The concentrations of submicron aerosol particles in maritime regions around
Antarctica are influenced by the extent of sea ice. This effect is two ways:
on one side, sea ice regulates the production of particles by sea spray
(primary aerosols); on the other side, it hosts complex communities of
organisms emitting precursors for secondary particles. Past studies
documenting the chemical composition of fine aerosols in Antarctica indicate
various potential primary and secondary sources active in coastal areas, in
offshore marine regions, and in the sea ice itself. In particular,
beside the well-known sources of organic and sulfur material originating
from the oxidation of dimethylsulfide (DMS) produced by microalgae, recent
findings obtained during the 2015 PEGASO cruise suggest that
nitrogen-containing organic compounds are also produced by the microbiota
colonizing the marginal ice zone. To complement the aerosol source
apportionment performed using online mass spectrometric techniques, here we
discuss the outcomes of offline spectroscopic analysis performed by nuclear
magnetic resonance (NMR) spectroscopy. In this study we (i) present the
composition of ambient aerosols over open-ocean waters across bioregions,
and compare it to the composition of (ii) seawater samples and (iii) bubble-bursting aerosols produced in a sea-spray chamber onboard the ship. Our
results show that the process of aerosolization in the tank enriches primary
marine particles with lipids and sugars while depleting them of free
amino acids, providing an explanation for why amino acids occurred only at
trace concentrations in the marine aerosol samples analyzed. The analysis of
water-soluble organic carbon (WSOC) in ambient submicron aerosol samples
shows distinct NMR fingerprints for three bioregions: (1) the open Southern
Ocean pelagic environments, in which aerosols are enriched with primary
marine particles containing lipids and sugars; (2) sympagic areas in the
Weddell Sea, where secondary organic compounds, including methanesulfonic
acid and semivolatile amines abound in the aerosol composition; and (3)
terrestrial coastal areas, traced by sugars such as sucrose, emitted by land
vegetation. Finally, a new biogenic chemical marker, creatinine, was
identified in the samples from the Weddell Sea, providing another
confirmation of the importance of nitrogen-containing metabolites in
Antarctic polar aerosols.
Introduction
The Antarctic continent is one of the last pristine areas of our planet, but
its natural ecosystems are now threatened by an acceleration of the effects
of global warming. Although at the beginning of the 21st century the signals
of climate change looked still weak in the region, the ice-sheet mass loss
in Western Antarctica has greatly accelerated in the last 10 years as the
Southern Ocean waters have experienced a clear warming trend (The IMBIE team, 2018). The consequences for Antarctic maritime and coastal environments
encompass strengthening of westerly winds, reduction of summer sea-ice
extent, shifting geographical ranges of bird communities, expanding
terrestrial vegetation, increasing glacier melt, and freshwater formation
over land, etc. (Rintoul et al., 2018). As all these specific ecosystem
impacts involve factors deemed important for aerosol production in
Antarctica (Davison et al., 1996; Schmale et al., 2013; Kyrö et al.,
2013; Barbaro et al., 2017), a significant effect of climate change on
atmospheric concentrations of aerosols and cloud condensation nuclei (CCN)
must be expected to occur by the end of this century. Field studies
performed in maritime and coastal areas around Antarctica in the austral
summer since the 1990s (Davison et al., 1996) enable us to gather precious
information on the multiple feedbacks between atmospheric composition and
ecosystems in a warming climate. In summer, the sea ice recedes, allowing
wind stress over the oceanic surface and sea spray to occur closer to the
continent, hence increasing the production of primary marine aerosols. At
the same time, the thinning of sea ice in its marginal zone and the
increased intensity of solar radiation allow microalgae to colonize the ice
(Fryxell and Kendrick, 1988; Roukaerts et al., 2016). The microbiota produce
low molecular weight metabolites as cryoprotectants and osmoregulators, like
dimethylsulfoniopropionate (DMSP) and quaternary nitrogen compounds
(Dall'Osto et al., 2017). Once released in seawater, such compounds become
precursors of atmospheric reactive volatile reactive compounds, such as
dimethylsulfide (DMS) and methylamines, which eventually can lead to the
formation of secondary aerosols. Indeed, Davison et al. (1996) observed
concentrations of DMS south of 60∘ S of more than 4 times higher
than in the Atlantic Ocean. DMS and other reactive volatile species are
known precursors to the secondary marine aerosol that contribute to the
aerosol population in the marine boundary layer together with primary
sea-spray particles. Marine aerosols impact global climate by reducing the
amount of solar radiation reaching dark surface of the ocean, both directly
(through scattering) and indirectly (by modulating cloud formation and
lifetime; O'Dowd and de Leeuw, 2007). Furthermore, in polar regions, cloud
seeding by marine aerosols transported over glaciated regions also affects
the longwave radiation budget (Willis et al., 2018).
During the 2015 PEGASO cruise (Dall'Osto et al., 2017, 2019; Fossum et al.,
2018), we conducted continuous atmospheric observations for over 42 d,
providing one of the longest shipborne aerosol measurement records in this
area of the world. We contrasted the composition of seawater north and south
of the Southern Boundary of the Antarctic Circumpolar Current (SBACC), which
represents the approximate boundary between the open Southern Ocean and the
waters directly affected by sea-ice formation and melt around Antarctica.
Dall'Osto et al. (2017) showed that not only did DMSP and DMS occur in
greater concentrations in sympagic waters (south of the SBACC), but
quaternary nitrogen compounds and methylamines did as well. By contrast, other
biological parameters of seawater, like chlorophyll a, total organic carbon
(TOC), and transparent exopolymeric particles (TEPs), showed higher
concentrations in the open Southern Ocean north of the SBACC. Results of
bubble-bursting experiments conducted on nascent seawater as well as using
melted sea ice showed that organic nitrogen and organic carbon were more
abundant in the aerosol in the latter case. Moreover, the production of
organic-rich particles was better traced by markers of the ice biota, such
as mycosporines, than by macro-tracers of biological productivity
(chlorophyll). These results indicate that not only productivity per se but
also the composition and ecophysiological state of the microbiota affect the
production of aerosol precursors in seawater. Indeed, the observations of
organic nitrogen in the aerosol – carried out using both online and offline
chemical methods – pointed to strong sources in the area of the Weddell Sea
which, at the time of the field campaign, was heavily covered by sea ice.
