Extinction of UV-visible radiation in wet midlatitude (maritime) snow: Implications for increased NO_x emission

A field and modeling study of the optical properties of wet midlatitude (maritime) mountainous snowpack is presented. The snowpacks were found to have greater UV penetration depths than polar (tundra) snowpacks and consequently may release more NO₂ gas to the atmosphere during the photolytic destruction of nitrate anions in the snowpack for a given spherical irradiance. Fluxes of NO₂ to the troposphere as a result of NO₃^- photolysis were calculated for different measured e-folding depths using tropospheric ultraviolet-visible (TUV)-modeled actinic flux data assuming all the NO₂ can leave the snowpack and the photolysis of nitrate is the rate-limiting step. These calculated fluxes ranged from 3.3 to 7.6 kg km^-2 yr^-1 (assuming 150 days of snow cover a year), significantly more than polar snowpacks by a factor of 4-10. The fieldwork measured liquid equivalent e-folding depths of 3.74-14.66 cm (e-folding depths of 7.25-32.4 cm^-1) at four sites in the Cairngorm mountain range, Scotland (57°07′N, 3°40′W), during the winter of 2003. The wavelength range studied was 300-450 nm. The snowpacks consisted predominantly of windblown rounded grains ranging from 0.1 to 1.5 mm in diameter. The liquid water content of the snowpacks varied between the sites, which were visited up to three times to observe temporal changes in the physical composition and optical properties of the snowpack. The snowpack was modeled using the TUV radiative-transfer model, calculating scattering cross sections (σ_scatt,) between 1 and 5 m² kg^-1. The absorption coefficient 2 due to impurities (σ⁺_abs) was also modeled and was found to be approximately 1.0 cm² kg^-1. Three optically different snowpack categories are suggested: cold, dry polar (tundra) snowpacks, σ_scatt = 20-30 in m² kg^-1; warmer polar coastal (maritime) snowpacks, σ _scatt= 6-13 in m² kg^-1; and melting midlatitude mountainous (maritime) snow, σ _scatt= 1-5 m² kg^-1. Thus for midlatitude wet snow, 85% of photochemistry is likely to occur in the top 15-60 cm.