Polar winter

During the dark, polar winter the temperature drops to extremely low values, on the order of-80°C. At these temperatures, water and nitric acid form polar stratospheric clouds. Polar stratospheric clouds are important because chemical reactions in the stratosphere are catalyzed on the surface of the crystals forming these clouds. The chemical primarily responsible for ozone depletion is chlorine. Most of the chlorine in the stratosphere is contained in the compounds hydrogen chloride, HCl, or chlorine nitrate, CIONO. Hydrogen chloride and chlorine nitrate undergo a number of reactions on the surface of the crystals of polar stratospheric clouds. Two important reactions are ... 

Figure 1.7 shows the total column ozone measured in October at one Antarctic location, Halley Bay, as a function of year. This includes the original Farman et al. (1985) data, as well as more recent data up to 1994 (Jones and Shanklin, 1995). It is clear that starting in the late 1970 s, there was a dramatic drop in total column ozone at the end of the polar winter when sunrise occurs. Observation of such a rapid change is unprecedented and quite remarkable. 

Reactions (1), (2) and (4) convert stable chlorine reservoir species, CIONO, and HC1, into the more easily photolyzable species Cl, HOC1, and C1NO, (nitryl chloride), respectively. This unique chemistry of CIONO, and N,0, on the cold surfaces of the PSC-surfaces is taking place due to the low temperatures of 180 to 200 K encountered in the lower stratosphere at altitudes between 15 and 25 km in the polar vortex. At sunrise, after the polar winter, these photolabile species release Cl atoms that initiate the chain destruction of ozone according to the mechanism, which is responsible for the fast ozone depletion event occuring within a few days to several weeks [34,35] ... 

There is evidence that polar air masses, one and half years after the eruption, carry a good deal less aerosols indicating that meridional mixing is incomplete. In an analysis of the SAM II data satellite extinction data for the Northern hemisphere winter of 1982, supplemented by airborne lidar observations, McCormick et al. (1983) found the polar vortex to be an area of substantially low aerosol content where the El Chichon cloud does not seem to have penetrated and that either an aerosol sink or a supply of clean air exists in the polar winter vortex. 

Halogen Chemistry on PSCs. The cold temperatures that occur in polar winter can lead to formation of clouds within the stratosphere, and there are visual sightings of such Arctic clouds dating back hundreds of years. In the unpopulated Antarctic, the earliest explorers noted unusually colorful high clouds in winter. The term polar stratospheric clouds (or PSCs) was coined by McCormick et al. (1982), who first presented satellite observations of high-altitude clouds in the Antarctic and Arctic stratospheres, but the clouds were considered little more than a scientific curiosity until the ozone hole was discovered. 

FIGURE 3.13 Heating rate from absorption of solar radiation by ozone for solar zenith angles 00 = 0°, 60°, 80°. The solid curve is for an ozone distribution representative of equatorial conditions the dashed curve is representative of polar winter conditions. This calculation accounts for the change in ozone mixing ratio with altitude, whereas the analytical formula (3.52) assumes mixing ratio is constant with altitude. 

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