These findings contribute to the growing observational dataset of aerosol
chemical compositions for coastal Antarctic and sub-Antarctic marine areas,
which hosts reports of chemical analysis performed on filter and impactor
samples (Davison et al., 1996; Virkkula et al., 2006), as well as the results
of online aerosol mass spectrometric techniques acquired in recent years
(Zorn et al., 2008; Schmale et al., 2013; Giordano et al., 2017). All the
measurements performed so far agree, showing a reduction of sea-salt
aerosols from the Southern Ocean to the coasts of Antarctica, while
secondary species including non-sea-salt sulfate and methanesulfonate (MSA)
occur at relatively higher concentrations at higher latitudes as a result of
the DMS emissions from marginal ice zone waters. Open questions remain about
(a) the amount of non-MSA organic matter in Antarctic air masses, and (b) its
origin (either primary or secondary). Recent studies also suggests that
blowing snow at high wind speeds may be an important yet hitherto
underestimated source (Giordano et al., 2018; Frey et al., 2020), adding
complexity to the source apportioning of organic aerosols. First
observations of organic carbon (OC) in size-segregated aerosol samples
collected at a coastal site in the Weddell Sea (Virkkula et al., 2006)
showed that MSA represented only a few percent of the total OC in the submicron
fraction. In contrast with these findings, aerosol mass spectrometric (AMS)
measurements showed that the organic matter in submicron aerosols
transported in Antarctic air masses was almost totally accounted for by MSA,
while non-MSA organic compounds were associated with aerosols originating from
highly productive waters in the Southern Ocean (Zorn et al., 2008). Non-MSA
OC can form also from insular terrestrial biomass emissions (Schmale et al.,
2013). In particular, organic particles emitted from seabird colonies
contain large amounts of nitrogen with MS spectral fingerprints overlapping
with those of natural amino acids. In the paper by Liu et al. (2018), Fourier-transform infrared (FTIR)
spectroscopy was employed to probe the sources of particulate organic
compounds at another coastal Antarctic site, and the results point to a
contribution of marine polysaccharides transported in sea-spray aerosols.
Finally, detailed organic speciation using offline analytical techniques
with high sensitivity and selectivity suggest further contributions from
marine proteinaceous material, terrestrial lipids, and secondary organic
compounds (Bendle et al., 2007; Barbaro et al., 2015, 2017), but it is unclear
how much the concentrations of compounds occurring at pg m-3 relate to
that of bulk organic matter. We present here the organic characterization of
Antarctic aerosol employing proton nuclear magnetic resonance (1H-NMR)
spectroscopy. NMR spectroscopy has been used for decades in several fields
of biogeochemistry for its ability to fingerprint several classes of
biomolecules and natural organic matter in aquatic and terrestrial
environments (e.g., Pautler et al., 2012; Hertkorn et al., 2013). In this
study, which focuses on the analysis of samples collected during the PEGASO
2015 cruise, we contrast the NMR composition of submicron aerosol samples
with that of seawater samples and bubble-bursting aerosols. The results
provide new hints on the origin of non-MSA aerosol organic matter in fine
aerosol particles in the Antarctic and sub-Antarctic marine environment.
ExperimentAmbient aerosol sampling on filters
The PEGASO (Plankton-derived Emissions of trace Gases and Aerosols in the
Southern Ocean) cruise was conducted onboard RV Hesperides in the regions of
Antarctic Peninsula, South Orkney Islands, and South Georgia Island from 2 January
to 11 February 2015 (Dall'Osto et al., 2017). A high volume sampler (TECORA
ECO-HIVOL, equipped with Digitel PM1 sampling inlet) collected ambient
aerosol particles with Dp<1µm on pre-washed and pre-baked
quartz-fiber filters, at a controlled flow of 500 L min-1. Sampling was allowed
only when the samplers were upwind the ship exhaust with a relative wind
speed threshold of 5 m s-1. Due to the necessity of collecting
sufficient amounts of samples for detailed chemical analyses, sampling time
was of the order of ∼50 h for each sample. A total of eight
PM1 samples were collected during the cruise (Fig. 1). The samples were
stored at -20∘C until extraction and NMR analysis.
Cruise of RV Hesperides. The colors indicate the duration of the
single aerosol samplings (short interruptions undertaken to avoid
contamination from ship emissions are not indicated in the figure). The
average time spent by air masses traveling over land (brown), marginal ice
zone (1 %–99 % surface coverage; grey), compact sea-ice (100 % coverage;
white), and open ocean (dark blue) is indicated for each sample.
For high-performance liquid chromatography mass spectrometry (HPLC-MS) analyses, aerosol samples were collected on PTFE fiber filters
(70 mm diameter, Pallflex T60A20, Pall Life Science) with flow rates of 2.31–2.41 m3 h-1 through a PM2.5 inlet. Sampling times ranged from 12 to 24 h, resulting sampling volumes of 28.1–56.1 m3 of air. As outlined
above, sampling was only allowed when the sampler was upwind the ship
exhaust.
Seawater sampling and tank experiments
Seawater samples were collected from a depth of 4 m using either the
uppermost Niskin bottle of the CTD rosette casts or the ship's flow-through
underway pumping system. The samples were filtered with a Millipore
filtration apparatus on quartz-fiber filters (Whatman, Ø=47 mm) after a
previous cut off at 10 µm performed with a polycarbonate filter
(Millipore, Isopore, porosity = 10 µm, Ø=47 mm). In total 45
samples were collected for subsequent quantification of the particulate
organic carbon (POC) and 20 mL of the filtrates were stored for subsequent
analysis of dissolved organic carbon (DOC). All the samples were stored at
-20 ∘C until the chemical analyses. Three samples of sea ice
from the marginal ice zone in the northern Weddell Sea were also collected
using the methodology described in Dall'Osto et al. (2017). The samples,
once melted, were filtered and treated similarly to the seawater samples.
