Introduction
Refractory (low-volatile) aerosol particles that are released into the
atmosphere due to natural processes (e.g., desert dust storms, volcanism,
pyrocumulus clouds and meteoric ablation) or
stem from anthropogenic sources (e.g., space debris, aircraft/rocket exhaust
and fossil fuel burning) are generally assumed to contribute significantly to
stratospheric aerosol composition (Carslaw and Kärcher, 2006). The kind
and strength of the impact of refractory particles of specific sources to the
overall stratospheric aerosol load vary with altitude and latitude. The
respective contribution of the refractory species depends on their fluxes
into the stratosphere, which are to a large extent not yet quantifiable.
One major source region of stratospheric refractory particles is assumed to
be the interplanetary space (Murphy et al., 1998, 2007a; Plane, 2012), where
cosmic dust is incorporated into the Earth's atmosphere. Additionally, when
larger cosmic bodies enter the upper atmosphere, a trace of ablation smoke is
left behind (also referred to as meteoritic smoke particles, hereafter MSPs)
due to high-temperature vaporization caused by the friction with air
molecules. Estimates of the global influx of material from space are highly
uncertain and vary within 2 orders of magnitude between ∼ 3 and
300 t day-1 (Plane, 2012). Apart from cosmic influx, aerosol particles
from the troposphere which are lifted into the stratosphere due to effective
vertical transport mechanisms (cf. Gettelman et al., 2011) also presumably
contribute to the stratospheric aerosol composition. Some contribution to the
stratospheric aerosol load may be provided by moderate tropical or
subtropical volcanism (e.g., Soufrière Hills or Nabro) even though the
volcano's vertical expulsion energy is too weak to directly inject material
into the stratosphere, in contrast, for example, to the 1991 Pinatubo
eruption. The ejected particulate matter accumulates in the upper troposphere
(Mattis et al., 2010) or lower stratosphere (Vernier et al., 2011; Borrmann
et al., 2010), where the Brewer–Dobson circulation likely lofts these
particles to higher altitudes within the stratosphere (Vernier et al., 2011).
Of course, aerosol material from other terrestrial sources (fine-mode desert
dust or biomass burning material from boreal fires) could also follow this
pathway into the stratosphere unless these species vanish due to in-cloud
processes (solution in the liquid phase and chemical transformation) or
precipitation (Fromm et al., 2000; Jost et al., 2004)
The present work focusses on analyses of particles sampled within or nearby
the Arctic winter vortex. The transport pathways of material in the upper
stratosphere and mesosphere converge towards the polar regions. The poleward
transport at high atmospheric altitudes (i.e., in the upper stratosphere and
mesosphere) is driven by the development of a polar vortex (Megner et al.,
2008; Saunders et al., 2012; Bardeen et al., 2008; Hervig et al., 2009)
occurring alternatingly twice per year, in the Arctic and the Antarctic. The vertical transport inside the vortices due to large-scale
air mass subsidence provides a fast, and likely most effective, removal
pathway of mesospheric and stratospheric material into the lowermost
stratosphere and upper troposphere, which takes place over timescales of
weeks to months during polar winter (Curtius et al., 2005; Weigel et al.,
2014). Once the vortex dissolves in late polar spring, the included aerosol
material is released towards lower latitudes within the lowermost
stratosphere. Indications for this process are provided by MSPs identified in
the midlatitude lowermost stratosphere by in situ laser ablation mass
spectrometry (Murphy et al., 1998, 2013; Cziczo et al., 2001). Though these
studies do not unambiguously link the transport history of observed MSPs to
previous vortex subsidence, it is likely that these MSPs underwent the polar
downward transport pathway prior to their effusion into the midlatitude
lowermost stratosphere. Due to the vertical transport in connection with
subsidence of mesospheric air over the Arctic winter, the contribution of
MSPs is expected to dominate the in-vortex refractory aerosol composition.
However, if tropospheric material is lofted from the tropics lower
stratosphere by the Brewer–Dobson circulation into the upper stratosphere, a
certain contribution of terrestrial material (e.g., silicates, soot)
can be expected, but this needs to be proven.
In the Arctic mesosphere the abundance of refractory particles, presumably
mostly dominated by MSPs, is demonstrated by detection of charged ultrafine
particles (< 100 nm diameter) at concentrations of 40–1000 of positive
or negative charges per cubic centimeters (Rapp et al., 2007; Rapp and
Strelnikova, 2009). Rocket-borne detection of charged particles with
concentrations of the order of thousands per cm3 at an altitude between 80
and 90 km (Amyx et al., 2008; Rapp et al., 2012; Friedrich et al., 2012;
Plane et al., 2014) confirms the previous remote sensing studies. At lower
altitudes in the Arctic vortex stratosphere, the abundance of non-volatile
aerosol material was observed by means of airborne in situ investigations at
up to 21 km (Curtius et al., 2005; Weigel et al., 2014), yielding about 100
particles per milligram of air, providing a fractional contribution of up to
75 % to the total concentration of in-vortex particles. Further in situ
observations at 24–27 km altitude also indicate in the Antarctic
stratosphere (Campbell and Deshler, 2014) that there is a relative increase
of refractory aerosol, contributing a fraction of up to 80 % to total
concentrations.
