The German Ultrafine Aerosol Network (GUAN) is a cooperative
atmospheric observation network, which aims at improving the
scientific understanding of aerosol-related effects in the
troposphere. The network addresses research questions dedicated to
both climate- and health-related effects. GUAN's core activity has
been the continuous collection of tropospheric particle number size
distributions and black carbon mass concentrations at 17 observation sites in Germany. These sites cover various
environmental settings including urban traffic, urban background,
rural background, and Alpine mountains. In association with partner
projects, GUAN has implemented a high degree of harmonisation of
instrumentation, operating procedures, and data evaluation
procedures. The quality of the measurement data is assured by
laboratory intercomparisons as well as on-site comparisons with
reference instruments. This paper describes the measurement sites,
instrumentation, quality assurance, and data evaluation procedures in
the network as well as the EBAS repository, where the data sets can be obtained (10.5072/guan).
Introduction
Atmospheric aerosol particles, or particulate matter (PM), are essential
constituents in the atmosphere influencing issues such as atmospheric
visibility, global climate, and human health. A climate-relevant effect is
their interaction with solar shortwave radiation . Two
major aerosol effects influencing the terrestrial radiation budget have been
distinguished: direct radiative forcing – scattering and absorption of
upwelling and downwelling radiation in the absence of clouds
– and indirect radiative forcing – the modification of
cloud radiative properties through the activation of additional particles as
cloud condensation nuclei . The magnitude of direct
radiative forcing depends, in general, on various properties of the aerosol
particles including particle diameter and chemical composition
but also shape, state of mixture, and hygroscopicity
. Black carbon (BC) is among the species
contributing to light absorption and, thus, atmospheric warming. According to
a recent survey, the radiative forcing due to BC can, at present, only be
bound to the uncertainty of a factor of 2 . The particle
number size distribution and the light absorption coefficient are useful
parameters to predict the direct radiative forcing on the basis of in situ
measurements.
On the other hand, ambient aerosol particles have been recognised to
affect human health e.g.. Recent projections of health effects yield drastic
numbers of morbidity and premature deaths due to particulate pollution
worldwide . In the European Union, the mass
concentration of \chem{PM_{{10}}}PM10 and \chem{PM_{{2.5}}}PM2.5 (particles smaller
than 10 and 2.5 \mathrm{{µ}m}µm in aerodynamic diameter, respectively)
currently serve as legal metrics to assess a population's exposure to
ambient particles (European Council, 2008/50/EC). A rationale for
using \chem{PM_{{10}}}PM10 and \chem{PM_{{2.5}}}PM2.5 has been the large body of
epidemiological evidence of adverse health effects based on these
metrics.
Some studies, however, have suggested that the mass-based metrics
might not be the most favourable parameter to characterise PM-induced
health effects . Some epidemiological studies have
associated health endpoints with the number of ultrafine particles or
the particle surface area rather than particle mass . Ultrafine particles are ubiquitous in
urban atmospheres , and their ability to penetrate
deep into the human body after inhalation has been forwarded as
a rationale for their adverse health effects. A recent overview by the
world health organisation WHO also counted
atmospheric soot particles (BC) among the relevant environmental risk
factors for human health. While there seems little doubt about the
potential adverse health effects of ultrafine particles and BC, their
relatively low mass concentration makes them hardly accessible by
total-mass-based measurements. Particle number size distribution and
BC mass concentration have consequently been recommended as exposure
parameters for future epidemiological studies .
International observation networks for in situ atmospheric aerosol
measurements include WMO-GAW (World Meteorological Organization Global
Atmosphere Watch) and EMEP (European Monitoring and Evaluation
Programme). European research infrastructure programmes have
contributed to the systematic collection of in situ atmospheric
aerosol data as well: EUSAAR (European Supersites for Atmospheric
Aerosol Research) and ACTRIS (Aerosols, Clouds, and Trace gases
Research InfraStructure Network). In the early 2000s the Nordic
Aerosol Network carried out particle number size distribution
measurements in a number of rural locations in Scandinavia
. The nature of most of these networks, however, has been
to measure aerosol abundance and characteristics on a continental and
global scale. Accordingly, the measurement sites are predominantly
located in rural settings where direct anthropogenic influence is
weak. Observation networks including urban sites are, for example, the
Black Carbon and Particle Numbers and Concentrations Networks in the
UK, operated by the National Physical Laboratory .
Government air quality networks in Europe operate many stations that
collect data on \chem{PM_{{10}}}PM10 and \chem{PM_{{2.5}}}PM2.5 mass
concentrations, which are relevant to European air quality
legislation. In the view of limited financial resources, however,
there is usually a limited incentive to measure aerosol and PM metrics
that go beyond legal requirements, although such activities might
provide enhanced scientific insights into climate-relevant or
health-related processes.
In 2008, the German Federal Environment Agency (UBA) and the Leibniz
Institute for Tropospheric Research (TROPOS) founded a new
Germany-wide network for the characterisation of fine and ultrafine
particles in the atmospheric aerosol. Several of UBA's manned
background monitoring stations and numerous other legal and research
institutions with their personnel and existing infrastructure have
been involved. Notable institutions have included the Saxon State
Office for Environment, Agriculture and Geology (LfULG), the Helmholtz
Zentrum Munich (HMGU), the Institute of Energy and Environmental
Technology (IUTA), and the German Meteorological Service (DWD). As
a result, continuous measurements of sub-\mathrm{{µ}m}µm particle number
size distributions and equivalent BC mass concentrations have been
installed and maintained at a total of 17 observation
sites. This paper serves to describe in detail the characteristics of
the measurement sites, the instrumentation deployed for continuous
particle measurements, and the location and properties of the data
files.
Concept for long-term measurements
To date, there are wide experimental options to characterise
atmospheric aerosol particles in much physical and chemical detail
e.g.. The
requirements of long-term deployment in a network, however, reduce
these options to experimental methods that are sufficiently stable and reproducible but also financially viable. When designing the German
Ultrafine Aerosol Network (GUAN) in 2008, it was decided to implement
a limited number of aerosol parameter measurements only but with
enhanced spatial coverage and operational reliability
. The measurements include, in particular
sub-\mathrm{{µ}m}µm particle number size distributions;
sub-\mathrm{{µ}m}µm particle number size distributions of
non-volatile particles;
equivalent black carbon (eBC) mass concentrations.
Number size distributions of particles in dry conditions are measured
by mobility particle size spectrometers. Depending on their individual
set-up, these instruments are called scanning mobility particle sizer
(SMPS), twin differential mobility particle sizer (TDMPS), or twin
scanning mobility particle sizer (TSMPS). Number size distributions of
non-volatile particles are measured after passage through
a thermodenuder at a temperature of 300 {}^{{\circ}}∘C. (Thermodenuders
remove particulate compounds that are volatile at this temperature.)
eBC mass concentrations are mainly measured by multi-angle
absorption photometers (MAAPs). During GUAN's first operation
phase (2009–2014), these instrumental methods proved robust and
yielded reproducible results so that they warrant successful
deployment in an observation network.
Location of the atmospheric observation sites in the German
Ultrafine Aerosol Network (GUAN), currently consisting of 17 sites. See Table for the full names and
characteristics of the sites.
Atmospheric observation sites
Figure illustrates the location of GUAN's 17 ground-based atmospheric measurement sites in Germany. Figure provides maps and illustrations of the immediate
surroundings of each measurement. Several Tables supply detailed
information: Table , the characteristics of the
measurement sites; Table , instrumental
features; Table , co-location with other
particle and air pollutant measurements; Table ,
associations with other networks, infrastructure, and research
projects; Table , the list of institutions
involved. The following text gives a brief description of each GUAN
site, its measurement programme, and references for published results.
Annaberg-Buchholz
Annaberg-Buchholz is a site of the Air Quality Monitoring Network of
Saxony (AQMNS) run by the Saxon State Office for the Environment,
Agriculture and Geology (LfULG, Dresden). Like all AQMNS sites,
technical operations are conducted by the State Department for
Environmental and Agricultural Operations in Saxony (BfUL,
Radebeul). The site is located in the city of Annaberg-Buchholz
(population ca. 21 000) in the Ore Mountains (Erzgebirge), distant
about 10 \mathrm{km}km from the German–Czech border. Particle number
size distributions and eBC mass concentrations have been measured
continuously since 2012 in the framework of UltraSchwarz, a research
project dedicated to ultrafine particles and health research in the
German–Czech border region . The measurements
are complemented by a suite of basic particulate and gaseous
pollutants (cf. Table ).
Augsburg
Augsburg is an urban background monitoring station operated by the
Helmholtz Zentrum Munich (HMGU), Institute of Epidemiology II, and the
University of Augsburg. The site was established in 2004 in the city
of Augsburg (population ca. 270 000) in southern Germany with a main
purpose to provide input to epidemiological studies of respiratory and
cardiovascular disease (KORA, cooperative health research in the
Augsburg region). The station is located on the university premises,
about 1 \mathrm{km}km south-east of the city centre. Particle number size
distributions and eBC mass concentrations have been collected
continuously since 2004 .
