AEROSOL PHYSICO-CHEMICAL AND CCN PROPERTIES IN AND AROUND
ANTARCTICA DURING THE AUSTRAL SUMMER
K.N. FOSSUM1, J. OVADNEVAITE1, D. CEBURNIS1, S. MARULLO3, M. DALL’OSTO2, R.
SIMO2, C. O’DOWD1
1
School of Physics & Centre for Climate and Air Pollution Studies, Ryan Institute, National
University of Ireland Galway
2
Institut de Ciéncies del Mar (CSIC), Barcelona, Catalonia, Spain
Agenzia nazionale per le nuove tecnologie, l'energia e lo sviluppo economico sostenibile, ENEA —
Centro Ricerche Frascati, Frascati, Italy
3
Keywords: Cloud condensation nuclei, maritime polar and continental Antarctic air mass, physicochemical properties
INTRODUCTION
The marine environment is an important source of aerosol primary [C. D. O'Dowd and Leeuw, 2007;
G. de Leeuw et al., 2011; Vignati et al., 2010] and secondary production [Myriokefalitakis et al.,
2010; Rinaldi et al., 2010]. In marine environments isolated from anthropogenic activity, natural
biological components can alter the chemical and physical composition of those aerosols [O’Dowd
et al., 2015]. In the Antarctic Ocean, algal blooms occur in the open ocean but also under permanent
and temporary sea ice in the Weddell Sea [Arrigo, 2014]. The temporary ice, will often break and
separate during the summer season, although remain unmanageable for ships to navigate. Cold water
algae will have high quantities of DMSP [Simó, 2001] (Usually more DMSP found in biological
blooms in colder water [Gabric et al., 2005] which is precursor to MSA, a chemical component
associated with new particle formation (NPF) [Charlson et al., 1987; Dawson et al., 2012; Leaitch
et al., 2013] leading many to believe that these regions may have inject large contributions of
organically rich sea spray into regional aerosol.
Organic enrichment has been seen to increase CCN activation efficiency of marine particulates, even
those that are already efficient CCN [Ovadnevaite et al., 2011]. Findings like this has led to the
expectation of increased CCN in areas of biological enrichment, such as algal blooms in otherwise
“clean” (black carbon < 2 ng/m3) or anthropogenically unaffected areas. Earth system models
(ESM) use biological enrichment parametrisations of CCN activation to better resolve cloud
formation over the marine environment. To do this they need a way to track biological activity with
good correlation to biological enrichment. Tracking biological enrichment has, therefore, been a hot
topic. There is evidence to suggest that satellite retrievals of chlorophyll-a (chl-a) density in marine
waters might be the best available way to mark areas where organic enrichment can be expected
[O’Dowd et al., 2015; Rinaldi et al., 2013]. Algal blooms, once thriving, enrich sea-spray with
organic matter as th bloom enters its demise phase and in fact a time lag of this chl-a retrieval and
organic enrichment has been shown [O’Dowd et al., 2015; Rinaldi et al., 2013]. However, it has also
been suggested that a persisting carbon pool on the marine surface layer is responsible for organic
carbon enrichment, without influence from local biological events. This would suggest chl-a as a
poor source for tracking biological enrichment [Quinn et al., 2014], but is in stark contrast with the
results of O’Dowd et al., [2015].
Previous studies [Davison et al., 1996] have shown polar air masses arriving from the Antarctic
continent to the South Atlantic and Antarctic Oceans were shown to consist primarily of H2SO4 in the
accumulation mode size range, with inferred NH+4 to SO=4 molar ratios close to zero. By comparison,
air masses of temperate maritime origin were significantly neutralized with molar ratios of 1. Events
of new particle formation were identified in the Weddell Sea and occurred under conditions of high
DMS flux and low aerosol surface area.
This region is particularly of interest due to its minimal influence of anthropogenic inputs and
probably is the most pristine and biologically-rich, region of the planet. Equally so, the impacts of
anthropogenic influence can be clearly quantified and the aerosol-cloud system is also quite
susceptible to perturbations, making it a unique natural laboratory to study this system.
The primary objective of this study is to characterise aerosol properties as a function of air mass
history, or origin.
METHODS
The PEGASO cruise, on board the Research Vessel Hesperides, took place as a joint
National University of Ireland Galway – Barcelona Marine Institute of Science effort to
elucidate climatological processes through the investigation of biological, physical, and
chemical components of the Antarctic marine waters. The Hesperides, departed from
Barcelona in September, 2015, and arrived in the Southern Ocean in December, 2015, after
which it traversed the Southern Ocean in the Austral summer (January – February, 2015).
The ship path can be seen in red in figure 1. An extensive suite of aerosol instrumentation
for a combination of in-situ ambient measurements, sea water extraction, and bubble-tank
experiments were employed during the cruise. The study reports on the physico-chemical
properties of marine and (Antarctic) continental aerosol encountered in the region over
January and February, 2015.
