Biogenic aerosol in central East Antarctic Plateau as a proxy for the ocean-atmosphere interaction in the Southern Ocean

Ten years of data of biogenic aerosol (methane sulfonic acid, MSA, and non-sea salt sulfate, nssSO₄^2-) collected at Concordia Station in the East Antarctic plateau (75° 06' S, 123° 20' E) are interpreted as a function of the Southern Annular Mode (SAM), Chlorophyll-a concentration (Chl-a; a proxy for phytoplankton biomass), sea ice extent and area. It is possible to draw three different scenarios that link these parameters in early, middle, and late summer. In early summer, the biogenic aerosol is significantly correlated to sea ice retreats through the phytoplankton biomass increases. Chl-a shows a significant correlation with nssSO₄^2- in the finest fraction (< 1 μm). In contrast, only Chl-a in West Pacific and Indian Ocean sectors correlates with MSA in the coarse fraction. The transport routes towards the inner Antarctic plateau and aerosol formation processes could explain the different correlation patterns of the two compounds both resulting from the DMS oxidation. In mid-summer, Chl-a concentrations are at the maximum and are not related to sea ice melting. Due to the complexity of transport processes of air masses towards the Antarctic plateau, the MSA concentrations are low and not related to Chl-a concentration. In late summer, MSA and nssSO₄^2- present the highest concentrations in their submicrometric aerosol fraction, and both are significantly correlated with Chl-a but not with the sea ice. In early and mid-summer, the enhanced efficiency of transport processes from all the surrounding oceanic sectors with air masses traveling at low elevation can explain the highest concentrations of nssSO₄^2- and especially MSA. Finally, considering the entire time series, MSA shows significant year-to-year variability. This variability is significantly correlated with SAM but with a different time lag in early (0-month lag) and late summer (4-months lag). This correlation likely occurs through the effect of the SAM on phytoplankton blooms.

Effect of sea ice retreat on marine aerosol emissions in the Southern Ocean, Antarctica

Sea ice retreat in the polar region is expected to increase the emissions of sea salt aerosols and biogenic gases, which may significantly impact the climate by increasing cloud condensation nuclei (CCN) population and changing solar radiation. In this study, aerosol compositions were measured at high-time-resolution (1 h) with an in-situ gas and aerosol composition monitoring system in polynya regions of the Southern Ocean (SO) to access the effects of sea ice concentrations on the sea salt aerosol (SSA) and secondary biogenic aerosol (SBA) in the SO. SSA emissions increased by more than 30% as sea ice concentration decreased from 85% to 29%. However, SSA emissions did not increase monotonically as the sea ice concentration decreased. The highest SSA concentration occurred in drifting sea ice region. Sea ice melting increased SBA concentrations by enhancing the air-sea exchanges of SBA precursor gases and the release of algae from sea ice. Positive correlations between SSA and wind speed were present in different sea ice regions, while SBA didn't reveal an obvious correlation with wind speed in the SO. The impact of wind speed on the SSA emissions were very different, Higher slope value of 41.83 and 35.81 were present in the DSI and SIF region, while the value was only about 16.74 in the SIC region. The results extended the knowledge of the effect of future sea ice retreat on marine aerosol emissions and potential climate changes in the polar region.

Atmospheric sea-salt and halogen cycles in the Antarctic

Atmospheric sea-salt and halogen cycles play important roles in atmospheric science and chemistry including cloud processes and oxidation capacity in the Antarctic troposphere. This paper presents a review and summarizes current knowledge related to sea-salt and halogen chemistry in the Antarctic. First, presented are the seasonal variations and size distribution of sea-salt aerosols (SSAs). Second, SSA origins and sea-salt fractionation on sea-ice and ice sheets on the Antarctic continent are presented and discussed. Third, we discuss SSA release from the cryosphere. Fourth, we present SSA dispersion in the Antarctic troposphere and transport into inland areas. Fifth, heterogeneous reactions on SSAs as a source of reactive halogen species and their relationship with atmospheric chemistry are shown and discussed. Finally, we attempt to propose an outlook for obtaining better knowledge related to sea-salt and halogen chemistry and their effects on the Antarctic and the Arctic.

