Passive sampling for O3 and NO2 was carried out at 15 birch sites
during the one-week period from May 11th to 18th 2010, i.e.
roughly two weeks after the last birch trees started to flower. Most
of the sites were equipped with two samplers (in total N ¼ 24) and
mean values were calculated from these measurements.
Passive samplers for O3 were provided and analyzed by PASSAM
AG (M€annedorf, Switzerland). The NO2 concentration was
measured according to Palmes’ principle (Palmes et al., 1976): a
triethanolamine-aceton mixture was applied to stainless steel
meshes which were subsequently air-dried for ten minutes. For
each location three coated meshes were brought into an air-tight
tube and fixed at the tree at a height of 3 m. Since NO2 binds to
the coated meshes by forming a triethanolamineeNO2ecomplex,
the adsorption of NO2 could be determined photometrically after
the exposure to ambient air.
These site-specific pollution data were only recorded at birch
sites. Since we expected a high spatial variation of O3 and NO2
concentrations, we did not allocate these values to hazel or horse
chestnut locations.