Refractory aerosol particles have particular importance in the stratosphere
as they may generally provide surfaces for heterogeneous chemical reactions
(e.g., chlorine activation). Furthermore, the presence of surfaces promotes
the condensation of saturated gaseous compounds. It can be assumed that the
presence of refractory aerosols such as MSPs or volcanic material impacts the
maintenance of the stratospheric aerosol layer (Junge layer). Bigg (2012)
even suggests that MSPs play a major role in controlling the stratospheric
background aerosol. Whenever refractory particles in the atmosphere concur
with saturated gaseous compounds (e.g., H2SO4, HNO3 or
H2O), the particles provide surfaces onto which vapors nucleate
heterogeneously (cf. Saunders et al., 2010, 2012; Bigg, 2012; Frankland et
al., 2015). Compounds of refractory aerosol may dissolve in acidic solutions
which could impact the freezing properties of, for example, binary
HNO3–H2O or ternary HNO3–H2SO4–H2O solution
droplets. Thus, solutes of an refractory particle in a
HNO3–H2SO4–H2O droplet could have crucial impact on
formation of polar stratospheric clouds (PSCs) (Chang et al., 1999; Wise et
al., 2003). Due to condensation of vapors the refractory cores grow, which
has direct implication for the particles mass and sedimentation speed, thereby
promoting a redistribution or removal of the condensed material.
The number of terrestrial supermicron refractory aerosol particles in the
middle atmosphere is low as micrometer-sized particles are removed quickly
due to sedimentation. The model study of Dhomse et al. (2013) concerning
atmospheric redistribution of 238PuO2 particles revealed that
submicron particles reside about 4 years in the atmosphere before reaching
the surface.
Some of the first scanning electron microscopy (SEM) images of aerosol
particles that were sampled at altitudes of 20–37 km were provided by Bigg
et al. (1970, 1971). Presumably, meteoritic ablation particles are shown here
as agglomerates of small spherical particles. Due to the gap of chemical
analysis techniques at these times it can only be assumed that the collected
spheroids were previously exposed to high (melting point) temperatures.
Further SEM/EDX (energy-dispersive X-ray detector; EDAX,
Tilburg, the Netherlands) analysis of particles sampled in the stratosphere was shown
by Mackinnon et al. (1982). These authors were able to attribute large
amounts of collected particles to their cosmic origin.
Knowledge of the physicochemical properties of refractory aerosol in the
stratosphere is still sparse. Large uncertainties remain concerning the
relative contribution of different sources such as meteoritic ablation
material, terrestrial crustal particles or ablation of space debris. During
two Arctic airborne missions with the Russian high-altitude research aircraft
M-55 Geophysica, located in Kiruna, Sweden, particles samples were
taken in the vortex stratosphere, exclusively above 410 K potential
temperature. The major aim of the present study is to gain insight into the
contribution of refractory particles of different origin to the overall
stratospheric aerosol composition. Furthermore, several samples were
collected when penetrating extended PSC fields. Thus, chemical analysis of
these particles may help to better understand their role in PSC formation.
Methods
Sampling campaigns
Particles were collected during two polar stratospheric campaigns in arctic
winters 2010 and 2011. Both campaigns
were scheduled for flights within the polar vortex and carried out under
potential PSC conditions. Flights were operated from Kiruna, Sweden, with the
high-altitude aircraft M55-Geophysica. This research aircraft can reach a
maximum altitude of 21 km and has a total range of about 2500 km
(Stefanutti et al., 1999). The RECONCILE (Reconciliation of essential process
parameters for an enhanced predictability of Arctic stratospheric ozone loss
and its climate interactions) campaign (von Hobe et al., 2013) in arctic
winter 2010 (January–March) focused on central questions regarding polar
ozone depletion: among others, the role of cold binary aerosols in
heterogeneous chlorine activation and polar stratospheric cloud processes
including heterogeneous nucleation of nitric acid trihydrate (NAT) and ice on
non-volatile background aerosol. The suite of instruments merged within
Geophysica during the RECONCILE campaign was the most comprehensive one to
date with respect to PSC processes flown on a single aircraft (von Hobe et
al., 2013). While the main objective of the ESSenCe (ESA Sounder Campaign)
campaign in December 2011 was the in-flight testing of the GLORIA (Gimballed
Limb Observer for Radiance Imaging of the Atmosphere) instrument and thereby
the retrieval of three-dimensional small-scale structures in
upper-tropospheric/lower-stratospheric composition (Kaufmann et al., 2015),
ESSenCe provided a second opportunity to collect stratospheric aerosol under
partly comparable conditions to RECONCILE.
Sampling technique
All aerosol samples during the RECONCILE campaign were taken from the inlet
line of COPAS (COndensation PArticle counting System; Curtius et al., 2005;
Weigel et al., 2009; Borrmann et al., 2010) with a Y-shaped manifold.
According to Weigel et al. (2009) the inlet efficiency can be compared with
the similarly designed and operated inlet system
presented by Hermann et al. (2001). For submicron particles, the
transmission efficiency of the COPAS aerosol inlet is 90 % or higher. The
inlet performance rapidly deteriorates for particles of micrometer size
(about 30–40 % for particles with 4 µm diameter). The
transmission efficiency of the COPAS inlet for particles with diameter larger
than about 6µm is below 5 %.
Particle collection was performed with a dual-stage cascade impaction system
(MINI; Micro Inertial Impactor, Kandler et al., 2007) with orifices of 0.75
and 0.25 mm and a jet-to-plate distance of 1.5 times the orifice diameter
(Fig. 1 left). Air velocity in the second nozzle is at speed of sound and,
thus, controlling the impactor flow, which was calculated to be
7.7 cm3 s-1 at stratospheric conditions. Temperature in the COPAS
system varied between 272 and 290 K and pressure between 50 and 67 hPa.
Efficiency cutoffs of 50 % for these values calculated according to Raabe
et al. (1988) are 400 nm aerodynamic diameter for the first impactor stage
and smaller than 10 nm for the second. In the present paper, particles in
the first impactor stage are referred to as coarse fraction (> 400 nm),
while those in the second stage are fine fraction (10–400 nm). Note that
under stratospheric conditions the strictness of the impactor size
discrimination is expected to be lower than under tropospheric ones.
