Calculations of in-snow NO₂ and OH radical photochemical production and photolysis rates: A field and radiative-transfer study of the optical properties of Arctic (Ny-Ålesund, Svalbard) snow

Depth-integrated production rates of OH radicals and NO₂ molecules from snowpacks in Ny-Ålesund, Svalbard, are calculated from fieldwork investigating the light penetration depth (e-folding depth) and nadir reflectivity of snowpacks during the unusually warm spring of 2006. Light penetration depths of 8.1, 11.3, 5.1, and 8.2 cm were measured for fresh, old, marine-influenced, and glacial snowpacks, respectively (wavelength 400 nm). Radiative-transfer calculations of the light penetration depths with reflectivity measurements produced scattering cross sections of 5.3, 9.5, 20, and 25.5 m² kg⁻¹ and absorption cross sections of 7.7, 1.4, 3.4, and 0.5 cm² kg⁻¹ for the fresh, old, marine-influenced, and glacial snowpacks, respectively (wavelength 400 nm). Photolysis rate coefficients, J, are presented as a function of snow depth and solar zenith angle for the four snowpacks for the photolysis of H₂O₂ and NO₃-. Depth-integrated production rates of hydroxyl radicals are 1270, 2130, 950, and 1850 nmol m⁻² h⁻¹ (solar zenith angle of 60°) for fresh, old, marine-influenced, and glacial snowpacks, respectively. Depth-integrated production rates of NO₂ are 32, 56, 11, and 22 nmol m⁻² h⁻¹ (solar zenith angle of 60°) for the fresh, old, marine-influenced, and glacial snowpacks, respectively. The uncertainty of repeated light penetration depth measurement was determined to be ~20%, which propagates into a 20% error in depth-integrated production rates. A very simple steady state hydroxyl radical calculation demonstrates that a pseudo first-order loss rate of OH radicals of ~10²-10⁴ s⁻¹ is required in snowpack. The snowpacks around Ny-Ålesund are thick enough to be considered optically infinite.