What are airborne ultra-fine and nano particles?

Although we can’t see them, the air we breathe is full of microscopic particles. These particles are health hazardous and are thus considered a specific type of air pollution. Often this type of air pollution is called fine dust. The size of these particles is in the order of several nanometers to several micrometers. Currently regulation focuses primarily on the measurement and reduction of fine particles. Fine particles are often identified by Particle Matter (PM) ratings. PM10 rating as an example represents the weight of particles that have a diameter smaller than 10 micrometer.

Ultra-fine Particles

However, a very large fraction of particles in urban air (>90%) has minute particles of around 100 nanometers (nm) and smaller. These we call ultra-fine particles or nano-particles. The picture above clearly demonstrates the difference is dimensions of fine and ultra-fine particles. Ultra-fine particles range below the currently monitored levels. In other words, there is an important actually invisible factor in the air around people.

Airborne particles originate from many natural and man-made sources (e.g. sand dust, fires, diesel smoke, sea salt). The scheme below shows a number of particle types from well-known sources. Ultra-fine particles are normally only generated at very high temperatures, such as combustion processes. One can think of wood fires, industry, engines, cooking fumes, or cigarette smoke. Toner (carbon black) from copiers, laser printers and welding-fumes or nano-materials are important sources as well.

Overview of particle sources
The most important source of ultra-fine particles in urban air however is car traffic. Especially diesel exhaust consists of large amounts of ultra-fine particles. Such particles are generally formed by a basically insoluble core of carbon of 10-20 nm, often covered with chemicals like sulphates, metals and hydrocarbons. These extremely small particles tend to conglomerate in the air into particles of around 100 nm.