Seawater was pumped from a depth of 4 m to fill an airtight high-grade
stainless steel tank (200 L) designed for aerosol generation experiments. Sea-ice samples were also introduced and melted in the tank for dedicated
experiments. Water was dropped from the top of the tank as a plunging jet at
a flow rate of 20 L min-1. The entrained air formed bubbles that, upon
bursting, produced sea-spray aerosol, as reported in O'Dowd et al. (2015).
Particle-free compressed air was blown into the tank headspace (120 L min-1), which had outlet ports leading to samplers for the collection
of filters and the subsequent offline chemical characterization of the
produced sea spray. In particular, nine sea-spray aerosol samples were
collected for approximately 72 h by a PM1 sampler (flow rate 40 L min-1) equipped
with pre-washed and pre-baked quartz-fiber filters (PALL, Ø=47 mm). In
six cases, bubble-bursting experiments were conducted in the tank
continuously flushed with fresh seawater conveyed form the ship's pumping
system. In the three sea-ice experiments, bubble bursting was carried
out in a closed-loop system instead because of the limited amount of water volume available from the melted sea-ice samples. In this case, the bubble-bursting
process could lead to chemical and biological modifications in the samples
like a progressive depletion of surfactants on the film. Quantification of
such artifacts is unavailable. Nevertheless, past studies carried out in
different geographical region of the northeast Atlantic but with the same
apparatus showed no evidence of decreasing organic enrichment in the
generated sea spray when operated in a closed-loop system (O'Dowd et al.,
2015).
Parallel bubble-bursting aerosol generation experiments with the same
seawater and sea-ice samples were carried out using a smaller glass tank (10
L) continuously flushed with particle-free air (11 L min-1; Schwier et al., 2015) and were dedicated to sea-spray aerosol characterization using
online mass spectrometers (HR-ToF-AMS and ATOFMS). The results from the
bubble-bursting experiments in the small tank are already reported in
Dall'Osto et al. (2017).
1H-NMR spectroscopy
Quartz-fiber filters from both ambient POC filter samples and sea-spray
generation experiments were extracted with deionized ultrapure water
(Milli-Q) in a mechanical shaker for 1 h and the water extract was filtered
on PTFE membranes (pore size: 0.45 µm) in order to remove suspended
particles. The water-soluble organic carbon (WSOC) content was quantified
using a TOC-TN thermal combustion analyzer (Multi N/C 2100 by Analytik Jena; Rinaldi et al., 2007). Aliquots of the aerosol extract were dried under
vacuum and re-dissolved in deuterium oxide (D2O) for organic functional
group characterization by 1H-NMR spectroscopy, as described in Decesari
et al. (2000). The 1H-NMR spectra were acquired at 600 MHz in a 5 mm
probe using a Varian Unity INOVA spectrometer, at the NMR facility of the
Department of Industrial Chemistry (University of Bologna). Sodium
3-trimethylsilyl-(2,2,3,3-d4) propionate (TSP-d4) was used as an
internal standard by adding 50 µL of a 0.05 % TSP-d4 (by weight)
in D2O to the standard in the probe. To avoid the shifting of
pH-sensitive signals, the extracts were buffered to pH ∼ 3 using a
deuterated-formate/formic-acid (DCOO-/HCOOH) buffer prior to the
analysis. The speciation of hydrogen atoms bound to carbon atoms can be
provided by 1H-NMR spectroscopy in protic solvents. On the basis of the
range of frequency shifts, the signals can be attributed to H-C containing
specific functionalities (Decesari et al., 2000, 2007). A total of eight
HiVol PM1 ambient aerosol samples (plus one blank), four POC samples from
seawater, two POC samples from melted sea ice, three samples from the
tank experiments (from aerosolization of one seawater sample and two melted
sea-ice samples) plus one blank for the 47 mm filters were characterized by
NMR spectroscopy.
UHPLC-HESI-Orbitrap-MS
One-half of each filter sample was extracted according to the following
protocol: sonication repeated three times in 1.5, 1, and 1 mL ACN/H2O (9:1,
v/v) for 30 min. The extracts were filtered through PTFE membranes (pore
size: 0.45 µm), combined, dried at 50 ∘C under a gentle
stream of N2, resuspended in 200 µL ACN/H2O (1:4, v/v), and
stored at -20∘C until analysis. Samples were analyzed in
triplicate by UHPLC-HESI-HRMS using an Orbitrap mass analyzer (Q-Exactive
hybrid quadrupole Orbitrap mass spectrometer, Thermo Scientific, Germany)
equipped with an UHPLC-System (Dionex UltiMate 3000 UHPLC system, Thermo
Scientific, Germany) and a Hypersil Gold, C18, 50×2.0 mm column with 1.9 µm particle size (Thermo Scientific, Germany). The injection volume
was 20 µL and the eluents were ultrapure water with 2 %
acetonitrile and 0.04 % formic acid (eluent A), and acetonitrile with
2 % water (eluent B). The gradient of the mobile phase with a flow rate of
0.5 mL min-1 was as follows: starting with 2 % B isocratic for 1 min,
increasing to 20 % B in 0.5 min, isocratic for 2 min, increasing to 90 %
B in 2.5 min, isocratic for 4 min, and decreasing to 2 % B in 0.5 min. Mass
spectrometric analyses were performed using an electrospray ionization (ESI) source under the following
conditions: 30 ∘C ESI temperature, 4 kV spray voltage, 40 psi
sheath gas flow, 20 psi auxiliary gas flow, and 350 ∘C capillary
temperature. Mass resolution was 70 000 and the acquired mass range was m/z
80–550. Creatinine calibration results are shown in Table S2 of the
Supplement.
Air mass back-trajectories
Five-day back-trajectories arriving at the ship's position at 03:00, 09:00,
16:00, and 21:00 GMT every day were calculated using the HYSPLIT model (Draxler
and Rolph, 2010) with GDAS data. In
total, 140 air mass back-trajectories were obtained. A polar stereographic
map was used to classify 24×24 km grid cells as land, sea, and ice. From this
information we calculated the percentage of time spent by each trajectory
over each surface type, and particularly over sea ice. Daily maps of sea-ice
percentage concentration measured on a 12.5 km grid were used for this
calculation. Sea-ice abundance was derived from satellite microwave data
(Ezraty et al., 2007) available at IFREMER. This analysis allowed also
assigning air mass trajectories (and percentages of surface type flown over)
to the aerosol samples collected on the filters (Fig. 1).