Bösel (Südoldenburg)
Bösel (Südoldenburg) is a regular site in the government air
quality monitoring system of Lower Saxony operated by the Labour
Inspectorate of Lower Saxony (Staatliches Gewerbeaufsichtsamt
Hildesheim, GAA). To the south, the sampling site borders agricultural
areas, while to the north, it is adjacent to residential areas of the
village of Bösel (population ca. 7400). The station is situated in
an area where livestock production is most intense. The gaseous
ammonia and organic emissions related to these activities are
anticipated to have an impact on the regional budget of secondary
aerosols. Bösel lies at a distance of about 100 \mathrm{km}km from the North
Sea, so that maritime air masses can be sampled with a relatively
minor impact of continental sources. Particle number size
distributions and eBC mass concentration measurements were carried out
on a continuous basis between 2008 and 2015.
Dresden-Nord
Dresden-Nord is another AQMNS station, located along the roadside in the city
of Dresden (population ca. 500 000). Continuous particle number size
distribution measurements since 2001 have confirmed a pronounced
diurnal cycle of traffic-related pollutants
. Around 36 000 vehicles pass by the site per day, including 3.5 % heavy-duty vehicles. The horizontal distances to
the traffic flows range between 7 \mathrm{m}m (minor traffic flow;
southerly direction) and 80 \mathrm{m}m (major traffic flow, westerly
direction). The site borders the railway station Dresden-Neustadt,
with mainly electrified trains passing by at a distance of
200 \mathrm{m}m to the north. Basic features of the particle number size
distributions are presented in and
.
Dresden-Winckelmannstrasse
Dresden-Winckelmannstrasse is another AQMNS station, located in the
urban background of Dresden, about 1.7 \mathrm{km}km south of the city
centre. The next major road passes by the site at a distance of
100 \mathrm{m}m in easterly direction. Particle number size
distributions measurements were established in 2010 and eBC measurements
in 2012. A major purpose of the site has been the provision of
representative urban background concentrations for particle size
distributions and particle mass within the UFIREG (Ultrafine Particles – an evidence based contribution to the development of regional and European environmental and health policy) project
(Table ). The measurements are complemented by
a suite of basic particulate and gaseous pollutants
(Table ).
Hohenpeißenberg
The Meteorological Observatory Hohenpeißenberg (MOHp) has a long tradition
of meteorological and climatological observations. It is operated by the
German Meteorological Service (DWD) and contributes to WMO-GAW, ACTRIS, and
EMEP. MOHp is located on a solitary hill in the rural countryside of southern
Bavaria (980 \mathrm{m{\,}a.s.l.}ma.s.l.), approximately 40 \mathrm{km}km north of the
Alpine mountain range. The observatory is located around 300 \mathrm{m}m above the
surrounding rural areas, which are composed mainly of agricultural pasture (70 %) and forests (30 %). MOHp hosts a wide range of atmospheric
aerosol and gas phase measurements. Some highlights include continuous
observations of volatile organic compounds (VOCs), gaseous sulfuric acid
(\chem{H_{2}SO_{4}}H2SO4), and numerous remote-sensing parameters. MAAP measurements
started in December 2003, while SMPS measurements started in 2008. The
observatory has delivered atmospheric data as part of GAW since 1994,
starting with elemental carbon (EC), total suspended particulate matter
(TSP), and total particle number. For the basic characteristics of particle
number size distributions and their relation to trace gas and meteorological
parameters, see .
Langen
Langen is an urban background measurement site located on the premises
of the German Federal Environment Agency (UBA) in Langen. The site
is located 15 \mathrm{km}km south of the city of Frankfurt am Main and
5 \mathrm{km}km south-east of Frankfurt Airport. Aerosol
particles are sampled at a height of 14 \mathrm{m}m on the rooftop of
the UBA building. Continuous particle number size distribution
measurements started in 2008, complemented since 2009 by measurements
of total particle number concentrations (ultrafine condensation particle counter (UCPC), TSI model 3776) and
lung-disposable surface area (nanoparticle surface area monitor, TSI
model 3550) .
Leipzig-Eisenbahnstrasse
Leipzig-Eisenbahnstrasse is a roadside observation site in the city of
Leipzig (population ca. 500 000), operated by TROPOS since 2002. The site is
located in a street canyon within a densely built-up residential area,
characterised by multistorey period buildings. The street canyon is regular
in that its aspect ratio is close to unity (height: 18 \mathrm{m}m, width:
20 \mathrm{m}m), and no building gaps are present. The street experiences
traffic of about 12 000 motor vehicles per working day. Ambient aerosol is
sampled 6 m above street level on the northern side of the street. Due to
the formation of a vortex inside the street canyon, northerly winds have been
identified as the condition that favours high particle number concentrations
. For an account of the spatial and temporal
variability of particle number size
distributions in this area of Leipzig, see .
Leipzig-Mitte
Leipzig-Mitte is another AQMNS station, located at the roadside in the
city of Leipzig. The site borders the inner-city ring road, in close
vicinity to the central train station. Immediately north of the site,
three main roads merge at an intersection with daily average traffic
volumes around 44 000 vehicles (48 000 on working day). Among all GUAN
sites, Leipzig-Mitte exhibits the greatest exposure to traffic-related
pollutants. \chem{PM_{{10}}}PM10 mass and ultrafine particle number
concentrations were discussed by and
, respectively. Leipzig-Mitte was added to GUAN for the
purpose of monitoring possible changes in ultrafine particle number and eBC mass
concentrations along with the introduction of the low-emission zone
(Umweltzone) in Leipzig . Particle number size distribution and eBC
measurements started in 2010. The station's portable cabin borders a tributary
road connected to the ring road by traffic lights. Construction activities in
the vicinity of the site have occasionally disturbed the measurements between
2010 and 2012. Days on which the impact of construction work was significant
were documented in .
Leipzig-Tropos
Leipzig-Tropos (simply called “Leipzig” in some databases) is an
atmospheric research station operated by TROPOS since 1997. The
station is situated on the roof of the TROPOS institute
building. Aerosol particles are sampled at a height of 16 m above the
ground. High-traffic roads border the premises at distances of at
least 100 \mathrm{m}m. Comparisons of particle number size distributions
at multiple sites in Leipzig have confirmed Leipzig-Tropos as an urban
background station . A cross-sectional
study suggested a total particle number concentration mean of
9400 \mathrm{cm^{{-3}}}cm-3, which proved to be higher than at comparable
sites in Helsinki and Copenhagen . During the
heating season, the site can be influenced by a gas heating stack
50 \mathrm{m}m south of the aerosol inlet. Screening of the data
showed that a perturbation of the measurements is likely under
southerly winds and at temperatures below 0 {}^{{\circ}}∘C. The size
distribution profile of the gas heating stack only affects particle number
concentration below 30 \mathrm{nm}nm.
Leipzig-West
Leipzig-West is another AQMNS site in Leipzig, located in the western suburbs
of Leipzig. The purpose of the site is to provide a second set of measurements for
urban background concentrations in Leipzig. The distance to Leipzig-Mitte is
about 6 \mathrm{km}km and that to Leipzig-Tropos about 10 \mathrm{km}km. The residential
area around the station consists of multistorey apartment blocks that are
heated by district heating.
Hot water is generated in a central power
plant 20 km away and delivered to the area through pipelines. Due to this
distance, it is expected that any emissions generated at the power plant will
have no influence on the local measurements at Leipzig-West.
The station's cabin
is placed in a park area on the premises of a hospital. A minor
road passes by the station around 30 \mathrm{m}m west of the site but has shown
to have negligible influence on the measurements. Particle number size
distribution and eBC measurements started in 2010 .
Illustration of the GUAN measurement sites: (a) sites 1–3, (b) sites 4–6, (c) sites 7–9,
(d) sites 10–12, (e) sites 13–15, (e) sites 16–17. Pictures illustrate the surroundings
of each station. The maps show the geographic area, illustrating major types of land use (Source: OpenStreetMap,
processed by Maperitive V.2.3.22). The location of each measurement site is marked by a diamond.
Melpitz
Melpitz is an atmospheric research station operated by TROPOS since
1992. The station is located in eastern Germany near the city of
Torgau and is ca. 50 \mathrm{km}km northeast of Leipzig. The site is
surrounded by flat and seminatural grasslands without any obstacles
in any directions. Besides GUAN, Melpitz contributes to WMO-GAW as
a regional background site as well as ACTRIS and EMEP. Measurements at Melpitz
can be taken as representative of the central European background
atmosphere . Melpitz hosts a wide suite of physical
and chemical measurements of atmospheric aerosols
. The site has proved useful to detect long-range
pollution transport from continental areas in an easterly direction
. The distance to the North Sea is about
400 \mathrm{km}km in a north-westerly direction and that to the Atlantic Ocean
about 1000 \mathrm{km}km in a westerly direction. An account of the basic
aerosol characteristics can be found in ,
, and . The effects of hygroscopic particle
properties on atmospheric light scattering were discussed in
. The formation of new atmospheric particles from
gaseous precursors has been analysed here since the 1990s
.