Left: single two-stage impactor used during
RECONCILE.
Right: multi-impactor system (allows 12 samples per flight) with purging
system used during ESSenCe.
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During ESSenCe, a newly constructed cascade impaction system (Multi-MINI) was
used (Fig. 1 right). While the basic impaction geometry remains unchanged
compared to the MINI, 12 dual-stage impactor units were integrated into a
single housing. Particle sampling of the single units is controlled by a set
of valves. A 12-fold symmetrical manifold delivers the aerosol to the
separate units. In addition, a purge flow system was added to the Multi-MINI.
The purge flow system fills the tubing and the interior of the manifold prior
to sampling with the current outside aerosol to avoid any carryover of
particles from previous measurements. The purge flow extends to the front of
the first impaction nozzle, so the potential volume affected by carryover is
minimal. The purge time (7 min) was chosen so that the tube and manifold
volume could be filled at least 10 times with the current aerosol. During
RECONCILE and ESSenCe, the separate impactors were operated in the
stratosphere for 20 min each. Particles were collected on self-constructed
highly polished boron substrates (Choël et al., 2005), which are well
suited for particle analysis by SEM and X-ray
microanalysis. During RECONCILE, in the time period from 17 January to
10 March 2010, 11 samples were collected during 11 mission flights
(Table 1). Twelve samples were collected during the only successful flight of
the ESSenCe campaign on 16 December 2011.
Aerosol samples with corresponding flight date, potential
temperature (K) and the vortex indicator based on dynamic tracer measurements
and CTM modeling for RECONCILE and ESSenCe (data from Weigel et al., 2014).
Aerosol
Flight
Potential
Vortex
Flight
sample
date
temperature
index
campaign
(K)
1
17 Jan 2010
410–425
0.6
R
2
20 Jan 2010
410–420
0.2
E
3
22 Jan 2010
415–430
0.9
C
4
24 Jan 2010
425–430
0.3
O
5
25 Jan 2010
440–450
0.9
N
6
28 Jan 2010
440–450
0.2
C
8
2 Feb 2010
425–445
–
I
9
27 Feb 2010
480–515
0.9
L
10
2 Mar 2010
460–470
1.0
12
5 Mar 2010
480–490
0.9
E
13
10 Mar 2010
480–490
0.8
A–F
16 Dec 2011
400–440
0.7–1
ESSenCe
An overview of the stratospheric conditions during sampling is given in
Fig. 2. The main goal of the present study was the identification and
characterization of refractory (non-volatile) particles. As these particles
are supposed to be extremely rare in the stratosphere, long sampling times of
20 min were chosen, leading to overloading of the substrates with the more
abundant typical Junge layer aerosol (mainly of volatile sulfuric compounds
and nitrates). Therefore, the refractory particles are generally embedded in
a matrix of sulfate / nitrate material, above all in the fine stage.
Potential temperature range, vortex situation and presence of polar
stratospheric clouds for the 11 aerosol samples for RECONCILE (2010) and the
ESSenCe sample (2011).
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Secondary electron images of refractory particles in the coarse
fraction: (a) silicatic spheres; (b) Fe-rich particle;
(c) complex metallic mixture (Al / Cr / Mn / Fe);
(d) Ca-rich particle; (e) carbon / silicate mixture;
(f) silicate particle.
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Sample analysis by electron microscopy
Individual particle analysis was performed in a FEI (Eindhoven, the
Netherlands) Quanta 200 FEG Environmental Scanning Electron Microscope
(ESEM) equipped with an EDX.
As a detailed analysis of the sulfates and nitrates was not intended, the
presumable refractory particles were selectively analyzed using backscattered
electron (BSE) images. The use of a boron substrate provides a high contrast
between the substrate (Z=5), the low-Z (Z=7 to 10) and high-Z (Z>10) particles in the BSE image. All particles with an average
atomic number higher than for the Junge layer (Z<10) can easily be
identified as they appear brighter (white spots within the Junge layer
particles). The elemental composition and morphology of all 759 detected
high-Z particles larger than 500 nm projected area diameter (from 11
RECONCILE flights) were studied in detail.
In contrast to the dominating sulfates and/or nitrates in the Junge layer,
the high-Z particles are non-volatile under electron bombardment and the
high vacuum conditions in the ESEM. In the following, the term “refractory”
is used for these particles. Low-Z refractory particles (e.g., soot) with
diameters above 500 nm were not observed on the coarse fraction stage. It
must be noted, however, that some of the refractory particles were stable
under a short electron bombardment only but showed slight signs of beam
damage after considerably longer exposure times (e.g., the Ca-rich
particles). The overloading of the fine fraction stage with sulfates and
nitrates prevented a systematic quantitative analysis of smaller refractory
particles. Thus, the abundance ratio of refractory to non-refractory
particles could not be determined quantitatively for this size-range.
Nevertheless, the fine stage was analyzed for refractory material in order to
obtain at least a qualitative estimate of the composition.
Secondary electron images of an agglomerate found during RECONCILE, containing (a) silicate sphere, (b) aluminium oxide
particle and (c) complex metal mixture.
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Results
Volatile matrix
Refractory particles, which are the focus of the present study, are only a
small fraction of the total collected stratospheric aerosol particles. In all
samples, the volatile Junge aerosol particles are the dominant particle group
(visible in Fig. 5 and 7). These volatile particles are mainly composed of
ammonium and sodium sulfates (Junge, 1963), which was also confirmed by the
present study. These particles, however, are not considered within the scope of this work
and are not regarded further.