ResultsOrganic composition of seawater: POC samples
The composition of seawater in terms of pigments, metabolites, fluorescent
organic matter, and other organic constituents from the PEGASO cruise has
been characterized in great detail (Dall'Osto et al., 2017; Nunes et al.,
2019; Zamanillo et al., 2019). Marine organic substances are found in the ocean
in dissolved and particulate form. Particulate organic carbon (POC) is
defined operationally by a filtration cutoff at 0.45 µm, and recovers
phytoplankton cells, bacteria, and large colloids, such as transparent
exopolymeric particles (“TEPs”; Passow et al., 2002). Dissolved organic
carbon (DOC) is mostly contributed by the excreta and metabolites of the
marine biota but it also accounts for a pool of refractory compounds,
resistant to microbial degradation, and is well mixed in the water column
(Hertkorn et al., 2013). Past studies have extensively characterized the NMR
features of labile and refractory organic constituents of marine organic
matter (Repeta, 2015). However, the NMR characterization of the dissolved
organic substances was limited to desalted fractions of DOC isolated by
solid-phase extraction or ultrafiltration (Koprivnjak et al., 2009).
Therefore, the NMR analysis of low molecular weight polar organic
constituents of marine DOC remains elusive. In our study, we screened the
NMR features of POC in phytoplankton bloom areas. In addition, samples of
aerosolized seawater and melted sea ice were used as a proxy of primary
marine aerosol (Dall'Osto et al., 2017). During the process of bubble-bursting performed in the tank experiments, aerosol particles became
depleted in sea salt with respect to seawater and enriched in surface-active
DOC components and in buoyant POC substances. The chemical characterization
of the smallest POC component (0.45–10 µm) aims to provide
information about the composition of the buoyant particles, while the
contribution from DOC to the surface film composition could not be
determined in this study.
Figure 2 shows the proton NMR spectra of three POC samples, one from
seawater (POC W3101) and two from melted sea ice (POC SeaIce-1, and POC
SeaIce-3) as examples. It is worth noting that the samples were pre-filtered
through a polycarbonate membrane of 10 µm porosity, hence the
analyzed POC fraction represents only the fine fraction (between ∼0.45 and 10 µm). During PEGASO, the concentration of fine POC
fraction (0.45–10 µm) ranged between 8 and 12 µmolC L-1 in bloom areas. The subset of samples analyzed by 1H-NMR
spectroscopy exhibited a concentration of 10.6±0.7µmolC L-1 (n=4). Sample POC W3101 originated from the bloom area west of
South Georgia Island, while the two sea-ice samples were collected in the
marginal ice zone of the Weddell Sea. The interpretation of the spectra was
carried out by comparison with the datasets and spectra provided by the
literature on metabolomics (e.g., Bertram et al., 2009; Matulova et al.,
2014; Li et al., 2015; Upadhyay et al., 2016) as well as by means of NMR
analysis of commercial standard compounds. Characteristic patterns of NMR
resonances for specific compounds (e.g., patterns in multiplicity) enabled
an accurate identification, while only a tentative attribution of the most
simple NMR resonances (singlets) was attempted when standards were not
available, because deviations with respect to published NMR data are
possible when different experimental conditions (e.g., in respect to pH of
the sample) are used. Nevertheless, the POC extracts show several NMR
features overlapping with typical ones for other biological matrices. In
particular, the occurrence of most common aliphatic amino acids was observed
in all three samples analyzed and particularly in sample POC SeaIce-1.
Acidic amino acids dominated over the basic ones, while aromatic residuals
were detected only in trace amounts (Fig. S1). The identification of
modified amino acids among the most typical natural products of the Antarctic
microbiota, such as mycosporines (Oyamada et al., 2007), could not be
carried out in detail because of the lack of suitable spectral libraries.
The presence of metabolites such as low molecular weight nitrogen-containing
compounds (choline, betaine, etc.) is confirmed by the singlets in the
chemical shift range 3.1–3.3 ppm from methyls bound to nitrogen atoms
(H3C-N-). Resonances at higher chemical shift ranges, between 3.4 and 4.2,
recovered the -NCHRCO- groups of alpha-amino acids and the H-C-O
groups of sugars and polyols: traces of glycerol were found in all three
samples analyzed, while glucose was found in trace amounts in samples POC
W3101 and as a major component in sample POC SeaIce-3 (Fig. S2). These
results confirm the potential of 1H-NMR spectroscopy for the
characterization of marine metabolites and natural products. The small set
of POC samples analyzed in this study is, however, mainly aimed at providing
spectral fingerprints useful for the interpretation of the results of the
aerosol sample analyses discussed in the following sections.
Aliphatic region of the 1H-NMR spectra of three POC sample
extracts: one for the seawater sample (POC W3201) and two from melted sea
ice (POC SeaIce-1 and POC SeaIce-3). Specific NMR resonances were assigned
to the following: the residuals of amino acids (Ala, Thr, Val, Ile, Leu, Glu and Asp) and
their alpha hydrogen atoms, isobutyric acid (IsoBu), acetic acid (Ace),
dimethylamine (DMA), N-osmolytes (Bet: betaine; Cho: choline), glycerol
(Glc), and glucose (Gls).
