Polar air mass

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. 

Eventually, as the polar air mass warms through breakup of the polar vortex and by absorption of sunlight, the PSCs evaporate, releasing HNO3. The nitric acid vapor photolyzes and reacts with OH to restore gas-phase NO ... 

Fig. 18-6 Characteristic air-mass trajectory and corresponding per mil isotopic composition of precipitation, along a transect from the subtropics to a polar ice sheet. This is a highly schematic view of the true atmospheric system.

Knowledge about the isotopic variations in precipitation is increased when single rain events are analyzed from local stations. Especially under mid-latitude weather conditions, such short-term variations arise from varying contributions of tropical, polar, marine, and continental air masses. 

After the first reports of this phenomenon, major field campaigns were launched, which clearly established a relationship between ozone destruction and chlorine chemistry. For example, Fig. 1.8 shows simultaneous aircraft measurements of ozone and the free radical CIO as the plane flew toward the South Pole. As it entered the polar vortex, a relatively well-contained air mass over Antarctica, 03 dropped dramati-... 

Second, the northern polar vortex is much less stable and hence less isolated from mixing with external air masses compared to the Antarctic case events in January and February in which there was substantial mixing of air from midlatitudes into the vortex have been reported (e.g., see Browell et al., 1993 Plumb et al., 1994). This makes it particularly important to make both measurements and model predictions with sufficient resolution (Edouard et al., 1996). In addition, the Arctic polar vortex tends to break up earlier than the Southern Hemisphere polar vortex since ozone destruction is determined to a large degree by the extent of exposure to sunlight, the earlier breakup and mixing with air external to the vortex cuts the ozone loss short. 

FIGURE 12.40 Loss of O, 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). 

Of course, it is not just the chemistry but also the dynamics that determine the net effect on total column ozone in midlatitudes. Transport of air from the tropics to midlatitudes was discussed earlier in Section A.l. There is also evidence for the influence of high-latitude air on ozone at midlatitudes. It has been proposed, for example, that the Arctic polar vortex acts more like a flowing processor than an isolated air mass. In this scenario, air flows through the polar vortex and as it does, undergoes the chemistry described earlier for the... 

If there had been similar linear trends of ozone outside and inside the vortex and linear dependence between mixing ratio of ozone inside and outside the vortex, it would have been a piece of evidence in favour of significant mixing of ozone poor air masses through a strong polar vortex edge. This nevertheless was not the case. 

Case Study IV - BrO in the springtime Arctic - In the spring time in both the Arctic and Antarctic large clouds of BrO-enriched air masses are observable from space (see Figure 22).These clouds cover several thousand square kilometres over the polar sea ice with BrO levels up to 30 pptv. The BrO is always coincident with low levels of ozone in the MBL. In order to observe these events, there is a requirement for meteorological conditions that stop mixing between... 

Finally, it is interesting to note that, since potential vorticity is a conserved quantity, air parcels that are displaced to an area with different PV values develop a differential motion which tends to restore them to their original PV area. Thus, strong PV gradients act as dynamical barriers to transport. Barriers against horizontal mixing are observed in the lower stratosphere near the polar vortex and at the boundary between tropical and extratropical air masses. Wave events distort PV surfaces often leading to the formation of thin filaments which are pulled away from the barriers. 

In many cases, the NOx family is formed as the sum of NO and N02, and accounts for the most reactive nitrogen species. The NOx/ NOy concentration ratio, which is often reported from field observations, is an indicator of the reactivity of odd nitrogen and its ability to destroy stratospheric ozone (or to affect other chemical families including chlorine and bromine compounds). The value of this ratio increases with altitude above 30 km to reach a value of nearly one in the upper stratosphere and mesosphere. It decreases substantially when the stratospheric aerosol load is enhanced, for example, after large volcanic eruptions (Fahey et al, 1993), and substantial amounts of nitrogen oxides are converted to nitric acid by heterogeneous reaction (5.152). It is also low in the polar regions, especially in air masses processed by polar stratospheric clouds. 

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