A total of 10 years of hourly aerosol and gas data at four rural German stations have been combined with
hourly back trajectories to the stations and inventories of the European Emissions Database for Global
Atmospheric Research (EDGAR),
yielding pollution maps over Germany of PM10, particle number concentrations, and
equivalent black carbon (eBC). The maps reflect aerosol emissions modified with atmospheric
processes during transport between sources and receptor sites. Compared to emission maps, strong
western European emission centers do not dominate the downwind concentrations because their
emissions are reduced by atmospheric processes on the way to the receptor area. PM10,
eBC, and to some extent also particle number concentrations are rather controlled by emissions
from southeastern Europe from which pollution transport often occurs under drier conditions. Newly
formed particles are found in air masses from a broad sector reaching from southern Germany to
western Europe, which we explain with gaseous particle precursors coming with little wet scavenging
from this region.
Annual emissions for 2009 of PM10, BC, SO2, and NOx were
accumulated along each trajectory and compared with the corresponding measured time series. The
agreement of each pair of time series was optimized by varying monthly factors and annual factors
on the 2009 emissions. This approach yielded broader summer emission minima than published values
that were partly displaced from the midsummer positions. The validity of connecting the ambient concentration and
emission of particulate pollution was tested by calculating temporal changes in eBC for subsets of
back trajectories passing over two separate prominent emission regions, region A to the northwest
and B to the southeast of the measuring stations. Consistent with reported emission data the
calculated emission decreases over region A are significantly stronger than over region B.
Introduction
Atmospheric aerosol is known to influence the Earth's radiation budget because it directly
scatters and absorbs solar radiation (Schwartz, 1996; Bond et al., 2013) and acts as cloud
condensation nuclei, thus modulating the optical properties and lifetimes of clouds (Twomey, 1974;
Penner et al., 2004). In many regions of the globe that underwent industrialization early on,
anthropogenic aerosol concentrations are currently in decline (Leibensperger et al., 2012; Zanatta
et al., 2016). With respect to declining concentrations and emissions, Samset al. (2018) suggest
that removing present-day anthropogenic aerosol emissions, assuming constant greenhouse gas
emissions, could lead to a global mean surface heating as high as 0.5–1.1 ∘C.
Besides climate, atmospheric aerosol has been acknowledged to influence human health through
respiratory and cardiovascular health end points (Anderson et al., 2012). Lelieveld et al. (2015)
quantified the worldwide burden of disease (premature mortality) due to outdoor pollution, a large
part of which was attributed to airborne particulate matter. It is apparent that the distribution of
adverse health effects is very uneven among the worldwide population, depending on the local level
of outdoor pollution.
In view of the described human-driven effects it seems imperative to develop instruments to reliably
monitor changes in anthropogenic aerosol concentrations as well as an understanding of the balance
between aerosol sources and measured concentrations. Researchers have strived to obtain a spatial
picture of the distribution of pollutants and to achieve a connection between the sources of
pollution and concentrations downwind. A widely used method has been the extrapolation of
concentrations measured in one or several locations into two-dimensional space through the use of
meteorological dispersion approaches: the first maps of particulate air pollutants over Europe were
constructed in the 1970s with the help of coarse emission data and simple trajectory models
(Eliassen, 1978). Statistical methods were developed to connect pollution sources and ensuing
aerosol concentrations at receptor sites (Miller et al., 1972; Friedlander, 1973; Cass and McRae,
1983). By combining statistics with back-trajectory data sectorial information about sources
controlling the composition of the aerosol over southern Sweden was derived by Swietlicki et
al. (1988). Later the approach of using back trajectories to map aerosol sources was refined by
Stohl (1996) and tested with 1-year sulfate data from the cooperative program for monitoring and
evaluation of the long-range transmission of air pollutants in Europe (EMEP,
http://www.emep.int, last access: 18 September 2020). In a similar approach with 5 years of aerosol data from a single
Siberian receptor site, Heintzenberg et al. (2013) identified potential source regions over Eurasia
with aerosol data from four Swedish icebreaker expeditions over the central Arctic (Heintzenberg
et al., 2015). Charron et al. (2008) constructed concentration field maps to identify the source
regions of specific types of aerosol particle size distributions arriving in England. All these
works share the approach that time-dependent information on concentrations measured at receptor
site(s) is transformed into space, thus allowing for conclusions on the potential source regions of
gaseous and/or particulate emissions.
With more comprehensive air quality models, concentrations of specific
aerosol were mapped over Europe together with short temporal developments
(e.g., Schell et al., 2001). For specific episodes high-spatial-resolution aerosol concentration maps in urban and nonurban
European areas have been generated with sophisticated chemistry transport
models (e.g., Beekmann et al., 2015; Riemer et al., 2004; Wolke et al.,
2004). For the years 2002 and 2003 Marmer and Langman (2007)
analyzed the spatial and temporal variability of the aerosol distribution
over Europe with a regional atmosphere–chemistry model. They found that
meteorological conditions play a major role in spatial and temporal
variability in the European aerosol burden distribution. Regionally, the year-to-year variability of modeled monthly mean aerosol burden reached up to
100 % because of different weather conditions.
In the present study 10 years of hourly aerosol data at four German
stations were available for the identification of potential source regions.
As it appears unrealistic to analyze such a large database with advanced
chemical transport models we resorted to the well-proven approach of
utilizing back trajectories as cited above and connected the results to
emission fields. We define the resulting concentration maps of particulate
and gas parameters as ambient pollutant concentration maps because they represent long-term
average emissions of air pollutants modified by the controlling atmospheric
processes along the pathways to the receptor sites. In Charron et al. (2008) this approach is termed the “concentration field map
method”. With a much larger dataset spanning a much tighter network of
1500 stations, Rohde and Muller (2015) used the kriging
interpolation approach (Krige, 1951) to construct air
pollution maps over China. Another approach to constructing pollution maps over
the province Henan, China, was used by Liu et al. (2018). They
combined an emission inventory with chemical modeling and back trajectories
to derive high-resolution maps of particulate and gaseous pollution
components and find that emissions from neighboring provinces are important
contributors to local air pollution levels.
Recent political, economic, and technological developments in Europe have
caused substantial changes in the emission of air pollutants. Lavanchy et
al. (1999) deduced a trend in atmospheric black carbon
from preindustrial times to 1975. Strong downward trends in major aerosol
components before and after the German reunification (1983–1998) over rural
East Germany were reported by Spindler et al. (1999). For the
years 2003–2009 Kuenen et al. (2014) published
trends in the development of aerosol emissions as elaborated from reported
emissions. The German Environmental Agency (GEA) publishes trends in air
pollution as measured at a number of ca. 380 federal and state air quality
stations (Minkos et al., 2019). According to these records,
PM10 mass concentrations declined by approximately 25 % over the
period 2000–2019.
