Trace metal characterization of bulk and size-resolved aerosol and cloud
water samples were performed during the Hill Cap Cloud Thuringia (HCCT)
campaign. Cloud water was collected at the top of Mt. Schmücke while
aerosol samples were collected at two stations upwind and downwind of Mt.
Schmücke. Fourteen trace metals including Ti, V, Fe, Mn, Co, Zn, Ni, Cu,
As, Sr, Rb, Pb, Cr, and Se were investigated during four full cloud events
(FCEs) that fulfilled the conditions of a continuous air mass flow through the
three stations. Aerosol particle trace metal concentrations were found to be
lower than those observed in the same region during previous field
experiments but were within a similar range to those observed in other rural
regions in Europe. Fe and Zn were the most abundant elements with
concentration ranges of 0.2–111.6 and
1.1–32.1 ng m
Aerosol trace metals play important roles in aerosol–cloud interactions as
they can serve as good catalysts for aqueous-phase reactions. They are useful
in understanding chemical processes and in the identification of parameters
that are important in controlling aerosol behavior. Dissolved trace metals
can form complexes with water, sulfate, and organic compounds and can thereby
influence their redox cycles (Zuo and Hoigne, 1994; Jacob and Hoffmann,
1983). They play important roles in the oxidation of S (IV) to S (VI), which
has been shown to substantially influence the sulfate budget and thus the
formation of cloud droplets (Harris et al., 2013). Transition metal ions such
as iron and copper significantly influence the OH radical budget in aqueous
media as they can react efficiently with many oxidizing and reducing agents
such as HO
Trace metals in continental aerosols originate from extremely complex
mixtures of gaseous and particulate components comprising of motor vehicle
emissions, abrasion of tires or brake linings, road dust, fly ashes from
wood, coal, lignite and oil combustion, refuse incineration, and also crustal
weathering. This mixture influences the chemical composition and properties
of the aerosol as well as the state at which trace metals are available in
the aerosol particles. Depending on the location and meteorological
conditions, the sources of trace metals can differ significantly, leading to
variations in the trace metal and chemical composition of the aerosol. Trace
metal concentrations vary according to their sources. Crustal sources are
usually found to strongly influence concentrations of trace metals such as
Fe, Al, Ti, and Mn with concentration ranges of up to a few
During the Hill Cap Cloud Thuringia (HCCT) experiment that took place in autumn 2010, trace metals were characterized both in aerosol particles at two valley stations, upwind of and downwind from the Schmücke mountain, as well as in cloud water collected at the top of the mountain. The aim was to characterize the trace metal content in the aerosol before and after the particles interact with the cloud and also in cloud water in order to investigate the role of trace metals in altering the chemical composition of the cloud and the properties of the particles after passing through the cloud. Four full cloud events (FCEs) during which the air mass was found to have traveled through all three stations and the strict criteria of the experiment were fulfilled as described in Tilgner et al. (2014) were chosen. In this study the temporal variation of the trace metals during the selected FCEs and the changes in trace metal particle size distribution and their properties after the passage of the particles through the cloud will be presented.
The HCCT-2010 experiment was carried out in the Thüringer Wald, Germany. Based on the experience from the previous FEBUKO (Herrmann et al., 2005) field studies at the same site, sampling was done in the months of September and October in 2010. Three measurement stations were built. One in-cloud mountain station, Mt. Schmücke, and two valley stations, Goldlauter (GL; upwind of the summit station) and Gehlberg (GB; downwind of the summit station) were set up. Cloud water collection was carried out only at Mt. Schmücke while aerosol particle sampling was conducted at the valley stations. A detailed description of the experiment, sampling site, and meteorological conditions during the experiment has been reported by Tilgner et al. (2014).