Refractory particles
Based on morphology, chemistry and beam stability the following eight
refractory particle groups were distinguished: Ca-rich particles, Fe-rich
particles, silicates, C / Si mixed particles, mixed metal / metal
oxides, Pb-rich particles, aluminum oxide spheres and soot.
All particles, with Ca as the main peak in the EDX spectrum, were classified as
Ca-rich. A typical secondary electron image is given in Fig. 3d. In most of
these particles no elements besides calcium, carbon, nitrogen and oxygen are
detectable. Under electron bombardment, some of the particles show slight
beam damage (visible in the center of the particle, Fig. 3d). The exact
chemical compound (e.g., CaCO3, Ca(NO3)2, Ca(OH)2) cannot
be determined by EDX. In total, 96 Ca-rich particles larger than 500 nm
(13 % of all analyzed large refractory particles) were detected with
highly varying abundance in 8 of the 11 RECONCILE samples. In 6 samples
(nos. 1, 3, 8, 10, 13) only a low abundance of 1–14 %
(equivalent to 1–8 particles sample-1) was observed, while in two
samples (2 and 5) almost 70 % of all detected large refractory
particles were Ca-rich. The high relative abundance in samples 2 and 5 also results
from a higher absolute abundance (50 and 24 Ca-rich
particles, respectively) and a lower abundance of other refractory particle types in these
samples.
Particles with iron and oxygen as major X-ray peaks were classified as
Fe-rich. A secondary electron image of a typical iron-rich particle is shown
in Fig. 3b. More than 50 % of these particles are single spheres or
agglomerates of spheres, tellingly described by Bigg et al. (1970) as
reminiscent of “a bunch of grapes”, indicating an origin by
high-temperature processes. In total 148 particles (20 %) of all detected
large refractory particles were classified as iron-rich.
Left: three secondary particles containing different amounts of
sulfates/nitrates/organics at the beginning of electron microscopic
investigation. Right: under electron bombardment the material evaporates
after a few seconds exhibiting a lead-rich inclusion.
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Particles with silicon and oxygen as major elements were classified as
silicates. Approximately 80 % of silicate particles are spheres or
agglomerates of spheres (Fig. 3a), while the rest shows irregular shapes
(Fig. 3f). In total 243 particles (32 %) of all detected large refractory
particles were classified as silicates. They were found in all 11 samples
with an abundance between 13 and 55 % (6–45 particles sample-1).
Si-rich particles with additional high-carbon contents were classified in a
C / Si-mixed group. These particles are often agglomerates of smaller
irregular and/or spherical particles. In total 44 of all detected large
refractory particles were classified as C / Si-mixed particles
(∼ 6 %). This particle type was only observed in 6 of the 11
RECONCILE samples.
Mixed metal / metal oxide particles are rich in the elements Fe, Al, Cr,
Mn, Ni and/or Zn. Most particles of this group are dominated by Fe and Al
but often with variable abundance of the other metals. All agglomerates
that contain these metals are classified into this mixed group, even, for example, when
silicates were attached to the agglomerates. Within agglomerates
of this group, Fe-rich particles, Al-rich particles, silicates, alloy-like
particles (Cr, Mn, Fe, Ni, Zn) and internal mixtures of all these components
are found. The morphology of these particles is highly variable (Fig. 3c).
These particles have a composition which shows similarities to certain
anthropogenic metal alloys (steel). Around 75 % of the mixed particles
are spheres, contain spheres or show at least rounded structures (Fig. 4),
which indicates a high-temperature origin. In 30 of the mixed particles, K
and F were detected. Of all detected large refractory particles during
RECONCILE, 228 were classified as mixed metal / metal oxide particles
(30 %). The mixed metal / metal oxide particles were a major
component in 7 of 11 RECONCILE samples (17–62 %) but completely absent
in the 4 other samples (2, 4, 5, 9).
All 759 analyzed refractory particles larger than 500 nm belong to the five
particle types presented so far. These five particle types were also found in
the fine stage (particle diameters smaller than 500 nm), together with three
further refractory particle types, which were not found in the coarse stage.
A small number of Pb-rich inclusions within volatile particles from the Junge
layer aerosol (Figs. 5 and 6) were observed in the RECONCILE and ESSenCe
samples. Due to their small size (often around 50 nm) and the fact that
these particles are often embedded in the secondary matrix, the chemical
composition cannot be determined in more detail by SEM/EDX analysis.
Secondary electron images of refractory particles in the fine
fraction: (a) lead-rich agglomerate (bright) within a secondary
particle; (b) soot particle; (c) agglomerate of spherical
silicate particles; (d) soot inclusions (marked by arrows)
within volatile material.
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Varying amounts of soot particles were observed in all RECONCILE samples (but
not during ESSenCe). Soot can clearly be identified by its characteristic
morphology as agglomerates of spherical primary particles. Most observed soot
particles were internally mixed with sulfates (Fig. 6d) and become visible
during the evaporation of volatile material under electron bombardment.
Secondary electron images of refractory particles in the fine
fraction of ESSenCe samples: (a) aluminum oxide spheres;
(b) Fe-rich particles; (c) Pb-rich particles;
(d) Mg-rich silicates.
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Additionally, some aluminum oxide spheres (AOSs) were detected in the fine
stages of the ESSenCe campaign.
Abundance during RECONCILE
The particle group relative number abundance (PGNA) determined for the 11
flights is given in Fig. 8.
