SEASONAL-VARIATION OF ATMOSPHERIC NITRIC-ACID OVER THE SOUTH-POLE IN 1992

We conducted year-round measurements of mid infrared atmospheric emiss ion over the South Pole in 1992. We were able to observe a large seaso nal change of the total column of nitric acid (HNO3) vapor. During the summer, the HNO3 column abundance was about 2x10(16) molecules/cm(2). There was a small increase in the fall. A rapid decrease of 50% was o bserved in late June, soon after the stratospheric temperature reached the threshold for formation of type I polar stratospheric clouds (PSC s). By early July, the stratospheric temperature was cold enough to fo rm type II PSCs, and the HNO3 column decreased to 7x10(15) molecules/c m(2). 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 HNO3 by gravitational settling, or l ong-term sequestering in large particles. Normal summer values were no t observed until the vortex dissipated and allowed the lateral transpo rt of HNO3 to the south polar region.