HONO emissions from snow surfaces
Photochemical production of NO_x and HONO from surface snow can significantly impact the NO_x, OH, and O_3 budgets in the overlying atmosphere. NO_x production is driven by the solar photolysis of NO_3^− within or at the surface of snowpacks. HONO, however, is a secondary species that involves H-atom transfer between natural donors and photogenerated NO_2. Here we investigate the mechanism of HONO generation in snowpacks by exploring how its emissions respond to on-and-off illumination and temperature cycles, and to the addition of various snow dopants. The presence of humic substances within or at the surface of the snowpack significantly enhances, and may be an essential requisite for HONO production. 

Emission fluxes of NO, NO_2, and HONO from snow surfaces were measured under controlled temperature, ozone mixing ratio and actinic flux conditions. We used natural mid-latitude surface snow as the snow substrate. Their combined peak emission fluxes reached up to ~3 × 10^(10) molecules cm^(−2) s^(−1), ~10^3 times larger than typical emissions from polar snowpacks. Less than 1% of available N was released in these experiments. We report significant post-irradiation HONO emissions from the snow. Present results indicate a strong, direct correlation between HONO emissions and the HULIS (humic-like substances) content of the snow surface.