The PGNA is highly variable for the different particle groups without any
clear visible time trend. However, the following patterns are obvious:
samples 2, 4, 5 and 9 show no mixed metal / metal oxide
particles in contrast to all other samples. While samples 2 and 5 are
dominated by Ca-rich particles with a relative number abundance of
approximately 70 %, samples 4 and 9 are dominated by silicates
plus C / Si mixed particles. Samples 3, 6, 8 and 10 have
high relative number abundances of mixed metal / metal oxide particles
(> 40 %). The other samples (1, 12, 13) have comparable
high relative abundances of several particle groups. In addition, the
relative number abundance of C / Si mixed particles is always low
(0–20 %).
When the PSC samples (1–5) are compared to non-PSC samples it becomes
obvious that the former have (on average) a clearly higher abundance of
Ca-rich particles and a lower abundance of Fe-rich and mixed
metal / metal oxide particles. This is not true for each individual
PSC flight, but it has to be assumed that even if samples 1–5
were sampled within a PSC (depending on the specific conditions during the
20 min sampling time), a varying amount of the deposited particles will be
derived from “out of PSC” air masses.
The average diameter of the 759 detected refractory particles is 1.0 µm. A size
distribution for all detected particle groups for the four particle diameter
intervals 0.5–1, 1–2, 2–3 and > 3 µm (equivalent projected
area diameter) is given in Fig. 9. In Sect. 4 the calculated mass mixing
ratio for the refractory particles will also be discussed. As expected, all
particle groups show strongly decreasing particle numbers with increasing
particle diameters; only the mixed metal / metal oxide groups show
highest values for the size interval 1–2 µm, which will be
discussed in Sect. 4.1.5.
A systematic quantitative analysis of the refractory particles smaller than
500 nm on a flight-by-flight basis was not possible, because the fine
fraction stage was overloaded with volatile particles, preventing analysis of
individual particles. Nevertheless, some general findings can be reported.
All refractory particle types for a certain flight, which were found in the
coarse fraction, were also found in the fine fraction. Additionally, soot and Pb-rich particles were observed in the
fine fraction. The Pb-rich
particles are rare in general but seem to be present ubiquitously. In
contrast, the soot abundance seems to be linked with the abundance of the
mixed metal / metal oxide group. The samples without these particles show
no soot at all or only few soot particles (< 10), while all samples with
mixed metal / metal oxide particles show a considerable soot abundance.
The samples with highest mixed metal / metal oxide abundance (flights 6,
8 and 10) also show the highest soot abundance.
Abundance during ESSenCe
Within the single flight of 16 December during the ESSenCe campaign, which
was conducted in early winter (young vortex) within a PSC, no refractory
particles larger than 500 nm were observed. Consequently, only the
observations regarding the fine-fraction refractory particles can be reported
here. Due to the low numbers of observed refractory particles and the
inherent statistical uncertainty, all samples received during this flight
were merged. In total, 116 refractory particles were detected and analyzed
for
the fine stages of all ESSenCe samples: 41 spherical silicates, 30 AOSs, 27 spherical Fe-rich particles and 18 Pb-rich
particles. Ca-rich particles, mixed metal / metal oxide particles,
C / Si-mixed particles and soot, which were observed during RECONCILE,
were not detected within the ESSenCe samples.
Discussion
In Fig. 9 the total number of detected refractory particles for the size
ranges 0.5–1, 1–2, 2–3 and larger than 3 µm is shown. In
principle, the number of detected particles can be used to assume an average
number and mass concentration of the refractory particles during RECONCILE.
As a variety of assumptions has to be made, the uncertainty for the received
mass and number concentrations is large. For particles smaller than 500 nm
no stratospheric concentration can be received on the basis of the electron
microscopic samples because the large amount of volatile material prevents the
determination of the total number of refractory particles in this stage.
Considering the total sampled air volume, the average mass mixing ratio of the
refractory particles (larger than 500 nm) during RECONCILE will be in the
range of 0.1 ppbm. The dN/dlogDP value for particle
diameters of 0.75 µm is around 10-2 cm-3. This
number / mass loading can be compared with other studies under
non-volcanic conditions in the stratosphere. Deshler et al. (2003) published
the results of 30 years of in situ stratospheric aerosol size distribution
measurements from Wyoming using balloon-borne instruments. They found an average
mass mixing ratio for the large mode of 0.02, 5 times lower than
the values determined in this study in late polar winter. Wilson et
al. (2008) report an average mass mixing ratio of 0.4 to 0.8 ppbm for the
aerosol sulfate for the non-volcanic influenced time period 1999–2004 and
for measurements in the polar vortex only a very low volume (close to
0 / linear ordinate) of particles larger than 500 nm. Deshler (2008)
reports in his review of global stratospheric aerosol measurements
observations of local stratospheric aerosol layers. He mentioned that in the
polar regions NAT forms readily on a small fraction (10-4–10-5) of
stratospheric aerosol particles through heterogeneous nucleation. Following
this value the fraction of nonvolatile stratospheric particles, which can
act as heterogeneous nuclei, is at least 100 times lower as the total number
of refractory particles, which we have observed during RECONCILE. He also
reports balloon-borne measurements above the tropopause in Australia,
where aerosol layers with particle diameters above 1 µm and number
concentrations as high as 10-2 cm-3 were observed. Size and number
distribution of this layer seems comparable to our findings, although the
possible sources are completely different (most probably thunderstorms)
to our study. In Weigel et al. (2014) a variety of different model size
distributions of stratospheric aerosol are shown. It can be seen that the
high number of large particles (> 500 nm) is on the upper limit of most
model distributions. However, for such a comparison it must be taken into account that
all presented size distributions do not refer in particular to the situation
in the late polar winter vortex. In this way there is the need for further
measurements of the stratospheric aerosol characteristics in late polar
winter to confirm the observed findings.
