HR: 0800h
AN: A11B-0056    [Abstracts]
TI: Light Penetration in the Snowpack at Summit, Greenland: Measurements of Chemical Actinometry J-Values and Spectrally Resolved Actinic Flux
AU: * Galbavy, E S
EM: esgalbavy@ucdavis.edu
AF: Department of Land, Air, and Water Resources, University of California at Davis, One Shields Ave, Davis, CA 95616 United States
AU: Anastasio, C
EM: canastasio@ucdavis.edu
AF: Department of Land, Air, and Water Resources, University of California at Davis, One Shields Ave, Davis, CA 95616 United States
AU: Lefer, B
EM: blefer@uh.edu
AF: Department of Geosciences, University of Houston, 312 SRB-1, 4800 Calhoun Road, Houston, TX 77204 United States
AB: Rates of photochemical reactions within the snowpack, both on snow grains and in the firn air, depend on how actinic flux is attenuated as a function of depth. This snowpack photon flux can either be measured directly (e.g., with spectral radiometers in the snow) or indirectly (e.g., by chemical actinometry where the rate of a photochemical reaction is measured). The advantage of the former method is that it gives wavelength-resolved photon fluxes that can be used to calculate the rate constant for any chromophore whose quantum yield and molar absorptivity are known. In contrast, the advantage of chemical actinometry is that it is a more direct measure of the photolysis rate constants that is unaffected by uncertainties in quantum yields or molar absorptivities. Here we describe results from both chemical actinometry experiments and spectral radiometric measurements in the snowpack at Summit during spring and summer. For the chemical actinometers we used three chromophores that photolyze to form hydroxyl radical: hydrogen peroxide, nitrate, and nitrite. For the spectral radiometer measurements, we used a scanning double monochromator spectroradiometer with a bialkali photocathode that measured the photon flux from 280 to 560 nm every 30 seconds. Both techniques generally showed good exponential decays in j-values with depth. Chemical actinometry measurements of e-folding depths (the depth at which the actinic flux is 1/e of its surface value) show that nitrate consistently had the smallest values, with an average of 9 cm for a mid-day value during spring, and 12 cm for summer mid-day. Hydrogen peroxide and nitrite were much more sensitive actinometers than nitrate, with surface j values (spring mid-day) that were approximately 40 and 150 times greater, respectively. While e-folding depths for hydrogen peroxide were similar to those of nitrate, those for nitrite were significantly larger, a result of the fact that nitrite absorbs at longer wavelengths where the snow is less absorbing. Preliminary analyses indicate that j-values derived from the spectral radiometer measurements are often similar to the chemical actinometry values, but that there were sometimes differences. These comparisons, and the potential reasons for the observed differences, will be discussed.
DE: 0360 Transmission and scattering of radiation
DE: 0365 Troposphere--composition and chemistry
SC: Atmospheric Sciences [A]
MN: 2004 AGU Fall Meeting