Abstract
Abstract. We use observations of the absorption properties of black carbon and non-black carbon impurities in near-surface snow collected near the research stations at South Pole and Dome C, Antarctica, and Summit, Greenland, combined with a snowpack actinic flux parameterization to estimate the vertical profile and e-folding depth of ultraviolet/near-visible (UV/near-vis) actinic flux in the snowpack at each location. We have developed a simple and broadly applicable parameterization to calculate depth and wavelength dependent snowpack actinic flux that can be easily integrated into large-scale (e.g., 3-D) models of the atmosphere. The calculated e-folding depths of actinic flux at 305 nm, the peak wavelength of nitrate photolysis in the snowpack, are 8–12 cm near the stations and 15–31 cm away (>11 km) from the stations. We find that the e-folding depth is strongly dependent on impurity content and wavelength in the UV/near-vis region, which explains the relatively shallow e-folding depths near stations where local activities lead to higher snow impurity levels. We calculate the lifetime of NOx in the snowpack interstitial air produced by photolysis of snowpack nitrate against wind pumping (τwind pumping) from the snowpack, and compare this to the calculated lifetime of NOx against chemical conversion to HNO3 (τchemical) to determine whether the NOx produced at a given depth can escape from the snowpack to the overlying atmosphere. Comparison of τwind pumping and τchemical suggests efficient escape of photoproduced NOx in the snowpack to the overlying atmosphere throughout most of the photochemically active zone. Calculated vertical actinic flux profiles and observed snowpack nitrate concentrations are used to estimate the potential flux of NOx from the snowpack. Calculated NOx fluxes of 4.4 × 108–3.8 × 109 molecules cm−2 s−1 in remote polar locations and 3.2–8.2 × 108 molecules cm−2 s−1 near polar stations for January at Dome C and South Pole and June at Summit suggest that NOx flux measurements near stations may be underestimating the amount of NOx emitted from the clean polar snowpack.
Highlights
Earth System Sciences e-folding depth is strongly dependent on impurity content 1 Introduction and wavelength in the UV/near-vis region, which explains the relatively shallow e-folding depths near stations where Research over the past two decades has provided ample evlocal activities lead to higher snow impurity levels
While we cannot quantify the concentration of nonBC (CnonBC), we can infer that the station is a source of both black carbon (BC) and nonBC because the change in the fraction of nonBC absorption (fnonBC) is smaller than the change in CBC
The minimum flux of NOx (FNOx) from snowpacks near the stations at South Pole and Dome C are similar because South Pole calculate the FNOx, we assume that the NOx lifetime against wind pumping is always shorter than the chemical lifetime of NOx in the snow at all polar locations considered, and that all NOx produced by nitrate photolysis from has higher [NO−3 ] but Dome C has a larger e-folding depth
Summary
Earth System Sciences e-folding depth is strongly dependent on impurity content 1 Introduction and wavelength in the UV/near-vis region, which explains the relatively shallow e-folding depths near stations where Research over the past two decades has provided ample evlocal activities lead to higher snow impurity levels. Nitrate deposited to the snowpack can be re-released to the atmosphere both by photolysis (as NOx) (Davis et al, 2008; Honrath et al, 1999, 2002) and evaporation (as HNO3) (Mulvaney et al, 1998) at depths below the snow surface followed by re-deposition to the surface snowpack (Rothlisberger et al, 2000) This is supported by observations of surface snow nitrate concentrations roughly an order of magnitude larger than nitrate concentrations at 10 cm depth in continental Antarctica snowpack (Dibb et al, 2004; Frey et al, 2009; Mayewski and Legrand, 1990; Rothlisberger et al, 2000) and by observations of an upward flux of NOx out of the snowpack in polar regions on the order of 1.3–6.7 × 108 molec cm−2 s−1 (Beine et al, 2002; Davis et al, 2004; Honrath et al, 1999; Jones et al, 2001; Oncley et al, 2004). Factors such as snow accumulation rate, snow acidity, and the gradient of temperature within the snow may influence the redistribution of snowpack nitrate (Rothlisberger et al, 2000)
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