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SUMMARY:Laboratory study of nitrate photolysis in Antarctic snow: quantum 
 yield\,  domain of photolysis\, isotope effects and wavelength dependence 
 - Dr. Carl Meusinger\, University of Copenhagen
DTSTART:20131112T100000Z
DTEND:20131112T110000Z
UID:TALK48386@talks.cam.ac.uk
CONTACT:Dr. Ailsa Benton
DESCRIPTION:Carl Meusinger1\, Tesfaye A. Berhanu2\, Joseph Erbland2\, Joel
  Savarino2 and Matthew S. Johnson1\n#1 Department of Chemistry\, Universit
 y of Copenhagen\, Copenhagen\, Denmark\n#2 LGGE (UMR5183)\, CNRS/Univ. Gre
 noble Alpes\, Grenoble\, France\n\nPost-depositional processes alter nitra
 te concentration and nitrate isotopic composition in the top layers of sno
 w at sites with low snow accumulation rates\, such as Dome C\, Antarctica.
  Available nitrate ice core records can provide input for studying past at
 mospheres and climate if such processes are understood. It has been shown 
 that photolysis of nitrate in the snowpack plays a major role in nitrate l
 oss. For unclear reasons the range of reported quantum yields for the main
  reaction spans orders of magnitude\, constituting the largest uncertainty
  in models of snowpack NOx emissions. Here a laboratory study is presented
  that uses snow from Dome C and minimizes effects of desorption and recomb
 ination by flushing the snow with pure N2 at water vapor equilibrium durin
 g irradiation with UV light from a Xenon lamp. A selection of UV filters a
 llowed examination of the effects of the 200 and 300 nm absorption bands o
 f nitrate and to emulate actinic fluxes similar to those in Dome C. \nIrra
 diated snow was sampled in 1 cm sections and analyzed for nitrate concentr
 ation and isotopic composition (δ15N\, δ18O and Δ17O)\; the actinic flu
 x was measured at similar sections in the snow. The quantum yield was obse
 rved to decrease with increasing exposure to UV radiation. Observed values
  for the quantum yield lie in the middle of the range of previously report
 ed values. The superposition of photolysis in two photochemical domains of
  nitrate in snow is proposed: one of photolabile nitrate and one of trappe
 d or buried nitrate. The difference lies in the ability of reaction produc
 ts to escape the snow crystal\, versus undergoing secondary (recombination
 ) chemistry. Modeled NOx emissions may be increased significantly due to t
 he observed quantum yield in this study influencing predicted boundary lay
 er chemistry significantly including ozone concentrations. For the tested 
 snow\, the quantum yield changes from 0.44 to 0.05 within what corresponds
  to weeks of UV exposure in Antarctica. \nAn average photolytic isotopic f
 ractionation of 15ε = -15 ± 1.2 ‰ was found for the experiments withou
 t a wavelength filter. These results are ascribed to excitation of the int
 ense absorption band of nitrate around 200 nm. An experiment with a filter
  blocking wavelengths shorter than 320 nm\, approximating the actinic flux
  spectrum at Dome C\, showed a photolytic fractionation constant of 15ε =
  -47.9 ± 6.8 ‰ in good agreement with the fractionation determined for 
 the East Antarctic Plateau ranging from -40 to -74.3 ‰. The isotopic fra
 ctionations obtained from this study are compared to theoretical estimates
  derived by applying the zero point energy shift model coupled with measur
 ed actinic fluxes at each depth. The results confirm that the photolytic f
 ractionation of nitrate isotopes in snow is very sensitive to the actinic 
 flux spectrum and indicate limitations in previous laboratory studies. Thi
 s work demonstrates that the spectrum of the excitation source is a key pa
 rameter determining nitrogen isotope fractionation in the photolysis of ni
 trate in snow.
LOCATION:room 307\,  British Antarctic Survey\, High Cross\, Cambridge\, C
 B3 0ET
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