MILLIMETER-WAVE SPECTROSCOPIC MEASUREMENTS OVER THE SOUTH-POLE .3. THE BEHAVIOR OF STRATOSPHERIC NITRIC-ACID THROUGH POLAR FALL, WINTER, AND SPRING

We present data from a 9-month series of ground-based measurements of stratospheric nitric acid, made over the South Pole from mid-April 199 3 to mid-January 1994. Observations were typically made at 3- to g-day intervals. Both profiles and column densities have been retrieved fro m pressure-broadened millimeter-wave emission spectra. These measureme nts provide the first quasi-continuous series of vertical mixing ratio profiles for this species in the heart of the south polar voter. Conv ersion of NOx to nitric acid by heterogeneous reactions, and its incor poration into polar stratospheric cloud (PSC) particles, along with su bsequent gravitational settling, is considered to be the main denitrif ying mechanism in the Antarctic stratosphere, setting up conditions fo r ozone destruction at the end of winter. In our observations, a small increase in HNO3 was seen between April and the end of May, after whi ch a rapid loss took place below 25 km. Column density above similar t o 15 km decreased to less than or equal to 1/4 its maximum within 30 d ays, and depletion continued until middle to late July, by which time the nitric acid column above 15 km had diminished by more than a facto r of 10. The initial depletion was coincident with the onset of a rapi d increase in lidar backscatter from polar stratospheric cloud formati on at the same altitude range. Gas-phase depletion was tracked as a fu nction of altitude and temperature and found to be consistent with the temperature and partial pressure relationship for formation of ternar y mixtures of HNO3, H2SO4, and H2O. Depletion occurred similar to 3 we eks earlier in 1993 than was seen in 1992 column density measurements by Van Alien et al. [1995]. In late June a new layer of HNO3 was gener ated in the vicinity of 40-km altitude and, subsequently, appeared to be carried downward with general vertical transport of air within the vortex. In spring, as temperatures increased, no rapid increase of gas -phase HNO3 was seen, indicating that gravitational settling had carri ed PSC-accreted nitric acid to low altitudes. By the end of observatio ns in January 1994, mixing ratios and column densities above similar t o 15 km had not yet reached more than about half the values seen the p revious April, indicating that a rather large increase in stratospheri c HNO3 occurs in the early austral fall over the south polar region.