Size-resolved aerosol particle sampling was conducted using a humidity
controlled low-pressure five-stage Berner impactor with a PM
Bulk cloud water was collected using four stainless steel compact Caltech Active
Strand Cloudwater Collectors (CASCC2) (Demoz et al., 1996). As will be
further discussed below, due to the utilization of the stainless steel
collector, high blanks of some trace metals were observed. For size-resolved
cloud water collection, a three-stage plastic CASCC (Raja et al., 2008) cloud
water collector with nominal cloud droplet cutoffs (50 % lower cut size
specified as drop diameter) at 4, 16, and 22
Total trace metal analysis was performed using total reflection X-ray fluorescence (TXRF) analysis. Size-resolved trace metals were analyzed from
the polycarbonate foils that were placed on the aluminum foils on each
impactor stage. Only filters from impactor stages 1 to 4 were used for trace
metal analysis since impaction spots on the fifth stage were mostly not
visible. Thus, the trace metal concentration at the valley stations is
representative of PM
For cloud water analysis, aliquots were spiked with 69 % conc. HNO
Cloud water trace metal blanks were measured from blank samples that were collected after cleaning of the stainless steel bulk cloud water collector with deionized water after each cloud event. The blanks were not subtracted since they were considered to be influenced by residual materials in the collectors from previous cloud events that were not fully removed during the cleaning process. Therefore, the first collected sample of each event was not considered in the data analysis due to likely strong influence of the wash-off of the collectors. However, most trace metal blanks were lower than 25 % of the hourly cloud water concentrations except for Fe, Cu, Ni, and Zn, whose blanks were mostly higher (up to 65 %) than the hourly concentrations. The results of these four trace metals were thus considered to be very inaccurate and were, therefore, not considered in the further analysis of the results.
For the analyses of the soluble transition metal ions (TMIs) redox states, a DIONEX ICS 900 ion chromatography instrument, equipped with a CS5A column and a UV–VIS detector was used. Using this instrument, transition metal ions such as Fe(III), Fe(II), Cu(II), and Mn(II) can be determined via post-column derivatization simultaneously. This is performed via a procedure similar to the one reported by Oktavia et al. (2008), however using a 4-(2-pyridylazo) resorcinol (PAR, P/N 039672) as post-column reagent. This combination renders a good separation of the abovementioned ions making the analysis easier to handle. TMI redox state measurements were only performed on size-resolved cloud water samples. As indicated above, similar redox state measurements on bulk cloud water samples were not considered for the data interpretation due to their high blank values.
The presented results are focused on data obtained during four FCEs): FCE 1.1 (14 September 2010 11:00 to 15 September 2010, 02:00), FCE 11.3 (2 October 2010, 14:30 to 19:30), FCE 13.3 (6 October 2010, 12:15 to 7 October 2010, 03:15), and FCE 22.1 (19 October 2010, 21:30 to 20 October 2010, 03:30). All times in the manuscript are given in CEST (Central European Summer Time). The results will be presented in two sections. The first section will focus on the aerosol particle analysis and the second on cloud water analysis.
Aerosol trace metal concentration averages
IQR is inter-quartile ranges; all concentrations are in
ng m
The aerosol trace metal concentrations during HCCT-2010 at the valley
stations are presented in Table 1. Reported trace metal concentrations in
other rural regions in Europe and during previous experiments in this region
reported by Rüd (2003) are also presented in Table 1 for comparison.
Amongst the 14 investigated elements, Fe was the most abundant element.
Similar findings were obtained by Rüd (2003) during the FEBUKO
experiment. The next most abundant trace metals were Zn, Cr, and Pb. Iron
concentrations were within the range of 0.2–111.6 ng m
Se and Rb mean concentrations were lower than those reported at Bertiz, Spain
(Aldabe et al., 2011), and Puy de Dôme, France (Vlastelic et al., 2014).
For trace metals such as Ti, Co, Fe, Ni, Cu, and Pb, the mean concentrations were
lower than those reported in the above-stated sites except for Fe and Cu
which were higher than the values reported at Bertiz, Spain (Aldabe et al.,
2011), and at Hyytiälä, Finland (Maenhaut et al., 2011), respectively.
However, the concentration ranges of these metals were within the same order
of magnitude as those reported at the other rural sites. The lower average
values obtained during HCCT-2010 than those reported at other rural sites are
likely due to the different geographical settings of the sampling sites and
the different particulate matter size ranges sampled at some of these sites.
