Bulk and size-segregated chemical composition
In Dronning Maud Land cyclone-driven marine air intrusions are infrequent,
sporadic events that are often associated with high precipitation rates
(Birnbaum et al., 2006; Schlosser et al., 2010; Welker et al., 2014; Kurita
et al., 2016). Such a cyclone-induced advection of marine boundary layer air
masses towards the Antarctic Plateau had essentially coined the physical and
chemical properties of the aerosol on-site, most noticeably by a maximum of
biogenic sulfur concentrations and the occurrence of an NPF event. During a
previous similar general weather situation at Kohnen (10 and 11 January
2002; LPS02), Piel et al. (2006) reported even strikingly higher
nss–SO42- (1100 ng m-3) and MS- (350 ng m-3)
maxima, though about 48 h after the passing of the frontal system and the transition
from marine to continental air mass origin. In contrast to the LPS15, LPS02
came along with heavy snowfall (Birnbaum et al., 2006). Another peculiarity
of our recent observations was a preceding and well-defined MS- peak
pursued by a distinct nss–SO42- maximum (Fig. 9) compared to
their simultaneous emergence in the aftermath of LPS02 described in Piel
et al. (2006). Given that MS- should have been primarily formed by
heterogeneous liquid-phase chemistry prevalent in the marine boundary layer
(Legrand et al., 2001; Bardouki et al., 2002; Hoffmann et al., 2016), the
segregated MS- peak indicated a temporary and efficient advection of
such air masses. In addition, a striking peak of the sea salt tracer
Na+ appeared along with the MS- maximum (Supplement, Fig. S7), emphasizing the impact of marine boundary layer air masses during this
part of LPS15. Note that Na+ remained at typical mean concentrations
around 7 ng m-3 during the former biogenic sulfur maximum between 14 and
15 January 2002 (Piel, 2004).
Results from size-segregated (Berner impactor) sampling during
the campaign 2015. Bold reddish lines are the mass size distributions during
LPS15. Percentage entries in the legend denote the portion of the
corresponding ion mass in the size range > 1 µm
(super-micrometer or coarse mode) calculated from the inverted profiles.
The bold gray line is the median mass size distribution of the corresponding
ion.
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Results from size-segregated (Berner impactor) sampling during
the campaign 2016 with the following mass portion of the respective ions in
the super-micrometer range (> 1 µm): MS- 14 % (a),
nss–SO42- 3 % (b), Na+ 46 % (c), and NH4+
15 % (d). The bold gray line is the median mass size distribution of the
corresponding ion.
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The 5-day back trajectories confirmed these conclusions (Fig. 14; starting
point 100 m above Kohnen): air masses during LPS15 were generally marine 2
to 3 days before arrival at Kohnen. Trajectories representing doy 19
(NPF event; reddish and yellow traces in Fig. 14a) spent several hours
within the marine boundary layer close to the East Antarctic coast before
arrival at Kohnen, largely following the contour lines of the local
topography. All trajectories started under cyclonic curvature, finally
approaching Kohnen in an anticyclonic bow from northerly directions (Fig. 14).
In order to estimate the reliability of this trajectory-based finding,
we repeated the calculations with an initial height of 10 m above Kohnen,
calculated trajectory ensembles, and made an extra attempt using the
isentropic approach instead of the 3-D wind field from GDAS data
(Figs. S8–S10). Though appreciable differences
regarding the geographic location of the corresponding source regions and
trajectory course were obvious, in the end the basic aforementioned
implications appeared consistent. In contrast to this finding, air masses
originated and stayed within continental Antarctica during the biogenic
sulfur peaks observed during 14 and 15 January 2002 (Piel et al., 2006),
which could be confirmed by a reanalysis with HYSPLIT trajectories
based on NCEP meteorological data (Fig. S11). Similar
air mass trajectories (not shown) were observed during and in the aftermath
of a short stormy period LPS16, which was again characterized by enhanced
Na+ and biogenic sulfur loadings (Figs. 10 and S7b).