Significant Underestimation of Gaseous Methanesulfonic Acid (MSA) over Southern Ocean

Methanesulfonic acid (MSA), derived from the oxidation of dimethylsulfide (DMS), has a significant impact on biogenic sulfur cycle and climate. Gaseous MSA (MSA_g) has been often ignored in previous studies due to its quick conversion to particulate MSA (MSA_p) and low concentrations. MSA_g, MSA_p, and nss-SO₄^2- were observed simultaneously for the first time with high-time-resolution (1 h) in the Southern Ocean (SO). The mean MSA_g level reached up to 3.3 ± 1.6 pptv, ranging from ∼24.5 pptv in the SO, contributing to 31% ± 3% to the total MSA (MSA_T). A reduction of the MSA to nss-SO₄^2- ratios by about 30% was obtained when MSA_g was not accounted for in the calculation, indicating that MSA_g was very important in the assessment of the biogenic sulfur contributions in the atmosphere. Mass ratios of MSA to nss-SO₄^2- increased first and then decreased with the temperature from -10 to 5 °C, with a maximum value at the temperature of -3 °C. Positive correlations between MSA_g to MSA_T ratios and temperature were presented, when the temperature was higher than 5 °C. This study highlights the importance of MSA_g for understanding the atmospheric DMS oxidation mechanism and extends the knowledge of MSA formation in the marine atmosphere.

Homogeneous sulfur isotope signature in East Antarctica and implication for sulfur source shifts through the last glacial-interglacial cycle

Sulfate aerosol (SO₄^2-) preserved in Antarctic ice cores is discussed in the light of interactions between marine biological activity and climate since it is mainly sourced from biogenic emissions from the surface ocean and scatters solar radiation during traveling in the atmosphere. However, there has been a paradox between the ice core record and the marine sediment record; the former shows constant non-sea-salt (nss-) SO₄^2- flux throughout the glacial-interglacial changes, and the latter shows a decrease in biogenic productivity during glacial periods compared to interglacial periods. Here, by ensuring the homogeneity of sulfur isotopic compositions of atmospheric nss-SO₄^2- (δ³⁴S_nss) over East Antarctica, we established the applicability of the signature as a robust tool for distinguishing marine biogenic and nonmarine biogenic SO₄^2-. Our findings, in conjunction with existing records of nss-SO₄^2- flux and δ³⁴S_nss in Antarctic ice cores, provide an estimate of the relative importance of marine biogenic SO₄^2- during the last glacial period to be 48 ± 10% of nss-SO₄^2-, slightly lower than 59 ± 11% during the interglacial periods. Thus, our results tend to reconcile the ice core and sediment records, with both suggesting the decrease in marine productivity around Southern Ocean under the cold climate.

Reduced marine phytoplankton sulphur emissions in the Southern Ocean during the past seven glacials

Marine biogenic sulphur affects Earth’s radiation budget and may be an indicator of primary productivity in the Southern Ocean, which is closely related to atmospheric CO₂ variability through the biological pump. Previous ice-core studies in Antarctica show little climate dependence of marine biogenic sulphur emissions and hence primary productivity, contradictory to marine sediment records. Here we present new 720,000-year ice core records from Dome Fuji in East Antarctica and show that a large portion of non-sea-salt sulphate, which was traditionally used as a proxy for marine biogenic sulphate, likely originates from terrestrial dust during glacials. By correcting for this, we make a revised calculation of biogenic sulphate and find that its flux is reduced in glacial periods. Our results suggest reduced dimethylsulphide emissions in the Antarctic Zone of the Southern Ocean during glacials and provide new evidence for the coupling between climate and the Southern Ocean sulphur cycle.

Ice core derived marine biogenic sulphate does not agree with marine sediment records. Here based on new ice core records spanning the past 720,000 years obtained from Dome Fuji the authors propose that dust contributed a higher percentage of sulphate aerosols than previously thought.