Particle group number abundance (PGNA) within 11 samples collected
during RECONCILE flights. PGNA refers to refractory particles larger than 500 nm (* the
number of analyzed particles is shown above each column).
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During RECONCILE Weigel et al. (2014) had observed that the Arctic vortex in
winter 2009/2010 showed an unusually strong subsidence. They found up to 11
submicron particles per cm3 and 8 of these 11 particles were found to
contain thermo-stable (250 ∘C) residuals with diameters of 10 nm to
1 µm. Particle mixing ratios (up to 150 of non-volatile particles
per milligram air) and fractions of non-volatile particles (up to 75 % of
totally detected particles) exhibited the highest values in air masses with the
lowest content of nitrous oxide (70 nmol mol-1 N2O). This
indicates that refractory aerosol originates from the upper stratosphere or
the mesosphere. They concluded that the impact of refractory aerosol material
in the Arctic polar winter is a regular feature. Weigel et al. also estimate the
total mass of mesospheric particles deposited over the 2009/2010 winter in
the entire Arctic vortex to range between 77 × 103 and
375 × 106 kg. This estimate yields an enhancement of the
refractory aerosol mass by a factor of 4.5 at the lowermost vortex
(100–67 hPa) toward the end of an Arctic winter. He also compares this
value with the expected influx of meteoric material
(110 ± 55 × 103 kg day-1) and discusses the
meaning of non-meteoric sources to the vortex refractory aerosol composition.
In the following subchapters we will present the different refractory
particle groups that were detected during the RECONCILE and ESSenCe
campaigns by our electron microscopic investigations. For each particle
group a discussion about the possible origin and the specific contamination
potential is included.
Size distribution of refractory particles during RECONCILE (coarse stage).
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Origin of the observed particle types
Ca-rich particles
Ca-rich particles in the upper stratosphere were recently described by Della
Corte et al. (2013). These authors specify Ca–C–F particles,
calcite / aragonite particles and a porous aggregate of Ca(O) nanograins,
sampled at 38 km height. They also discovered a beam damage of these
particles under SEM analysis, which was also observed in this study. They
hypothesize meteoric dust smoke as source for these particles. While this
cannot be proven based on the available data, Ca-rich particles are at a low
risk to be an instrumental artifact, as Della Corte et al. (2013) used a
totally different sampling setup.
Fe-rich particles
In many airborne upper troposphere/lower stratosphere measurements,
“iron-rich/metallic” particles were reported (e.g., Targino et al.,
2006). However, most studies give no further details about these particles,
not even which actual metals were detected. Zolensky et al. (1989) found two
different Fe-rich particle types, one with high contents of sulfur and one
type without sulfur. In the present study, this differentiation is not
possible because of the sample overloading and the subsequent strong
agglomeration of the sulfur-containing volatile material with most of the
refractory particles (e.g., visible in Fig. 3b and f). Xu et al. (2001) also
applied SEM/EDX for the characterization of individual stratospheric
particles. They detected no particles larger than 500 nm particle diameter
but, in the size range 100–500 nm, iron-rich particles were the main
component of the refractory particles. Mossop (1965) had already suggested in 1965
that small Fe-containing particles may be of extraterrestrial
origin. For example, Hua and Buseck (1998) report on Fe3O4 spherules
observed in carbonaceous chondrites. Another possible source for these
particles is anthropogenic space debris or MDP (Zolensky et al., 1989).
Iron and iron oxide have some risk to be instrumental artifacts, as these
particles could be produced by abrasion of iron-containing material, which is
used for aircraft and inlet design. For example, Murphy et al. (2004)
describe this effect. However, abrasion particles show in principle no
spherical morphology. Furthermore, they show often other alloy components
such as Cr, Mn or Ni, usually derived from stainless steel components. As
consequence, we do not regard the Fe-rich particles as artifacts.
Silicates
In stratospheric samples silicates are often described as the main refractory
component. Most often chondritic material from
cosmic dust is discussed as the source for these silicates. The original chondritic material is characterized
by specific elemental ratios, e.g., a high Mg / Si ratio (∼ 1;
Rietmeijer, 1998). Similar values (∼ 0.85) were determined for the
small (particle diameter < 500 nm) silicate spheres found during ESSenCe
and RECONCILE. The larger silicate spheres (particle diameter > 500 nm),
which were only found in the RECONCILE samples, show lower Mg / Si ratios
(∼ 0.4). It is known that high-temperature processing leads to a loss
of more volatile materials. Volatility studies have shown that within the
residues up to a remaining fraction of 20 % Mg and Si acts similar, but
for even higher ablation rates Mg becomes depleted in contrast to Si (Floss
et al., 1996). In this way the residual spheres, as the remains of heated but
not fully ablated material, can show completely different elemental ratios. In
this way Rietmeijer (2000) states the following: “We may consider the possibility that
non-chondritic element abundances are a norm rather than an exception.”
Twenty percent of the large silicate particles during RECONCILE are not
spheres but show irregular morphologies (Fig. 3f). Even when this fact does
not allow an unambiguous assignment to a high-temperature history, these
particles can originate from cosmic dust. A different source for such
particles is discussed by Zolensky et al. (1989). They suggested that such
silicates with non-chondritic composition (which were the majority in the size
range 1–5 µm in their study) may originate from aerospace
activities. They listed as sources solid rocket fuel exhaust, solid rocket
motor ablation, thermal reflective paint from the outer hull of spacecraft
and ablating hardware from satellites and discarded rocket sections in
low-Earth orbit.