Combining long-term aerosol and gas data at the four stations in the present
study provides an excellent database for identifying both the most important
source regions and possible temporal changes. During the 10 recent years
covered by our data we expected noticeable systematic changes in our time
series that can be interpreted in terms of emissions. As a side result in
the process of deriving long-term emission trends of major air pollutants
over Germany, information on the monthly disaggregation of annual aerosol
emissions can be derived.
Aerosol and trace gas data
The core data for the present study have been measured at the stations
Melpitz (ME), Neuglobsow (NG), and Waldhof (WA) of the German Ultrafine
Aerosol Network (GUAN) network (Birmili
et al., 2016) and at Collmberg (CO) station operated by the Saxonian
Environment Agency. These four rural background stations lie in the
northeastern lowlands of Germany at distances between 30 and 205 km from
each other. The 10-year average particle mass concentrations under 10 µm
particle diameter (PM10) and their standard deviations at the four
stations are rather similar: 15±13, 22±12, 14±10, and
15±11µgm-3 at CO, ME, NG, and WA, respectively. The
corresponding long-term average particle number concentrations between 10
and 800 nm particle diameter (N10-800) and their standard deviations at
the three GUAN stations are 5400±4100, 3600±2300, and
4300±2800cm-3, respectively. Basic statistics on particle
number and equivalent black carbon (eBC) mass concentrations of the three GUAN stations were presented
in Sun et al. (2019), whereas details
about instrumentation and their maintenance can be found in Birmili et al. (2016). The ensemble of hourly data at
the four stations is the basis of the pollution maps derived in this work.
TROPOS-type mobility particle size spectrometers (MPSSs; Wiedensohler et al., 2012) were used to
record particle number size distributions across the particle size range 10–800 nm. Quality
assurance of the long-term measurements followed the recommendations of Wiedensohler et al. (2018),
including weekly inspections as well as monthly and annual maintenance intervals. Once a year the
MPSSs were intercompared against a reference MPSS from the WCCAP (World Calibration Center for Aerosol
Physics) on-site and/or at the calibration facility. The lower detection limit of the MPSS is
around 30 cm-3 for a time resolution of 30 min. Equivalent black carbon (eBC) was
determined by multi-angle absorption photometers (MAAPs) using a mass absorption cross section of
6.6 m2g-1 (Petzold et al., 2013; Nordmann et al., 2013; Birmili et al., 2016). An
intercomparison of multiple MAAP instruments resulted in an inter-device variability of less than
5 % (Müller et al., 2011). While the MAAP deployed at the TROPOS station Melpitz was
biannually compared to the reference absorption photometer at the WCCAP in Leipzig, the instruments
at the UBA stations Waldhof and Neuglobsow were serviced by the manufacturer. For hourly
measurements of PM10, continuous oscillating microbalances (Thermo Scientific TEOM 1400)
were utilized at stations CO, NG, and WA. At station ME PM10 was determined in daily
filter samples (00:00 to 24:00 CET; Spindler et al., 2013). The TEOM1400 instrument and gravimetric
filter sampling are different methods for particle mass concentrations. The TEOM collects
particulate mass on a vibrating substrate (tapered element) and registers the change in the
oscillation frequency that is decreasing with mass loading (Patashnick and Rupprecht, 1991). The
TEOM operates at a constant temperature setting above ambient (typically 30–50 ∘C)
to prevent contraction and expansion of the tapered element and reduce interferences from water
vapor condensation. However, heating the ambient air enhances volatilization of particle-bound
semivolatile compounds (e.g., ammonium nitrate and some organic species), resulting in an
underestimation of PM when semivolatile material dominates the particulate phase during cold
seasons. The condensation and evaporation of ammonium nitrate and organic species can also influence
the filter sampling under ambient conditions. Here the effect can be partly balanced by the
temperature variation during the daily filter sampling. However, the results of both methods are mostly
in good agreement (e.g., Zhu et al., 2007).
Hourly aerosol data from the three GUAN stations during 2009–2015 (NG ≥ 2011) have been
utilized in a previous study (Heintzenberg et al., 2018) to understand aerosol processes during air
mass transport between the stations. In the present study the dataset was enlarged to include the
additional station Collmberg and data at all stations from the year 2016 through 2018. The integral
aerosol parameters particle number concentration (N10-800, cm-3), light-absorption-equivalent mass concentration of black carbon (eBC, µgm-3), and particle
mass concentrations under 10 µm particle diameter (PM10,
µgm-3) were utilized. N10-800 is based on the integral over measured particle
size distributions from 10 to 800 nm.
NOx and SO2 emitted by anthropogenic combustion processes are
transformed in the atmosphere and add to the anthropogenic aerosol. At the three GUAN stations both
are measured with the same temporal resolutions as the aerosol data. Additionally, at Collmberg
NOx data could be utilized in the interpretation of the aerosol data. The trace
gas analyzers for NOx and SO2 were calibrated with test gases for NO
(NO in N2) and SO2 (SO2 in N2, both Air Liquide,
Germany). NO2 was produced in a gas-phase titration device (GPT APMC370, Horiba, Germany)
by quantitative oxidation of NO test gas (Rehme, 1976). The trace gas analyzers were used in an
optimal range, and all registered values (also below the detection limit) were used for this
long-term study. As is the case for most particle numbers in polluted continental environments, tropospheric ozone is
a secondary atmospheric pollutant. We utilized hourly ozone data taken at all four stations
throughout the studied time period as ancillary information in the discussion of particle-number-related results. For the ozone measurements a common trace gas ozone monitor was used (Horiba
APOA-350). This device quantifies tropospheric ozone by UV absorption and uses the cross-flow
modulation principle. Ambient air with and without ozone (elimination by a selective scrubber) was
used alternatively in the measuring cuvette, yielding a very stable ozone signal. The devices were
calibrated using an ozone standard (Ozone Calibrator, Thermo Environmental Instruments 49PS).
Characteristics of the four stations in the present study; see the text
for instrumental details. The number of validated data hours is given for
each component.
