Photolysis frequency measurements at the South Pole during ISCAT-98

Spectrally resolved actinic flux measurements from 280 to 420 nm were made on the roof of the South Pole Atmospheric Research Observatory during Decem ber 1998. Photolysis frequencies Of O-3 --> O-2 + O(D-1) and NO2 --> NO + O (P-3) were calculated from the measured actinic flux spectra and establishe d absorption cross-section and quantum yield data. The average values of jN O(2) and jO(D-1) were observed to be 1.1 x 10(-2) +/- 1.3 X 10(-3) s(-1) an d 9.0 x 10(-6) +/- 1.4 x 10(-6) s(-1), respectively. The variability of jNO (2) was due to decreases in direct UV-A radiation during overcast condition s, while the larger variability observed for jO(D-1) was primarily the resu lt of changes in the overhead ozone column. Modeled photolysis frequencies were obtained using the NCAR TUV radiation model. The downwelling modeled a nd measured photolysis frequencies agree well during clear sky conditions. As a consequence of the high albedo and 24 hours of sunlight at this time o f the year, the daily averaged jNO(2) measured at the South Pole was three times higher than predicted for a tropical location.