The measurements at the UK sites were based on total suspended particulate
matter (TSP) while the measurements at Payerne in Switzerland (Hueglin et
al., 2005) and the measurements from Rüd (2003), Puy de Dôme
(Vlastelic et al., 2014), and K-Puska (Maenhaut et al., 2008) were done on
PM
Figure 1 shows the size distribution of the trace metals during four full cloud events (FCE 1.1, FCE 11.3, FCE 13.3, and FCE 22.1). In principle, the aerosol mass may decrease during the passage of the air mass parcel through the cloud and over the mountain due to particle and/or cloud droplet deposition during air mass transit across the forested mountain. Entrainment of cleaner air from above might also reduce concentrations or, in the case of more polluted air mass from above, increases the concentration. However, during HCCT, the meteorological analysis as reported by Tilgner et al. (2014) did not show strong entrainment from more polluted air masses especially during the selected events. In Fig. 1, such a trend is observed for FCEs 1.1, 13.3, and 22.1 wherein the total trace metal mass decreased from the upwind side (GL) to the downwind side (GB) of the mountain. During FCE 11.3, there was no significant difference in the total particle trace metal mass, suggesting there may have been limited deposition or an influence from local sources such as traffic, as seen in the increase in the Cr, Cu, and Ni concentrations. During FCE 1.1 Fe was mostly found in the coarse mode, while Zn and Pb were mostly found in the fine modes (stages 2 and 1) at Goldlauter. In Gehlberg, the trace metal size distribution changed with the trace metals concentrated in the fine mode as compared to the coarse mode observed in Goldlauter. The different concentrations and size distributions are indicative of preferential particle loss during this event as a significant part of the concentration difference is largely due to the decrease in the Fe concentration by a factor of 4 in the fourth stage. The decrease in the mass concentration is related to loss of Fe-containing material after passage of the air mass through the cloud, especially as iron is mostly found in the coarse mode. Coarse mode particles are usually emitted from mechanical processes such as re-suspension of soil particles in the atmosphere or abrasion of car parts due to friction or from other industrial processes. These particles are larger and heavier and can therefore be easily lost through deposition.
Aerosol particle size-resolved trace metal concentrations during
four full cloud events at Goldlauter (GL) and Gehlberg (GB). The PM stage
cutoffs for St. 1 to St. 4 are 0.05, 0.14, 0.42, 1.2, and
3.5
During FCE 11.3 the size distribution did not vary largely. Although a decrease in the fine mode fraction was apparent especially due to loss of Zn- and Pb-containing particles, the changes in the coarse mode fraction were not significant. Cr, Cu, and Ni concentrations were found to be higher at GB. Although entrainment of more polluted air masses may lead to an increase in the concentration of trace metals in the downwind site, meteorological studies by Tilgner et al. (2014) revealed a more stable stratification during this event with no large differences observed in the coefficient of divergence of the particles, suggesting that the possibility of entrainment of polluted air masses from above was rather low. These increased trace metals are often found from traffic emissions relating to car brake wear (Maenhaut et al., 2005) or from fuel combustion (Lee and Vonlehmd, 1973). The GB measurement site was close to the main road linking GB and Mt. Schmücke and could be influenced by traffic especially during weekends (period of this event) when more touristic activities are present in this region, thus suggesting that traffic emissions may have influenced these concentrations. A different trend was observed during the FCE 13.3 during which the total mass of the trace metals decreased. No significant changes in the shape of the size distribution of most elements were observed. The major elements observed were also Fe, Zn, and Pb. A similar trend was observed during FCE 22.1 with the trace metal concentration decreasing from GL to GB and the size distribution of these metals not changing significantly. Slight increases in Cr and Cu were observed in the coarse mode fraction at GB.
Scatter plots of levoglucosan, a biomass burning tracer, and K, Zn,
As and Pb, in PM
During most of the FCEs the total trace metal concentration dropped from GL to GB preserving the size distribution. An exception was observed during FCE 11.3 where a significant difference was not observed in the coarse mode fraction. Fe, Rb, Sr, Mn, and Ti were mostly found in the coarse mode while Pb, Zn, As, Se, Cr, and Ni were mostly found in the fine mode aerosol particles. Fe and Zn were the most deposited trace metals especially during FCE 1.1 and FCE 11.3. Although back trajectory analysis showed slight differences between the air mass origins during FCEs 1.1, 11.3, and 13.3, this difference was not strongly reflected in the trace metal distributions and no significant difference in the relative contribution of a particular metal was observed. Details on the likely sources of these trace metals will be further discussed in Sect. 3.3 below.
Figure 2 shows scatter plots of levoglucosan with potassium, zinc, lead, and
arsenic in the particulate matter at the upwind (GL) and downwind (GB)
stations in PM
Table 2 shows the correlation coefficients of levoglucosan and other trace
metals in PM
Correlation coefficients (
The combination of these losses may lead to differences in the correlations observed between the upwind and downwind stations. In principle, no differences in this tendency were observed for the coarse and fine mode concentrations of these components, indicating that the fine mode aerosols carried most of the observed levoglucosan. Similar observation has been reported elsewhere (Herckes et al., 2006).
