Atmos. Chem. Phys., 13, 6421–6428, 2013
www.atmos-chem-phys.net/13/6421/2013/
doi:10.5194/acp-13-6421-2013
© Author(s) 2013. CC Attribution 3.0 License.
EGU Journal Logos (RGB)
Advances in
Geosciences
Open Access
Natural Hazards
and Earth System
Sciences
Open Access
Annales
Geophysicae
Open Access
Nonlinear Processes
in Geophysics
Open Access
Atmospheric
Chemistry
and Physics
Open Access
Atmospheric
Chemistry
and Physics
Open Access
Discussions
Atmospheric
Measurement
Techniques
Open Access
Atmospheric
Measurement
Techniques
Open Access
Discussions
Biogeosciences
Open Access
Open Access
Biogeosciences
Discussions
Climate
of the Past
Open Access
Open Access
Climate
of the Past
Discussions
Earth System
Dynamics
Open Access
Open Access
Earth System
Dynamics
Discussions
Geoscientic
Instrumentation
Methods and
Data Systems
Open Access
Geoscientic
Instrumentation
Methods and
Data Systems
Open Access
Discussions
Geoscientic
Model Development
Open Access
Open Access
Geoscientic
Model Development
Discussions
Hydrology and
Earth System
Sciences
Open Access
Hydrology and
Earth System
Sciences
Open Access
Discussions
Ocean Science
Open Access
Open Access
Ocean Science
Discussions
Solid Earth
Open Access
Open Access
Solid Earth
Discussions
The Cryosphere
Open Access
Open Access
The Cryosphere
Discussions
Natural Hazards
and Earth System
Sciences
Open Access
Discussions
222
Rn-calibrated mercury fluxes from terrestrial surface of southern
Africa
F. Slemr
1
, E.-G. Brunke
2
, S. Whittlestone
3
, W. Zahorowski
3
, R. Ebinghaus
4
, H. H. Kock
4
, and C. Labuschagne
2
1
Max-Planck-Institut f
¨
ur Chemie, Hahn-Meitner-Weg 1, 55128 Mainz, Germany
2
South African Weather Service c/o CSIR, P.O. Box 320, Stellenbosch 7599, South Africa
3
ANSTO Environment, PMB 1, Menai, NSW 2234, Australia
4
Helmholtz-Zentrum Geesthacht, Institute of Coastal Research, Max-Planck-Strasse, 21502 Geesthacht, Germany
Correspondence to: F. Slemr (franz.slemr@mpic.de)
Received: 18 February 2013 – Published in Atmos. Chem. Phys. Discuss.: 26 March 2013
Revised: 4 June 2013 – Accepted: 6 June 2013 – Published: 8 July 2013
Abstract. Gaseous elemental mercury (GEM) and
222
Rn, a
radioactive gas of primarily terrestrial origin with a half-life
of 3.8 days, have been measured simultaneously at Cape
Point, South Africa, since March 2007. Between March
2007 and December 2011, altogether 191 events with high
222
Rn concentrations were identified. GEM correlated with
222
Rn in 94 of the events and was constant during al-
most all the remaining events without significant corre-
lation. The average GEM/
222
Rn flux ratio of all events
including the non-significant ones was −0.0001 with a
standard error of ±0.0030pgmBq
−1
. Weighted with the
event duration, the average GEM/
222
Rn flux ratio was
−0.0048± 0.0011 pg mBq
−1
. With an emission rate of 1.1
222
Rn atoms cm
−2
s
−1
and a correction for the transport
time, this flux ratio corresponds to a radon-calibrated flux
of about −0.54ngGEMm
−2
h
−1
with a standard error of
±0.13 ng GEM m
−2
h
−1
(n = 191). With wet deposition,
which is not included in this estimate, the terrestrial sur-
face of southern Africa seems to be a net mercury sink of
about −1.55 ng m
−2
h
−1
. The additional contribution of an
unknown but presumably significant deposition of reactive
gaseous mercury would further increase this sink.
