Airborne Particulate Matter (PM) contributes significantly to climate change and has been linked to adverse health effects. From a legislation point of view the most important metric to monitor particulate air pollution is the mass concentration1-3, more specifically the total mass per unit volume of air of particles with aerodynamic diameter smaller than 10 μm or 2.5 μm, commonly referred to as PM10 and PM2.5, respectively.
The reason behind making PM mass concentration the default metric of PM was the idea that mass measurements can easily be made in a traceable manner. The reference method for measuring PM is thus based on the manual weighing of filters after sampling for 24 hours. However, in the past few years it has become apparent that measurements of particulate matter deposited on filters are associated with artifacts due to semi-volatile particles, humidity, chemical reactions during and after sampling etc4. Practical experience has shown that measurements can vary up to 50% depending on the method and instrument employed, making thus compliance with legislation a controversial issue5.
PM itself cannot be clearly defined as it does not refer to a pollutant with a distinct chemical composition, such as carbon or nitrogen oxides, but rather to a highly variable mixture of combustion particles, salts, metal oxides, organic substances and other materials. As a con-sequence, a suitable standard calibration aerosol does not exist. To date, automated PM instruments are tested for equivalence with the manual reference method in monitoring sites using real ambient air6. This requires long and expensive trials. The development of a versa-tile generator of aerosols representing ambient aerosols of different chemical compositions will enable calibration of automated PM instruments in the laboratory according to standard-ized procedures. Hence it is a major step towards simplification, cost-efficiency, and higher precision in the calibration of automated PM measurements.
The Laboratory of Particles and Aerosols at METAS has long experience in the generation of aerosols, such as soot, salt particles and oil droplets to name but a few, in a wide size and concentration range. Moreover, the Sector of Analytical Chemistry possesses the expertise and the necessary infrastructure to perform qualitative and quantitative chemical analyses in complex samples. Given this background, the Laboratory of Particles and Aerosols can make a highly innovative contribution to the field by constructing an ambient-aerosol generator. The generator will be tunable with respect to the particle number concentration, size distribution and chemical composition in order to produce reference aerosols representative of real atmospheric aerosols.
The production of stable and reproducible calibration aerosols that mimic the properties of atmospheric aerosols will find direct applications not only in the field of PM measurements, but also in the fields of filter testing and gas turbine electrical power generation. Moreover, it will help us improve services which are currently being offered, such as the calibration of condensation particle counters. For calibrations at small particle diameters, where particle chemistry plays an important role, the generator will enable the choice of a suitable composition of the calibration aerosol for individual instruments. In a broader perspective, improving the measurement accuracy of airborne particulate matter will have a major social impact as it will contribute to the protection of human health and the environment.