Mülheim-Styrum
Mülheim-Styrum is part of the air quality
monitoring network of North Rhine–Westphalia (LUQS). The site is
operated by the State Agency for Nature, Environment, and Consumer
Protection (LANUV). IUTA Duisburg operates an additional portable cabin for research measurements. The site is situated within a residential
area but is also within reach of a motorway (around 250 \mathrm{m}m to
the north), a national road (B223, around 400 \mathrm{m}m to the west),
and industrial premises (around 600 \mathrm{m}m to the east and
south). Overall, the site qualifies as an urban background monitoring
station and has been used for exposure assessment in
health-related studies (ESCAPE, European Study of Cohorts for Air
Pollution Effects) and source apportionment studies
. Mülheim-Styrum was added to GUAN as
a representative for the Ruhr district, the largest urban agglomeration in
Germany. Continuous particle number size distribution measurements
started in 2008. Lung-disposable surface area has been measured by
a nanoparticle surface area monitor (TSI model 3550) as an additional
measure of quality assurance.
Neuglobsow
Neuglobsow is one of the permanently manned stations within UBA's continuous
observation network . The sampling site is surrounded by lakes
and forested areas in all directions and is therefore only influenced very
little by local sources. Neuglobsow contributes to EMEP. Measurements here
can be taken as representative of the atmospheric background in north-eastern
Germany. Particle number size distribution and eBC measurements started in
2010.
Schauinsland
Schauinsland is another of UBA's manned observatories. The station is
located at 1205 \mathrm{m{\,}a.s.l.}ma.s.l. near the Schauinsland peak in the
Black Forest in south-western Germany. Measurements started as early
as 1965 as part of research programmes funded by the German Research
Foundation (DFG). The station is well-suited to characterise air
masses that approach central Europe from westerly directions. In
winter, the site tends to lie in relatively clean air above the
Rhine valley's inversion layer. Observations at Schauinsland have
focused on the detection of long-term trends. For instance, the site hosts the longest continuous observation of carbon dioxide
(\chem{CO_{2}}CO2) observations in Europe (since 1972; ). Particle number size distribution measurements
started in 2005 and eBC measurements in 2008.
Waldhof
Waldhof is another manned station within UBA's observation
network. The sampling site is surrounded by forest in all directions
and is therefore only influenced very little by local
sources. Measurements here are representative of the
background in the north German lowlands. Waldhof is the only German
atmospheric station contributing to GMOS (Global Mercury Observation
System) . Particle number size distribution and eBC
measurements started in 2008.
Zugspitze (Schneefernerhaus)
Zugspitze (Schneefernerhaus) is part of the global WMO-GAW station
Zugspitze/Hohenpeissenberg and jointly operated by the German Federal
Environment Agency (UBA) and the German Meteorological
Service (DWD). The observatory is located at 2670 \mathrm{m{\,}a.s.l.}ma.s.l.,
about 300 \mathrm{m}m below the Zugspitze summit and on the southern
slope of the mountain massif. The high altitude leads to
a significant annual cycle in observed aerosol particle number and
mass concentration, caused by different boundary layer heights in
summer and winter The station's elevated position
allows us to sample air masses that had only little contact with the
local boundary layer, especially in the cold season from October to
March. Zugspitze occasionally receives lofted aerosol layers from
remote source regions, such as North America or
the Eyjafjallajökull volcanic eruption
. Besides the GUAN measurements, the Karlsruhe
Institute of Technology conducts long-term measurements of total
particle number concentration (condensation
particle counter – CPC), particle number size distribution
(optical particle counter – OPC; aerodynamic particle sizer – APS), and primary biological aerosol particles. Particle number
size distribution measurements started in 2004 and eBC measurements in
2008.
Atmospheric measurement sites in GUAN, in alphabetic
order.
No.NameEBAS CodeSite operatorTypeAltitudeLocationSite and/or dataset description1Annaberg-BuchholzDE0061BLfULGurban background545 m50{}^{{\circ}}∘34{}^{{\prime}}′18{}^{{\prime\prime}}′′ N, 12{}^{{\circ}}∘59{}^{{\prime}}′56{}^{{\prime\prime}}′′ E2AugsburgDE0062BHMGU/UAurban background485 m48{}^{{\circ}}∘21{}^{{\prime}}′29{}^{{\prime\prime}}′′ N, 10{}^{{\circ}}∘54{}^{{\prime}}′25{}^{{\prime\prime}}′′ E, 3Bösel (Südoldenburg)DE0056RGAArural17 m52{}^{{\circ}}∘59{}^{{\prime}}′53{}^{{\prime\prime}}′′ N, 07{}^{{\circ}}∘56{}^{{\prime}}′34{}^{{\prime\prime}}′′ E4Dresden-NordDE0063KLfULGroadside116 m51{}^{{\circ}}∘03{}^{{\prime}}′54{}^{{\prime\prime}}′′ N, 13{}^{{\circ}}∘44{}^{{\prime}}′29{}^{{\prime\prime}}′′ E, 5Dresden-Winckelmannstr.DE0064BLfULGurban background120 m51{}^{{\circ}}∘02{}^{{\prime}}′10{}^{{\prime\prime}}′′ N, 13{}^{{\circ}}∘43{}^{{\prime}}′50{}^{{\prime\prime}}′′ E6HohenpeißenbergDE0043GDWDrural (mountain)980 m47{}^{{\circ}}∘48{}^{{\prime}}′06{}^{{\prime\prime}}′′ N, 11{}^{{\circ}}∘00{}^{{\prime}}′34{}^{{\prime\prime}}′′ E7LangenDE0065BUBAurban background130 m50{}^{{\circ}}∘00{}^{{\prime}}′18{}^{{\prime\prime}}′′ N, 08{}^{{\circ}}∘39{}^{{\prime}}′05{}^{{\prime\prime}}′′ E, 8Leipzig-Eisenbahnstr.DE0066KTROPOSroadside120 m51{}^{{\circ}}∘20{}^{{\prime}}′45{}^{{\prime\prime}}′′ N, 12{}^{{\circ}}∘24{}^{{\prime}}′23{}^{{\prime\prime}}′′ E, 9Leipzig-MitteDE0067KLfULGroadside111 m51{}^{{\circ}}∘20{}^{{\prime}}′39{}^{{\prime\prime}}′′ N, 12{}^{{\circ}}∘22{}^{{\prime}}′38{}^{{\prime\prime}}′′ E, 10Leipzig-TroposDE0055BTROPOSurban background126 m51{}^{{\circ}}∘21{}^{{\prime}}′10{}^{{\prime\prime}}′′ N, 12{}^{{\circ}}∘26{}^{{\prime}}′03{}^{{\prime\prime}}′′ E, 11Leipzig-WestDE0068BLfULGurban background122 m51{}^{{\circ}}∘19{}^{{\prime}}′05{}^{{\prime\prime}}′′ N, 12{}^{{\circ}}∘17{}^{{\prime}}′51{}^{{\prime\prime}}′′ E, 12MelpitzDE0044RTROPOSrural84 m51{}^{{\circ}}∘31{}^{{\prime}}′32{}^{{\prime\prime}}′′ N, 12{}^{{\circ}}∘55{}^{{\prime}}′40{}^{{\prime\prime}}′′ E, 13Mülheim-StyrumDE0069BLANUV/IUTAurban background37 m51{}^{{\circ}}∘27{}^{{\prime}}′17{}^{{\prime\prime}}′′ N, 06{}^{{\circ}}∘51{}^{{\prime}}′56{}^{{\prime\prime}}′′ E14NeuglobsowDE0007RUBArural70 m53{}^{{\circ}}∘08{}^{{\prime}}′28{}^{{\prime\prime}}′′ N, 13{}^{{\circ}}∘01{}^{{\prime}}′52{}^{{\prime\prime}}′′ E15SchauinslandDE0003RUBArural (mountain)1205 m47{}^{{\circ}}∘54{}^{{\prime}}′49{}^{{\prime\prime}}′′ N, 07{}^{{\circ}}∘54{}^{{\prime}}′29{}^{{\prime\prime}}′′ E, 16WaldhofDE0002RUBArural75 m52{}^{{\circ}}∘48{}^{{\prime}}′04{}^{{\prime\prime}}′′ N, 10{}^{{\circ}}∘45{}^{{\prime}}′23{}^{{\prime\prime}}′′ E, 17Zugspitze (Schneefernerhaus)DE0054RUBA/DWDAlpine mountain2650 m47{}^{{\circ}}∘25{}^{{\prime}}′00{}^{{\prime\prime}}′′ N, 10{}^{{\circ}}∘58{}^{{\prime}}′47{}^{{\prime\prime}}′′ EBirmili et al. (, )
Technical features of GUAN instrumentation. Mobility particle
size spectrometers follow the TROPOS design
(Sect. ) unless stated otherwise. Two types of
thermodenuders are used to measure non-volatile size distributions (Sect. ).
