Arctic air masses

Cantrell, W., G. Shaw, R. Benner, and D. Veazey, Evidence for Sulfuric Acid Coated Particles in the Arctic Air Mass, Geophys. Res. Lett., 24, 3005-3008 (1997). 

The atmospheric situation is complicated by varying conditions of temperature, relative humidity, and concentrations of other gases such as NH3 which can enhance nucleation rates over those expected for a well-mixed air mass at a fixed temperature and RH (e.g., see Nilsson and Kulmala, 1998). However, there is a general consensus that the observed rates of nucleation of H2S04 often, indeed usually, exceed those expected from classical binary homogeneous nucleation theory. (Note that this is not always the case. For example, Pirjola et al. (1998) reported that the measured formation of nuclei in the Arctic boundary layer... 

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 12.37 (a) CIO concentrations and (b) HCI deficit at various minimum temperatures experienced by the air masses in the Arctic stratosphere during October 1991-February 1992 (adapted from Toohey et al. (1993) and Webster et al. (1993a)). 

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... 

O, in a given air mass is destroyed over the Arctic, compared to near-total ozone destruction over Antarctica. As a result, bromine does make a significant contribution to the total O, destruction in the Arctic, and as a result, control of brominated organics is expected to have a greater effect on minimizing ozone destruction in this region. 

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... 

An interesting issue concerns the existence of interannual variability in bromine activation and ODEs. At Spitsbergen, Tuckermann et al. (1997) measured up to 30 pmol mol of BrO during the spring of 1995, however, much less in 1996 when there was also much less ozone destruction. The reason is probably less transport to the site of air masses that had been in contact with the Arctic Ocean in 1996 than in 1995. 

Thus, the most frequently observed trajeetories are types II, III, IV. The trajeetories transporting the dry and warm tropical air mass are referred to as types I, II, V, VI, and those with rather wet and cold air mass from moderate or arctic latitudes as type IV. In most cases the air flows transverse a rather long path over desert and steppe territories, so the extent of its transformation and pollution is very great. BT of types I and VI are seldom observed... 

FIGURE 4.21 NOv concentrations during flights to the Antarctic (August 23, 1987) and the Arctic (February 7, 1989) (Fahey et al., 1990). The shaded areas highlight the difference between measured gas-phase NO, and calculated NO, based on unperturbed air and represent denitrification in the air masses. The dashed line indicates the boundary of the polar vortex. Reprinted by permission from Nature, 344, Fahey, D. W., et al. Copyright 1990 Macmillan Magazines Limited. 

Concentrations of cis-chlordane <0.0054 ng/m in the Norwegian Arctic are believed to originate in the Soviet Union, thousands of miles away (Pacyna and Oehme 1988). Similarly, the source of chlordane-related compounds in brown snow in the central Canadian Arctic, based on back-trajectories of air masses, was probably western China (Welch et al. 1991). It is estimated that -3,300 kg of chlordane are deposited annually in the Arctic regions (from 60° N. latitude to the pole) (Cotham and Bidleman 1991). 

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