Table 1 gives an overview of the instrumental characteristics of all stations and the total number
of validated data hours for each utilized component. The minimum is 57 962 h for validated
MPSS data at the three GUAN stations, and the maximum is 88 838 validated data hours for
NOx at all four stations. Strictly concurrent numbers (by the hour) are less validated
data hours. For MPSS, eBC, and SO2 data at the GUAN stations, these numbers are 48 533, and
48 114, and 47729 h, respectively, for PM10 and NOx data at all
four stations. However, these reduced strictly concurrent numbers do not substantially affect the
10-year average maps discussed below.
Back trajectories
With the HYSPLIT4 model (Stein et al., 2015) and based on the meteorological fields from the Global
Data Assimilation System with 1∘ resolution (GDAS1,
https://www.emc.ncep.noaa.gov/gmb/gdas/, last access: 18 September 2020), three-dimensional trajectories were calculated
arriving every hour at a height of 500 m above ground level at the four stations. The
trajectories were calculated backward for up to 5 d using the meteorological fields from the
server at the Air Resources Laboratory (ARL), NOAA (http://ready.arl.noaa.gov, last access: 18 September 2020), where more
information about the GDAS dataset can be found. In the pollution maps constructed with extrapolated
measurements at the stations and in any comparisons with emissions along the back trajectories, only
trajectory points under 1000 m of altitude above the ground were utilized. Turbulent atmospheric
mixing is included in parameterized form in HYSPLIT4. The present study utilizes the default version
of this parameterization according to Draxler and Hess (1998). The back trajectories are calculated
with the base version of HYSPLIT4 that does not include any specific dispersion and scavenging of
atmospheric trace substances. Precipitation along the trajectories was used in the interpretation of
the pollution maps. The precipitation values mapped in the present study and the temperature values
used in the trend discussion of N10-800 are those listed by HYSPLIT4 at each point of a
trajectory. They are meteorological parameters at the nearest grid cell of the assimilated global
meteorological fields provided by the US National Weather Service's National Centers for
Environmental Prediction (NCEP) (Kanamitsu, 1989). Average horizontal wind speeds between two
1 h trajectory steps were calculated from the distance covered by a trajectory between two
successive steps. With the 350 593 hourly back trajectories from the four stations the time series of
N10-800, PM10, and eBC were extrapolated over Germany and part of the neighboring
countries. At Melpitz PM10 data were only available as daily averages. Thus, the daily
average concentrations were extrapolated along each hourly trajectory of the respective day.
Emission data
For the interpretation of the pollution maps we used the emission dataset version 4.3.2 for 2009 of
the component particle mass concentrations below 10 µm (PM10), BC,
NOx, and SO2 as compiled in the Emissions Database for Global
Atmospheric Research (EDGAR, https://edgar.jrc.ec.europa.eu/overview.php?v=432_AP, last access: 18 September 2020, DOI
10.2904/JRC_DATASET_EDGAR, Janssens-Maenhout et al., 2011). This dataset concerns primary
emissions only and has been introduced by Crippa et al. (2018). All human activities, except large-scale biomass burning and land use, land use change, and forestry, are included in the database. Emissions of coarse particles from agricultural surfaces are not included. They are, in fact,
very sensitive to soil and weather conditions and thus not trivial to quantify. Primary aerosol
emission data are generally characterized by rather high uncertainties. For EDGAR
Crippa et al. (2018) report a range of variation in 2012 between 57.4 % and 109.1 % for
PM10 and between 46.8 % and 92 % for BC. Even higher uncertainties in PM
emissions might come from super-emitting vehicles that are not considered in this database (Klimont
et al., 2017). In our maps and trend calculations we applied the grid values of emission data that
were listed in the EDGAR inventories no more than 30 km away from any trajectory time step.
Results and discussionAerosol concentration maps (pollution maps)
The trajectory-extrapolated N10-800, PM10, and eBC from the four stations yielded
pollution maps averaged over the period 2009–2018, which are collected in Figs. 1 and 2. Both the
particle-number-related N10-800 and the particle-mass-related PM10 and eBC exhibit
systematic seasonal variations. Most events of new particle formation (NPF) over the continents
occur during the photochemically active summer months (Kulmala et al., 2004), whereas the
particle-mass-related aerosol parameters due to combustion processes exhibit the highest concentrations
during the winter months (Matthias et al., 2018). Consequently, we constructed two maps for each
discussed component: one of averages over the months April through October and one of averages over
the months November through March. Only map cells with at least 300 trajectory hits are
discussed. Interpreting these hits in terms of Poisson statistics would then yield a maximum
uncertainty of 5.8 % per cell. In terms of a Gaussian statistic the arithmetic cell averages
displayed in the maps exhibit standard deviations of cell averages that are less than 6 %.
Maps of particle number concentration N10-800 (cm-3)
extrapolated under 1000 m of height along 5 d back trajectories from
hourly data at the four stations from 2009 to 2018; left: months April
through October; right: months November through March. The GUAN stations are
marked with colored diamonds. The Collmberg station lies 30 km southeast of
Melpitz station. Here and in the following maps the black dots represent
cities larger than 100 000 inhabitants, with the size of the dots being
proportional to the number of inhabitants.
As Fig. 1 but for particle mass concentrations (top, PM10;
µgm-3) and black carbon concentrations (bottom, eBC;
µgm-3).
The maps of N10-800 in Fig. 1 show distributions of air masses over Germany and adjacent
countries related to particle numbers instead of particulate mass. There are two arguments for
showing maps of number-related results. First, particle number concentrations are connected with
cloud processes, their formation (Pruppacher and Klett, 1978), radiative effects, e.g., albedo
(Twomey, 1974), and precipitation (Li et al., 2011). Second, in the area of aerosol health issues,
ultrafine particles (<100 nm diameter) have been gaining attention in recent years
(Wichmann and Peters, 2000); i.e., an increasing number of health effects is attributed to
particle number rather than to particle mass. The fact that NPF events occur concurrently in or near the
top of the continental planetary boundary layer over wide geographical regions (e.g., Wehner et al.,
2007) is partly due to concurrent advantageous photochemical conditions allowing for the formation
of condensable vapors, in particular global radiation (Birmili et al., 2001). Two other factors
constraining NPF are the availability of gaseous particle precursors and the concurrent preexisting
aerosol.