C <inline-formula><mml:math display="inline"><mml:mo>/</mml:mo></mml:math></inline-formula> Si mixed particles
Detailed analysis of the C / Si-mixed agglomerates shows, that carbon and
silicon are often distributed heterogeneously within the analyzed
agglomerates (Fig. 3e). At least half of the mixtures were agglomerates of
carbon-rich grains and silicate particles. This is a hint for carbonaceous
chondrites. In this way, a cosmic origin is the most likely source for the
C / Si mixed particles.
Mixed metal <inline-formula><mml:math display="inline"><mml:mo>/</mml:mo></mml:math></inline-formula> metal oxides
As mentioned above, these particles show a composition which is similar to
certain anthropogenic metal alloys. In the NASA cosmic dust catalog (Warren
et al., 2011), such “alloy-like” stratospheric particles are partly
classified as cosmic and partly as artificial terrestrial contamination.
As metallic inlet abrasion particles are a well-known artifact of
stratospheric particle collection (Murphy et al., 2004) this process cannot
be excluded as a possible source for at least some of these particles. This is
supported by the fact that the average diameter of this mixed group is
1.65 µm, the largest one of all observed groups. Furthermore,
two-thirds
(62 of 93) of all observed refractory particles larger than 2 µm
belong to this group (see also size distribution in Fig. 9). Thus, on the one
hand, with regard to the high number of such large particles in such a small
sample volume, contamination is not an unlikely source.
On the other hand, there is also a variety of findings supporting the
particles' non-artifact origin. First, 75 % of the mixed particles are
spheres or contain spheres or other rounded structures, pointing to a high-temperature history. This is a strong hint that the particles do not
originate from the inlet or the sampling line of the sampling system, as
abrasion particles would not show such spherical structures. Second, these
particles were observed neither in all RECONCILE samples nor in blind samples
taken before and during RECONCILE. As sampling conditions inside the aircraft
were the same for all RECONCILE flights, contamination should thus be
expected to be found in all samples similarly.
Thus, we consider these particles in the present work not to be instrumental
artifacts of the aircraft sampling.
The size, mixing state and chemical composition of these particles fit to
particles described by Mackinnon et al. (1982) in the Johnson Space Center
Stratospheric Dust Collection and classified as “Al prime”. Of all detected agglomerates in this study, 31 % belong to this group.
Mackinnon et al. describe variable abundance of Al, Fe and Si in these
supermicron agglomerates, which is in accordance with the complex mixing state
of Si-rich, Al-rich and Fe-rich primary particles, which we determined within
the complex agglomerates. As the samples during the Johnson Space Center
Stratospheric Dust Collection were collected on flat-plate dust collectors
mounted on the wings of the aircrafts, it is highly unlikely that the
particles in these samples are instrumental artifacts. Mackinnon et al.
assume that the source of the majority of the Al prime particles is
anthropogenic (partly solid rocket fuel exhaust) and that a minority has an
extraterrestrial origin.
A possible origin of these particles might be, similar to the case of the
Fe-rich particles, aerospace activities (rocket exhaust, space debris),
according to Zolensky et al. (1989). Fe and Al, but also other metals, are
among the most abundant elements in such particles which also have a high
chance of having undergone high-temperature processes. A space debris source is
also indicated by the findings of Weigel et al. (2014). During the RECONCILE
campaign they observed a strong subsidence; i.e., most particles will have
their origin at high altitudes (e.g., extraterrestrial, MDP). The
contribution of meteoric particles is discussed controversially, but recent
investigations suggest that meteoric material contributes a significant
fraction of the total stratospheric aerosol mass (Carslaw and Kärcher,
2006).
For example, Jackman et al. (1998), Danilin et al. (2001) and Deshler (2008)
have discussed the contribution of space debris and rocket exhaust particles
for the abundance of refractory stratospheric particles. While they assume
that the abundance of such particles may be on average low, Newman et
al. (2001) has found that the exhaust plume of a single rocket could have
been observed over relatively large time- and spatial scales. In this way
such an individual event (either a rocket plume or a plume of space debris or
meteoric material) could lead to an drastically higher abundance of such
particles in individual stratospheric particle samples and could be one
possible explanation for the large number of observed large refractory
particles during the RECONCILE experiment. The Fe- and Al-containing
particles are commonly observed in stratospheric aerosol particles. Cziczo et
al. (2002) describe Fe and Al signatures in their mass spectrometry
measurements; Kordesh et al. (1983), Sheridan et al. (1994) and Chen et
al. (1998) also describe particles with high Fe and Al contents. In this way,
we may hypothesize that their source must be widespread.
In some of the observed mixed metal particles K and F were also determined.
These elements are often used as fingerprints for welding processes (Berlinger
et al., 2011), as potassium fluoride is used as flux melting agent. Thus, at
least a fraction of the mixed metal / metal oxide particles will have an
anthropogenic (space debris, rockets) and not a cosmic origin.
Pb-rich particles
In both field campaigns small (∼ 50 nm) Pb-rich particles were
observed. Most of the small Pb-rich particles were embedded in volatile
material. The Pb-rich inclusions become visible under electron bombardment
in the ESEM, when the volatile material is evaporating (Fig. 5). It has to be
noted that because of the high average atomic number these particles are
extremely bright in SE image in the electron microscope (see Fig. 6a) and thus may be over-represented in the qualitatively analysis of the fine
stage. The mass abundance of this component can be very low. Nevertheless, by
in situ mass spectrometry measurements Murphy et al. (2007b) found lead in 5 to
25 % of all analyzed particles in the upper troposphere. They
discussed aviation fuel and coal combustion as a source for these particles. A
distinct internal contamination source for lead could not be discovered, but
because of the ubiquitous distribution of trace amounts of lead in the
environment (Murphy et al., 2007b) it cannot completely be excluded.