Organic composition of bubble-bursting aerosols
The three primary marine aerosol samples collected in the tank and analyzed
by 1H-NMR spectroscopy included the following samples. One sample was
collected from bubble bursting of seawater (BB W1101) obtained
during almost 4 d of navigation west and north of the South Orkney
Islands. Seawater was continuously flushed onboard RV Hesperides maintaining
continuous sea-spray production in the tank. The other two samples (BB
SeaIce-1 and BB SeaIce-3) were obtained from two of the three sea-ice
samples melted in the tank and run in a closed-loop system. Sea ice was
collected from the marginal ice zone around 100 km south of the South
Orkney Islands by using small inflatable boats and clean laboratory ware. The
chemical information obtained for these bubble-bursting aerosols is,
therefore, representative for primary marine particles in the northern
sector of the Weddell Sea. The natural process of sea spray – mimicked by
the experiments carried out in the tank onboard RV Hesperides – selectively transfers
organic compounds from seawater into the aerosol depending on the ability of
the specific pools of organic substances to enrich in the surface microlayer
and/or to be scavenged by rising air bubbles. The selective nature of such
process is witnessed by our NMR data, showing that the seawater composition
dominated by amino acids, osmolytes, and sugars/polyols differs quite
substantially from that of bubble-bursting aerosols from the tank
experiments (Fig. 3, Fig. S3). Bubble-bursting aerosol was characterized
by the occurrence of low molecular weight metabolites like lactic acid and
amines (dimethylamine, DMA, and traces of monomethylamines and trimethylamines),
which likely originated from DOC components of seawater. The most
characteristic feature of the spectra is, however, the bands at 0.9 and 1.3 ppm of chemical shift. These correspond to aliphatic chains with terminal
methyl moieties typical of lipids. Their occurrence in the aerosolized
seawater and not in the POC samples can be explained by an enrichment of
surface-active compounds from DOC in the surface microlayer. Lipid
enrichment in aerosol from bubble-bursting experiments has already been
documented by the two previous studies reporting NMR composition data
(Facchini et al., 2008; Schmitt-Kopplin et al., 2012). Nevertheless, our
findings clearly show that, beside lipids, there are specific constituents
of POC taking part in the formation of primary aerosol particles in the tank
experiments. In particular, the spectral region for sugars and polyols in
bubble-bursting aerosols is completely consistent with the spectral features
of POC (Fig. S4), although the contribution of the -NCHRCO-
groups of amino acids in the same spectral window is clearly missing in the
aerosol. The presence of nitrogen-containing metabolites (betaine) is
confirmed in the aerosol samples from the tank. It is plausible that
betaine, glycerol, and other sugars have a chemical bond to lipids, making
glycolipids and phospholipids, which could explain their preferential
enrichment during the aerosolization process with respect to other POC
constituents like amino acids. It is a matter of fact that amino acids
could be detected only in very trace amounts (the doublet of alanine at 1.45 ppm is barely visible) in the sea-spray samples. Other molecular tracers
found in previous sea-spray experiments in other geographical regions, such
as acrylic acid (Schmitt-Kopplin et al., 2012), which is also product of
DMSP degradation, were not found in our experiment.
The same as Fig. 2 but for the three bubble-bursting aerosols:
from seawater sample W1101 (BB W1101) and melted sea ice no. 1 and no. 3 (BB
SeaIce-1 and BB SeaIce-3). Specific resonances were assigned to lactic acid
(Lac), acetic acid (Ace), isobutyric acid (IsoBu), alanine (Ala),
dimethylamine (DMA), glycerol (Glc), N-osmolytes (Bet: betaine; “N-osms”:
unidentified, possibly phosphocholine), and blank contaminations (b).
Unresolved mixtures of aliphatic compounds were identified as lipids.
Organic composition of ambient submicron WSOC samples
The eight ambient PM1 HiVol samples analyzed for organic composition include
six that were collected in parallel to the impactor samples discussed in
Dall'Osto et al. (2017). The proton NMR spectra of the eight samples are
reported in Figs. S5–S7. Air mass origin varied largely during the cruise,
with transport from the Weddell Sea prevalent during the first half of the
cruise turning into open-ocean prevailing air masses during the second half
(Fig. 1). Two samples (A-0701 and A-0102) of mixed origin were omitted
by Dall'Osto et al. (2017), who focused on the comparison between aerosols
from the sympagic regions and those from the open ocean. We applied
hierarchical cluster analysis to investigate if a dual classification also
held with the NMR spectra (Fig. 4). The original spectra were normalized
to their integrals and binned to 354 points before clustering. Two main
clusters were indeed identified: a first one recovering three samples
collected downwind the Weddell Sea during the first half of the cruise, and
a second cluster with samples representative of a greater diversity of
conditions, from the Drake Channel to the Antarctic Peninsula and the
productive waters around South Georgia Island. This second cluster corresponds to
the samples characteristic for the open-ocean conditions in Dall'Osto et al. (2017) plus samples A-0701 and A-0102. Unexpectedly, sample A-0701, whose
air mass spent most of time over sympagic waters (Fig. 1) clustered together
with the samples from the open ocean according to NMR composition. It is
noticeable, however, that binned NMR spectra can only trace the distribution
of the major organic functional groups while the information carried by fine
spectral features, which is critical to detect the presence of specific
molecular markers, is not taken into account in the cluster analysis. In the
following sections, we will show that sample A-0701 exhibits a peculiar NMR
composition which must be put in relation to terrestrial sources of organic
compounds. On the basis of the back-trajectories (Figs. 1 and 8), the likely
land sources were located in the Antarctic Peninsula. In summary, the
variability in the distribution of NMR functional groups in ambient PM1
samples (Table 1) was primarily driven by the air mass origin over sympagic
(Weddell Sea) or pelagic waters, in agreement with the results on inorganic
compounds, WSOC, and amines reported by Dall'Osto et al. (2017, 2019).
Nevertheless, the analysis of fine NMR spectral features supports the
existence of a third source area over land. In the following discussion, we
will provide an in-depth description of the NMR compositions for these three
source sectors.
Cluster analysis of the 1H-NMR spectra of the PM1 HiVol
samples of ambient aerosol. Units for correlation distance are dimensionless.
Concentrations of 1H-NMR functional groups and of molecular markers determined in the ambient aerosol samples. ND: not detected. NA: not available because of the interference of organic compounds with overlapping chemical shifts.
