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3. Removal of Stratospheric NO_y during the Antarctic Winter:

Our measurements of HNO₃ from its peak in mid-fall to its mid-winter minimum (April-May to July-August) show that the column density of HNO₃ decreases by about 2x10¹⁶ molecules/cm² during this period, and that very little returns to the gas phase as temperatures rise above the PSC maxima. The latter observation indicates that gravitational settling has removed most PSCs from the lower stratosphere by this time. UARS/MLS satellite mapping of HNO₃ depletion show that the area involved is typically at least as large as the continental area of Antarctica. A simple calculation thus yields a value of at least 3x10⁸ kg of NO_y removed from the stratosphere over Antarctica each winter, given the rather accurate assumption that nearly all stratospheric NO_y is in the form of HNO₃ by late fall or winter. This estimate does not include additional smaller quantities of NO_y (primarily NO₂) which undergo transformation to HNO₃ in the mid-to-upper stratosphere in winter, and are transported downward far enough to encounter PSC formation temperatures and condense out. I estimate that at perhaps an additional 0.3x10⁸ kg.

The overall amount of NO_y removal has probably been increasing in recent decades, as well-documented increases in stratospheric CO₂ and water vapor lead to faster radiative cooling, lower winter temperatures, and an earlier, longer period of PSC formation in the lower stratosphere. Greatly diminished ozone also delays springtime warming, leading to longer gravitational settling times that continue to move small as well as larger HNO₃-bearing PSCs out of the lower stratosphere.