The air quality standards, such as the European Union 2008/50/EC Directive, set maximum atmospheric concentrations for specific pollutants. In particular, the EU 24-hour-mean limit value for particles with aerodynamical diameter less than 10 µm (PM10) is 50 µg/m3, with 35 permitted exceedences each year, and the annual-mean limit value is 40 µg/m3.
This legislation has forced the deployment of measurement stations, which usually yield concentration values of total suspended particles (TSP), PM10 or PM2.5. This information is very useful for monitoring mineral dust events, but it is necessary to bear in mind that air quality stations measure the overall concentration of particles, not just dust. Moreover, it is important to consider the station site, since the abundance of anthropogenic particulates close to cities, large industrial plants or motorways can mask the presence of mineral dust.
The reference method to measure concentrations of particulate matter is the gravimetric or filter-based sampling. Gravimetric instruments collect particulates on ventilated filters, which are later weighed at special laboratory facilities to determine the mass concentration of particles in the air. Gravimetric monitoring techniques have been used for years, but filter-based sampling is labour intensive. Filters must be conditioned, weighed before sampling, installed and removed from the instrument, and reconditioned and weighed again at a special facility. Results may not be available for days or weeks. Furthermore, filter-based techniques integrate samples over a long period of time, usually 24-hours, to obtain the required minimum mass for analysis.
With the increasing concerns about the effect of PM on human health, limitations of the time integrated filter approach are becoming apparent. Further, the delay involved in sampling and determining PM concentration is also a concern. Continuously operating sampling methods like oscillating microbalances (TEOM) or beta-attenuation can detect the deposited mass almost in situ, but require operating conditions different to the environmental situation or are not completely specific to mass. It is therefore necessary to introduce correction factors in the measurements.
This section also includes backtrajectories ending at the different measuring stations. The backtrajectories provide information on the origin of the airmass so they can be used to make assumptions about the nature of the particles found in the air.