Hours exposed to sunlight

FIGURE 12.40 Loss of O3 in Arctic polar vortex as a function of hours of exposure of the air mass to sunlight (adapted from von der Gathen et al., 1995).

evident, which they attribute to condensation of gaseous HNO3 onto particles as the temperature falls, followed by reevaporation into the gas phase at higher temperatures.

While there is a variety of evidence from these and other measurements (e.g., see Notholt, 1994) that denitrification is more episodic and less widespread in the Arctic compared to the Antarctic, it does not mean that it does not occur. Indeed, there is good evidence for denitrification of the Arctic polar vortex under some conditions (e.g., see Schlager et al., 1990; Kondo et al., f992; Wilson et al., 1992; Dye et al., 1992; Pueschel et al., 1992b; Tuck et al., 1994; and Hopfner et al., 1996). For example, direct measurements of stratospheric water and NO,, at the edge of the Arctic polar vortex in one study showed that in that case, both dehydration and denitrification had occurred (Hintsa et al., 1998a).

When the Arctic polar vortex is not denitrified, more gas-phase HN03 is available as the sunlight intensity increases, and this photolyzes, regenerating NOz:

The production of NOz then assists in terminating the

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