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Climatology of aerosol composition (organic versus
inorganic) at nonurban sites on a west-east transect
across Europe
Casimiro Pio, Michel Legrand, Tiago Oliveira, Joana Afonso, C. Santos,
Alexandre Caseiro, Paulo Fialho, Filipe Barata, Hans Puxbaum, Asunción
Sánchez-Ochoa, et al.
To cite this version:
Casimiro Pio, Michel Legrand, Tiago Oliveira, Joana Afonso, C. Santos, et al.. Climatology of aerosol
composition (organic versus inorganic) at nonurban sites on a west-east transect across Europe. Jour-
nal of Geophysical Research: Atmospheres, American Geophysical Union, 2007, 112 (D23S02), 1 à 15
p. �10.1029/2006JD008038�. �insu-00377242�
Climatology of aerosol composition (organic versus inorganic) at
nonurban sites on a west-east transect across Europe
C. A. Pio,
1
M. Legrand,
2
T. Oliveira,
1
J. Afonso,
1
C. Santos,
1
A. Caseiro,
1,4
P. Fialho,
3
F. Barata,
3
H. Puxbaum,
4
A. Sanchez-Ochoa,
4
A. Kasper-Giebl,
4
A. Gelencse´r,
5
S. Preunkert,
2
and M. Schock
6
Received 15 September 2006; revised 11 December 2006; accepted 6 July 2007; published 1 September 2007.
[1] In the framework of the European CARBOSOL project (Present and Retrospective
State of Organic versus Inorganic Aerosol over Europe: Implications for Climate),
atmospheric aerosol was continuously sampled for 2 years at six sites along a west-east
transect extending from Azores, in the mid-Atlantic Ocean, to K-Puszta (Hungary),
in central Europe. Aerosols were analyzed for
210
Pb, inorganic ions, elemental (EC) and
organic (OC) carbon, water soluble organic carbon (WSOC), macromolecular type
(humic-like) organic substances (HULIS), C
2
–C
5
diacids, cellulose, and levoglucosan.
Pooled aerosol filters were also used for the identification of different families of organic
compounds by gas chromatography/mass spectrometry, GC/MS, as well as
14
C
determinations. The data resulted in a climatological overview of the aerosol composition
over Europe in the various seasons, from west to east, and from t he boundary layer to the
free troposphere. The paper first summarizes the characteristics of the sites and
collected samples and then focuses on the aerosol mass partitioning (mass closure,
inorganic versus organic, EC versus OC, water soluble versus insoluble OC), giving an
insight on the sources of carbonaceous aerosol present in rural and natural background
areas in Europe. It also introduces the main role of other companion papers dealing
with CARBOSOL aerosol data that are also presented in this issue.
Citation: Pio, C. A., et al. (2007), Climatology of aerosol composition (organic versus inorganic) at nonurban sites on a west-east
transect across Europe, J. Geophys. Res., 112, D23S02, doi:10.1029/2006JD008038.
1. Introduction
[2] Atmospheric aerosols influence climate directly
through scattering and absorbing radiation, and indirectly
by acting as condensation nuclei for clouds, with modifi-
cation of the clouds optical properties and lifetime, probably
resulting in a negative forcing. Furthermore, aerosols have
deleterious health effects as they often contain toxins and/or
carcinogens that contribute to cardiopulmonary diseases and
mortality [Pope, 2000; Po¨schl, 2005].
[
3] Model estimates of the global cooling due to man-
made aerosols suggest that anthropogenic aerosols could
counterbalance the warming due to g rowing levels of
greenhouse gases by some 30%, possibly up to 50–80%
[Charlson et al., 1992; Penner et al., 1994; Jacobson, 2001;
Bellouin et al., 2005]. Thus aerosols may have weakened
the rate of the gl obal warming during the last century
[Andreae et al., 2005]. However, numerous uncertainties
still exist in estimating the climatic impact of aerosols,
which is thought to be larger over industrialized regions
[Menon, 2004]. This is mostly because the spatial distribu-
tion of aerosols is very heterogeneous, requiring numerous
investigations in time and space to serve as inputs and
constraints for climate models.
[
4] The most important inorganic aerosol component in
terms of radiative forcing is sulfate which has well known
optical properties. The sources of sulfate are rather well
quantified and SO
2
anthropogenic emissions are well docu-
mented, at least for the regions of interest in this study
(western and central Europe) [Lefohn et al., 1999; van
Aardenne et al., 2001]. The present-day spatial distribution
of sulfate is documented throughout Europe at numerous
low-level sites, as well as at some sites located above 1000 m.