Multiple instruments were employed for in-situ ambient measurements during the
campaign. Meteorological activity was monitored and recorded throughout the cruise. There
was then sampling off of laminar flow lines to an HR-Tof-AMS, a TSI standard SMPS, a
TSI standard CPC (range 10nm -3um), and the continuous-flow streamwise thermalgradient CCN counter (CCNC) commercially available from Droplet Measurement
Technologies, Inc.[Roberts and Nenes, 2005; Rose et al., 2008]. The CCNC was set-up
using a DMA system to size segregate the aerosol samples [Paramonov et al., 2013; Rose et
al., 2010]. Eleven sizes were scanned, each for 60 s, with mobility diameters ranged from
20-244 nm against five supersaturations ranging from 0.08 -1.48%. The first and second size
scan were both 20 nm to allow CCNC temperatures to stabilize. Critical diameters could
then be extracted at a minimum time resolution of one hour. Critical diameters were later
used in conjunction with the SMPS to find the average hourly total CCN at varying
supersaturations. Origin of arriving aerosols were resolved from 120 hr back trajectories
from HYSPLIT [Stein et al., 2015], using a starting point of 100 m AGL on ship’s GPS
position.
RESULTS AND CONCLUSIONS
Air masses are large bodies of air that have been influenced by large scale homogeneous
surfaces (land or sea). They generally reach characteristic temperatures and humidities after
sustained influence of surface fluxes over many days. Their meteorological properties are
typically invariant within the air mass and the air masses are also associated with synoptic
scale meteorological systems. Notable changes in the meteorological parameters are
normally only associated with frontal passage, representing a change in air mass. Similar to
pseudo-steady-state meteorological parameters being characteristics of air mass origin,
atmospheric composition parameters (e.g. reactive gases and aerosols) are also expected to
possess pseudo-steady-state characteristics. In classifying air masses, we filter the dataset for
extended periods of relatively stable or invariant meteorological and atmospheric
composition characteristics, setting stability over 4 hours as the minimum requirement for
selection and inclusion into the air mass characterisation database.
Four main air mass influences were observed; three maritime polar, (mP), air masses, and
one continental AntarcticA (cAA) air mass. The first mP comes from the region of the
Southern Ocean, West of Argentina, the second from the region East of Argentina, and the
third is a modified mP influenced by the Weddell Sea. The cAA air is also modified due to
Weddell Sea influence. One example of each is presented in Figure 1.
Weddell Sea modification seems to play a heavy role in the aerosol chemistry of observed
cases, with a heavy emphasis on non-sea-salt-SO4, (nss-SO4), domination. Activation
efficiency for CCN follows a slope indicative of nss-SO4, with low levels of sea salt
contributing to increase activation efficiency at larger sizes. The lack of sea salt
contributions can be seen in a lower mass-volumetric mode event in the larger particle sizes.
The mP air masses are distinct in that, in many of the cases, particle number in the Aitken
mode was almost doubled to that associated with the accumulation mode. However, due to
the larger fractional contribution of sea salt, the mass-volumetric contribution increases at
larger diameters.
The results also indicated that while organic mass fraction, nss-SO4, NH4, and organic
nitrogen can be correlated to CCN numbers, chlorophyll-a (chl-a) mass satellite retrievals do
not correlate to CCN number. However, there is a strong correlation between enriched
organic fraction [O’Dowd et al., 2015] and chl-a.
Case 1,
cAA,
Weddell
Sea
modified
Case 6,
East mP,
Weddell
Sea
modified
Case 12,
West mP
Case 9,
East mP
A
B
C
D
Figure 1. Aerosol chemical and physical characterization, broken down by case. Each case has 4
columns, A, B, C, and D. [A] Air mass back trajectories extracted from HYSPLIT [Rolph, 2016;
Stein et al., 2015]. In red, PEGASO cruise ship path. In blue, 120 hr back trajectory ending 100m
AGL directly above location of ship. On trajectories, (A) denotes the trajectory from start of the case
period, (B) denoting the middle, and (C) the end. The larger the letter the greater the air mass height
(0-1000m above sea level). [B] Particle number (black) and volume (blue) size distributions. On left,
number distribution, with case variability shown in grey. On top are the total average number of
particles cm-3 (N), and total average particulate volume cm-3 (V). [C] Pie chart of chemical mass
fractions in cases. On top, total mass m-3 is listed, followed by black carbon mass m-3 (BC). [D] CCN
activation efficiency slopes. Graph shows critical supersaturation against critical dimeter, case slope
shown in black, and expected/accepted values of ammonium sulphate shown in red and sodium
chloride shown in green from AP3 model [Rose et al., 2008].
ACKNOWLEDGEMENTS
The research leading to these results has received funding from the European Union’s Seventh
Framework Programme (FP7/2007-2013) project BACCHUS under grant agreement n_ 603445;
Spanish Ministry of Economy and Competitiveness (MINECO) as part of the PEGASO (Ref.:
CTM2012-37615) and BIO-NUC (Ref:CGL2013-49020-R); HEA-PRTLI4.
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