1 Introduction
Mercury poses a serious environmental issue because of its
transformation to methyl mercury, which is a potent toxin
to humans and animals (Mergler et al., 2007; Scheuhammer
et al., 2007). Of primary concern are thus the emissions of
mercury, which due to the long atmospheric residence time
of elemental mercury (Lindberg et al., 2007) can be dis-
tributed all over the world. According to the current emis-
sion inventories and models, anthropogenic emissions repre-
sent the largest mercury source with 2880tyr
−1
, followed by
2680 t yr
−1
from the oceans and 1850 t yr
−1
from the terres-
trial surfaces (Mason, 2009; Pirrone et al., 2010). Whereas
anthropogenic emissions are believed to be known with an
uncertainty of ±30 %, the uncertainties of the emissions from
oceans and terrestrial surfaces are considered to be ±50%
and more (Lin et al., 2006; Lindberg et al, 2007).
The uncertainties related to emissions from terrestrial sur-
faces originate mostly from the poor knowledge of the emis-
sion mechanisms, the worldwide up-scaling of a small num-
ber of field measurements made in a few geographic re-
gions, and the measurement challenges (Lindberg et al.,
2007; Gustin et al., 2008; Mason, 2009; Smith-Downey et al.,
2010). Mercury emission from terrestrial surfaces is depen-
dent on meteorological conditions, type of soil and vegeta-
tion, and historical atmospheric deposition (Zhang and Lind-
berg, 1999; Gustin et al., 2000, 2008; Gustin, 2003; Song and
Van Heyst, 2005; Bash, 2010; Smith-Downey et al., 2010).
The influence of these parameters has been studied in the
laboratory and in the field, but the underlying mechanisms
are still not well understood (Mason, 2009). The flux can
be bi-directional depending on the mercury concentration in
ambient air: deposition at higher concentrations and emis-
sion at lower concentrations with a cross-over point termed
“compensation point” (e.g. Hanson et al., 1995; Lindberg et
al., 1998; Zhang et al., 2009). An intercomparison of field
Published by Copernicus Publications on behalf of the European Geosciences Union.
6422 F. Slemr et al.:
222
Rn-calibrated mercury fluxes
flux measurement techniques revealed substantial disparities
between the chamber and the micrometeorological methods
(Gustin and Lindberg, 2000). In addition to all these chal-
lenges, field flux measurements have so far been carried out
almost exclusively in temperate regions of North America
and Europe. Their up-scaling to other regions in the North-
ern and Southern Hemisphere is thus necessarily fraught with
large additional uncertainties.
222
Rn is a radioactive gas of predominantly terrestrial ori-
gin with a half-life of 3.8 days. Its emission rate from soil
is relatively evenly distributed (Zhang et al., 2011 and ref-
erences therein) making
222
Rn a good tracer for studies of
emissions from terrestrial surfaces (Zahorowski et al., 2004).
According to Jacob et al. (1997), the assumption of a uni-
form
222
Rn emission rate of 1 atom cm
−2
s
−1
is accurate to
roughly 25% globally, or by a factor of 2 regionally.
222
Rn
has been successfully used to derive regional emissions of
CO
2
, CH
4
, and N
2
O (e.g. Gaudry et al., 1990; Wilson et al.,
1997; Zahorowski et al., 2004; Hirsch, 2007). To the best of
our knowledge, its only application to mercury flux estima-
tions has been reported by Obrist et al. (2006). They found
good agreement between fluxes estimated from the accumu-
lation of Hg and
222
Rn in the stable nocturnal boundary layer
and those measured by the modified Bowen ratio micromete-
orological technique. The major advantage of the Hg/
222
Rn
method is its capability to estimate regional fluxes and by this
its capability to avoid shortcomings related to up-scaling of
point measurements in the field (Wilson et al., 1997; Obrist et
al., 2006). In this paper we use concurrent measurements of
gaseous elemental mercury and
222
Rn at Cape Point, South
Africa, to derive the regional mercury flux from southern
Africa.
2 Experimental
The Cape Point station (34
◦
21
0
S, 18
◦
29
0
E) is part of the
World Meteorological Organization’s (WMO) Global Atmo-
sphere Watch (GAW) network. Cape Point is about 60 km
south of Cape Town, and located on top of a coastal cliff
230 m above sea level at the southernmost tip of the Cape
Peninsula. The site is located in a nature reserve and ex-
periences moderate temperatures, dry summers with occa-
sional biomass burning episodes in the surrounding area and
increased precipitation during austral winter. The dominant
wind direction is from the south-eastern sector, which is rep-
resentative of clean maritime air from the Southern Ocean
(Brunke et al., 2004). The site is occasionally also subjected
to air from the northern to north-eastern sector (mainly dur-
ing austral winter), which is influenced by anthropogenic
emissions from the greater Cape Town area and/or by other
continental sources (both local and regional).