Elemental carbon (EC) is often considered as soot particles that are emitted
from diesel engines, or from other combustion processes as well as from
industrial emissions, while organic carbon (OC) has both anthropogenic and
biogenic sources (Herrmann et al., 2006; Saarikoski et al., 2008). In rural
areas such as GL and GB, EC could originate from
combustion processes or long-range transport from industrial and urban
regions. Figures 3 and 4 show scatter plots of Ti, Mn, and Fe against OC as
well as Pb, As, and Zn against EC at the upwind stations. The
scatter plots reveal good correlation between these components in PM
PM
EC correlation with Pb, Se, As, and Zn remained stable after passage of air
masses through the cloud, implying that the Se-, Pb-, or As-containing material
might have not been strongly deposited or lost during their transport
through the cloud, or the deposition of these metals and EC were similar.
Apparently, these elements are mostly present in particles in the fine mode.
OC also showed good correlation with Fe after passage of air mass through
the cloud but poor correlation with Ti and Mn. The poor correlation after
passage of air mass through the cloud as already explained above is probably
related to preferential loss via wet or dry deposition of Ti or Mn as well
as of OC
At the upwind station, oxalate showed good correlation with iron
(
Enrichment factor (EF) analysis was performed on PM
PM
Size-resolved enrichment factor analysis of aerosol trace metals at the valley stations. Plotted values are averaged over all FCEs with their respective standard deviations plotted as error bars.
Despite these changes, clear trends with increasing enrichment factors from
Rb to Se could be observed. Trace metals were identified and grouped
according to their similar EF. One group of trace metals including Mn, Fe,
Co, Rb, and Sr showed little to no enrichment at all the stages at both the
upwind and downwind stations with EF
The average concentrations and the range of the various metal concentrations observed at Mt. Schmücke are presented in Table 3. Reported concentrations from previous FEBUKO experiment performed at the same location in 2002 and also reported concentrations from fog and cloud measurements done at other mountain sites in the world are also presented. For comparison of the cloud data with the valley stations, and for the understanding of the changes from sample to sample, and to obtain the equivalent air loading, the variation of the liquid water content (LWC), which alters dilution of solutes in the cloud water, was removed by multiplying the cloud water aqueous concentration with the LWC.
Trace metal concentrations decreased from Ti, Mn, Cr, to Co in cloud water.
The average Ti concentration was observed at 9.8
Figure 6 shows the temporal variation of the trace metals during the four FCEs and the variations between the cloud events. Often, higher trace metal concentrations were observed during the first hours of the events and subsequently decreased during the course of the event. This trend was mostly observed during FCE 1.1 and FCE 11.3, while during FCE 1.1 very small differences were observed between the temporal variations of the metals. During FCE 1.1 the trace metal concentrations show uniform variation with slight increase in concentrations observed after 17:00. While As, Pb, Se, V, and Mn showed similar trends, Ti and Cr showed very different patterns. Similar trends were observed during FCEs 11.3, 13.3, and 22.1 for As, Pb, and Mn, respectively. Ti trends were unique throughout the FCEs. During FCE 13.3 and FCE 22.1, Co, V, and Cr showed similar temporal variations. The temporal variations in the trace metal concentrations are likely due to the temporal changes in the trace metal concentrations in the activated cloud condensation nuclei (CCN) and changes in air mass origin. The similar trend in the temporal variation of the trace metals is indicative of their similar origin. Ti is a good tracer for crustal sources while Pb and V are good tracers of anthropogenic activities. Thus, the origins of the trace metals were similar throughout most of the events and were of anthropogenic and crustal origins.
Trends in total trace metal cloud water concentrations during selected full cloud events: FCE 1.1: 14–15 September 2010, FCE 11.3: 2 October 2010, FCE 13.3: 6–7 October 2010, and FCE 22.1: 19–20 October 2010.
Soluble Fe(III), Fe(II), Cu(II), and Mn(II) were measured from
0.45
Temporal variation of size-resolved soluble transition metal ions,
Fe(III), Fe(II), Cu(II), and Mn(II) in cloud water on 14–15 September 2010
during FCE 1.1. The nominal stage cutoffs were 22, 16,
and 4
Mean and range of trace metal concentrations in cloud water at Mt. Schmücke during HCCT 2010.