Regarding the chemical composition of the aerosol during the final stage of
LPS15, subsequent increasing nss–SO42- along with declining
MS- concentrations indicated a minor importance of liquid-phase
chemistry (Hoffmann et al., 2016). We may speculate that intrusions of
marine boundary layer into the so-called buffer layer were responsible for
the efficient advection of gaseous DMS photooxidation products like SO2
and DMSO (Davis et al., 1998; Russel et al., 1998). Russel et al. (1998)
assumed that the buffer layer typically extends from the turbulent marine
boundary layer (400 to 700 m) up to a capping inversion (1400 to 1900 m). While
transported to continental Antarctica, gas-phase photooxidation
processes should have dominated, leading finally to a preferred formation of
H2SO4 at the expense of MSA (Preunkert et al., 2008). This
plausible but subtle transport route could not be unequivocally deduced from
respective backward trajectory analyses because in the case at hand the
presence and the extent of a buffer layer could not be ascertained from
available meteorological data.
Mean size-segregated results for the
MS- / nss–SO42- mass ratio βMS determined for each of
the five impactor stages for both seasons and for the event LPS15.
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Another conspicuous point was the comparatively large part of MS-,
most notably NH4+, found in coarse-mode aerosol during LPS15
(11.5 and 39 %, respectively; Fig. 11). Apart from post-sampling
reactions on the first two impactor stages, which could not be entirely
excluded, preceding heterogeneous processes in the atmosphere, i.e.,
the acidification of sea salt particle surfaces and subsequent chemisorption of
(basic) NH3, should be a more realistic explanation. Though MS-
added up to merely 0.03 compared to 0.072 nmol m-3
NH4+ in coarse aerosol during LPS15, other acidic gases (probably
HNO3, but according to our data only to a minor extent H2SO4)
reacted with coarse-mode sea salt particles and scavenged gaseous NH3.
Previous size-segregated aerosol sampling at Concordia revealed that coarse-mode ammonium was primarily present as sulfate or methane sulfonate salts,
while nitrate was of minor importance (Becagli et al., 2012). During the
Japanese–Swedish joint Antarctic expedition (JASE), on the other
hand, Hara et al. (2014) reported H2SO4- and MSA-modified sea
salt particles close to the coast, while farther inland most probably the
reaction with HNO3 dominated. According to recent year-round
investigations at Concordia, however, it appears that the competing role of
HNO3 vs. acidic sulfur aerosol is more complicated: regarding coarse-mode sea salt particles, only during midsummer and late summer does sulfuric aerosol
and not HNO3 become the preferred acidic reactant (Legrand et al.,
2017b).
The 5-day backward trajectories during the NPF event calculated
with a trajectory starting height of 100 m above Kohnen at the points in
time given in the legend (a). Below, the travel height above the ground (local
topography) is illustrated in a color-coded scale (point interval 1 h) (b).
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In conclusion it is worthwhile to consider the results from Neumayer. This
coastal site was governed by the same low-pressure system (LPS15) provoking
a blizzard there around 19 January 2015. From there, daily low-volume
sampling (Teflon–nylon filter combination), CP concentration (measured with
a CPC 3022A, TSI; Dp(50%)= 7 nm), and meteorological data were
available. Again, biogenic sulfur (particularly MS-), Na+, and CP
concentrations showed distinctive maxima though about 1 day after the
LPS15 (Fig. S12), similar to the previous situation
described by Piel et al. (2006). Consequently we can assume that this
characteristic weather situation transported marine aerosol throughout DML.
But in contrast to Kohnen the impact of LPS15 on the delayed particle
concentration and ionic composition maxima at Neumayer was less pronounced.
Pant et al. (2010) concluded from previous particle concentration and size
distribution data measured at coastal Maitri that during the impact of
cyclones, coarse-mode sea salt aerosol increased by an order of magnitude
compared to calm weather conditions, which is similar to our results during LPS15
(Table 2).