Within the framework of the WMO-GAW programme,
continuous trace gas measurements of CO
2
, CH
4
, CO and
O
3
have been made at Cape Point for more than 30 yr
now (Scheel et al., 1990). The
222
Rn measuring programme
started in 1999 and serves mostly to classify air masses
into maritime, continental or mixed (Brunke et al., 2004).
Gaseous mercury concentrations had been measured inter-
mittently (about 200 samples per year) since September 1995
until December 2004 (Slemr et al., 2008) and have been
continuously with a resolution of 15 min since March 2007
(Brunke et al., 2010). Only the high-resolution data until the
end of 2011 were used in this work.
Continuous measurements of gaseous mercury are made
using a Tekran 2537A vapour-phase mercury analyser
(Tekran Inc., Toronto, Canada). The Tekran 2537A is capa-
ble of measuring low-level mercury concentrations typically
observed at background locations (Ebinghaus et al., 1999;
Munthe et al., 2001). The analyzer is operated in an air-
conditioned laboratory and run with a sampling air flow rate
of 1 L min
−1
at 15 min sampling intervals. The span of the
analyzer is checked by an internal permeation source once
every 25 h. The permeation rate of the internal permeation
source was determined by repeated injections of mercury sat-
urated vapour from a primary mercury source (Tekran Model
2505) and was found to be stable within 2% over the period
of the measurements. The air sample intake was attached to
a 30 m high aluminium sampling mast at a height of approx-
imately 5 m above the rocky surface and about 235 m above
sea level. A Teflon filter (pore size 0.2 µm; ID = 45 mm)
upstream of the instrument protects the analyzer against con-
tamination by particulate matter. The filter was replaced once
every two weeks. The 15 min TGM data have been converted
to 30 min averages so that comparisons with
222
Rn, other
trace gas and meteorological data being measured simultane-
ously at Cape Point could be made. Under the prevailing at-
mospheric conditions at Cape Point (higher temperature and
air humidity, in addition to hygroscopic sea salt aerosols),
we assume that reactive gaseous mercury (RGM) will be
adsorbed by the inlet tubing and the aerosol filter and that
the measured atmospheric mercury concentration thus repre-
sents exclusively gaseous elemental mercury (GEM) (Brunke
et al., 2010). All GEM concentrations are given in ng m
−3
(STP, i.e. at 273.2 K and 1013 hPa). The precision of the 30
min GEM measurements was ∼ 0.035 ng m
−3
and their over-
all uncertainty including the uncertainty of the permeation
rate and the sampling flow calibrations ∼ 5 %.
Since 1999 a
222
Rn detector designed by the Australian
Nuclear Science & Technology Organisation (ANSTO) and
manufactured by AGH Industries (Riverwood, Australia) has
been installed at Cape Point. The so-called two-filter instru-
ment is described in detail by Whittlestone and Zahorowski
(1998) and Brunke et al. (2002) and was run with 30 min
resolution. Briefly, radon and thoron decay products are re-
moved from the air by the first filter. Decay products newly
formed under controlled conditions in the instrument de-
lay tank are then retained by a second filter. Their alpha
radiation is determined by a zinc sulfide scintillator. The
Atmos. Chem. Phys., 13, 6421–6428, 2013 www.atmos-chem-phys.net/13/6421/2013/
F. Slemr et al.:
222
Rn-calibrated mercury fluxes 6423
20
Figures
Fig. 1: Monthly frequency of events with
222
Rn concentrations > 1000 mBq m
-3
(“all”)
and those with significant GEM vs
222
Rn correlations (“significant”).
Fig. 1. Monthly frequency of events with
222
Rn concentrations
> 1000mBq m
−3
(“all”) and those with significant GEM vs.
222
Rn
correlations (“significant”).
detection limit of the instrument at Cape Point is quoted to
be 33 mBq m
−3
(Brunke et al., 2002).
Hg vs.
222
Rn was correlated using orthogonal regression
(Cantrell, 2008), which takes the uncertainties of both corre-
lated parameters into account. Factors affecting the sensitiv-
ity and accuracy of the Cape Point
222
Rn detector have been
discussed by Brunke et al. (2002) and by references therein.