Concentrations are in
Similar soluble trace metal concentrations have been reported in fogs (Straub
et al., 2012) and cloud water (Li et al., 2013; Parazols et al., 2006;
Hutchings et al., 2009) in other regions. The differences in the size
distribution of these trace metals suggest that these TMIs might have had
different source origins especially for Mn and Fe. Although Mn as well as Fe
has high crustal abundance, both also have different anthropogenic sources
such as waste incineration or metallurgical industries for Mn and fly ash for
Fe. Fine mode iron can also be emitted from fuel combustion processes
(Sholkovitz et al., 2012), which can thus explain its occurrence in this size
fraction. Thus, the differences in the size distribution of these metal ions
could be due to their source origins during this event or the differences in
the efficiency of activation of the particles containing these metals. Fe(II) was observed mostly during evening hours during which photochemical
processes are expected to be low due to lower solar radiation. This indicates
that the observed nighttime Fe(II) concentrations were not directly related
to photochemical processes. Fe(III) is known to be the most stable form of
iron, but it can be reduced to Fe(II) via HO
Due to the fast oxidation of Cu(I) to Cu(II), it is easier to measure Cu(II) than Cu(I). During FCE 1.1, Cu(II) was successfully measured with results revealing comparable concentrations as those reported elsewhere (Deguillaume et al., 2005; Li et al., 2013; Hutchings et al., 2009). The Cu(II) temporal variation is also shown in Fig. 7. The observed Cu(II) concentration in the same size fraction as soluble iron indicates that the Fe(II) at night could have been related to aqueous-phase reductions of Fe(III) to Fe(II) by Cu(I) species. Fe(III) catalyzed oxidation of sulfur (IV) has been suggested to be another important source of Fe(II) in cloud water during nighttime (Schwanz et al., 1998; Millero et al., 1995). However, with no data available on sulfur (IV) measurements during FCE 1.1, conclusions about the contribution of this pathway cannot be made. Anthropogenic sources of Fe in the environment, e.g., in fly ash, biomass burning emissions and other combustion sources, have been found to contain considerable concentration of Fe(II) (Trapp et al., 2010). This suggests that, although aqueous-phase reduction processes could have contributed to the observed nighttime Fe(II) concentrations, the source of the aerosol cannot be neglected. Nighttime concentrations of Fe(II) have also been observed in different studies, but no unique reason for their occurrence has been presented yet (Schwanz et al., 1998; Kotronarou and Sigg, 1993; Siefert et al., 1998; Parazols et al., 2006). The variation of the soluble trace metal concentrations with cloud drop size suggests that the metal-catalyzed oxidation of S(IV) may be strongly drop size dependent since the concentrations of TMIs influence the in-cloud S(IV) oxidation rates (Rao and Collett, 1998).
PM
Concentrations are in ng m
As shown in Tables 1 and 3, Mn, Rb, and Ti in cloud water were higher than at
the valley stations. However, although the average values for Mn and Ti were
higher in cloud water as compared to those from the valley stations, the
concentration ranges were similar. One reason for the observed difference in
the Mn, Ti, and Rb concentrations could have been the analyzed particle size
from the impactor samples collected from the valley stations. Mn, Ti, and Rb
are often of crustal origin and also often found in the coarse aerosol
fraction. As explained above, the aerosol trace metal analysis was performed
only on PM
Soluble aerosol trace metal concentrations were measured from
0.45
Although it is known that cloud processes enhance the solubility of trace metals, especially for elements such as Fe via acid dissolution or complexation with organic acids such as oxalic acid, no clear trend and significant difference was observed between the upwind station (GL) and the downwind station (GB) soluble iron concentrations. Since most of the FCEs were observed in the evening period, the possibilities for photochemical processes to occur were limited. As with iron, no clear trend in the variation of the soluble trace metal concentrations was observed for the other metal ions during all the FCEs. The variation in the observed concentrations during the different FCEs is also related to influences in air mass origin and the loss due to preferential deposition of larger or more efficiently cloud-scavenged particles.
The characterization of trace metals in size-resolved and bulk aerosol
particles at the valley upwind (Goldlauter, GL) and downwind (Gehlberg, GB)
stations as well as in cloud water at Mt. Schmücke during four cloud
events during HCCT has been presented. The concentrations of the 14
investigated trace metals showed variations between the cloud events with Fe
and Zn being the most abundant observed trace metals in the aerosol. The most
deposited trace metals between the upwind and downwind stations were also Fe
and Zn. Aerosol particle trace metal concentrations were lower than those
observed in the same region during the FEBUKO field experiments about 9 years
ago. The decrease in metal concentrations could be related to decreases in
emission as well as differences in the PM size fraction analyzed (PM
The authors acknowledge the support of several TROPOS staff members during cloud water sampling and aerosol sampling, the German Federal Environmental Agency (UBA) for their support and cooperation at Schmücke field site and the administrations of the villages Suhl-Goldlauter and Gehlberg for their help. HCCT-2010 was partially funded by the German Research Foundation (DFG) under contract HE 3086/15-1. Partial additional support for Colorado State University was provided by the US National Science Foundation (AGS-846 0711102 and AGS-1050052). Edited by: C. George