The summer map (4-10) of N10-800 exhibits high values in the
southwest to northeast sector of the map. The highest values are concentrated in a belt reaching from
Burgundy through Switzerland, southern Germany, and the Czech Republic to southwestern
Poland. Interestingly, this belt of high N10-800 is collocated to large extent with a belt of
high summer ozone concentrations (see Fig. S1 in the Supplement). This photochemically controlled
pollutant (Monks et al., 2015) exhibits the highest summer concentrations in air masses from
southwestern Poland and the northern Czech Republic, a region from which high ozone values are reported
(Struzewska and Jefimow, 2013; Hůnová, 2003; Hůnová and Bäumelt, 2018). However,
the summer map of N10-800 does not show the highest values in air masses from the region with the
highest ozone pollution. High particle numbers in air masses coming over the Alps from northern
Italy may be related to the high emissions of air pollutants in the Po Valley that are known to
be reached frequently through so-called alpine pumping (Winkler et al., 2006; Lugauer and Winkler, 2005;
Reitebuch et al., 2003) over the mountains. The high NOx concentrations in air
masses from northern Italy in both the summer and winter maps (see Fig. S2 in the Supplement) indicate
that pollution from south of the Alps can even reach northeastern Germany. In the winter map of
N10-800 (11-3 in Fig. 1) the belt of highest summer values is apparently complemented by more
transalpine pollution transport and by transport from the southeast. The lower photochemical
activity in winter is reflected in the lower winter ozone concentrations in Fig. S1 in the
Supplement, although transalpine pollution transport is still visible in the winter map of
NOx in Fig. S2 in the Supplement. Northwestern Italy also shows up as an emission
hot spot in the maps of trajectory-summed emissions in Fig. S4 in the Supplement.
In both summer and winter the maps of PM10 and eBC in Fig. 2 exhibit a clear
northwest-to-southeast structure with the cleanest sector being in the northwest, covering the
coastal area of the North Sea and the BENELUX countries Belgium, the Netherlands, Luxembourg, and
northwestern Germany. The strongest contrast between the cleanest northwesterly and the most
polluted southeasterly map sectors is seen in the winter map of eBC. The highest average concentrations
are measured in air masses from the southeastern half of the map, most strongly expressed in
PM10 and eBC with maxima in a region leading from southwest Poland through the Czech
Republic, Slovakia, Austria, and former Yugoslavia to northeastern Italy. The back trajectories in
the southeastern sector of the maps for PM10 and eBC point towards countries in which
emissions of air pollution in the past 20 years developed very differently compared to those in
western Europe. According to the European Environment Agency
(https://www.eea.europa.eu/data-and-maps/dashboards/air-pollutant-emissions-data-viewer-2, last access: 18 September 2020)
parts of western Europe experienced a strong and nearly monotonous decrease in emissions of
PM10, whereas the emissions in Poland, the Czech Republic, Slovakia, Austria, former
Yugoslavia, and Italy stayed nearly constant or even increased in recent years after dramatic
decreases in the course of political developments of the 1990s. The seasonal maps of
combustion-derived SO2 in Fig. S3 in the Supplement look very similar to the those of the
particle-mass-related maps of PM10 and eBC, again with the strongest NW–SE contrast visible in
winter.
Pollutant emissions and atmospheric processes
In Fig. 3 annual average emissions of PM10, BC, SO2, and NOx
are mapped for 2009 according to EDGAR. Except for the
absolute numbers, the maps for SO2 and NOx look rather similar to
those for particulate emissions. They all emphasize highly populated and
industrialized emission centers. Beyond that, the SO2 map accentuates
individual large combustion sources such as conventional power plants.
Whereas the strong emissions in northern Italy are seen in the maps of
PM10, BC, and NOx, emissions in the countries in the southeastern
sector of the maps by no means reflect the high concentrations of
particulate components seen in the pollution maps in Figs. 1 and 2.
As Fig. 1 but (a) for PM10 emissions (t(0.1×0.1∘)-1yr-1), (b)
BC emissions, (c)SO2 emissions, and (d)NOx emissions
(t(0.1×0.1∘)-1yr-1) according to EDGAR
(https://doi.org/10.2904/JRC_DATASET_EDGAR) for 2009 averaged over the geogrid of the
present study.
The seeming discrepancy between the pollution maps in Figs. 1 and 2 and the
emission maps in Fig. 3 can be resolved. For that purpose, the
EDGAR emissions of PM10, BC, SO2, and NOx along all 350 593
hourly back trajectories to the four stations during the 10 studied years
were summed up. Then the sums were extrapolated back along each trajectory.
In Fig. S4 in the Supplement 10-year average maps of these extrapolated emission sums are
displayed. As in Fig. 3, except for the absolute numbers, there is a strong
similarity between the four mapped component sums. Because of the integral
nature of the mapped results one cannot expect the maps in Fig. S4 to correctly locate
specific emission centers. However, they certainly indicate the
map sectors from which the most substantial emissions could have reached the
stations. As in Figs. 1 and 2 the southeastern sectors of the maps of
integrated emissions most prominently show up. Interestingly, the maps in
Fig. S4 in the Supplement also indicate the highly polluted region of northwestern Italy
(Diémoz et al., 2019a, b). Emissions from
the emission centers in northwestern Europe are hardly discernible in Fig. S4 in the Supplement. They do show up (most strongly in Fig. S4c in the Supplement for SO2 emission sums)
as apparent emissions over the adjacent North Sea. We interpret the
“misplaced” emissions over the North Sea as air mass transport from the
North Sea via the emission region in the BENELUX countries to the receptor
sites that was not compensated for by other low-pollution air transport from the
North Sea to the stations that had not passed over the northwestern European
emission centers.
(a) Map of horizontal wind speed (u, kmh-1) as reported by
HYSPLIT along hourly 5 d back trajectories to the four stations marked
in the graph averaged over the time period 2009 to 2018; (b) as (a) but for
precipitation (RR, mmh-1).
Two major atmospheric processes will reduce the concentrations of emitted or
in situ formed aerosol particles: dilution through mixing with cleaner air
masses and wet scavenging through in-cloud and sub-cloud processes. As a
tracer of the first of these two processes, Fig. 4a gives the long-term
average geographical distribution of trajectory-derived wind speed over the
study area. The highest average wind speeds and ensuing atmospheric mixing are
seen over the major emission centers of northwestern Germany, the BENELUX
countries, and adjacent seas, whereas the lowest wind speeds are seen over
northern Germany and the southeastern neighboring countries. The long-term
average geographical distribution of precipitation as taken by HYSPLIT from
the GDAS meteorological fields in Fig. 4b corroborates the results for
atmospheric cleaning processes indicated in Fig. 4a. The small absolute
numbers in Fig. 4b are due to the episodic nature of precipitation: most of
the time it does not rain or snow. The blue crescent reaching from the North
Sea through the BENELUX countries, eastern France, Switzerland, and the
alpine region exhibits maximum precipitation values, while southern and
eastern Germany with the adjoining countries to the east and southeast show
minimum precipitation values. Thus, in the long term we expect much of the
high western European emissions to be substantially scavenged by wet
processes. In addition, air masses arriving from western and northwestern
directions at the stations usually cross the western European emission
centers with much lower pollution burdens than air masses coming from the
polluted countries of southeastern Europe arriving at the corresponding map
borders (see the figure labeled PM10 – 36th maximum daily average value (µgm-3) for 2005; EEA, 2009).