For the correlations here, the GEM and
222
Rn uncertainties
were set to 0.05ngm
−3
and 50mBqm
−3
, respectively. As
222
Rn is always emitted, the positive slope sign stands for
mercury emissions and the negative one for mercury depo-
sition. The mean values throughout the paper are given with
the standard errors of the mean instead of the more common
standard deviations of individual measurements.
The regions of origin for the pollution events were in-
terpreted using ten-day isentropic back trajectories from
NOAA ESRL (Earth System Research Laboratory of Na-
tional Oceanic and Atmospheric Administration, http://www.
esrl.noaa.gov/gmd) and seven-day back trajectories calcu-
lated by NILU (Norwegian Institute for Air Research) us-
ing the FLEXTRA model (http://www.nilu.no/projects/ccc/
trajectories/).
3 Results and discussion
Altogether 191 events with
222
Rn concentrations above
1000 mBq m
−3
, which lasted usually for more than a day,
have been identified between March 2007 and December
2011. Their seasonal occurrence frequency is shown in
Fig. 1. Most of them occur in the months March–September,
in agreement with the seasonal variation of wind direction
at Cape Point (Brunke et al., 2004). The events can extend
up to 7 days, but most of them last 2–4 days. Their duration
21
Fig. 2: Frequency distribution of all Hg/
222
Rn slopes and only of those which are
significant.
Fig. 2. Frequency distribution of all Hg/
222
Rn slopes and only of
those which are significant.
is thus substantially longer than that of the depletion events
or the typical pollution plumes observed at Cape Point,
which generally last only several hours (Brunke et al., 2010,
2012). Using time series and scatter plots, this difference al-
lows us to discriminate against the depletion events, the an-
thropogenic emissions and emission from biomass burning.
Fifty-six events with enhanced
222
Rn concentrations coin-
cided with such depletion and pollution events. These short
depletion and pollution events were eliminated for the subse-
quent analysis of the relationship between Hg and
222
Rn in
the
222
Rn events.
Figure 2 shows the frequency distribution of the
GEM /
222
Rn slopes from the correlations. In 94 events
the correlations were meaningful at least at the 95 % sig-
nificance level. The insignificant correlations for the re-
maining events may either imply that there is no relation
whatsoever or that the GEM concentration remains con-
stant during the
222
Rn event. Figure 2 shows that the lat-
ter is the case, i.e. that the largest difference between the
frequency of all and significant GEM/
222
Rn slopes is in
the bin with the central value of 0.00 pg mBq
−1
(−0.01
to +0.01 pg mBq
−1
), followed by the bins with the central
values −0.02, +0.02, and +0.04 pg mBq
−1
. In the remain-
ing bins almost all correlations are significant. Thus the 97
events with insignificant GEM vs.
222
Rn correlations and
a slope close to zero still provide meaningful information
about the net GEM flux between the surface and the atmo-
sphere, and we have included them in subsequent analyses.
The average GEM /
222
Rn slope of all 191 events is −0.0001
±0.0030 pg mBq
−1
, which is statistically indistinguishable
from the average of −0.0057 ± 0.0051 pg mBq
−1
for 94
events with significant correlations. Both averages cannot be
statistically distinguished from zero flux.
www.atmos-chem-phys.net/13/6421/2013/ Atmos. Chem. Phys., 13, 6421–6428, 2013
6424 F. Slemr et al.:
222
Rn-calibrated mercury fluxes
22
Fig. 3: Seasonal variation of the GEM/
222
Rn slopes (upper panel) and the intercepts
(bottom panel).
Fig. 3. Seasonal variation of the GEM /
222
Rn slopes (upper panel)
and the intercepts (bottom panel).
Figure 3 shows the slopes and the intercepts of all GEM
vs.
222
Rn correlations in the upper and lower panel, re-
spectively. The intercepts represent the background mercury
concentrations at Cape Point. They vary between 0.69 and
1.15 ng m
−3
and average 0.92 ±0.01ngm
−3
for all corre-
lations and 0.93 ± 0.01 ng m
−3
for the significant ones. The
intercepts do not show any apparent seasonal variation. The
slopes vary between −0.105 and +0.178 pg mBq
−1
, and they
also do not show any pronounced dependency on season.