Co-location of GUAN measurements with other continuous
aerosol and air pollutant measurements. The core of GUAN
measurements include sub-\mathrm{{µ}m}µm particle number size
distributions (PNSD), sub-\mathrm{{µ}m}µm particle number size
distributions of non-volatile particles (NV-PNSD), and equivalent
black carbon mass concentrations (eBC). Additional continuous
measurements may include total particle number concentration (TNC)
– measured by condensation particle counters; coarse particle
number size distribution CPNSD – using an aerodynamic particle
sizer (APS) or an optical particle counter (OPC); nanoparticle
surface area (NSA) – using an NSAM monitor; aerosol scattering
coefficient (\sigma _{p}σp) – using a nephelometer; PM{}_{{x}}x
particle mass concentrations; and basic meteorological parameters
(Meteo) including TT, RH, wind speed, wind direction, global
radiation, and precipitation. These additional data need to be obtained
directly from the operator of the respective measurements.
{}^{\text{a}}a WMO-GAW: World Meteorological Organization/Global Atmosphere Watch (http://www.wmo.int/pages/prog/arep/gaw/gaw_home_en.html, http://gaw.empa.ch/gawsis).{}^{\text{b}}b EMEP: European Monitoring and Evaluation Programme (http://www.emep.int).{}^{\text{c}}c ACTRIS: Aerosols, Clouds, and Trace gases Research InfraStructure Network (http://www.actris.eu).{}^{\text{d}}d AQMNS: Air quality monitoring network of Saxony (Luftgütemessnetz Sachsen), coordinated by LfULG, operated by BfUL.{}^{\text{e}}e UFIREG: Ultrafine particles – an evidence based contribution to the development of regional and European environmental and health policy (http://www.ufireg-central.eu).{}^{\text{f}}f LLEZ: Leipzig low-emission zone studies .{}^{\text{g}}g UltraSchwarz: Ultrafine particles and health in the Ore Mountains district and the region of Usti (Ultrafeinstaub und Gesundheit im Erzgebirgskreis und Region Usti) (http://www.ultraschwarz-ziel3.de).{}^{\text{h}}h KORA: Cooperative health research in the Augsburg region (http://www.helmholtz-muenchen.de/kora).{}^{\text{i}}i LÜN: Air quality monitoring system in Lower Saxony (Luftüberwachungssystem Niedersachsen), operated by GAA.{}^{\text{j}}j VAO: Virtual Alpine Observatory, coordinated by the Bavarian Research Alliance
(http://www.bayfor.org).{}^{\text{k}}k LUQS: Air quality monitoring network of North Rhine–Westphalia (Kontinuierliches Luftmessnetz), operated by LANUV.{}^{\text{l}}l GMOS: Global Mercury Observation System
(http://www.gmos.eu).
Institutions involved in GUAN, in alphabetic order.
AcronymNameLocation (all in Germany)BfULState Dept. for Environmental and Agricultural Operations in Saxony (Betriebsgesellschaft für Umwelt und Landwirtschaft)RadebeulDWDGerman Meteorological Service (Deutscher Wetterdienst)HohenpeißenbergGAALabour Inspectorate of Lower Saxony (Staatliches Gewerbeaufsichtsamt)HildesheimHMGUHelmholtz Zentrum Munich, Institute of Epidemiology IINeuherbergIUTAInstitute of Energy and Environmental Technology e.V.DuisburgLANUVState Agency for Nature, Environment and Consumer Protection in North Rhine–Westphalia (Landesamt für Natur, Umwelt und Verbraucherschutz Nordrhein-Westfalen)EssenLfULGSaxon State Agency for the Environment, Agriculture and Geology (Sächsisches Landesamt für Umwelt, Landwirtschaft und Geologie)DresdenTROPOSLeibniz Institute for Tropospheric ResearchLeipzigUAUniversity of AugsburgAugsburgUBAGerman Federal Environment Agency (Umweltbundesamt)Dessau-RosslauInstrumental set-upParticle number size distributions
Particle number size distributions are recorded on a continuous basis
using mobility particle size spectrometers. Depending on the specific
configuration, these can be TDMPSs (typical particle size range 5–800 \mathrm{nm}nm), TSMPSs (size range 5–800 \mathrm{nm}nm), or SMPSs (typical size range
10–800 \mathrm{nm}nm). The core component of each mobility particle size
spectrometer is a differential mobility analyser (DMA).
Note that in GUAN, similar to WMO-GAW, particle number size distributions are
measured in a “dry state” i.e. at relative humidities (RHs) below 50 %.
The reason is that particle size distributions from different sites should
remain comparable. Atmospheric particles are known to grow significantly in
diameter above RH 50 % . To prevent such effects in
the instrumentation, aerosol dryers are installed upstream of each mobility
particle size spectrometer and also within the sheath flow circuit
. The actual values of RH and temperature in every
instrument are stored at the EBAS database and can be accessed through the
database interface there.
SMPS (TROPOS design)
The principle of the custom-built SMPS is described in detail in
. Briefly, the instrument uses a Vienna-type
DMA (electrode length 28 \mathrm{cm}cm) with a condensation particle
counter (CPC model 3772, TSI Inc.) to measure particles between 10 and
800 \mathrm{nm}nm. Before entering the DMA, ambient aerosol is conveyed
to a bipolar charge equilibrium using a \chem{{}^{{85}}Kr}85Kr
neutraliser. The sheath flow rate is 5 \mathrm{L{\,}min^{{-1}}}Lmin-1 at an aerosol
flow rate of 1 \mathrm{L{\,}min^{{-1}}}Lmin-1. The high-voltage supply provides
positive polarity, thus collecting negatively charged particles. The
sheath air is generally circulated in a closed loop. Both the aerosol
sample flow and the sheath air flow are actively dried in this
instrument, thus ensuring a relative humidity during size
classification below 40 % at most times. Temperature, relative
humidity, and pressure inside the instrument are continuously
monitored. The typical time resolution for one combined upscan and
downscan is 5 \mathrm{min}min. TROPOS-designed SMPS instruments are
currently deployed at the GUAN stations 1, 3, 5, 6, 14, 15, and 16
(see Table ).
SMPS (TSI design)
Commercial SMPS instruments (model 3936, TSI Inc., Shoreview, USA)
are deployed at the GUAN
stations 7, 13, and 17. The sheath air to aerosol flow ratio is
5:1\,{\mathrm{L{\,}min^{{-1}}}}5:1Lmin-1 at stations 7 and 17 – yielding
a particle size range of 10–600 \mathrm{nm}nm – and
3:0.3\,{\mathrm{L{\,}min^{{-1}}}}3:0.3Lmin-1 at station 13 – yielding a particle size
range of 14–750 \mathrm{nm}nm. Time resolution is 5 \mathrm{min}min. The
instruments at all stations were upgraded to meet the quality criteria
recommended for ambient aerosol measurements by the EUSAAR and ACTRIS
initiatives . The upgrades concern dryers for
the aerosol sample flow and sheath air, as well as additional sensors
for temperature, relative humidity, and pressure.
At stations 7 and 13, these upgrades took place prior to the start of
measurements. At station 17, the modifications took place at the end of 2008.
In addition, the high-voltage supply at station 17 was changed from negative
to positive polarity and a range of up to 12.5 \mathrm{kV}kV. This effect was
considered in the processing of the data by the use of a different
corresponding bipolar charge distribution.
TDMPS/TSMPS
Mechanically, the TDMPS and TSMPS are dual DMA versions of the SMPS
described in Sect. . The first
subsystem combines an ultrafine Vienna-type DMA (electrode length
11 \mathrm{cm}cm) with a UCPC (model 3025, TSI Inc., Shoreview (MN), USA) to measure particles across
the range 3–80 \mathrm{nm}nm. The second subsystem combines another DMA
(electrode length 28 \mathrm{cm}cm) with a condensation particle counter
(CPC model 3010 or 3772, TSI Inc.) to measure particles between 10 and
800 \mathrm{nm}nm. Due to enhanced measurement uncertainties below
5 \mathrm{nm}nm, only the diameter range 5–800 \mathrm{nm}nm is further
analysed and fed into the EBAS database. Like with the SMPS, sheath air
is circulated in a closed loop at relative humidities ranging mostly
between 10 and 40 %. The typical time resolution of the
instrument is 10 \mathrm{min}min. TDMPS instruments are deployed at sites 2, 4, and 12, while TDMPS instruments are operated at sites
8–11. TSMPS and TSMPS have, in principle, no major differences in
hardware. In software, TSMPS use a continuous ramping of the high
voltage, like in the SMPS, rather than the stepwise change used in the
TDMPS.
