Visible and near‐ultraviolet spectroscopy at McMurdo Station, Antarctica: 8. Observations of nighttime NO₂ and NO₃ from April to October 1991

Lunar absorption spectra have been used to determine the vertical column abundances of NO 2 and NO 3 above McMurdo Station, Antarctica (77.8°S), during the fall, winter, and spring seasons in 1991. The observed nighttime NO 2 and NO 3 amounts during the fall and spring were broadly consistent with model predictions and with daytime measurements. During winter, local photochemistry would imply a long polar night, with associated gradual removal of NO 2 and buildup of NO 3 and N 2 O 5 . The winter observations of both species differ from such simple predictions by factors greater than 10, with the measured NO 2 being far larger than predicted while the measured NO 3 is much smaller than predicted. The concurrent measurements of both NO 2 and NO 3 strongly constrain possible solutions for these discrepancies and suggest that air parcel excursions to sunlit, lower latitudes likely take place quite regularly throughout the winter (approximately every 3–7 days). Thus the concept of an extended period of polar night as often applied in modeling studies of polar photochemistry appears inconsistent with these observations.