A plot of the slopes against the intercepts (not shown) also
does not reveal any dependence of the flux on background
GEM concentration. However, the average of slopes for the
austral autumn–winter months (April to September) is nega-
tive and with −0.0091 ± 0.0032 pg mBq
−1
(n = 72) signifi-
cantly lower (at > 99.9 % confidence level) than the positive
average for the spring–summer period (October to March)
of +0.0150 ± 0.0056 pg mBq
−1
(n = 119). This is consistent
with the expected temperature dependence of fluxes, but the
stimulation of the flux by seasonally variable precipitation in
the interior of southern Africa with its maximum in the sum-
mer months may also contribute.
Two backward trajectories for the
222
Rn events are shown
in Fig. 4: one for 12:00 UTC of 10 February 2008 (left
panel), and the other for 12:00 UTC of 30 March 2007
(right panel). Both look similar and are typical for most of
the
222
Rn events presented here. They encompass usually
South Africa and the neighbouring countries of Namibia,
Botswana, Zimbabwe, and Mozambique. The GEM /
222
Rn
flux ratio was +0.077 ± 0.008 pg mBq
−1
for the event on 10
February 2008. However, the event on 30 March 2007 (the
lowest of all events with significant correlations) had a flux
ratio of merely −0.026± 0.005pgmBq
−1
. This and the tra-
jectory analysis of other events could not reveal any system-
atic dependence of the terrestrial flux ratios on backward tra-
jectories.
Precipitation is known to stimulate the emission of mer-
cury from soils, especially in arid regions (e.g. Song and Van
Heyst, 2005; Cobbett et al., 2007; Xin et al., 2007). There-
fore, the occurrence of precipitation along the backward tra-
jectories was investigated for 7 of the events with the high-
est emission and 5 events associated with the highest depo-
sition. The events with the highest emission were more fre-
quently connected to intermediate rain over southern Africa
(4 events) than those with highest deposition (1 event), sug-
gesting indeed some degree of stimulation of mercury emis-
sions by precipitation.
The terrestrial surface of southern Africa is presumed
to emit about 1.1
222
Rn atoms cm
−2
s
−1
corresponding
to 23.1mBq m
−2
s
−1
(Zhang et al., 2011). With this
emission rate, the radon-calibrated GEM flux of south-
ern Africa varied between −8.7 and +14.8 ng m
−2
h
−1
.
The un-weighted average GEM /
222
Rn flux ratio of all
events of −0.0001± 0.0030 pg mBq
−1
corresponds to a
flux of −0.01 ± 0.25 ng m
−2
h
−1
. The event duration
weighted average GEM /
222
Rn flux ratio of all events was
−0.0048± 0.0011 pg mBq
−1
corresponding to the GEM flux
of −0.40 ±0.09ngm
−2
h
−1
.
222
Rn decay has not been con-
sidered in these estimates. Assuming an average transport
time of 2 days (corresponding to a transport distance of
∼ 1000 km), the absolute flux value would increase by about
36 % to −0.01 ± 0.34 ng m
−2
h
−1
for an un-weighted mean
and −0.54± 0.13ngm
−2
h
−1
if weighted with event dura-
tions.
To the best of our knowledge, we are not aware of any
long-term measurements of mercury species over southern
Africa. The reactive gaseous mercury (RGM) concentra-
tion in the marine boundary layer around southern Africa is
smaller than 7 pg m
−3
(Soerensen et al., 2010a) representing
less than 1 % of the GEM concentration. Because of much
lower halogen concentrations in the continental boundary
layer, even lower RGM concentrations can be expected over
southern Africa. Assuming that the concentration of particu-
late mercury is within the same range (Slemr et al., 1985), the
contribution of RGM and particulate mercury dry deposition
could still be significant because of their much higher de-
position velocities (Selin et al., 2007). In fact, modelled dry
deposition of RGM for southern Africa is comparable to that
for GEM, each ranging from about 1 to 5ngm
−2
h
−1
(Smith-
Downey et al., 2010). The occurrence of GEM depletion
events at Cape Point was reported by Brunke et al. (2010), but
their mechanism remains obscure. With some 50 events per
year lasting on average ∼ 5 h, they are unlikely to contribute
substantially to the mercury flux even if all GEM were con-
verted to RGM and/or to particulate mercury and deposited.