Non-volatile size distributions (thermodenuder)
Upstream of some mobility particle size spectrometers, a thermodenuder
(TD) is deployed as a way of removing volatile aerosol
components. The standard operation mode for these extended instruments
is to record size distributions upstream and downstream of the TD in
alternating sampling intervals . This procedure
provides a steady flow of size distributions both with and without the
TD and an effective time resolution of half the original instrumental
time resolution.
One thermodenuder type follows the design of
. Volatile particle material is evaporated at
a temperature of 300 {}^{{\circ}}∘C and subsequently removed with the
assistance of active carbon in a cooling section. The temperature of
300 {}^{{\circ}}∘C was selected with the aim of evaporating the
overwhelming mass of volatile and semi-volatile material, particularly
ammonium nitrate, ammonium sulfate, and most organic carbonaceous
compounds. Major compounds not removed include elemental carbon,
crustal material, and sodium chloride
cf.. Meanwhile, 300 {}^{{\circ}}∘C is a temperature
at which charring (i.e. the incomplete combustion of oxygenated
hydrocarbons) of organic compounds is avoided. A mass closure for the
non-volatile particle fraction at the research station Melpitz
suggested that the non-volatile fraction at 300 {}^{{\circ}}∘C contains
not only refractory black carbon but also a comparable share of
low-volatility organic aerosol compounds
LVOCs;.
The second, simplistic thermodenuder type consists of a simple
steel tube that is heated to 300 {}^{{\circ}}∘C by a laboratory
furnace. Excess vapours are only adsorbed by the tube walls. Such
a simplistic TD has been used, for example, by
. Experiments with ambient aerosol in Leipzig suggested
that both thermodenuder designs produced equivalent results for
non-volatile particle size distributions across the diameter range
10–800 \mathrm{nm}nm. The tubing acting as a TD is usually cleaned once
per year.
In the TD, the aerosol sample is subject to enhanced particle
losses. One can think of, e.g., diffusional as well as thermophoretic
losses, which are both a function of particle size. In the Wehner-type
thermodenuder, part of the sample flow passes through the active
carbon filter. Therefore, this instrument was calibrated for particle
losses by using spherical silver particles (solid at 300 {}^{{\circ}}∘C) and by measuring the particle counts upstream and downstream of the
TD. The penetration of solid particles through the Wehner-type
thermodenuder was about 0.34 at 3 \mathrm{nm}nm, 0.66 at 10 \mathrm{nm}nm
and 0.85 for particles bigger than 100 \mathrm{nm}nm at a flow rate of
2.5 \mathrm{L{\,}min^{{-1}}}Lmin-1. All data collected from this thermodenuder
type were corrected for these losses.
Aerosol absorption and eBC
The aerosol absorption coefficient is measured by MAAPs. The MAAP converts the light attenuation
and reflection by a particle-laden quartz fibre filter into an
absorption coefficient by calculating the radiative transfer through
this two-layer system (Petzold and Schönlinner, 2004). The
wavelength (\lambda=637λ=637\mathrm{nm}nm) corresponds to the region of the
solar spectrum where BC is the prime absorber, thus
minimising interferences with “brown carbon” and mineral dust. Brown
carbon and mineral dust tend, in fact, to absorb light more
efficiently towards the ultraviolet wavelengths .
Because there is no unique relationship between aerosol absorption and black
carbon mass concentration, black carbon is typically reported as eBC (Petzold et al., 2013). For this purpose, the aerosol
absorption coefficient \sigma _{{\text{abs}}}σabs is converted into an eBC mass
concentration using an experimentally determined mass absorption cross
section (MAC). The manufacturer of the MAAP instrument reports a MAC value of
6.6 \mathrm{m^{2}{\,}g^{{-1}}}m2g-1, which is automatically applied to all data records.
An assessment of aerosol absorption using Raman spectroscopy and EC measurements as reference methods yielded a mean MAC value of
5.3 \mathrm{m^{2}{\,}g^{{-1}}}m2g-1, with a range of variability between 3.9 and
7.4 \mathrm{m^{2}{\,}g^{{-1}}}m2g-1 across a selection of seven GUAN sites
. An intercomparison of multiple
instruments showed that different MAAP instruments produce comparable results
with less than 5 % inter-device variability .
Besides eBC mass concentrations, the sample flow rate, temperature, and
pressure, the MAAP yields the raw signals of loaded and blank filter material
at scattering angles of 0, 135, and 165{}^{{\circ}}∘. The latter parameters are
stored internally in a so-called “extended data format”.
The extended
version of the data, i.e. the raw signals at various scattering angles could
come to use if at some point in the future, the radiative transfer algorithm
to retrieve eBC from the MAAP measurement might be improved. The raw signals
would give the possibility to recalculate eBC according to such a different
scheme.
To provide accurate and comparable measurements under dry sample
conditions, the MAAP aerosol flow is usually conditioned by a membrane dryer.
At Augsburg, an aethalometer (Type 8100, Thermo Fisher Scientific
Inc.) is deployed using a cut-off of 2.5 \mathrm{{µ}m}µm. This
instrument yields mean eBC mass concentrations that are comparable to
those from a MAAP instrument. Details on this instrument can be seen
in Appendix A.
Unfortunately, not all eBC measurements in GUAN use the same inlet
configuration. Inlets for \chem{PM_{{10}}}PM10,
\chem{PM_{{2.5}}}PM2.5 and \chem{PM_{{1}}}PM1 are used throughout the network
(Table ). In order to harmonise the eBC values,
the data recorded downstream of the \chem{PM_{{1}}}PM1 cyclone inlets may
be adjusted to the corresponding level of a \chem{PM_{{10}}}PM10 inlet using
suitable correction factors. The multiplication factors recommended
here are 1.10 for rural sites, 1.08 for urban background sites, and
1.05 for roadside sites. These values were determined by a direct
intercomparison of the readings of two MAAP instruments using
a \chem{PM_{{10}}}PM10 and a \chem{PM_{{1}}}PM1 inlet at the sites
Leipzig-Eisenbahnstrasse (roadside), Leipzig-Tropos (urban
background), and Melpitz (rural background). Because very high
correlations were found during those intercomparison experiments
(R^{2}=0.99R2=0.99), a post correction appears justified
.
Quality assurance
Quality assurance (QA) in GUAN consists of procedures to ensure that
measurements remain stable both instrument to instrument (or site to site) and instrument to standard. Temporal or spatial deviations in atmospheric
pollutants often occur in the range of a few per cent of the absolute
concentration of a specific parameter. The issue of QA is therefore vital to
successfully address scientific questions on the basis of such data. It has
been an aim in GUAN to ensure an accuracy of within a few per cent for the eBC mass concentration
measurements, of a few per cent for the particle sizing accuracy in number
size distributions, and of \pm±10 % for particle number
concentrations over the entire measurement period.
The following paragraphs describe the state-of-the-art QA procedures for
mobility particle size spectrometers, which were followed at the majority of the stations. Due to the different
degree of access and availability of manpower, they may not be valid in all
details at every single station and for all historical parts of the data
collection. Most of these measures were co-developed within the framework of
previous infrastructure projects, such as WMO-GAW, EUSAAR, ACTRIS, and
research projects initiated by the Saxon State Office for Environment,
Agriculture and Geology (LfULG).
MaintenanceWeekly or biweekly inspection
At unmanned GUAN stations, the mobility particle spectrometers and MAAPs are
inspected in person at least every 2 weeks, preferably every week. At the
manned GUAN stations (sites 6 and 14–17), the instrumentation is usually
inspected more often, up to once per working day. At unmanned sites, remote
data access has helped to check instrumental performance. Unfortunately,
remote data access to unmanned stations is currently available only for a few
stations (e.g. 1, 2, 4, 5, 12). The weekly or biweekly inspection of mobility
particle size spectrometers includes visual checks whether all instrumental
components are switched on and working correctly: high-voltage power supply,
sheath air flow, and aerosol flow; CPCs; supply of CPC working liquid
(butanol or water); data acquisition program; flow status and operation of
the MAAP.
Monthly maintenance
A full maintenance check is typically carried out with mobility particle size
spectrometers every 4 weeks. Here, the following instrumental flow rates are
verified using an external reference flow meter, usually a bubble flow meter:
aerosol flow rate, sheath air flow rate, flow rate of the aerosol dryer's
counter flow, flow rate of the sheath air dryer counter flow. Aerosol and
sheath air flow meters in the mobility particle size spectrometers are
recalibrated if they deviate by more than 5 and 2 % from their set point
values, respectively. Instruments are also checked for leaks using a total
particle filter. The performance of the mobility particle size spectrometers
is deemed satisfactory if the total particle number collected by the
instrument after a waiting time of 15 \mathrm{min}min does not exceed
10 \mathrm{particles{\,}cm^{{-3}}}particlescm-3.
The high-voltage supply for each DMA is checked with a digital multimeter,
involving a verification of the high voltage between 0 and 1000 \mathrm{V}V.