Sample ID A-0701A-0901A-1301A-1801A-2401A-2801A-0102A-0602Sampling times 7 Jan 20:00–9 Jan 14:50–13 Jan 19:20–18 Jan 13:30–24 Jan 15:00–28 Jan 13:30–1 Feb 14:50–6 Feb 22:00–9 Jan 09:0013 Jan 13:5018 Jan 12:2021 Jan 23:5528 Jan 05:1531 Jan 13:506 Feb 03:1510 Feb 11:00Average air mass type Weddell Sea/WeddellWeddellWeddellOpenOpenOpen ocean/OpenAntarctic PeninsulaSeaSeaSeaoceanoceanmixedoceanWater-soluble organic carbon (µgC m-3) WSOC0.140.070.120.130.090.140.050.111H-NMR functional groups (nmolH m-3) H-C2.602.162.283.033.272.812.072.82H-C-C=O2.401.581.802.101.911.861.281.78H-C-O2.150.570.690.832.060.990.991.41O-CH-O0.200.070.050.040.080.090.070.09Ar-H0.120.050.000.100.090.100.110.07MSA2.131.952.634.541.722.902.221.53Alkylamines0.300.790.531.320.340.490.130.15Molecular markers (ng m-3) MSA68628414555937149methylamines2.315.53.799.02.533.560.921.20creatinine0.091.651.522.21∼0.051.000.290.41glycerolNA1.10.70.73.00.80.71.3sucrose11NDNDNDNDNDNDNDalanineNDNDtraces1traces10.6NDND0.7betaineNDNDNDNDtraces2NDNDND
1 below the limit of quantification (0.3 ng m-3). 2 below the
limit of quantification (0.2 ng m-3).
Ambient aerosols from the Weddell Sea
Sample A-0901 was collected in the marginal ice zone of the Weddell Sea. Its
spectrum shows a complete absence of aromatic compounds and alkenes (Fig. S7). The aliphatic region (Figs. 5, S8) exhibits broad similarity to that
of the primary marine particles generated in the sea-spray tank, but with a
major difference in the chemical shift range between 1.7 and 3.0 ppm where
the background broad NMR bands are much more intense in the ambient sample.
This is also the region recovering the signals from acyl groups
(RCH-(C=O)-) in aliphatic carboxylic acids and keto acids,
which are formed by volatile organic compound (VOC) oxidation in the atmosphere (Barbaro et al., 2017).
The most abundant individual compounds detected in these samples were,
however, MSA (Fossum et al., 2018) and the low molecular methylamines (MMA,
DMA, TMA). The predominance of semivolatile C1–C3 alkylamines (Ge
et al., 2011) indicates that the amines form in the ambient aerosol by
secondary processes involving volatilization from the ocean surface and
recondensation onto acidic aerosol particles (Dall'Osto et al., 2019). The
aliphatic bands at 0.9 and 1.3 ppm in sample A-0901 show a partial overlap
with the resonances of the lipids in the aerosolized seawater. However, the
bands at 1.6 ppm and 2.2–2.3 ppm which, in lipids, correspond to methylenes
in beta and alpha position to a C=O group, are much more intense in the
spectrum of A-0901 than in BB SeaIce-3 (Fig. S8), indicating that
aliphatic chains are shorter and more substituted in the ambient aerosol
than in nascent primary aerosol particles. The pattern of bands at 0.9, 1.3,
1.6, 2.2, 2.4, and 2.6 ppm follows the structure elucidated by Suzuki et al. (2001) and is attributed to C7–C9 aliphatic dicarboxylic acids and
oxo- acids. This class of organic compounds, clearly characterizing the
aliphatic composition of the ambient samples in the Weddell Sea area, could
originate from degraded (oxidized) lipids (Kawamura et al., 1996), or from
gas-to-particle conversion of carbonyls produced by the photochemical
oxidation of lipids at the air–sea interface (Bernard et al., 2016; Alpert
et al., 2017). Support for the latter hypothesis (secondary formation) is
given by the fact that the N-osmolytes (betaine, choline) present in the sea
spray generated in the tanks were completely absent in the ambient sample.
Nevertheless, the resonances in the spectral window 3.5–3.8 ppm in sample
A-0901 are completely consistent with the occurrence of glycerol, indicating
that in fact primary aerosol particles contributed to the composition of the
ambient aerosol in this region (Fig. S9). There is another striking
difference between the composition of the ambient aerosol and sea-spray
particles: the former contains significant levels (1.65 ng m-3) of
creatinine. This compound is responsible for the two singlets at 3.12 and
4.27 ppm of chemical shift and was identified by the comparison with a
standard under identical NMR experimental conditions (Fig. S11). The
concentration of creatinine clearly follows that of low molecular weight
amines (Fig. 6) and shows a maximum in the three samples collecting most
of the air masses that traveled over the Weddell Sea. Creatinine was also
determined by HPLC/-MS analysis in a parallel set of filter samples
collected onboard Hesperides during the PEGASO cruise (see Sect. 2.4).
Identification was based on MS/MS fragmentation patterns and retention time.
Quantification was based on chromatographic peak area. Figure 7 shows
extracted ion chromatograms for m/z 114.0655–114.0667, corresponding to
creatinine, of the filter extract of sample 0119N obtained during the PEGASO
campaign and the neat creatinine standard. The HPLC/MS analysis indicates
that creatinine occurred in concentrations of 20–50 pg m-3 in the
samples from the Weddell Sea area (Table S1 in the Supplement), much less than the
concentrations determined by 1H-NMR spectroscopy (1.6–2.5 ng m-3, Table 1). Such discrepancy can be due to the different
extraction protocols and to non-ideal chromatographic conditions in HPLC/MS
for creatinine quantification (elution close to the void volume).
Nevertheless, our findings demonstrate that high-field NMR methods can
integrate HPLC/MS analysis for the identification of molecular markers in
atmospheric aerosol complex organic mixtures.
The same as Fig. 2 but for the three ambient submicrometer
aerosol samples. Specific resonances were assigned to lactic acid (Lac),
isobutyric acid (IsoBu), alanine (Ala), monomethylamine (MMA), dimethylamine
(DMA), trimethylamine (TMA), glycerol (Glc), sucrose (Suc), creatinine (Cra),
and blank contaminations (b). Unresolved mixtures of linear aliphatic
compounds (A), including possible contributions from lipids, are indicated
in the spectra. Other NMR signals were only tentatively attributed to
cadaverine (X4).
Concentrations of creatinine and methylamines in the PM1 samples.
The concentrations are expressed as contributions to WSOC (mol % of
carbon). “Weddell Sea” and “Open ocean” labels indicate the sampling
periods identified by Dall'Osto et al. (2017) to characterize the aerosol
composition in air masses traveling over sea ice and in the Southern Ocean,
respectively.