Current evaluations of the present-day direct radiative
forcing by sulfate over Europe are i n self agreement, within
a factor of 2, with the main uncertainties related to the
assumed sulfate distribution which depends on deployed
chemistry-transport schemes, the assumption on size distri-
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, D23S02, doi:10.1029/2006JD008038, 2007
1
Centre for Environmental and Marine Studies and Department of
Environment, University of Aveiro, Aveiro, Portugal.
2
Laboratoire de Glaciologie et Ge´ophysique de l’Environnement,
Centre National de la Recherche Scientifique, Saint Martin d’He´res, France.
3
Department of Agrarian Sciences, University of the Azores, Angra do
Heroı´smo, Portugal.
4
Institute for Chemical Technologies and Analytics, Vienna University
of Technology, Vienna, Austria.
5
Air Chemistry Group at the Hungarian Academy of Sciences,
University of Pannonia, Veszpre´m, Hungary.
6
Institut fu¨r Umweltphysik, Universita¨t Heid elbe rg, Heidelb erg,
Germany.
Copyright 2007 by the American Geophysical Union.
0148-0227/07/2006JD008038
D23S02 1of15
bution and the degree of internal or external mixing
[Boucher et al., 1998]. Much larger uncertainties exist in
evaluating the indirect effect [Penner et al., 1998; Lohmann
et al., 2000; Menon, 2004].
[
5] Although carbonaceous particles are now recognized,
together with sulfate, as major components of fine aerosol
[Heintzenberg, 1989; Zappoli et al., 1999], there is a large
uncertainty on their sources (biogenic versus anthropogenic,
primary versus secondary), their complex chemical nature
and optical properties [Gelencse´r, 2004]. Total carbon in
atmospheric aerosol can be routinely determined. Analytical
methods are able to differentiate between organic (OC, a
scattering material), and elemental carbon (EC), or black
carbon (BC), a light absorbing material with a graphite-type
crystalline structure. Usually the term BC is used for
measurements based purely on optical methods, while EC
is associated with thermal analytical methods and the
direct determination of carbon [Gelencse´r, 2004]. The split
between OC and EC is rather arbitrary and has been a
subject of controversy for the last decades [Schmid et al.,
2001]. EC lies generally in the submicron range whereas
OC exhibits a wider size distribution, which remains poorly
documented. The hygroscopic properties of OC and EC are
not well established, although laboratory studies have
suggested hydrophobic behavior f or fresh particles and
enhancement of hygroscopicity after contact with oxidants
[Gelen cse´r, 2004]. Bulk OC can be divided into water
soluble (WSOC) and water insoluble (WinOC) parts. The
hygroscopic properties of bulk OC are widely unknown
although some WSOC species like dicarboxylic acids are
known to be very hygroscopic [Saxena et al., 1995].
[
6] In the existing data, EC and OC are mainly separated
as carbona ceous fractions, and only very recent studies
started to discriminate WSOC and WinOC. On the other
hand, organic speciation using recent analytical methods
have failed to assign more than 15% of total carbon to
individual organic compound classes such as n-alkanes,
carboxylic acids, terpenoids, etc. [Alves et al., 2002; Pio
et al., 2001]. It seems that failure in organic speciation
results from the fact that most OC in aerosol is contained in
oligomeric or polymeric matter. Spectroscopic data on
WSOC fraction shows that its properties present large
similarities to those of terrestrial or aquatic humic and
fulvic acids, and recently it was demonstrated that they
were made up of similar structural units [Simoneit and
Mazurek, 1982]. Another important fraction was shown to
consist of other biopolymers such as cellulose [Puxbaum
and Tenze-Kunit, 2003]. Given that, the organic oligomers
and polymers may constitute a major part of OC, but their
origins are still an open question.
[
7] Until now most of the research on carbonaceous
aerosol has been focused on urban areas, where traffic
represents a main source of pollution, or on areas highly
impacted by biomass burning. In contrast, only a few
studies have investigated remote, rural or semirural environ-
ments, and most of them were restri cted to short-time
campaigns (see Putaud et al. [2004] for a recent review
focused on Europe).
[
8] This paper gives an overview of the network sampling
scheme deployed to measure inorganic and carbonaceous
aerosol constituents at rural and background sites in Europe,
in the framework of the European CARBOSOL project
[Legrand and Puxbaum, 2007]. It focuses on major frac-
tions of aerosol, namely the rather well-known inorganic
ions, EC and OC, and its different subfractions versus
volatility, and the water soluble organic fraction. The
climatology of these aerosol fractions is discussed, with
backup of information from
210
Pb, to highlight the effect of
continentality along the west to east transect, as well as the
vertical mixing, particularly at mountain sites. The paper
also introduces several companion papers of thi s issue
dedicated to more specific organic species investigated in
parallel during the CARBOSOL project.