The terrestrial surface of southern Africa might be
quite unique due to its arid characteristics and as a re-
sult of its location in the Southern Hemisphere. Our un-
weighted average flux of −0.01 ± 0.34 ng m
−2
h
−1
is smaller
than 0.4± 0.5ngm
−2
h
−1
measured over a period of 1 yr
on the forest floor in Standing Stone State Forest in
Atmos. Chem. Phys., 13, 6421–6428, 2013 www.atmos-chem-phys.net/13/6421/2013/
F. Slemr et al.:
222
Rn-calibrated mercury fluxes 6425
23
24
Fig. 4: Backward trajectory for 12:00 of February 10, 2008 (left panel), and 12:00 of
March 30, 2007 (right panel). The GEM/
222
Rn flux ratio was +0.077 ± 0.008 pg mBq
-1
for the event on February 10, 2008, and -0.026 ± 0.005 pg mBq
-1
for the event on March
30, 2007.
Fig. 4. Backward trajectory for 12:00 of 10 February 2008 (left panel), and 12:00 of 30 March 2007 (right panel). The GEM /
222
Rn flux
ratio was +0.077± 0.008 pg mBq
−1
for the event on 10 February 2008, and −0.026± 0.005pg mBq
−1
for the event on 30 March 2007.
Tennessee (Kuiken et al., 2008a) but within the uncertainty
of 0.2 ± 0.9 ng m
−2
h
−1
, measured at six forested sites in
different states of the eastern USA (Kuiken et al., 2008b).
Substantially larger average net emissions of 1.71 (esti-
mate from 1.14 to 4.55) and 1.60 (estimate from 0.86 to
3.20) ng m
−2
h
−1
can be derived from Table 7.5 of the com-
pilation by Mason (2009) for deserts/metalliferrous zones
and savannah regions, respectively, in tropical/subtropical re-
gions.
The radon-calibrated GEM fluxes derived by us do not in-
clude mercury wet deposition. Precipitation measurements
at Cape Point and Pretoria in 2007–2009 yield an aver-
age wet deposition of −1.01 and −2.32ngm
−2
h
−1
, re-
spectively (Gichuki and Mason, 2013). The GEOS model
by Selin et al. (2008) predicts a wet deposition flux of
about −0.34 to −0.11 ng m
−2
h
−1
for pre-industrial times
in southern Africa and an enrichment factor of ∼ 4 due
to anthropogenic activities yielding a current deposition of
about −1.37 to −0.46 ng m
−2
h
−1
. A soil model by Smith-
Downey et al. (2010) predicts a wet deposition rate rang-
ing from ∼ −0.5 ng m
−2
h
−1
in the vicinity of Cape Point to
−4.6 ng m
−2
h
−1
in the industrial region around Johannes-
burg. An improved GEOS model by Soerensen et al. (2010b)
predicts a wet deposition flux of −1.10ngm
−2
h
−1
for Cape
Point. Thus the wet deposition predicted by models is in
reasonable agreement with the measurements of Gichuki
and Mason (2013). Assuming an average wet deposition
flux of −1.01ng m
−2
h
−1
to be representative of southern
Africa and the event duration weighted dry GEM flux of
−0.54± 0.13 ng m
−2
h
−1
from this work, the net deposition
over southern Africa would be about −1.55ngm
−2
h
−1
. Ad-
ditional unknown deposition of RGM and particulate mer-
cury would further increase the net deposition. The terres-
trial surface of southern Africa thus seems to be a net sink
for atmospheric mercury. The GEOS model by Selin et
al. (2008) predicts soils to be a net mercury sink of some
−0.61 ng m
−2
h
−1
, if re-emission by biomass burning is ex-
cluded and the flux to all terrestrial surfaces is considered
to be the same. The exclusion of biomass burning is justi-
fied, since we excluded the short pollution events from our
radon-calibrated fluxes. The model-predicted net deposition
rate is thus smaller than our radon-calibrated fluxes. Speci-
ated mercury measurements and more data on wet deposition
in southern Africa would further constrain the uncertainty of
the net mercury deposition in this area.
4 Conclusions
Radon-calibrated fluxes of mercury over the terrestrial sur-
face of southern Africa were derived from concurrent mea-
surements of GEM and
222
Rn at Cape Point between March
www.atmos-chem-phys.net/13/6421/2013/ Atmos. Chem. Phys., 13, 6421–6428, 2013