Ambient aerosols in the open ocean
Sample A-2401 was collected during the northern transit of the cruise, RV Hesperides just
west of South Georgia Island (55∘ S; Fig. 1). During sampling, the air
masses had a westerly component and can be considered representative of
Southern Ocean conditions. The 1H-NMR spectrum of A-2401 shares
similarities with that of A-0901 described above: (a) the resonances of MSA
and methylamines are much more intense than those of other low molecular
weight compounds (such as N-osmolytes); (b) the spectral region of acyls (1.8–3.0 ppm) accounting for unresolved carboxylic acids is clearly more
intense than in the spectrum of primary organic aerosols; (c) the pattern of
bands at 0.9, 1.3, 1.6, and 2.2–2.4 ppm highlights the presence of linear
aliphatic structures substituted with oxo- and carboxylic groups.
Nevertheless, MSA and the low molecular weight amines were less abundant in
A-2401 than in the sample from the Weddell Sea (Table 1). Also the ratio
between acyl (CH-C=O) and alkyl (CH-CH) groups
was smaller in A-2401 than in A-0901 (Fig. S8). The linear aliphatic
structures involved longer methylenic chains in A-2401 than in A-0901, so
that in the former case they were more similar to the aliphatic structures
of the aerosolized melted sea ice (Fig. S8). Another difference between
the two ambient aerosol samples is that the one from the Southern Ocean
contains much more alcoxy groups (HC-O, in the chemical shift range 3.4–4.2 ppm) of polyols than the one from the Weddell Sea (Fig. 5; Table 1).
When comparing the functional group distributions of the ambient aerosol
samples to that of the aerosol generated during the tank experiments,
clearly the samples from the Southern Ocean show a better match than the
samples from the Weddell Sea do. Other similarities between the composition
of A-2401 and the aerosol in the tank can be found in the fine structures of
the spectra, especially in the ranges of aromatics, acetals, and polyols
(Fig. S12). A-2401 clearly contains traces of organic markers of primary
aerosols and specifically glycerol, N-osmolytes (Fig. S10), and amino acids
(alanine). Finally, in contrast with A-0901, sample A-2401 contains only trace
amounts of creatinine.
Ambient aerosols influenced by coastal land sources
Sample A-0701 was collected in the western sector of the Weddell Sea. The
air masses showed several passes over the Antarctic Peninsula. The
1H-NMR spectrum shows unique features: isobutyric acid was found in
relatively high concentrations, together with an amine tentatively
identified as cadaverine (Fig. 5). The aliphatic chains are found in much
lower amounts than in the samples described above; the band of acyls is not
as pronounced as in A-0901 (Fig. S8), whereas alcoxyls are abundant,
especially due to the occurrence of sucrose at a remarkable concentration of
10 ng m-3. Finally, no creatinine was found in this sample. Clearly, the
composition of A-0701 is drastically different from that of the other
samples collected in the Weddell Sea. The presence of sucrose (Fig. S9)
points to a contribution from primary biological particles emitted from a
terrestrial biota, not a marine one. Vegetation cover (scarce but present)
in the Antarctic Peninsula can be responsible for such emissions. The NMR
composition of A-0701 provides evidence of the diversity of biogenic aerosol
sources active in this area of the world.
DiscussionSource apportioning of primary and secondary organic components in
different regions
The comparison of the NMR compositions of the ambient aerosol samples
collected onboard RV Hesperides (Fig. 8) supports the differentiation of aerosol sources
between the sympagic and pelagic environments introduced by
Dall'Osto et al. (2017). The higher abundance of alkyl (C-H) and alcoxy
(H-C-O) groups detected in the second half of the cruise points to a larger
fraction of primary organic compounds rich in lipids and polyols in the
aerosols of the open Southern Ocean. Analogous compositions were obtained
using FTIR spectroscopy at Ross Island (Liu et al., 2018). In our study, the
attribution of compound classes and molecular markers (such as glycerol and
N-osmolytes) to primary marine particles was supported by the comparison
with the analysis of tank-generated sea-spray particles. According to our
NMR datasets, primary marine organics were ubiquitous in the region as
witnessed by the presence of glycerol in all samples. However, glycerol
accounted for almost the entire polyol content in the three samples from the
eastern/northern Weddell Sea, while the samples from the open ocean contained
much larger and more complex mixtures of polyols/sugars. Free amino acids (alanine) in the sub-ng m-3 range and N-osmolytes in trace amounts
with a greater abundance of linear aliphatic structures similar to lipids
in the samples from the Southern Ocean point to a major contribution of
primary organics to submicron organic aerosols in this environment. These
findings provide further confirmation of the importance of sea spray as a
source of marine organic particles in oceanic regions characterized by high
productivity and strong wind stress.
(a) MS spectrum of a creatinine standard. (b) Extracted ion
chromatograms for m/z 114.0655–114.0667, corresponding to creatinine, of the
filter extract of sample 0119n obtained during the PEGASO campaign and the
neat creatinine standard. The retention time of creatinine was found to be
0.33 min using the conditions outlined in Sect. 2.4.
NMR functional group compositions of WSOC in the PM1 HiVol
samples. Functionalities: H-C (alkyls), H-C-(C=) (acyls), H-C-O (alcoxyl),
MSA, amines, anomeric, and Ar-H (aromatic).
In sympagic waters, other mechanisms of aerosol formation take place.
Sympagic waters are rich in S- and N-osmolytes produced by the algal
communities colonizing the sea ice. The osmolytes degrade to VOCs which are
then converted to secondary organic aerosol (SOA) components, such as MSA (Davison et al., 1996) and
low molecular weight methylamines (Facchini et al., 2008). Also the
distribution of the oxygenated functional groups was different between
sympagic and pelagic regimes. While alcoxyl groups (H-C-O) from polyols and
sugars account for almost 50 % of total alcoxyl (H-C-O) and acyls
(H-C-C=O) in the samples from the Southern Ocean, the fraction is less than 30 % in the three samples from the offshore areas of the
Weddell Sea (Fig. 8). The mixtures of organic compounds carrying acyls, like
carboxylic and oxocarboxylic acids, are not associated with primary marine
aerosols and are likely components of SOA. Carboxylic acids can form
photochemically (Cui et al., 2019) during the austral summer. The nature of
parent VOCs regarding carboxylic acids in our samples is unknown, but the
occurrence of linear aliphatic compounds containing oxo- and carboxylic
groups indicates that one of the possible sources if found in the oxidative
degradation of lipids – either in the aerosol or in the marine microlayer –
as suggested by past studies in Antarctica (Kawamura et al., 1996) and
consistent with recent AMS observations in the Arctic marginal ice zone
(Willis et al., 2017).
