Pollen traps and microscopy
Each of the pollen monitoring sites around Europe has a so-called Hirst-type volumetric spore trap. Traps are located usually on the roof of a suitably accessible building about two or three storeys high. Traps are located at this height to enable the general ambient airflow to be monitored which contains a good mix of the local and further distant pollen sources gathered on the wind. If the trap was at ground level then it would mainly collect pollen from the immediate vicinity and results between sites would not be comparable.
The air is sucked into the trap through a slit at a rate of 10 litres per minute and the pollen and other particles fall onto a prepared adhesive tape passing the slit at a set rate. Some sites have the adhesive tape mounted onto a rotating drum, others use a microscope slide.
The tape, after exposure, is put onto a microscope slide and covered with a gel mountant containing a stain to aid identification and the pollen is then counted. Counting is done using a standard sampling procedure.
The highest possible resolution goes down to two hourly intervals. A daily total is obtained by summing the counts for each species found in the twelve transects and these figures are then converted to grains per cubic metre of air using an equation called the correction (or conversion) factor. Counting each slide can take up to an hour or more depending on the amount of pollen on it. Nobody has yet developed a technological method for counting pollen.