2. Sampling
[9] Aerosol was collected at six sites along a west-east
European transect of 4000 km, extending from Azores
(Portugal) in the middle of the central-north Atlantic Ocean,
to K-Puszta (Hung ary) in the central European plains
(Figure 1). Referring to the classification given by Va n
Dingenen et al. [2004] for Europe, the sites can be classified
into marine background (Azores), rural background (Aveiro
and K-Puszta: lowland; Schauinsland: mountain), a nd
natural continental background (Puy de Dome and Sonnblick:
free troposphere in winter).
[
10] At Azores (AZO) (38°41
0
N, 27°21
0
W) sampling was
done at top of a 50 m high cliff over the sea, west of the
Terceira Island (397 km
2
, 60,000 inhabitants). The site area
is used for catt le grazing and horticulture. The Azores are in
the mid-north Atlantic Ocean, representative of background
marine atmosphere, with levels frequently influenced by
transport from North America and, to a lesser degree, from
Europe and Africa. Aerosol was sampled by sucking air at a
flow rate of 1.1 m
3
min
1
through quartz fiber filters
(Whatman QM-A, 10 8 inches). The sampler stands
2.5 m above the ground and was run with an Anderson
PM10 inlet to remove particles larger than 10 mm. The filter
holder had a Tisch 2.5 mm impactor stage which separates
particles larger and smaller than 2.5 mm. Only PM2.5 particles
were analyzed.
[
11] At Aveiro (AVE) (40°35
0
N, 8°38
0
W) sampling was
performed at 2.5 m above the ground using a system similar
to the AZO one. The sampling site is located in a rural area
used to grow maize in spring and summer. The site (47 m asl)
is located on the west coast of Portugal, 10 km from the
Atlantic Ocean, and 6 km southeast from the small town of
Aveiro (50,000 inhabitants). The region is characterized by
maritime pine and eucalyptus forests and small-scale agri-
cultural fields (horticulture and maize growing).
[
12] The Puy de Doˆme (PDD) (45°46
0
N, 2°57
0
E) site is
located at the Microphysics and Chemistry station run by
the Observatoire de Physique du Globe de Clermont Fer-
rand (OPGC) on the top of the Puy de Dome mountain
(1450 m asl), in central France. In winter the site is very
often under free tropospheric conditions [Sellegri e t al.,
2003]. Details on the station ca n be found at http://
wwwobs.univ-bpc lermont.fr/a tmos/pdd/visitepuyde dome/
accueil.htm. The region is characterized by intensive agri-
culture, cattle husbandry and forest management activities.
Aerosol was sampled at a flow rate of 1.1 m
3
min
1
on circular quartz filters (Gelman Pallflex Tissuquartz
2500QAT-UP) with 15 cm diameter (filter holder from
D23S02 PIO ET AL.: CLIMATOLOGY OF EUROPEAN AEROSOL
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D23S02
Digitel). The air inlet is located 6 m above the ground and
equipped with a heated rain/snow shelter (Digitel) which has
a cutoff size of 10 mm.
[
13] The Schauinsland (SIL) (47°55
0
N, 07°54
0
E) sam-
pling site is located at the Global Atmosphere Watch
(GAW) station run by the German Federal Environmental
Agency (http://www.empa.ch/gaw/gawsis/reports.asp?
StationID=93). The station is on a mountain ridge (1205 m asl)
in the Black Forest (southwestern Germany). It is sur-
rounded by coniferous forests (50%), meadows (40%)
and some agricultural fields (10%). The city of Freiburg is
located in the Rhine valley at 12 km southwest from the
site. The station is usually situated above the ground-
level atmospheric inversion layer of the densely populated
Rhine Valley. Howev er, during summer, strong thermal
convections may transport air masses from the Rhine valley
to the SIL station. Aerosol was sampled at a flow rate of
0.9 m
3
min
1
on circular quartz filters (Gelman Pallflex
Tissuquartz 2500QAT-UP) with a diameter of 15 cm. The
air inlet, similar to the PDD one, was located at 4 m above
the ground.