Pollution trends for air from specific source regions
As mentioned in the Introduction, the pollutant emissions reported by European and national
environment agencies represent a synthesis of known pollutant sources combined with assumed emission
factors. These emissions are typically used as input for air quality modeling and subsequent
assessment, as well as for trend analyses. However, it remains unclear to what extent these reported
emissions are realistic and whether their trends represent the trend in true emissions. Here, we
attempt to assess spatially resolved trends in real particulate emissions by an analysis of measured
concentrations (pollution) in air masses traveling over source-specific regions.
To test our method, we selected two pronounced source regions in Europe located within
1000 km of distance from our observation sites. These regions were defined by emission hot-spot
regions that can be seen in the EDGAR emission maps in Fig. 3a and b: region A
(Be-NL-NRW; comprising most of Belgium, southern parts of the Netherlands, and much of the German
state North Rhine-Westphalia) and region B (CZ-PL-SK; comprising the central parts of the Czech
Republic, southern parts of Poland, and adjacent areas of Slovakia). According to the European
Environment Agency (EEA) these are regions where reported particulate emissions have developed
differently during the past 10 years. Our goal is to verify this through an analysis of real
atmospheric observations over this period.
Temporal trends were computed using the customized Sen–Theil trend estimator (Sen, 1968; Theil,
1992). The Sen–Theil estimator is the median of many slopes calculated in a continuous or
noncontinuous time series, with its robustness against outliers being one of its main assets. For
a detailed description of this trend estimator we refer to Sun et al. (2020, Sect. 2.3.1). Here we
computed the Sen–Theil estimator for hourly observation data at stations ME, NG, and WA. Subsets of
back trajectories were selected that spent at least 1, 3, 6, or 12 h over source regions
A and B. Depending on that criterion, different subsets were analyzed. The difference in median eBC
mass concentration between air masses arriving from source region A and B is obvious, as could
already be determined in the corresponding pollution maps (Fig. 2c and d). As we learned from
Sect. 5.2 these pollution maps are strongly influenced by the different meteorological conditions
governing atmospheric dispersion in different wind directions, so these values allow no direct
conclusion on the strength of emission sources located upwind.
Median concentrations of eBC (µgm-3)
and temporal trends (2009–2018) of eBC in terms of Sen–Theil slope
(%yr-1) as determined for air masses passing over
regions A and B as analyzed at the stations Melpitz (ME), Neuglobsow
(NG), and Waldhof (WA). For comparison the national annual decreases
in BC emissions for 2009–2017 in percent according to the European
Environmental Agency are added.
DELTANo. of back Median eBC Sen–Theil slope Decrease in national BC T1trajectories (µmm-3) (%year-1) emissions (%yr-1) (hours)MENGWAMENGWAMENGWAThree stations2121 94117 51427 2180.380.400.41-6.40-6.80-4.80-5.85BelgiumNetherlandsGermanyRegion A318 60514 26822 1320.380.400.41-6.40-6.90-4.80-5.89-6.1%-6.1%-4.9%B-NL-NRW614 80210 08615 9360.390.400.42-6.40-7.60-5.10-6.19126817374661310.400.500.50-7.10-7.90-5.30-6.62111 096526441911.101.191.13-3.60-3.40-1.70-3.16Czech Rep.PolandSlovakiaRegion B39601433935411.081.181.12-3.40-3.40-2.10-3.14-2.8%0.5 %-2.3%CZ-PL-SK67000306225701.051.091.11-4.00-2.90-2.70-3.47123628141012771.001.001.00-3.70-3.00-2.70-3.34ALL85 84675 19078 3560.450.360.36-5.90-5.60-4.00-5.18Sun (2020)-4.40-7.80-3.20
1 Minimum time spent over the specified source region.
2 Weighted mean according to the available number of back trajectories.
We analyzed the temporal trends in eBC over the period 2009–2018 for the subsets belonging to
regions A and B – assuming that these systematic differences in meteorological conditions should
even out over such long observation periods. Table 2 shows that the Sen–Theil slope estimator for
region A is between -7.6% and -5.1% for the three observation sites and the
requirement of a back trajectory to have spent at least 6 h over region A. For region B, the
corresponding Sen–Theil slope estimators are between -4.0% and -2.7% for the
observation sites. As we can read from these results, the annual decrease in eBC is more pronounced
for air masses that have traveled over region A.
Percental decreases in anthropogenic emissions of PM10, BC,
SO2, and NOx relative to 2009 as reported by the European
Environment Agency (EEA,
https://www.eea.europa.eu/data-and-maps/dashboards/air-pollutant-emissions-data-viewer-2),
the German Environment Agency (GEA), and calculated in the present study.
The EEA and GEA only report data until 2017.
Between 2009 and 2017 for the EU member states of Belgium, the Netherlands, Germany, the Czech
Republic, Poland, and Slovakia the annual rates of decrease in reported emissions were between
-4.9% and -6.1% for the first three countries and between +0.5% and
-2.8% for the latter three
(https://www.eea.europa.eu/data-and-maps/dashboards/air-pollutant-emissions-data-viewer-2).
As compiled in Table 3 these reported trends are largely consistent with the rates of change
derived from our eBC pollution trends. Although we need to keep in mind that the six nations
only partially contribute to our regions A and B, it seems valid to conclude that BC emissions in
region A indeed decreased more rapidly in the past decade compared to region B. Our approach seems
able to differentiate between concentration trends in air masses that have passed over rather
different source regions. This might represent a step towards the assessment of changes in
real-world emissions allocated in specific source regions over multi-annual periods.