Prominent features in isotopic, chemical and dust stratigraphies from coastal East Antarctic ice sheet (Eastern Wilkes Land)

In this work we present the isotopic, chemical and dust stratigraphies of two snow pits sampled in 2013/14 at GV7 (coastal East Antarctica: 70°41' S - 158°51' E, 1950 m a.s.l.). A large number of chemical species are measured aiming to study their potentiality as environmental changes markers. Seasonal cluster backward trajectories analysis was performed and compared with chemical marker stratigraphies. Sea spray aerosol is delivered to the sampling site together with snow precipitation especially in autumn-winter by air masses arising from Western Pacific Ocean sector. Dust show maximum concentration in spring when the air masses arising from Ross Sea sector mobilize mineral dust from ice-free areas of the Transantarctic mountains. The clear seasonal pattern of sulfur oxidized compounds allows the dating of the snow-pit and the calculation of the mean accumulation rate, which is 242 ± 71 mm w.e. for the period 2008-2013. Methanesulfonic acid and NO₃^- do not show any concentration decreasing trend as depth increases, also considering a 12 m firn core record. Therefore these two compounds are not affected by post-depositional processes at this site and can be considered reliable markers for past environmental changes reconstruction. The rBC snow-pit record shows the highest values in summer 2012 likely related to large biomass burning even occurred in Australia in this summer. The undisturbed accumulation rate for this site is demonstrated by the agreement between the chemical stratigraphies and the annual accumulation rate of the two snow-pits analysed in Italian and Korean laboratories.

Particle size distribution of inorganic and organic ions in coastal and inland Antarctic aerosol

The concentration and particle-size distribution of ionic species in Antarctic aerosol samples were determined to investigate their potential sources, chemical evolution, and transport. We analyzed aerosol samples collected at two different Antarctic sites: a coastal site near Victoria Land close to the Italian Research Base "Mario Zucchelli", and another site located on the Antarctic plateau, close to Italian-French Concordia Research Station. We investigated anionic compounds using ion-chromatography coupled to mass spectrometry, and cationic species through capillary ion chromatography with conductometry. Aerosol collected close to the coast was mainly characterized by sea salt species such as Na⁺, Mg²⁺, and SO₄^2-. These species represented a percentage of 88% of the total sum of all detected ionic species in the aerosol samples from the coastal site. These species were mainly distributed in the coarse fraction, confirming the presence of primary aerosol near the ocean source. Aerosol collected over the Antarctic plateau was characterized by high acidity, with nss-SO₄^2-, NO₃^-, and methanesulfonic acid as the most abundant species. These species were mainly distributed in the <0.49 μm fraction, and they had a behavior of a typical secondary aerosol, where several chemical and physical processes occurred.

The Laschamp geomagnetic excursion featured in nitrate record from EPICA-Dome C ice core

Here we present the first direct comparison of cosmogenic ¹⁰Be and chemical species in the period of 38–45.5 kyr BP spanning the Laschamp geomagnetic excursion from the EPICA-Dome C ice core. A principal component analysis (PCA) allowed to group different components as a function of the main sources, transport and deposition processes affecting the atmospheric aerosol at Dome C. Moreover, a wavelet analysis highlighted the high coherence and in-phase relationship between ¹⁰Be and nitrate at this time. The evident preferential association of ¹⁰Be with nitrate rather than with other chemical species was ascribed to the presence of a distinct source, here labelled as “cosmogenic”. Both the PCA and wavelet analyses ruled out a significant role of calcium in driving the ¹⁰Be and nitrate relationship, which is particularly relevant for a plateau site such as Dome C, especially in the glacial period during which the Laschamp excursion took place. The evidence that the nitrate record from the EDC ice core is able to capture the Laschamp event hints toward the possibility of using this marker for studying galactic cosmic ray flux variations and thus also major geomagnetic field excursions at pluri-centennial-millennial time scales, thus opening up new perspectives in paleoclimatic studies.