[
14] The Sonnblick (SBO) (47°03
0
N, 12°57
0
E) site is the
Sonnblick Observatory (SBO) operated by the Central
Institute for Meteorology and Geodynamics. It is located
on a mountain peak (3106 m asl) in the main ridge of the
Austrian Alps and is frequently above the at mospheric
mixing layer [Kasper and Puxbaum, 1998]. The observa-
tory is supplied with electricity and has no local sources of
exhaust fumes. More details on SBO can be found at http://
www.amap.no/envinet/site.cfm?SiteID=4. Atmospheric
aerosol was sampled with a high-volume sampler (Digitel)
at a flow rate of 0.5 m
3
min
1
on quartz fiber (Gelman
Pallflex Tissuquartz 2500QAT-UP) 15 cm diameter filters.
The sampler is set up on the roof platform of the observa-
tory and is equipped with a PM2.5 inlet.
[
15] At K-Puszta (KPZ) (46°58
0
N, 19°35
0
E) aerosols were
collected at the station run by the Hungarian Meteorological
Service and the University of Veszpre´m, as part of the GAW
and EMEP (Convention on Long-range Transboundary Air
Pollution) (http://www.nilu.no/projects/ccc/sitedescriptions/
hu/index.html) networks. The site is in the middle of
the Hungarian Plain, 60 km southeast from Budapest
(1.9 million inhabitants). The largest nearby town (Kecskeme´t,
110,000 inhabitants) is located 15 km southeast from the
station. The sampling site is surrounded by forests (62%
coniferous trees) interspersed with clearings. Aerosol was
sampled at a flow rate of 0.6 m
3
min
1
(Sierra-Andersen
impactor) on quartz fiber filters (Whatman QM-A) of 20
25 cm size. The sampler is located at 7 m above ground and
was configured to remove particles larger than 2 mm.
[
16] As above mentioned, in contrast to AZO, AVE, SBO
and KPZ where a PM 2–2.5 inlet was deployed, at SIL and
PDD a PM 10 inlet was used. That has to be kept in mind
when comparing data related to coarse particles. Neverthe-
less, a size distribution study carried in different types of air
masses at PDD have shown that, except during sporadic
Saharan dust event, the aerosol mass (organic and inorganic)
is mainly present below 3 mm[Sellegri et al., 2003].
[
17] Samples were collected, almost continuously, during
2 full years at each site. At AZO, AVE and KPZ, sampling
was initiated beginning of July 2003. At the remaining sites
sampling started in October of the same year. At AVE and
SIL no weekly samples were lost. At other sites, such as
AZO, several samples could not be collected properly,
mainly in winter, as result of equipment breakdown associ-
ated with bad weather conditions and electricity supply
failures. The 538 weekly aerosol filters collected at the six
sites were analyzed for inorganic ions, EC and OC, WSOC,
HULIS, C
2
-C
5
dicarboxylic acids, cellulose, and levoglu-
cosan. Except at AZO all filters were investigated for
210
Pb. Weekly filter samples, collected from July 2002 to
Figure 1. Location of the six CARBOSOL sites in Europe.
D23S02 PIO ET AL.: CLIMATOLOGY OF EUROPEAN AEROSOL
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D23S02
September 2003, were pooled monthly (86 samples) for the
identification of different class species (alkanes, aldehydes,
ketones, alcohols, acids, and aromatics) by GC/MS. With
the exception of AZO, 65 weekly aerosol filters collected at
the 5 other sites were selected, by considering their season
and
210
Pb leve ls as related to the continentality of the
sampled air mass (total of 13 single filters and 5 pooled
samples) for
14
C determinations. Additional measurements
included a few size-segregated sampling with a high-
volume impactor at AVE and KPZ.
[
18] To collect enough material for conducting all analy-
sis, even at remote oceanic and mountain sites, weekly
sampling was applied to avoid problems related to detection
limits of the deployed analytical methods. Because of the
long sampling period, alteration of aerosol may have
occurred on filters during sampling in relation to either
the organic or inorganic constituents of aerosol. During the
long sampling period it is probable that filt ered organic
particulate matter suffers chemical transformation resulting
in partial oxidation of the more thermodynamically unstable
organic species, as result of attack by strong oxidants, such as
ozone, over the filter. This has to be taken into account in the
interpretation of the results. Also condensation/volatilization
processes may happen into and from the deposited particles
and on reactive sites on filter quartz fibers. Therefore results
for semivolatile compounds, such as low ring number PAHs
(poli-aromatic hydrocarbons) and ammonium nitrate, have
to be regarded with precaution.