Recalibrations are made if the voltages exceed defined thresholds at
0 \mathrm{V}V (\pm±3 \mathrm{V}V), 6.25 \mathrm{V}V (\pm±25 %),
100 \mathrm{V}V (\pm±10 %), and 1000 \mathrm{V}V (\pm±1 %).
Frequently checking the high voltage is essential to provide a correct sizing
of the DMA, particularly at the lower end of the particle size distribution.
A NIST (National Institute of Standards and Technology, US Department of
Commerce) certified particle size standard is used to verify the exact sizing
of the instrument. Currently, the most popular standard is 203 \mathrm{nm}nm
Polystyrene latex (PSL) spheres, which are certified within 2.5 % of
the nominal particle diameter. The particles are nebulised from aqueous
suspension using a jet nebulizer (e.g. PariBoy, Pari GmbH, Starnberg,
Germany). If the sizing of the mobility particle size spectrometer deviates
more than 2 % from the standard size (i.e. is outside the interval
200–206 \mathrm{nm}nm), the sheath air flow is adjusted until the DMA matches
the certified diameter of the PSL particles.
Annual maintenance
The annual maintenance event includes more extensive instrumental checks and
care. It is preferentially performed in a central laboratory. Many GUAN
instruments are returned to the World Calibration Centre for Aerosol Physics
WCCAP; at TROPOS in Leipzig once a year. The annual
maintenance includes check and calibration of the humidity sensors;
calibration of the sheath air flow rate zero offset; check and calibration of
the pressure transducer; disassembly and cleaning of the DMA(s); check of the
saturator sponge inside the CPC; check and calibration of the CPC(s) against
a particle number concentration standard. It has also been found important to
check the activity of the charge neutraliser. A \chem{{}^{{85}}Kr}85Kr beta source,
for example, degrades substantially in its ion production rate after about
10 \mathrm{years}years. The annual maintenance event is also used to perform
hardware improvements and software updates.
Comparison to reference instruments
Intercomparisons with reference instruments are essential in establishing
a relationship to a defined standard. In the case of mobility particle size
spectrometers, we use CPCs of a specific type (model 3010 and 3772, TSI Inc.,
Shoreview, USA) and an electrometer (model 3068B, TSI Inc.) as an
intermediate standard for particle number concentration. It is also
a standard procedure to compare a mobility particle size spectrometer once
per year to a reference instrument, which will reveal possible deviations
with respect to the size-dependent instrument response. In GUAN, these
intercomparisons are made in the central laboratory, at calibration
workshops, or in the field as part of a “round-robin test”.
Laboratory intercomparisons
Intercomparisons with the TROPOS reference mobility particle size
spectrometers can be made at the WCCAP “on the
fly”, i.e. at short notice at most times. However, it is common practice to
collect a pool of instruments, which are then examined collectively against
one or two reference instruments . Because of their
state close to the bipolar charge equilibrium, we generally prefer ambient
aerosols for all instrumental intercomparisons, even if the experiments are
conducted in the central laboratory. Three reference instruments are provided
by WCCAP for quality assurance, which are checked regularly against each
other and against total particle counters. (Reference instruments need to
agree within \pm±5 % for all particle sizes between 20 and
300 \mathrm{nm}nm.) Laboratory intercomparisons are preferentially accompanied
by a total particle counter (CPC). The deviation between the total particle
number concentration derived from any mobility particle size spectrometer and
the total particle counter must not exceed \pm±10 % for ambient
aerosols. On the basis of such laboratory intercomparisons, we estimate the
accuracy of the particle number concentration measured by mobility particle
size spectrometers to be \pm±10 % for the diameter range
20–300 \mathrm{nm}nm. Outside this range, larger uncertainties are possible.
Below 20 \mathrm{nm}nm, deviations between SMPS instruments may amount up to
50 % and above 300 \mathrm{nm}nm up to 30 %. TDMPS and TSMPS
instruments exhibit a higher accuracy at the lower particle size end:
\pm±10 % down to 10 \mathrm{nm}nm and ca. \pm±30 % at
5 \mathrm{nm}nm.
Round-robin intercomparisons
Ideally, every mobility particle size spectrometer undergoes at least one
calibration experiment per year. To bridge gaps between the scheduled WCCAP
calibration workshops, the QA measures may include a round-robin test on its
measurement site using a reference mobility particle size spectrometer. In
practice, such field intercomparisons usually involve (a) setting up
a reference instrument in parallel to the mobility particle size
spectrometers on site, (b) performing parallel size distribution measurements
of ambient aerosol with the reference instrument during one night
(introductory performance test), (c) maintaining and/or improving the
instrument in case of problems, and (d) repeating the ambient aerosol
comparison for several days as a final performance test. The round-robin test
usually involves checks of particle sizing and parallel measurements by
a total particle counter (CPC) as well. As in the case of the laboratory
intercomparisons, an agreement between the total number concentration of
a mobility particle size spectrometer and the reading of a total particle
counter of \pm±10 % is required. In most practical cases, we also
found a good agreement between the test and reference instruments within
\pm±10 % for the diameter range 20–300 \mathrm{nm}nm.
Enhanced quality assurance
For three stations in Saxony (sites 1, 4, and 5), enhanced quality assurance
measures have been developed . At these sites, the
mobility particle size spectrometers are equipped with an automatic function
control unit that performs unattended instrumental comparisons. First, a leak
check using a total particle filter is performed every 3 days. A dedicated
total particle counter (TSI model 3772, “transfer CPC”) is moved to each of
the three stations every 8 weeks. During a 2-week presence of the transfer
CPC, the control unit will perform comparison measurements between this CPC
and the particle size spectrometer every 23 h. To avoid uncertainties due to
nucleation mode particles present in the lower cut-off region of the CPC,
particles at the lower end of the number size distribution are removed by
a diffusion screen. For traceability, the transfer CPC is checked every 8
weeks against a dedicated reference CPC at WCCAP and once a year against
a calibrated electrometer. This procedure allows an even closer tracking of
the actual performance of the mobility particle size spectrometer than under
less infrequent intercomparisons. In our experience, the procedure can narrow
down the uncertainty with respect to particle number concentration across the
diameter range of 20–300 \mathrm{nm}nm from \pm±10 to \pm±5 %.
At several sites, additional instrumentation has been used as a support for
quality assurance and control: at sites 2, 6, and 17, CPCs have been deployed
on a continuous basis to measure total particle number concentration. As
total particle concentration can also be computed from the particle number
size distribution, a divergence of the two readings can help identify
instrumental problems. At sites 7 and 13, nanoparticle surface area monitors
(NSAM, TSI Inc.) have been deployed as a QA tool because the lung-deposited
particle surface area (LDSA) can be computed from the particle number size
distribution as well. At site 13, significant deviations between the measured
and calculated LDSA over several consecutive days indicated the failure of
one of the instruments which, in practice, almost always turned out to be the
SMPS or its CPC.
Data processing and validationParticle number size distribution processing
Particle number size distributions from all stations except station 13
were processed using the TropINV software package, written in LabVIEW
(Version 8.5, National Instrument, Austin, USA). This program evokes
a linear multiple charge inversion algorithm Muchain, Multiple
Charge Inversion; and performs various additional
corrections before yielding the final particle number size
distributions. TropINV performs the following steps:
reformatting the raw concentration and instrument diagnostics
data (optional);
condensing the number of size bins of the raw data to a fraction
of its original number (optional);
automatic flagging of periods exhibiting diagnostics data
outside the nominal range;
assimilation of the two DMA branches (only for TDMPS and TSMPS
instruments);
multiple charge inversion ;
adjustment of particle number concentrations for nonideal
aerosol flow rates (optional);
size-dependent corrections of particle number concentration
:
CPC counting efficiency
DMA transfer function
bipolar charger
aerosol dryer.
connecting tubes inside and outside of the instrument
normalisation of ambient concentrations to 0 {}^{{\circ}}∘C,
1013 \mathrm{hPa}hPa (optional);
re-binning the size channels to a predefined standard set of
standard channels.
Step 7 yields technically correct data that are ready for scientific use
(dubbed “Level-1” in Sect. ). Step 8 yields data that,
after averaging to 1 h resolution, form “Level-2” data
(Sect. ). GUAN has followed a practice that during step 9,
all particle number size distributions are re-binned to a uniform set of 40
channels between 10 and 800 \mathrm{nm}nm in the case of the SMPS instrument and
46 channels between 5.1 and 800 \mathrm{nm}nm by linear interpolation. It is
expected that in this way, data processing will be facilitated for the
scientific end users. The data at station 13 were processed using the Aerosol
Instrument Manager software (TSI Inc., Revision G, October 2006), which
performs all the necessary steps (5–7) in a manner equivalent to the TropINV
software .
