains aerosol physicist Paul Winkle.