Seasonal variation of atmospheric nitric acid over the South Pole in 1992

We conducted year‐round measurements of mid infrared atmospheric emission over the South Pole in 1992. We were able to observe a large seasonal change of the total column of nitric acid (HNO 3 ) vapor. During the summer, the HNO 3 column abundance was about 2×10 16 molecules/cm². There was a small increase in the fall. A rapid decrease of 50% was observed in late June, soon after the stratospheric temperature reached the threshold for formation of type I polar stratospheric clouds (PSCs). By early July, the stratospheric temperature was cold enough to form type II PSCs, and the HNO 3 column decreased to 7×10 15 molecules/cm². Measured nitric acid values in the spring remained extremely low even after the stratosphere warmed well above PSC temperatures. This may indicate permanent removal of HNO 3 by gravitational settling, or long‐term sequestering in large particles. Normal summer values were not observed until the vortex dissipated and allowed the lateral transport of HNO 3 , to the south polar region.