Comparison of pollution and emission trends
Besides the map comparison a second approach was used to connect emission data with the measured
aerosol time series. Along each of the hourly back trajectories the emissions according to EDGAR
were summed up. Then monthly medians of the emission sums and the measured parameters were
formed. EDGAR reports annual average emissions. PM10, black carbon, and other
combustion-related air pollutants show substantial annual variations, with high winter and low summer
values at nonurban sites (e.g., Heintzenberg and Bussemer, 2000). In emission modeling the temporal
variation of annually reported emissions is considered by disaggregating the annual values with
monthly, weekly, and daily factors (Matthias et al., 2018). For the time-resolved comparison of
PM10 and BC emissions with PM10 and eBC concentrations at the GUAN sites, monthly
medians of PM10 and eBC values at the stations were formed and plotted in Fig. 5. We
expected both seasonal variations and a long-term trend in the emissions. For M hours per month
of measured components at the four stations the annual average EDGAR emissions
EPM10,EBC,ESO2, and ENOx were summed up along the 121
trajectory steps leading to the stations. Then monthly medians Ẽi=1,4 were formed
according to Eq. (1) (exemplified for BC). Medians were chosen to reduce the effect of outliers due
to local emission and scavenging events.
ẼBC=Median∑n=1121EBCm=1,M
The monthly median emission sums Ẽi=1,4 were modified with a monthly (fm) and an
annual factor (gy) in order to simulate respective median monthly measured concentrations taken
over all stations. Thus, for each component 12 monthly and 10 annual trend factors determined the
agreement of modified summed emissions and measured concentrations. As an objective or utility function,
χ2, the sum of squared deviations between annually and monthly modified emission sums and
monthly median measured concentrations, was formed taken over the 120 months of the present study
(exemplified for BC in Eq. 2).
χBC2=∑j=1120fm=1,12×gy=1,10×ẼBC-eBC2χ2 was minimized with a generalized reduced gradient (GRG) solver (Lasdon et al., 1978) that
optimized the 12 monthly and 10 annual factors for each of the four measured components. We used
Excel's® implementation of the GRG solver procedure for the
optimization. After optimizing month and trend factors the average relative deviation between
emission-simulated and measured monthly median curves is 14 %, 21 %, 25 %, and 18 %
for PM10, eBC, SO2, and NOx, respectively. The optimized
monthly median emission sums for all four parameters are displayed in Fig. 5 together with the
measured monthly median concentrations.
(a) Monthly medians of PM10 concentrations at the four stations in the
present study (blue) and monthly medians of optimized sums of PM10 emissions along back
trajectories leading to the stations (red). Panel (b) is as (a) but for measured
eBC concentrations and BC emissions along back trajectories. Panel (c) is as (a) but for
measured SO2 concentrations and SO2 emissions along back
trajectories. Panel (d) is as (a) but for measured NOx concentrations
and NOx emissions along back trajectories.
A 10-year trend in emissions of PM10, BC, SO2, and NOx, as well as
average monthly factors for the respective parameters are the two essential results derived from the
optimization approach. The 10-year trends relative to 2009 are collected in Fig. 6. Annual averages
of the relative differences between the monthly median measured parameters and the corresponding
emission-derived parameters were formed and applied to the GUAN trend values displayed in
Fig. 6. The resulting error bars on the trends serve as estimates of the uncertainties of the
optimization approach. The general trend in Fig. 6 is downward to minima between 30 % and
70 % of the 2009 values in 2016–2017 after which all parameters exhibit increases, most strongly
PM10. SO2 shows the strongest decrease, whereas PM10 and
NOx emissions diminished the least. In 2010–2011 the trend curves of
PM10 and NOx in Fig. 6 show a slight increase that can be linked to a
recovery of economic activity after the worldwide financial and economic crisis during the period
2007-2009. The increase in PM10 is also visible in the trend curves relative to 2005
published by the German Environment Agency
(https://www.umweltbundesamt.de/daten/luft/luftschadstoff-emissionen-in-deutschland/emissionen-prioritaerer-luftschadstoffe, last access: 18 September 2020).
GUAN: trends in the emissions of (a)PM10, (b) BC, (c)SO2, and (d)NOx relative to 2009 as calculated by
optimizing the agreement between 2009 EDGAR emissions and concentrations measured at the four
stations in the present study. The error bars represent annual average relative deviations between
measured and simulated data. GEA: trends as reported for Germany by the German Environment
Agency. EEA: trends as optimized from combinations of trends over Germany and neighboring
countries (see the text for details).
The results of two comparisons of our trends with data reported by the
German and European environment agencies are added to Fig. 6. In general,
the trends reported by the German Environment Agency for all German
emissions exhibit weaker reductions than the results of the present study.
Only for PM10 in 2011 and 2013 does the present study yield higher values
than GEA. We note that primary PM10 emissions may have substantial
contributions from wind erosion of agricultural soils (Panagos et
al., 2015) that are not incorporated in present anthropogenic inventories.
SO2 exhibits the strongest trend discrepancies, with much stronger
reductions of the trend in the present study compared to GEA results. As
Germany has been reducing SO2 emissions systematically since the
1980s one would not expect any further strong trends during the
time period of the present study. As other studies have demonstrated before
(e.g., van Pinxteren et al., 2019), the maps in Fig. 1
indicate the possibility of imported pollution, in particular from the
southeast. Consequently, we searched for similar trends in emission data
reported by the EEA for neighboring countries until 2017 directly west, south,
and east of Germany, going east all the way to Romania. Excel's
solver optimized combinations of the EEA trends for Germany and neighboring
countries in order to fit the trends derived in the present study. The
solver did not choose German trends for any of the four parameters
PM10, BC, SO2, and NOx. For PM10 a combination of
emission trends for the BENELUX countries and France was optimum, albeit
without being able to simulate the relative maxima in 2011 and 2013 and the
minimum around 2016. For BC the emission trend for the BENELUX countries
came closest to the trend of the present study. For SO2,
emissions in Romania, with minor contributions from French and BENELUX trends,
simulated the trends observed over Germany best. NOx trends were best
simulated by emissions over the Czech Republic and Slovakia. Emissions
trends over Switzerland, Austria, Hungary, and Poland were not utilized by
the solver. All simulated trends are displayed as curves in Fig. 6. We
do not claim that these simulated trends numerically correspond to imported
pollution over Germany. However, the good fit of the SO2 trend to
emissions over Romania corroborates our finding of pollution import from
southeastern Europe to northeastern Germany, while the development of BC
appears to better follow emission trends over western neighboring countries
than over Germany.