In the Weddell Sea, under the influence of air masses that had traveled
over the Peninsula (sample A-0701), the contribution of the emissions from
the land biota became evident, therefore supporting the observations of
Schmale et al. (2013) on the contribution of primary biological particles
from the coastal land ecosystems. Our data suggest that beside animal
colonies, the land vegetation (grasses, mosses, lichens) of the
Antarctic Peninsula can also contribute to the emission of particles, and in
particular to the content of sugars. Other biological compounds of primary
origin, the amino acids, were not found in the Weddell Sea in our study.
These results contrast with the previous findings that a significant
fraction of the ambient PM1 mass was accounted for by proteinaceous material
at an island site in the Southern Ocean (Schmale et al., 2013). On the other
hand, the observations of Schmale et al. (2013) were carried out under the
direct influence of the emissions of seabird colonies, while our
observations were carried out offshore. More research is needed to quantify
the range and extent to which primary particles from the terrestrial biota
impact the marine aerosol composition in the Antarctic region.
A new potential marker: creatinine
The source of creatinine in ambient aerosol is controversial. On the
basis of its chemical structure, it is water soluble but clearly less
volatile than the methylamines and, as a consequence, its Henry coefficient
must be much less favorable for transferring this amine out of seawater into
the gas phase. A primary origin via sea spray is also doubtful because
creatinine is not a strong surfactant. On the other hand, Prather et al. (2013) showed that sea-spray aerosols encompass several classes of organic
particles, including some made of biological material: POC particles and
large colloids can be scavenged by rising bubbles and injected in the
atmosphere by jet drops. Jet drop emission represents a plausible mechanism
to transfer primary organic compounds which are not strong surfactants from
seawater to the atmosphere. If this happened to creatinine, it must have
occurred in source areas other than the algal blooms where we conducted the
tank experiments, since we did not detect any creatinine in the aerosolized
seawater and sea ice. Creatinine is a common metabolite of mammals;
therefore an alternative source via the excreta of sea lions in Antarctic
coastal areas can be postulated. However, a much more vast source
in seawater is also possible under the hypothesis that creatinine results
from the enzymatic conversion of creatine, which is a known metabolite of
the urea cycle in marine animals (Whitledge and Dugdale, 1972) and
phytoplankton (Allen et al., 2011) that contributes to pelagic DOC across
the world's oceans (e.g., Wawrik et al., 2017).
Conclusions
Our results demonstrate that, beside MSA, a complex mixture of biogenic
organic compounds contributes to the composition of submicron aerosol
particles in the Antarctic atmosphere. Although individual organic markers
encompassing sugars, amino acids, and carboxylic acids have been
identified in past studies, our results indicate that non-MSA biogenic
organic compounds impact the bulk composition of organic aerosol in this
environment (Fig. 8). The NMR analysis provides evidence for both
secondary (more important in sympagic regions) and primary (more
important in pelagic areas) marine sources. A third contribution from the
terrestrial biota in the Antarctic Peninsula was also identified. The
emission of sea-spray organics in offshore areas was unambiguously
demonstrated by the determination of molecular tracers for lipids and
polyols and by the comparison of the fine structures in the 1H-NMR
spectra of the ambient samples and of the aerosol generated in the tank
experiments. A new biogenic marker, creatinine, was identified for the first
time in the ambient aerosol, extending the list of reduced nitrogen-containing molecular tracers in the atmosphere. The discovery of creatinine
also exemplifies the usefulness of employing non-targeted analytical
techniques like NMR spectroscopy for screening the organic composition of
aerosol in remote environments where the sources of atmospheric
particulate matter are still poorly understood. The complexity of organic
composition illustrated in this study calls for more research on suitable
methodologies – both online and offline, and combinations of them – to
investigate the nature of non-MSA marine organic particles in offshore
regions around the Antarctic continent.
Data availability
The NMR data sets are available on request to the corresponding author.
The supplement related to this article is available online at: https://doi.org/10.5194/acp-20-4193-2020-supplement.
Author contributions
SD wrote the paper; MDO and RS coordinated the experimental activities in
the field; MP, MDO, and SG collected the aerosol samples; MDO, MP, and DC
collected the sea-ice samples; COD, JO, and DC set up the bubble-bursting
tank; MR, MP, MR, NZ, and FV performed the sample extraction and preparation
for WSOC and NMR analyses; NZ and MP performed the NMR analyses; SG and CJK
carried out the HPLC/MS analyses; SD, MP, and ET elaborated on the NMR data;
MDO, RS, and ET contributed to the interpretation of the analyses of the
seawater samples; SD, MP, MR, MDO, TH, CJK, and ET contributed to the
interpretation of the analyses of the aerosol samples; all authors
contributed to the general discussion and the main conclusions
of this study.
Competing interests
The authors declare that they have no conflict of interest.
Special issue statement
This article is part of the special issue “Marine organic matter: from biological production in the ocean to organic aerosol particles and marine clouds (ACP/OS inter-journal SI)”. It is not associated with a conference.
Acknowledgements
The cruise was funded by the Spanish Ministry of Economy through projects
PEGASO (CTM2012-37615) and Bio-Nuc (CGL2013-49020-R). 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 no. 603445. The research activities of CNR were also supported by the project
AirSEaLab: Progetto Laboratori Congiunti. We would like to thank
Andrea Mazzanti for his advice in performing the NMR experiments at the NMR
facility of the Dep. Industrial Chemistry, University of Bologna. We also
thank David Beddows (University of Birmingham) for help in drawing figures, in
particular air mass back-trajectories.
Financial support
This research has been supported by the European Commission (grant no. BACCHUS (603445)) and CNR project “AirSEaLab: Progetto Laboratori Congiunti”.
Review statement
This paper was edited by Manuela van Pinxteren and reviewed by two anonymous referees.
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