NPF, particle growth, and size-segregated chemical composition
Obviously the most striking feature during LPS15 was a distinctive NPF
event. The closed shape of PNSD, starting with a modal maximum not less
than 12 nm, implied that the actual particle nucleation event should have
occurred upwind of Kohnen. Assuming a constant growth rate of 0.6 nm
h-1 throughout advection to the measuring site, particle nucleation
thus happened about 20 h before, i.e., according to backward trajectories
roughly 700 km away from Kohnen at 73∘ S, 19∘ E and 160 m
above the ground (all virtually independent from the choice of the starting
height). Note that the observed particle growth was confined to the
nucleation mode and consequently did not reach a size range potentially
relevant for acting as CCN. As described in Weller et al. (2015), NPF events
at Neumayer generally showed almost comparable growth rates. Therein we used
a simple estimate based on Nieminen et al. (2010) and Yli-Juuti et al. (2011)
that demonstrated the need for condensable vapors other than sulfuric
acid to sustain the observed particle growth (Weller et al., 2015). We
employed the same approach in connection with the NPF event at Kohnen
(i.e., Eq. 3 in Weller et al., 2015, using T= 250 K), resulting in
1.4 × 107 molec cm-3 gaseous H2SO4 necessary for
the determined growth rate of 0.6 nm h-1. Again, this appeared at least
an order of magnitude too high compared to the respective values published
(e.g., Mauldin III et al., 2004), emphasizing the importance of other yet
unknown condensable vapors, most probably low-volatility organic compounds
(Metzger et al., 2010; Tröstl et al., 2016). Interestingly, Kyrö et
al. (2013) identified biogenic emissions by nearby melting ponds as a
potential source for condensable vapor. The surroundings of Kohnen,
however, are completely ice covered throughout as typical for the Antarctic
Plateau region. The nearest rocky outcrops are more than 200 km away.
In this context it is interesting to compare our results with previous PNSD
measurements conducted at the South Pole during two summer campaigns (Park et
al., 2004) and year-round observations at Concordia (Järvinen et al.,
2013). Results from the South Pole showed similar PNSDs with mean a Dp
between 60 and 98 nm (Park et al., 2004, Table 4 therein), while PNSDs at
Concordia were appreciably lower with Dp= 39 nm (Järvinen et
al., 2013). Järvinen et al. (2013) could detect NPF throughout the year
under condensation sink values of around 1.8 × 10-4 s-1, comparable
to the condensation sink range found at Kohnen. At Concordia, NPF events
with a determinable growth rate were not connected with low-pressure systems
but essentially observed under air mass advection from the upper atmosphere
(Järvinen et al., 2013). This fact indicates that also at Kohnen, NPF
may potentially occur during clear sky conditions, but could not be detected
within our admittedly limited observation period. Furthermore, the growth rates
determined so far at coastal sites (Virkkula et al., 2007; Asmi et al.,
2010; Kyrö et al., 2013; Weller et al., 2015) appeared comparable to GR
reported from continental Antarctica (Järvinen et al., 2013; Chen et
al., 2017) and were within a similar broad range between 0.2 and up to 8.8 nm h-1.
Pant et al. (2010) presented a detailed analysis about the impact of passing
cyclones on particle size distributions at coastal Maitri. The authors
observed bimodal PNSD with a coarse-mode maxima around 2 µm and a
broad Aitken mode between 0.04 and 0.1 µm when a storm
approached the site. Occasionally NPF occurred just after the passage of a
cyclone associated with particle growth rates between 0.2 and 0.6 nm h-1.
From meteorological data the authors conclude that the observed
NPF events were linked with the mixing of marine and continental air during
the subsidence of free tropospheric air after the storm (Pant et al., 2010).
Daily 10-day backward trajectories (3-D approach, starting height
100 m) during clear sky conditions in 2016 (doy 12 to doy 31, N=80).
Shown is the relative (percentage) number of trajectory intersections on a
given grid cell (resolution 1∘ × 1∘).
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Daily 20-day backward trajectories (3-D approach, starting height
100 m) during clear sky conditions in 2016 (doy 12 to doy 31, N= 80).
Shown is the relative (percentage) number of trajectory intersections on a
given grid cell (resolution 1∘ × 1∘).
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