In situ measurements
Condensation particles (CP) comprise all atmospheric particles which can act as nuclei for condensation of low volatile gaseous compounds like organic species (biogenic or from any combustion process), organic and mineral acids which are both reaction products of atmospheric photooxidation processses and most notably water vapour. Generally particles >3 nm in diameter can act as condensation nuclei. Especially particles within the accumulation mode (particle diameter 0.1-2.5 µm) can act as efficient cloud condensation nuclei and play an important role in cloud formation. The latter fact emphasizes the crucial role of aerosols in determining atmospheric radiation transfer. CP concentrations at Neumayer exhibit a stepwise increase from polar winter (below 100 particles cm-3) to a maximum in late austral summer of around 1000 particles cm-3.
During summer the chemical composition of these particles is mainly MSA and nss-sulfate, thus CN are formed in the marine troposphere by photooxidation of DMS emitted by the phytoplankton. During winter and stormy weather conditions, however, sea salt dominates the aerosol mass. Interestingly, the maximum of condensation particle concentration typically appears in late February to early March, i.e. shifted by around 4-6 weeks compared to the MSA and nss-sulfate maxima. Our measurements suggest that during late summer the concentration of very small particles between 3-5 nm diameter (nucleation mode) is significantly enhanced, indicating new particle formation. DMS could act via its photooxidation product sulphuric acid as gaseous precursor for nucleation mode particles, a process known as gas to particle conversion.
Due to the relatively short atmospheric lifetime (a few hours) of nucleation mode particles, regional sources should dominate the measured signal. We believe that following the retreat of sea-ice in the nearby Atka Bay during late February, considerable amounts of DMS are released by the now emerging phytoplankton bloom in this area. Note that nucleation mode particles do not contribute much to the total aerosol mass due to their small size, therefore nss-sulfate and MSA concentration maxima do not necessarily coincide with the particle number concentration maximum!
Weller, R., A. Minikin, D. Wagenbach, and V. Dreiling, Characterization of the inter‑annual, seasonal, and diurnal variations of condensation particle concentrations at Neumayer, Antarctica. Atmospheric Chemistry and Physics., Copernicus Publications, 11, pp. 13243‑13257. doi: 10.5194/acp‑11‑13243‑2011, 2011.
It is generally accepted that the photooxidation of trace gases to water soluble compounds, followed by rainout, is the major cleaning procedure of tropospheric air. Photooxidation in the troposphere consists of typical radical chain reactions which need to be initiated, mainly by OH radicals. OH in turn is primarily generated via ozone photolysis and subsequent reaction of the so produced O1D (i.e. electronically excited oxygen) atoms with water vapour. Thus tropospheric ozone is certainly a key trace gas in controlling the chemical composition of the troposphere. Surface ozone is continuously measured since 1982 at Neumayer (GvN) by electro-chemical concentration cells (ECC, until 1994) and uv-absorption from 1994 ongoing. Ozone mixing ratios measured at the former GvN Station by ECC seem to be significantly lower before 1987, a probably artificial peculiarity which is not yet clarified. Nevertheless from this record, covering now more than 30 years of observation (the figure to the left shows an overview, based on monthly mean values), no significant trend can be deduced. A more detailed section of this times series is depicted in the central figure, based on daily mean values. Maximum ozone values of about 32 ppbv are generally observed in August while during polar summer (December-January) a distinct minimum of around 13 ppbv is typical. In strong contrast to urban areas where nitrogen oxides (NOx) levels are about three orders of magnitude higher, photochemical ozone destruction and not formation occurs in summer leading to surface ozone minima in pristine regions like Antarctica. In addition, from August to September extraordinary tropospheric ozone depletion events can frequently be detected (figure to the right, enlargement of the red area, now based on hourly mean values) which are not visible in the monthly mean data series to the left. Comparable to stratospheric ozone depletion, reactive halogen compounds, are responsible for this anomaly. However, in contrast to the chemical processes occurring in the stratosphere, tropospheric ozone depletion in polar regions is a natural phenomenon most probably caused by release of reactive bromine compounds eventually derived from sea-salt over sea-ice.
Wessel, S., S. Aoki, P. Winkler, R. Weller, A. Herber, H. Gernandt and O. Schrems, Tropospheric ozone depletion in polar regions: A comparison of observations in the Arctic and Antarctic. Tellus, 50B, 34-50, 1998.
Frieß, U., J. Hollwedel, G. König-Langlo, T. Wagner, and U. Platt, Dynamics and chemistry of tropospheric bromine explosion events in the Antarctic coastal region, J. Geophys. Res., 109, D06305, doi:10.1029/2003JD004133, 2004.
Continuous black carbon (BC) observations were conducted from 1999 through 2009 by an aethalometer (AE10) and from 2006 through 2014 by a Multi-Angle Absorption Photometer (MAAP) in co-operation with the DLR at Neumayer Station under stringent contamination control. Considering the respective observation period, BC concentrations measured by the MAAP were somewhat higher (median ± standard deviation: 2.1±2.0 ng/m3) compared to the AE10 results (1.6±2.1 ng/m3). Neither for the aethalometer nor for the MAAP data set a significant long-term trend could be detected. Consistently a pronounced seasonality was observed with both instruments showing a primary annual maximum between October and November and a minimum in April. Occasionally a secondary summer maximum in January/February was visible.
Weller, R., A. Minikin, A. Petzold, D. Wagenbach, and G. König‑Langlo, Characterization of long‑term and seasonal variations of black carbon (BC) concentrations at Neumayer, Antarctica, Atmospheric Chemistry and Physics, COPERNICUS GESELLSCHAFT MBH, 13, pp. 1579‑1590. doi:10.5194/acp-13-1579-2013.
Rn activity concentrations are measured by on-line alpha spectroscopy of 222Rn decay products at Neumayer by the IUP-Heidelberg (PI: Ingeborg Levin). The radioactive isotope 222Rn is a decay product of 238U and 232Th with a radioactive half-life time of 3.82 days. The main source is emission of 222Rn from terrestrial soil, while the oceanic source strength (with two order of magnitude lower emission rates) can usually considered as negligible. However, in case of Neumayer continental sources are far away and our measurements indicate that oceanic emissions significantly contribute to the measured signal and especially to the seasonal maximum in late summer after retreat of the sea ice.
R. Weller, I. Levin, D. Schmithüsen, M. Nachbar, J. Asseng, and D. Wagenbach, On the variability of atmospheric 222Rn activity concentrations measured at Neumayer, coastal Antarctica, Atmos. Chem. Phys., 14, 3843-3853, doi: doi:10.5194/acp-14-3843-2014, 2014.