Sun et al. (2020) investigated trends
of size-resolved number and eBC mass concentrations at 16 observational
sites in Germany from 2009 to 2018, including the three GUAN sites in the
present study. Based on monthly median time series they report average
decreases for ME, NG, and WA of -5.5%, -6.1%, and -3.9%, respectively.
The corresponding result for eBC in the present study is -4.6%, albeit
with a high variability (see Fig. 6) of 20 % units expressed in terms
of an SD.
Over the polluted continent the particle-number-based parameter N10-800
is largely secondary in nature; i.e., its concentrations are controlled by
atmospheric constituents and processes. Thus, there is no primary emission
database with which a similar trend analysis as with PM10, BC,
SO2, and NOx could be attempted. Instead we chose the 10-year
grand averages (GAs) taken over the whole time period of the present
study as references from the deviations of annual averages are discussed.
Sun et al. (2020) report very minor
trends (between -3.5% and 0.1 %) for N20-800 at the three GUAN
stations used in the present study. The 10-year interannual variation of our
N10-800 in Fig. 7a bears out why only a minor trend if any can be
expected. For the first 4 years the annual averages are substantially
higher than average. Then annual values decrease down to a minimum in the
years 2016–2017 before they increase again to a level slightly above the
10-year average.
Trends in annual average deviations of (a)ΔN10-800, (b)ΔO3, and (c) temperature ΔT along the trajectories 5 d back
in time, as well as (d) precipitation rate ΔRR along the trajectories 3 d back in time. The deviations are taken relative to the respective 10-year grand average
(GA). The error bars represent the SDs of the annual averages.
In Fig. 7b–d annual deviations from the respective GAs are displayed that
can be connected to the 10-year course of N10-800. Ozone concentrations
averaged over the data from the three GUAN stations can be interpreted as an
indicator for photochemical activity that also controls NPF. The annual
deviations of O3 in Fig. 7b rather closely follow those of
N10-800. In Fig. 7c and d annual deviations of ambient temperature
and precipitation rates are displayed that have been averaged over the
meteorological data along the back trajectories leading to the four
stations. For the temperature an averaging period of 120 trajectory hours
yielded the highest (negative) correlation with N10-800 of r=-0.8.
After a dip in 2009 annual average trajectory temperatures reached a maximum in
2016 before returning to near average in 2018. For the precipitation rates
along the trajectories the highest (negative) correlation with N10-800
was found with an averaging period of 3 d (r=-0.6) before arrival
at the stations. The results displayed in Fig. 7c and d illustrate the
complexity of the processes and conditions controlling atmospheric particle
number concentrations. On one hand, a scavenging effect of precipitation can
be used as an argument for the high values of N10-800 in the years
2010–2013 and the low values in the years 2014 through 2018. On the other
hand, lower annual temperatures during years of relatively high N10-800
and higher-than-GA temperatures during years of relatively high N10-800
are harder to interpret. Possibly, the nucleation of condensable vapors is
furthered by lower air temperatures upwind of the stations.
An important result of trend analysis is the average monthly factors
disaggregating the annual emissions. In general the summer minima of the
month factors determined in the present study are broader than the curve
given by Matthias et al. (2018) for combustion emissions. The
decrease in the month factor of PM10, BC, and NOx in December and
the late winter maxima of PM10 and SO2 are not reflected in the
Matthias et al. (2018) results. Interestingly, both PM10
and SO2 show a minor secondary peak in June. As an example of the
seasonal variability of eBC within an urban source region we averaged the
relative annual variation of eBC concentrations at the station Leipzig
Eisenbahnstraße (plotted as curve L-EBS in Fig. 8), exhibiting a smaller
seasonal swing than all other curves. The curve for PM10 comes closest
to that for L-EBS, probably because of agricultural noncombustion emissions
in summer.
Month factors for emissions of PM10, BC, SO2, and
NOx as determined by optimizing the agreement between EDGAR emissions
and concentrations measured at the four stations in the present study. For
comparison the month factors of Matthias et al. (2018) for
combustion emissions are plotted, and the relative annual variation of eBC
concentrations measured at the station Leipzig Eisenbahnstraße (L-EBS)
are averaged over the time period of the present study.
In general the downward trends in particulate parameters determined in the present study are similar
to temporal trends of particle number and black carbon mass concentrations at 16 observational sites
in Germany from 2009 to 2018 (Sun et al., 2020). The long-term emission decrease in PM10
as determined in the present study from 2009 to 2018 is smaller than the corresponding number
published by the EEA as an average over all 28 EU member states but similar to the figures published by
the GEA through 2017 (see Table 3). For BC,
SO2, and NOx the present study yields substantially stronger
emission reductions than both the GEA and EEA. These findings are emphasized when considering 2017 to be the
end point of the trend calculation (see Table 3) at and after which our study shows consistent emission increases of
all studied parameters. Comparing the calculated trends with emission trends in neighboring
countries as published by the European Environment Agency supports the explanation that the observed
trends are to some extent due to changes in imported air masses. This holds most strongly for
SO2, the trend of which follows that of Romanian emissions rather well.
The last issue we take up in this discussion concerns the frequent residual difference between
measured and emission-simulated time series. In Fig. 5, e.g., in most winters, there are months when
optimized BC emissions remain substantially lower than the measured monthly medians of eBC. Some
information can be gleaned from the “Großwetterlagen” (GWL), representing 29 classifications
of large-scale weather types after Hess and Brezowsky for central Europe (Gerstengarbe and Werner,
1993), provided by the German Weather service for each day
(http://www.dwd.de/DE/leistungen/grosswetterlage/grosswetterlage.html, last access: 18 September 2020). During the winter
months with the strongest difference between measured and simulated time series the probabilities of
high-pressure systems over Fennoscandia with south-to-southeasterly flow to the four stations are
substantially higher than the respective probabilities averaged over the whole 10-year period of
the study. This GWL information is consistent with the back trajectories during the high-pollution
winter months coming predominantly from the southeasterly sector of the map. While the classified
large-scale weather situation with weak dilution of pollution during the winter months is conducive
to high particulate concentrations at the receptor sites, it does not explain the discrepancy. In
principle our simplistic approach of accumulating emissions along back trajectories may be flawed
during certain weather situations. However, an alternative explanation could be that the emissions
inventories over eastern and southeastern Europe in EDGAR are somewhat lower than the
real emissions.