Technical and manual data validation
The first step of quality control of mobility particle size
spectrometer data involves the automatic flagging of data records that
are associated with invalid instrument diagnostic data. For example,
the relative humidity in the instrument needs to remain below
40 %. Instrumental temperature needs to be within a range of
++10 to 30 {}^{{\circ}}∘C. Aerosol sampling flows are not
permitted to deviate from their set point by more than
\pm±10 %, and the sheath air flows must not diverge more than
\pm±5 % from their set point. These thresholds are motivated in
.
Exemplary contour diagrams of particle number size distributions at
six mainly urban GUAN sites (20–26 December 2009). Such diagrams assist the
visual quality control of the data.
Further quality control involves additional visual checks by the scientists
processing the data. One procedure is to carefully inspect contour diagrams
of the particle number size distribution. An example for such contour
diagrams at the highest available time resolution is given in
Fig. . The human eye is rather sensitive to unusual features in
such two-dimensional structures, and in our experience, this visual
inspection has proved very effective in detecting irregularities in the data,
such as
flash-over of high voltage between the two DMA electrodes (this
unwanted effect tends to produce artificial particles at the upper
end of the size distribution);
decrease in the counting efficiency of a CPC, as a result of
laser diode power degradation;
contaminations, such as room air leaking into the instrument
disturbing particle sources inside and outside of the
measurement station.
A second tool is the screening of the time series, spanning half a year or
more for the all-time maxima and minima. We look, for example, at the total
particle number and volume concentrations calculated from number size
distributions and review their time series and histograms for unusual
outliers. Individual values far outside the main frequency distribution are
then inspected more closely and, if judged as the result of technical faults,
deleted from the subsequent processing.
Data repository and formatEBAS (World Data Center for Aerosols)
Particle number size distributions and aerosol absorption coefficients from
all GUAN stations are stored at the World Data Center for Aerosols EBAS
database, where they are publicly available and free of charge . The GUAN data can be accessed by following the persistent identifier 10.5072/guan or
by visiting the EBAS user interface directly (http://ebas.nilu.no).
EBAS is a database infrastructure operated by NILU (Norwegian Institute for
Air Research), with basic funding secured through the EMEP protocol of the
Convention on Long-range Transboundary Air Pollution. EBAS handles, stores,
and disseminates atmospheric composition data generated by international and
national frameworks in long-term monitoring programmes and research projects
and has, for instance, been used by more than 50 European Union research
projects. Dedicated staff (ca. 10 persons) secure the reliability for
long-term data access.
Data integrity, interpreted as quality assurance and protection of the data
holdings, is secured by the following measures: each data submission to EBAS
undergoes manual quality assurance for the correctness of the accompanying
metadata, as well as an extra check of the data records for outliers and
implausible values. Errors are iterated with the data originators until the
submission passes quality assurance. Once inserted into the database, the
data are backed up together with the whole database at least twice per week
(two full and two incremental back-ups per week) at an off-site location.
With regard to copyright, any scientific user is free to download, copy,
distribute, transmit, and adapt the data sets as long as he/she gives credit
to the original authors (equivalent to the Creative Commons Attribution
License). Data users accept that they should make an offer of co-authorship
through personally contacting the data providers or owners whenever
substantial use is made of their data. It is desired that the data users
contact the data originators in order to create positive feedback and foster
scientific communication. In all cases, acknowledgement must be made of the
data originators and of the project name when these data are used within
a publication. illustrate the possibilities of
a cross-sectional study that makes extensive use of data stored at EBAS.
Data format
At EBAS, aerosol data are stored in three levels, reflecting an enhanced data
submission protocol for particle number size distribution and BC mass
concentration data. The objective of a three-level protocol is to enhance the
traceability of measurements and data processing, ranging from instrumental
raw data up to final hourly averages.
Level-0 data contain raw data, i.e. electrical particle mobility
distribution data and instrumental diagnostic parameters, as they are
measured directly by the instrument. The electrical particle mobility
distribution is provided as an array of measured particle number
concentrations vs. particle diameters (electrical particle mobility
for singly charged particles).
Level-1 data are scientifically correct particle number size
distributions after multiple-charge inversion and the correction for
particle losses. In the case of particle number size distributions,
the data refer to the original conditions during the measurement,
i.e. ambient pressure at the station, and typically 20 {}^{{\circ}}∘C
laboratory temperature. In the case of the aerosol absorption
coefficient, the data refer to standard temperature (273.15 \mathrm{K}K)
and pressure (1013.25 \mathrm{hPa}hPa) already. In any case, the time
resolution refers to the original instrumental time resolution.
Level-2 data are given as hourly averages and adjusted to a uniform standard
temperature (273.15 \mathrm{K}K) and pressure (1013.25 \mathrm{hPa}hPa). Moreover,
most of the particle number size distribution data sets have been re-binned
to the same standard set of 40 or 46 diameter channels (cf.
Sect. ). Level-2 data are presumed to be the main point
of interest to most data users. Level-2 data are available through the EBAS
user interface (see Supplement), while Level-0 and Level-1 data are available
on request from the administrators of the database.
Data are stored at EBAS in the NASA-Ames format, which is
based on the ASCII text NASA-Ames 1001 format but contains additional
metadata specifications ensuring instrumental documentation. For more
information on the format, we refer the reader to
and the EBAS website. An extract of NASA-Ames
1001 text is provided in the Supplement.
State of data submission
Although data are collected continuously, the body of GUAN data currently
requires some degree of manual quality control and semi-manual data post
processing. Data parcels encompassing half a calendar year are currently
submitted to EBAS at regular intervals. Feeding the data from all 17 GUAN
stations and also non-volatile particle number size distributions into EBAS
has not been completed and will require, especially in the case of some
retrospective data sets, additional time. It has been our first priority to
transfer all recent GUAN data spanning the years 2009–2014 into EBAS.
At the time of writing, data from 2015 have been published as well.
The Tables provided on GUAN's download page 10.5072/guan present the
current state of the data submission and availability. (A snapshot of that
page is also shown in the Supplement to this article.) The most
complete year of data availability is 2012, with particle number size
distributions available at 14 sites. The longest continuous time series are
available for particle number size distributions in Dresden-Nord and Melpitz.
It needs to be mentioned that certain data sets prior to 2009 were still
processed under previous data processing standards. This is a result of the
EBAS repository emerging from projects that go back to the early 2000s. These
older particle number size distribution data sets lack, particularly, the
corrections for the losses in the internal tubing of the mobility particle
size spectrometers. These losses come into effect mainly for particle
diameters below 30 \mathrm{nm}nm, and inconsistencies may occur when older and
modern data sets are directly compared. The older data sets are clearly
marked on the download page. It is planned that the old data sets will be
updated to modern standards.
Preliminary results
Preview of the mean annual cycles of total particle number
concentration (N_{{[20;800]}}N[20;800]) and eBC mass concentration. The data coverage
ranges between 2 and 6 years (see text and
Table ).
Mean values of total particle number concentration (N_{{[20;800]}}N[20;800])
and eBC mass concentration at 17 GUAN sites. The table indicates the
number of hourly samples nn, mean value \muμ, standard deviation \sigmaσ,
median (“med”), and the 95th percentile of the hourly statistics (“p95”).
Numbers are indicated at a precision of three significant digits. The
stations are sorted according the decreasing median of N_{{[20;800]}}N[20;800].
This section provides a brief overview of the data collected in GUAN so far.
Figure displays the total particle number concentration (diameter
range 20–800 nm) and eBC mass concentration at the various GUAN sites as a
function of the month of the year, while
Table indicates a brief overall statistics of
the two parameters based on hourly average values. The data coverage used in
this compilation ranged between 2 and 6 years: Annaberg-Buchholz – 3 years
(2012–2014); Langen – 3 years (2011–2013); Dresden-Winckelmannstr. and
Neuglobsow – 4 years (2011–2014); Leipzig-Mitte and Leipzig-West – 5 years
(2010–2014); and Augsburg, Bösel (Südoldenburg), Dresden-Nord,
Hohenpeißenberg, Leipzig-Eisenbahnstr., Leipzig-Tropos, Melpitz,
Mülheim-Styrum, Schauinsland, Waldhof, and Zugspitze (Schneefernerhaus)
– 6 years (2009–2014). The different periods of data coverage reflect,
above all, different operation periods at some stations.
Figure illustrates the mean concentrations at the various sites
but also their annual course. To facilitate the reading of the figure, the
legend has been arranged in descending order of mean concentration.
Observations at the roadside are distinct in that they yield an average
N_{{[20;800]}}N[20;800] above 9000\,cm{}^{{-3}}-3 most of the year, while urban
background and rural observations sites form a continuum of concentrations
below that region. Zugspitze (Schneefernerhaus) shows the lowest particle
concentrations, due to its altitude above 2500 m.