Tales of volcanoes and El-Nino southern oscillations with the oxygen isotope anomaly of sulfate aerosol

The ability of sulfate aerosols to reflect solar radiation and simultaneously act as cloud condensation nuclei renders them central players in the global climate system. The oxidation of S(IV) compounds and their transport as stable S(VI) in the Earth's system are intricately linked to planetary scale processes, and precise characterization of the overall process requires a detailed understanding of the linkage between climate dynamics and the chemistry leading to the product sulfate. This paper reports a high-resolution, 22-y (1980-2002) record of the oxygen-triple isotopic composition of sulfate (SO4) aerosols retrieved from a snow pit at the South Pole. Observed variation in the O-isotopic anomaly of SO4 aerosol is linked to the ozone variation in the tropical upper troposphere/lower stratosphere via the Ozone El-Niño Southern Oscillations (ENSO) Index (OEI). Higher (17)O values (3.3‰, 4.5‰, and 4.2‰) were observed during the three largest ENSO events of the past 2 decades. Volcanic events inject significant quantities of SO4 aerosol into the stratosphere, which are known to affect ENSO strength by modulating stratospheric ozone levels (OEI = 6 and (17)O = 3.3‰, OEI = 11 and (17)O = 4.5‰) and normal oxidative pathways. Our high-resolution data indicated that (17)O of sulfate aerosols can record extreme phases of naturally occurring climate cycles, such as ENSOs, which couple variations in the ozone levels in the atmosphere and the hydrosphere via temperature driven changes in relative humidity levels. A longer term, higher resolution oxygen-triple isotope analysis of sulfate aerosols from ice cores, encompassing more ENSO periods, is required to reconstruct paleo-ENSO events and paleotropical ozone variations.

Frequency comb based spectrometer for in situ and real time measurements of IO, BrO, NO₂, and H₂CO at pptv and ppqv levels

We report an instrument designed for trace gas measurement of highly reactive halogenated radicals, such as bromine oxide and iodine oxide, as well as for nitrogen dioxide and formaldehyde. This compact and robust spectrometer relies on an alternated injection of a frequency-doubled femtosecond radiation at 338 and 436 nm into two parallel high-finesse cavities, for measuring BrO + H(2)CO, and IO + NO(2), respectively. The transmission of the broadband radiation through the cavity is analyzed with a high resolution, compact spectrograph consisting of an echelle grating and a high sensitivity CCD camera. The transportable instrument fits on a breadboard 120 × 60 cm size and is suitable for in situ and real time measurements of these species. A field campaign at the Marine Boundary Layer in Roscoff (in the northwest of France, 48.7°N, 4.0°W) during June 2011 illustrates the outstanding performance of the instrument, which reaches a bandwidth normalized minimum absorption coefficient of 1.3 × 10(-11) cm(-1) Hz(-1/2) per spectral element, and provides detection levels as low as 20 parts per quadrillion of IO in 5 min of acquisition.

Single-particle characterization of summertime Antarctic aerosols collected at King George Island using quantitative energy-dispersive electron probe X-ray microanalysis and attenuated total reflection Fourier transform-infrared imaging techniques

Single-particle characterization of Antarctic aerosols was performed to investigate the impact of marine biogenic sulfur species on the chemical compositions of sea-salt aerosols in the polar atmosphere. Quantitative energy-dispersive electron probe X-ray microanalysis was used to characterize 2900 individual particles in 10 sets of aerosol samples collected between March 12 and 16, 2009 at King Sejong Station, a Korean scientific research station located at King George Island in the Antarctic. Two size modes of particles, i.e., PM(2.5-10) and PM(1.0-2.5), were analyzed, and four types of particles were identified, with sulfur-containing sea-salt particles being the most abundant, followed by genuine sea-salt particles without sulfur species, iron-containing particles, and other species including CaCO(3)/CaMg(CO(3))(2), organic carbon, and aluminosilicates. When a sulfur-containing sea-salt particle showed an atomic concentration ratio of sulfur to sodium of >0.083 (seawater ratio), it is regarded as containing nonsea-salt sulfate (nss-SO(4)(2-)) and/or methanesulfonate (CH(3)SO(3)(-)), which was supported by attenuated total reflection Fourier transform-infrared imaging measurements. These internal mixture particles of sea-salt/CH(3)SO(3)(-)/SO(4)(2-) were very frequently encountered. As nitrate-containing particles were not encountered, and the air-masses for all of the samples originated from the Pacific Ocean (based on 5-day backward trajectories), the oxidation of dimethylsulfide (DMS) emitted from phytoplanktons in the ocean is most likely to be responsible for the formation of the mixed sea-salt/CH(3)SO(3)(-)/SO(4)(2-) particles.