Summary and conclusions
A total of 10 years of hourly aerosol and gas data at three stations of the German Ultrafine Aerosol Network
(GUAN) and one station of the Saxonian Environment Agency have been combined with hourly back
trajectories to the stations and emission inventories. Measured PM10, particle number
concentrations between 10 and 800 nm, and equivalent black carbon were extrapolated along
the trajectories. This process yielded what we termed pollution maps of these aerosol parameters
over Germany. They reflect aerosol emissions modified with atmospheric processes along the air mass
transport between sources and the four receptor sites at which the potential effects of particulate
air pollution would be realized.
The 10-year average pollution maps do not simply show the distribution of pollution sources upwind
of the receptor sites. The comparison with emission data based on the European EDGAR shows that strong western European emission centers do not dominate the downwind
concentrations because their emissions often are reduced by wet scavenging and dilution processes on
the way to the receptor area. Maps of average precipitation and wind as they occurred along the
trajectories illustrate these processes. In the receptor region mass-related aerosol parameters, such
as PM10, equivalent black carbon, and to some extent also the particle number
concentration, are instead rather controlled by emissions from eastern and southeastern Europe from
which pollution transport often occurs under drier meteorological conditions in continental
high-pressure air masses. This finding corresponds to the air mass results derived for the
submicrometer particle number size distribution by Birmili et al. (2001), by Engler et al. (2007)
for the size distribution of non-volatile particles, by Ma et al. (2014) for optical particle
properties all evaluated at the Melpitz station, and by van Pinxteren et al. (2019) for
the transboundary transport of PM10 to 10 stations in eastern Germany from neighboring
countries. Newly formed particles, on the other hand, are found in air masses from a broad belt
reaching from Burgundy to the western Czech Republic and southern Poland, a region with high
photochemical activity in summer that is affected by emissions in northern Italy.
Annual EDGAR emissions for 2009 of PM10, BC, SO2, and NOx
were accumulated along each trajectory and compared to the calculated emission sums with the
corresponding measured time series on a monthly basis. With a generalized reduced gradient solver
the agreement of each pair of monthly time series, e.g., measured eBC and BC emissions, was optimized
by letting the solver determine both monthly emission factors disaggregating the annual EDGAR
emission fields and adjusting the emissions with annual factors modifying the 2009 fields. Relative
to 2009 the annual averages of the analyzed air pollutants were lower in 2018 by values between
6 % for PM10 and 60 % for SO2. In general, the 10-year reductions
determined in the present study were stronger than those reported by the German and the European
environmental agencies. N10-800 exhibited substantial interannual variability but no net
decrease over the 10 studied years.
The validity of the present approach of connecting the ambient concentration and emission of
particulate pollution was tested by calculating temporal changes in eBC for
subsets of back trajectories passing over two separate prominent emission
regions, region A to the northwest and B to the southeast of the measuring
stations. Consistent with reported emission data the calculated pollution
decreases over region A are significantly stronger than over region B.
Compared to published emission monthly factors by Matthias et al. (2018) the present approach yielded broader summer minima that
were partly displaced from the midsummer positions given by Matthias et al. (2018). As an aside we note that during the winter months with
extremely high particulate pollution the emissions accumulated along back
trajectories are often substantially lower than the measured concentrations,
which raises the question of the validity of the emission figures in eastern
and southeastern European source regions.
There are clear limits to the methodology in the present study. Air mass
trajectories have inherent uncertainties that increase with their distance
traveled (Stohl, 1998). Meteorological processes affecting
aerosol during air mass transport are only considered rather coarsely,
whereas aerosol dynamics are not considered at all. Possible future
improvements concern ensemble trajectories with higher resolution, better
meteorological information along the trajectories, e.g., radar-derived
precipitation as used in Heintzenberg et al. (2018), more
comprehensive emission inventories with higher spatiotemporal resolution, and
higher numbers of analyzed stations.
Data availability
The data from the stations Melpitz, Neuglobsow, and Waldhof are deposited at EBAS (http://ebas.nilu.no/default.aspx, last access: 18 September 2020; Tørseth et al., 2012)
The data from the Collmberg station have kindly been provided by Annette Pausch of the Saxon State Office for Environment, Agriculture and Geology (https://www.umwelt.sachsen.de/umwelt/klaps/state_agency_environment_agriculture_geology.htm, last access: 18 September 2020).
The supplement related to this article is available online at: https://doi.org/10.5194/acp-20-10967-2020-supplement.
Author contributions
JH initiated and conducted the study, did all calculations related to statistics, maps, and trends, wrote most of the text, and generated all figures. WB provided large parts of the trajectory calculations and parts of the statistical discussion. BH compiled, quality-controlled, and provided aerosol and gas data from the UBA stations Neuglobsow and Waldhof and participated in the statistical discussion. GS provided aerosol and gas data from Melpitz station. TT and AW maintained the measurements of particle size distributions (PSDs) at the UBA stations and at Melpitz and reduced and quality-controlled the PSD data.
Competing interests
The authors declare that they have no conflict of
interest.
Acknowledgements
This work was accomplished in the framework of the project ACTRIS-2 (Aerosols, Clouds, and Trace
gases Research InfraStructure) under the European Union–Research Infrastructure Action in the
frame of the H2020 program “Integrating and opening existing national and regional research
infrastructures of European interest” under grant agreement N654109 (H2020 – Horizon
2020). Additionally, we acknowledge the WCCAP (World Calibration Centre for Aerosol Physics) as part
of the WMO–GAW program base-funded by the German Federal Environmental Agency (UBA). Continuous
aerosol measurements and data processing at Melpitz, Waldhof, and Neuglobsow were supported by
the German Federal Environment Agency grants F&E 370343200 (German title: Erfassung der Zahl
feiner und ultrafeiner Partikel in der Außenluft) 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). We gratefully acknowledge receiving the emission dataset from
the European emission database for global atmospheric research (EDGAR). We acknowledge technical
support by Annette Pausch of the Saxon State Office for Environment, Agriculture and Geology at the
Collmberg station, Achim Grüner and René Rabe (TROPOS) at the Melpitz
station, Olaf Bath (GEA) at the Neuglobsow station, and Andreas Schwerin (GEA) at the Waldhof
station. Fabian Senf compiled the “Großwetterlagen” for the present study. We are most
grateful for the ideas provided by Peter Winkler in the interpretation of the data.
Financial support
This research has been supported by the European Union (grant no. N654109) and by the German Federal Environment Agency (grant nos. F&E 370343200 and F&E 371143232).The publication of this article was funded by the Open Access Fund of the Leibniz Association.
Review statement
This paper was edited by Veli-Matti Kerminen and reviewed by two anonymous referees.
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