Aerosol optical properties calculated from size distributions, filter samples and absorption photometer data at Dome C, Antarctica, and their relationships with seasonal cycles of sources

Optical properties of surface aerosols at Dome C, Antarctica, in 2007-2013 and their potential source areas are presented. Scattering coefficients (sigma(sp)) were calculated from measured particle number size distributions with a Mie code and from filter samples using mass scattering efficiencies. Absorption coefficients (sigma(ap)) were determined with a three-wavelength Particle Soot Absorption Photometer (PSAP) and corrected for scattering by using two different algorithms. The scattering coefficients were also compared with sigma(sp) measured with a nephelometer at the South Pole Station (SPO). The minimum sigma(ap) was observed in the austral autumn and the maximum in the austral spring, similar to other Antarctic sites. The darkest aerosol, i.e., the lowest single-scattering albedo omega(o) approximate to 0.91, was observed in September and October and the highest omega(o) approximate to 0.99 in February and March. The uncertainty of the absorption angstrom ngstrom exponent alpha(ap) is high. The lowest alpha(ap) monthly medians were observed in March and the highest in August-October. The equivalent black carbon (eBC) mass concentrations were compared with eBC measured at three other Antarctic sites: the SPO and two coastal sites, Neumayer and Syowa. The maximum monthly median eBC concentrations are almost the same (similar to 3 +/- 1 ng m(-3)) at all these sites in October-November. This suggests that there is no significant difference in eBC concentrations between the coastal and plateau sites. The seasonal cycle of the eBC mass fraction exhibits a minimum f (eBC) approximate to 0.1 % in February-March and a maximum similar to 4 %-5 % in August-October. Source areas were calculated using 50 d FLEXPART footprints. The highest eBC concentrations and the lowest omega(o) were associated with air masses coming from South America, Australia and Africa. Vertical simulations that take BC particle removal processes into account show that there would be essentially no BC particles arriving at Dome C from north of latitude 10 degrees S at altitudes < 1600 m. The main biomass-burning regions Africa, Australia and Brazil are more to the south, and their smoke plumes have been observed at higher altitudes than that, so they can get transported to Antarctica. The seasonal cycle of BC emissions from wildfires and agricultural burning and other fires in South America, Africa and Australia was calculated from data downloaded from the Global Fire Emissions Database (GFED). The maximum total emissions were in August-September, but the peak of monthly average eBC concentrations is observed 2-3 months later in November, not only at Dome C, but also at the SPO and the coastal stations. The air-mass residence-time-weighted BC emissions from South America are approximately an order of magnitude larger than from Africa and Oceania, suggesting that South American BC emissions are the largest contributors to eBC at Dome C. At Dome C the maximum and minimum scattering coefficients were observed in austral summer and winter, respectively. At the SPO sigma(sp) was similar to that observed at Dome C in the austral summer, but there was a large difference in winter, suggesting that in winter the SPO is more influenced by sea-spray emissions than Dome C. The seasonal cycles of sigma(sp) at Dome C and at the SPO were compared with the seasonal cycles of secondary and primary marine aerosol emissions. The sigma(sp) measured at the SPO correlated much better with the sea-spray aerosol emission fluxes in the Southern Ocean than sigma(sp) at Dome C. The seasonal cycles of biogenic secondary aerosols were estimated from monthly average phytoplankton biomass concentrations obtained from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) satellite sensor data. The analysis suggests that a large fraction of the biogenic scattering aerosol observed at Dome C has been formed in the polar zone, but it may take a month for the aerosol to be formed, be grown and get transported from the sea level to Dome C.

Automated summary

The study presented the optical properties of surface aerosols at Dome C, Antarctica from 2007 to 2013 and their potential source areas. The research showed fluctuations in absorption and scattering coefficients in different seasons, and similar black carbon mass concentrations at various Antarctic sites but varying at different times of the year.