Snow optical properties at Dome C (Concordia), Antarctica; implications for snow emissions and snow chemistry of reactive nitrogen
Other literature type
France, J. L.
King, M. D.
Frey, M. M.
(issn: 1680-7324, eissn: 1680-7324)
Measurements of <i>e</i>-folding depth, nadir reflectivity and stratigraphy of the
snowpack around Concordia station (Dome C, 75.10° S, 123.31° E) were
undertaken to determine wavelength dependent coefficients (350 nm to 550 nm)
for light scattering and absorption and to calculate potential fluxes
(depth-integrated production rates) of nitrogen dioxide (NO<sub>2</sub>) from the
snowpack due to nitrate photolysis within the snowpack. The stratigraphy of
the top 80 cm of Dome C snowpack generally consists of three main layers:- a
surface of soft windpack (not ubiquitous), a hard windpack, and a hoar-like
layer beneath the windpack(s). The <i>e</i>-folding depths are ~10 cm for the
two windpack layers and ~20 cm for the hoar-like layer for solar
radiation at a wavelength of 400 nm; about a factor 2–4 larger than previous
model estimates for South Pole. The absorption cross-section due to
impurities in each snowpack layer are consistent with a combination of
absorption due to black carbon and HULIS (HUmic LIke Substances), with
amounts of 1–2 ng g<sup>−1</sup> of black carbon for the surface snow layers.
Depth-integrated photochemical production rates of NO<sub>2</sub> in the Dome C
snowpack were calculated as 5.3 × 10<sup>12</sup> molecules m<sup>−2</sup> s<sup>−1</sup>,
2.3 × 10<sup>12</sup> molecules m<sup>−2</sup> s<sup>−1</sup> and 8 × 10<sup>11</sup> molecules m<sup>−2</sup> s<sup>−1</sup> for clear skies and solar zenith
angles of 60°, 70° and 80° respectively
using the TUV-snow radiative-transfer model. Depending upon the snowpack
stratigraphy, a minimum of 85% of the NO<sub>2</sub> may originate from the top
20 cm of the Dome C snowpack. It is found that on a multi-annual time-scale
photolysis can remove up to 80% of nitrate from surface snow, confirming
independent isotopic evidence that photolysis is an important driver of
nitrate loss occurring in the EAIS (East Antarctic Ice Sheet) snowpack.
However, the model cannot completely account for the total observed nitrate
loss of 90–95 % or the shape of the observed nitrate concentration depth profile. A more
complete model will need to include also physical processes such as
evaporation, re-deposition or diffusion between the quasi-liquid layer on
snow grains and firn air to account for the discrepancies.