Soot
Soot particles were also found in the stratosphere by other researchers
(e.g., Renard et al., 2008; Cziczo et al., 2002; Pueschel et al., 1992, 1997;
Sheridan et al., 1994; Strawa et al., 1999). As the most probable source
aviation fuel, rocket launches and forest fires are discussed. Bigg (2011)
also proposed MDPs as a possible source for soot particles in the
stratosphere. This would possibly explain the observation that during
RECONCILE the abundance of small soot particles and of the larger mixed metal
particles (also originating from upper stratosphere/mesosphere) in individual
flight samples seem to be partly correlated. A self-match of the Geophysica
aircraft can be excluded. During RECONCILE one self-match flight was
performed (30 January 2010; see Suminska-Ebersoldt et al., 2012) and it was
shown that such a self-match becomes directly visible in the data of diverse
Geophysica measuring devices like the COPAS system (Weigel et al., 2009).
Aluminium oxide spheres
Solid rocket motors are known to be a source for aluminum-rich
particles in the stratosphere (e.g., Cziczo et al., 2002). The impact of
these particles on the heterogeneous chemistry of stratospheric ozone has
been studied since the early 1970s as part of the Climatic Impact Assessment
Program (CIAP) (Hoshizaki, 1975). Detailed discussion can be found, for
example, in Danilin et al. (2001), Jackman et al. (1998) and Denison et
al. (1994). Cofer III et al. (1991) measured a bimodal size distribution of
AOSs in the Space Shuttle plume with peaks at 2 and < 0.3 µm.
Small AOS spheres (diameter around 100 nm) were observed during RECONCILE
and ESSenCe (see Fig. 7a). While no externally mixed supermicron AOS
particles were detected during both campaigns, supermicron AOSs were part of
the complex agglomerates within the mixed metal / metal oxide group,
observed during RECONCILE and described in Sect. 4.1.5. In Kiruna, rockets
are launched within the framework of the ESRANGE program. A local plume from
this activity cannot be expected as the last rocket launches from Kiruna
prior to RECONCILE were conducted on 22 and 29 November 2009 and therefore
around 7 weeks before the sampling flights. However, there is a constant
entry of particles from space launch activities in to the stratosphere and
mesosphere. For example, during the time period of RECONCILE at least
nine space launches took place, including Proton rockets, Soyuz-U, Delta
rockets and the Space Shuttle Endeavour (see
www.spacelaunchreport.com/log2010.html).
Atmospheric situation during sampling
Within the samples taken during the one single sampling flight of the ESSenCe
campaign, only a small number (106) of sub-µm and no
super-µm refractory particles were detected. In contrast some 100
large refractory particles were observed within the 11 RECONCILE flight
samples. Further on, for the 11 RECONCILE flights at least two different
situations were found. Flights 2, 4 and 5 deliver samples with Ca-rich
particles, silicates, carbon / silicate-mixtures and Fe-rich particles,
while the samples 1, 3, 6, 8, 10, 12 and 13 show an additionally high
abundance of mixed metal / metal oxide particles. A detailed discussion
of the flight conditions with respect to PSC occurrence and polar vortex
situation for all conducted flights can be found in Weigel et al. (2014) and
Molleker et al. (2014). The most important data for the different sampling
flights are summarized in Fig. 2 and Table 1. According to the classification
of Weigel et al. (2014), three sampling flights were performed outside of
vortex, five in the central vortex and three under mixed vortex conditions
(like vortex walls). An assessment of PSC occurrence during the specific
sampling times is based on the data from Molleker et al. (2014). These data
suggest a PSC occurrence for the samples 1, 2, 3, 4, 5 of RECONCILE and the
single flight A of ESSenCe. It has to be noted, however, that due to the long
sampling time and the unknown density and exact location of the PSCs, the
real impact of the PSCs on the single samples is difficult to estimate. For
example, a sample from this subset might originate from anywhere between a
PSC core to the region close to, but not within, a PSC. Thus, it cannot be
concluded that the PSC conditions during these six flights were completely
comparable nor that all sampled particles from these events originate from a
dense PSC.
Nevertheless, it is a clear result that all samples without any mixed
metal / metal oxide particles were taken within classified PSC events. An
exception is the RECONCILE flight 9, where no mixed
metal / metal oxide particles were observed. However, this flight took place
in the outer vortex and at (in contrast to the other flights) a high potential
temperature of ∼ 500 K (see Table 1 and Fig. 2), indicating a
different air mass origin. Within the PSC samples the highest abundance
of Ca-rich particles were also determined.
Further on, there is generally much variation in the potential temperature
(Θ) of the different flights, ranging from Θ=400 K up to
values of Θ=515 K, indicating potential different air masses as
source for the analyzed refractory particles. In general, it is difficult to
compare the ESSenCe situation with a young vortex in early winter with the
RECONCILE situation with advanced subsidence in late winter.
In all ESSenCe and in three of five RECONCILE samples, which were taken within
distinct PSC events, large non-volatile particles were almost completely
absent. During ESSenCe inside the PSC no refractory particles larger than
500 nm could be detected at all and only a small number (116) of small
(< 500 nm) refractory particles were determined. Besides around 35 %
Mg-rich silicate spheres and 25 % Fe-rich spheres, which have
predominantly cosmic origin, 25 % AOSs
presumably from rocket launches were also detected and 15 % lead-rich
particles from an unknown source.
Weigel et al. (2014) state that during RECONCILE air masses from the upper
stratosphere and mesosphere were subsiding inside the Arctic winter vortex,
thus initializing a transport of refractory aerosol into the lower
stratosphere (Θ < 500 K). The strength and extent of this
downward transport depend on the dynamical evolution of the vortex.