A new instrument to measure UV extinction profiles and photolysis frequencies in a snowpack environment

A new profiling irradiance spectroradiometer measured ultraviolet (UV) radiation at four to five depths in the surface snowpack during a seven-week campaign at Summit, Greenland (Summer 2003). The UV photons were collected by a small irradiance head with a Teflon diffuser and transmitted through a UV-VIS fiber optic into a scanning 1/8-meter double monochrometer and detected with a UV sensitive photomultplier. One complete scan of the UV-VIS radiation between 290-420 nm took less than 30 seconds. Five different fibers (i.e., five different depths) were serially sampled using a stepping motor and micropositioning slider to create a penetration profile every 2.5 minutes. Simultaneous atmospheric actinic flux data from a synchronized above snow scanning spectroradiometer was used to identify periods when the incident downwelling UV intensity changed during a depth profile. The UV penetration distance was calculated as an e-folding depth (the depth over which the monochromatic UV irradiance decreases by a factor of e). After measuring the angular response of the irradiance entrance optics and assuming that the in-snow radiation environment was isotropic, it is possible to scale up from irradiance to actinic flux. Fourteen different photolysis frequencies (including jNO₃^-_(aq)) were calculated from the snowpack actinic flux data. These data can then be used to add to our understanding of snowpack photolytic processes that produce gas phase NO and/or NO₂ and are speculated to result in the snowpack production of other important photochemical species (such as HONO, HCHO, HOOH, etc.) in snow-covered environments.