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 December 1998. Photolysis frequencies of O3→O2 + O(¹D) and NO2→ NO + O(³P) were calculated from the measured actinic flux spectra and established absorption cross‐section and quantum yield data. The average values of jNO2 and jO(¹D) were observed to be 1.1 × 10−2 ± 1.3 × 10−3 s−1 and 9.0 × 10−6 ± 1.4 × 10−6 s−1, respectively. The variability of jNO2 was due to decreases in direct UV‐A radiation during overcast conditions, while the larger variability observed for jO(¹D) was primarily the result of changes in the overhead ozone column. Modeled photolysis frequencies were obtained using the NCAR TUV radiation model. The downwelling modeled and measured photolysis frequencies agree well during clear sky conditions. As a consequence of the high albedo and 24 hours of sunlight at this time of the year, the daily averaged jNO2 measured at the South Pole was three times higher than predicted for a tropical location.