[
19] It can, however, been assumed that, during sampling,
equilibrium between gas and particulate phase would
govern the behavior of the filtered aerosol similarly to what
happens in the atmosphere. Furthermore the large mass of
particles collected would reduce potential adsorption of
semivolatile organic and inorganic compounds on active
sites of the quartz fibers surface, as result of their rapid
saturation.
[
20] To reduce contamination by organic material, quartz
filters for PDD, SIL and SBO were prefired in factory,
whereas for AZO, AVE and KPZ, filters were pretreated by
heating in a furnace during several hours at 500 – 700°Cin
laboratory. After that, filters were wrapped in thermally
treated and cleaned aluminum foil.
[
21] After sampling, filter samples were folded in two,
with the exposed side face to face, wrapped in aluminum
foil and immediately transported to the laboratory in charge
of the sampling site, where they were stored at 20°C.
Batches of sampled filters and filter blanks were divided
into several fractions, enclosed into heated treated alumi-
num cylinders, and sent by express mail to the various
laboratories participating in the analytical work.
3. Analytical Methods
[22] Filter samples were analyzed at the University of
Aveiro (OC, EC, and GC/MS analysis), Laboratory of Glaci-
ology in Grenoble (inorganic ions and light carboxylates),
Technical University of Vienna (levoglucosan, cellulose and
HULIS), University of Veszpre´m (WSOC and HULIS), and
University of Heidelberg (
210
Pb and
14
C). In this paper
details are given on the analysis of major organic fractions
(EC, OC, and WSOC) and inorganic ions, whereas for
specific organic compounds more details can be found in
corresponding dedicated papers of this issue.
3.1. OC and EC
[
23] Using thermal methods, the various carbon fractions
are volatilized by applying sequential heating at increased
temperatures. Separation between OC and EC is achieved
by initially heating an exposed filter punch under an inert
atmosphere, to evaporate first the OC fraction. The remain-
ing fraction is sequentially evaporated/burnt under a gas
flow containing O
2
. This last carbon fraction contains initial
EC plus OC that has pyrolyzed during heating under an
inert atmosphere, called pyrolytic carbon (PC). The inter-
ference between PC and EC can be controlled by continu-
ous evaluation of the blackening of filter using a laser beam
and a photodetector, measuring either the filter surface light
reflectance or the filter light transmittance. Usually, charring
control by transmittance results in lower EC values than by
reflectance [Chow et al., 2001]. That is because reflectance
only detects charring at the filter surface, while transmit-
tance also detects charring in the filter fiber structure.
However, even charring control by transmittance can pro-
duce quite different results, depending mainly on the
maximum temperature of the heating step under the inert
atmosphere. For instance, methodologies, such as the
NIOSH (National Institute for Occupational Safety and
Health) method, that preheat the filter under He at 900°C,
have a tendency to result in lower EC values than methods
that have a maximum temperature step at 550 – 6 00°C
[Chow et al., 2001]. This has to be kept in mind when
discussing EC/OC data.
[
24] The OC/EC analysis were done with a thermal-
optical technique [Pio et al., 1994; Castro et al., 1999]
based on the concept proposed by Huntzicker et al. [1982].
The system includes a quartz tube with two heating zones, a
laser, and a nondispersive infrared (NDIR) CO
2
analyzer.
Filter samples are first exposed to HCl vapors for several
hours to remove carbonates. The filter is then transferred
within the first heating zone, which can be heated up to
900°C. The second heating zone, filled with cupric oxide
(CuO), is maintained at 700°C. Quantitative combustion of
volatilized carbon to CO
2
is achieved in the second heating
zone, where O
2
is added. The control of the heating
program permits separation of OC into different subfrac-
tions, according to their volatility. During heating, the
blackness of the filter is monitored by measuring light
transmittance through the filter sample with a pulsed laser
beam. The analyzer is daily calibrated with standard atmos-
pheres and filters impregnated with known amounts of
potassium phthalate.
[
25] The program used to analyze the CARBOSOL
samples included the following steps: under N
2
, at 150°C
for 4 min, at 350°C for 4 min, at 600°C for 5 min, and from
600 to 250°C within 3 min; N
2
with 4% of O
2
, at 350°Cfor
1 min, from 350 to 500°C within 7 min, and at 850°Cfor
6 min (Figure 2). With that, the carbonaceous content of the
sample can be subdivided into five fractions denoted OC1
to OC3, PC, and EC. PC is organic carbon that has
pyrolyzed during heating under inert atmosphere, calculated
from the mass of CO
2
emitted during the second heating
phase under a gas flow containing O
2
, until the recovering
of filter light transmittance (Figure 2). Finally EC is the
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