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Notably, the instruments used in the European Supersites for Atmospheric
Aerosol Research (EUSAAR)/Aerosols, Clouds, and Trace gases Research Infra-
Structure Network (ACTRIS) and German Ultrafine Aerosol Network (GUAN)
measurements used in this chapter are from intercalibrated measurements, where
the abilities of the instruments were determined in common intercalibration
workshops [
15
]. Overall, the instruments agree well on particle sizes between
20 and 200 nm, with the differences above 200 nm still relatively minor for number
concentrations. In smallest particles sizes the instrument deviation is large, and
for this reason we only consider particles larger than 30 nm in diameter in this
chapter.
2.2 Networks of Measurements in Europe
Several European intensive short-term (“campaign-type”) projects have provided
important information on the atmospheric aerosol properties in Europe, usually by
concentrating on specific aerosol properties or interactions. However, these kinds of
campaign-type measurements do not necessarily represent the seasonal or annual
variations of the aerosol concentrations and can overestimate some properties of the
aerosol populations. Long-term measurements, especially with intercalibrated instru-
ments and common data handling and calibration protocols make the data compari-
son between stations much more reliable and provide the end users (e.g., atmospheric
modelers) good datasets to compare with.
The EUSAAR project of the Sixth Framework Programme of the European
Commission is one of the steps towards a reliable and quality-controlled network of
measurements [
16
]. The EUSAAR project improved and homogenized 20 European
sites for measuring aerosol chemical, physical, and optical properties following a
standardized protocol of instrument maintenance, measurement procedures, and data
delivery in common format to a common database. EUSAAR also provided inter-
comparison and calibration workshops as well as training for the station operators.
The work started in the EUSAAR is continued in ACTRIS infrastructure of the
Seventh Framework Programme of the European Commission.
GUAN is a network of multiple German institutes with an interest in submicron
aerosol properties, which was established in 2008 [
17
]. The methodologies of
particle number size distribution measurements and data handling procedures in
both GUAN and EUSAAR networks are very similar, and the size distribution
measurement results are comparable between the two networks. The EUSAAR
measurements were available (with some station-to-station variability) for the year
2008-2009 and the GUAN measurements were mostly from 2009. The locations of
the stations are shown in Fig.
2
.
A key feature of the EUSAAR and GUAN stations is that they are regional
background stations. This means in this context that the stations are expected to
represent the (possibly polluted) background air and long-range transport, not the
direct influence of any obvious local sources. This means that the levels of aerosol