These findings are also reflected in Table ,
where the sites and values have been sorted according to descending median
concentrations of N_{{[20;800]}}N[20;800]. One can clearly see how the stations
cluster according to their station type. The concentrations at the stations
can therefore be arranged almost completely in the order roadside >> urban
background >> rural >> rural (mountain) >> Alpine mountain. Overall, the
GUAN sites provide a continuum of atmospheric observations over 1 magnitude
of median values in total particle number concentration and eBC mass
concentration. N_{{[20;800]}}N[20;800] spans a range between 900 and 9000 cm{}^{{-3}}-3,
while eBC ranges between 0.15 and 2.3 \mathrm{µ}µg cm{}^{{-3}}-3. One notable
exception in the sequence of stations is Bösel (Südoldenburg) —
classified as “rural”, which seems, however, influenced by sources of
particle number that are likely to be found in the adjacent village (cf.
location of the site in Fig. ).
The annual cycle of total particle number concentration yields only a weak
annual cycle, whereas this is not the case for the eBC mass concentrations
(Fig. ). For eBC, the monthly maximum concentration occurs between
October and March at the urban and rural lowland sites. The summer values
(May–September), in contrast, are significantly lower. One explanation is
that the atmosphere tends to be less vertically mixed in the winter, leading
to the trapping of pollution at the urban and rural lowland sites. Second,
eBC emissions are enhanced in wintertime due to the more intensive power and
heat generation. In summer, vertical dilution is a common atmospheric
phenomenon that will decrease the corresponding mean values. It is worth noting that in summer, the particle mass concentrations of eBC at rural
sites are very close to each other, suggesting rather uniform eBC
concentrations in the rural atmosphere.
A more elaborate interpretation of the GUAN data, notably the 2009–2014
section, is currently underway and in preparation for publication in a
separate paper.
Conclusions
The co-operative German Ultrafine Aerosol Network (GUAN) delivers atmospheric
particle number size distributions and equivalent black carbon
concentrations. Data are transferred to the World Data Center for Aerosols
repository EBAS (10.5072/guan) at regular intervals. These continuously
measured data are expected to provide the basis for a better scientific
understanding of sub-\mathrm{{µ}m}µm aerosol processes in the troposphere,
addressing questions related to both human particle exposure and
climate-relevant effects. Particle number size distributions down to
5 \mathrm{nm}nm allow us to study the emission and formation processes of
atmospheric ultrafine particles e.g.. Black carbon, on the
other hand, is an essential parameter that is linked to atmospheric
particulate light absorption . Number size
distributions of refractory particles (300 {}^{{\circ}}∘C) represent particle
cores that are of likely relevance for health studies and which have been
associated with harmful soot particles .
The selection of GUAN's measurement sites provides
a continuum of exposure situations from roadside sites, urban background
sites, and rural background sites to a high Alpine mountain site that spans
approximately 1 order in mean concentrations. In association with partner
projects, GUAN has implemented a high degree of harmonisation of
instrumentation, operating procedures, and data evaluation procedures. The
GUAN data have already proved suitable for the validation of atmospheric
dispersion and process simulations involving atmospheric aerosols. Examples
for their successful use include the validation of a global chemical
transport model , a regional-scale radiative transfer
model , a street canyon aerosol dynamics model
, and statistical prediction tools for particle number
size distributions in rural and urban environments .
The high standards of the GUAN data make them suitable for use in
cross-sectional air quality and health studies, particularly alleviating the
lack of health studies using BC and ultrafine particles as exposure
variables. In the future, the data set may also serve as an excellent basis
for the future discussion of legislative regulation of (ultrafine) particle
number and/or BC mass concentrations in ambient air.
Data availability
This manuscript refers to data from the German Ultrafine Aerosol Network (GUAN)
that are stored at the World Data Center for Aerosols EBAS database .
Data access is possible through the persistent identifier 10.5072/guan or
by visiting the EBAS user interface directly (http://ebas.nilu.no). When accessing the
data set through the persistent identifier (10.5072/guan), visitors will find detailed
information on data availability, data acquisition and processing.
Scientific users are free to download, copy, distribute, transmit, and adapt the data
sets as long as they adhere to EBAS' data policy, which involves, amongst other things, giving credit
to the original authors (cf. Sect. ).
The GUAN data set is not static in that the length of the data records is augmented at regular intervals by fresh,
quality-controlled measurements – currently every 6 months (cf. Sect. ).
This will lead to a considerably longer, continuous data set in the foreseeable future.
Correction formula for the aethalometer in Augsburg
At Augsburg, an aethalometer (Type 8100, Thermo Fisher Scientific Inc.) is
deployed using a cut-off of 2.5 \mathrm{{µ}m}µm. Similarly to the MAAP, the
Type 8100 aethalometer evaluates the attenuation of transmitted light,
although at a different wavelength (\lambda=880λ=880\mathrm{nm}nm) and using
a different mass absorption cross section (16.6 \mathrm{m^{2}{\,}g^{{-1}}}m2g-1). To
validate the aethalometer's performance, we compared the instrument in
Augsburg to a MAAP instrument during 1 month
(9 September–8 October 2008). The intercomparison suggested that the
aethalometer values are biased by a value around
++0.3 \mathrm{{µ}g{\,}m^{{-3}}}µgm-3 at the zero end, while the mean concentrations
during the period matched fairly precisely (MAAP:
1.89 \mathrm{{µ}g{\,}m^{{-3}}}µgm-3; aethalometer: 1.87 \mathrm{{µ}g{\,}m^{{-3}}}µgm-3). The
relation to determine standardised (i.e. MAAP-based) black carbon
concentrations from the aethalometer is as follows:
8224\text{BC}_{{\text{MAAP}}}=1.111\cdot\text{BC}_{{\text{aeth}}}-0.18.BCMAAP=1.111⋅BCaeth-0.18.
For aethalometer concentrations below 1.36 \mathrm{{µ}g{\,}m^{{-3}}}µgm-3,
a squared fit represents the data more accurately:
8224\text{BC}_{{\text{MAAP}}}=-0.033\cdot\text{BC}_{{\text{aeth}}}^{2}+1.335\cdot\text{BC}_{{\text{aeth}}}-0.43.BCMAAP=-0.033⋅BCaeth2+1.335⋅BCaeth-0.43.
Both fits show a high measure of determination (R^{2}R2) of 0.96.
The Supplement related to this article is available online at doi:10.5194/essd-8-355-2016-supplement.
Acknowledgements
The establishment of GUAN was made possible by the German Federal
Environment Ministry (BMU) grants F&E 370343200 (German title:
“Erfassung der Zahl feiner und ultrafeiner Partikel in der
Außenluft”), 2008–2010, and F&E 371143232 (German title:
“Trendanalysen gesundheitsgefährdender Fein- und
Ultrafeinstaubfraktionen unter Nutzung der im German Ultrafine
Aerosol Network (GUAN) ermittelten Immissionsdaten durch
Fortführung und Interpretation der Messreihen”), 2012–2014. We
acknowledge additional funding within the infrastructure projects
EUSAAR (European Supersites for Atmospheric Aerosol Research; EU FP6
contract RII3-CT-2006-026140), and ACTRIS (Aerosols, Clouds, and Trace gases Research InfraStructure Network; EU FP7 grant
262254). WCCAP was funded by the German Federal Environment Agency
(UBA) grant FKZ 35101086 (The World Calibration Centre for Aerosol
Physics within WMO-GAW), 2012–2014. We acknowledge funding by the
LfULG grants UFP 2011 (German title: “Qualitätssicherung der
Messung ultrafeiner Partikel in Außenluft 2011”) and UFP 2012
(“Aufwandsreduzierung in der Qualitätskontrolle der Messung
ultrafeiner Partikel in der Außenluft im Luftgütemessnetz
Sachsen”). Measurements at Annaberg-Buchholz were supported by the
EU-Ziel3 project UltraSchwarz (German title: “Ultrafeinstaub und
Gesundheit im Erzgebirgskreis und Region Usti”), grant
100083657. Measurements at Dresden-Winckelmannstrasse were co-funded
by the European Regional Development Fund Financing Programme
Central Europe, grant No. 3 CE288P (UFIREG). Measurements in
Augsburg were financed by the Helmholtz Zentrum Munich and partly by
UFIREG (see above). IUTA e.V. acknowledges funding by the State
Agency for Nature, Environment, and Consumer Protection North
Rhine–Westphalia (LANUV), grant title: “Untersuchung ultrafeiner
Partikel zur räumlichen und zeitlichen Verteilung im
Ruhrgebiet”. Individual credits for TROPOS personnel: Thomas Tuch
and Achim Grüner (TROPOS station Melpitz), Stephan Nordmann
(MAAP measurements), and Johanna Rehn (map generation). We thank
Horst-Günther Kath (State Dept. for Environmental and
Agricultural Operations in Saxony, Betriebsgesellschaft für
Umwelt und Landwirtschaft – BfUL), Andreas Hainsch and Dirk Haase
(Labour Inspectorate of Lower Saxony, Staatliches
Gewerbeaufsichtsamt Hildesheim – GAA), and Dieter Gladtke (LANUV),
who made the GUAN measurements possible at the observations sites of
their air quality networks.Edited by:
A. Kokhanovsky
Reviewed by: two anonymous referees
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