T

20 40

Day of 1993

Figure 7.20 Time series of HNO, abundance at several equivalent latitudes in the high northern latitudes. MLS HNO, measurements (version 4) at 465 K in a ±1° equivalent latitude (derived from UKMO PV) bund are averaged to produce an average HNO, abundance for each day. The time series has been smoothed to reduce the day-to-day variability.

abundance, the dominant component of NOv in the lower stratosphere. There is little change of lower-stratospheric UNO, through the winter. Comparison with Figure 7.10, the comparable plot for the southern hemisphere, shows the dramatic difference between the hemispheres.

7.2.3 Chlorine activation in the Arctic

The presence of temperatures below the chlorine activation threshold (~196 K, see Figure 7.18) suggests that CI, in the Arctic should be enhanced; in other words, the abundance of CI, should be much higher than its normal abundance of a few tens of parts per trillion by volume. Figure 7.21 shows that this is indeed the case.

Also shown in Figure 7.21 is CI, in the Antarctic polar vortex. As one can see, CI, in the Antarctic is generally higher throughout this time period. The difference between the two hemispheres can be attributed to two factors, both related to the warmer temperatures in the Arctic polar vortex. First, warmer temperatures mean that there is less irreversible denitrification in the Arctic. As a result, abundances of HNO, are higher in the Arctic, and this in turn leads to a proportionately higher production rate of NO, from HNO, destruction. This NO, reacts with CIO to form ClONO,. In concert with production of HC1, this leads to a much faster deactivation of CI, in the Arctic than in the Antarctic [267,269]. Second, the warmer temperatures

Day of year

Figure 7.21 Time series of average CI, abundance at 465 K potential temperature for 1992, 1993, and 1994 in the Arctic polar vortex, and for 1992 in the Antarctic vortex (day of the year is shifted by 6 months). CI, is derived from daytime UARS MLS CIO data (version 4) assuming photochemical steady-state. (After Wu and Dessler [262|.)

Day of year

Figure 7.21 Time series of average CI, abundance at 465 K potential temperature for 1992, 1993, and 1994 in the Arctic polar vortex, and for 1992 in the Antarctic vortex (day of the year is shifted by 6 months). CI, is derived from daytime UARS MLS CIO data (version 4) assuming photochemical steady-state. (After Wu and Dessler [262|.)

in the Arctic mean that less frequent PSC processing, and therefore less chlorine activation, occurs during the Arctic winter, and stops earlier in the year, than in the Antarctic.

The upshot of this is that, in the Arctic, the rate of chlorine activation is lower and the rate of chlorine deactivation is higher than in the Antarctic. This leads to generally lower abundances of CI, in the northern hemisphere, as shown in Figure 7.21.

It is also noteworthy that the interannual variability of CI, abundance is higher in the north than in the south. In the Antarctic the temperatures are so far below the PSC-processing threshold temperature of 196 K that interannual variability in the Antarctic vortex temperatures has little effect on the amount of PSC processing and, therefore, on the abundance of CI,. Consequently, CI, shows little year-to-year variability (see Figure 7.12). In the Arctic, however, the temperatures are much closer to the PSC-processing threshold. As a result, a variability of a few degrees can have a dramatic effect on the amount of air exposed to temperatures below ~I96 K. This leads to significant year-to-year variability in the abundance of CI, in the Arctic (Figure 7,21). As we will see, this year-to-year variability in the abundance of CI, leads to significant year-to-year variability in the loss of O, [2761.

7.2.4 Odd Oxygen loss

Figure 7.22 shows O, loss per day calculated for the northern hemisphere vortex during the last month of winter. Comparison with Figure 7.13 reveals much lower O, loss rates in the Arctic than the Antarctic. The amount of sunlight the polar regions receive is comparable, so the difference in O, loss rates can be almost fully attributed to differences in CI,.

The rate of the ClO-dimer loss cycle scales approximately as ICIOJ", while the rate of the CIO-BrO loss cycle is linear in [CIO]. Thus, at the lower CI, abundances of the Arctic the CIO dimer cycle makes up about half of the total loss, compared to about two-thirds in the Antarctic. In both hemispheres the CIO-BrO cycle makes up most of the remainder, with the ClO-O cycle making up a few percent.

In Figure 7.23 we plot vortex-average O, on several potential temperature surfaces in the lower and mid-stratosphere. Unlike the Antarctic (see Figure 7,15), Arctic O, shows little change during the late winter and early spring. This might seem pu/./.ling because we have shown that O, is being lost during this period. The answer is that significant transport of 0,-rich air into the vortex is also occurring, and the net result of these two processes is that there was little change in vortex O, in this year.

To demonstrate the importance of this transport, in Figure 7.24 we plot a calculation of the evolution of vortex-averaged lower stratospheric O, in the absence of chemical O, loss. In this case, all changes in O, in the lower stratosphere are due to transport (solid line)—e.g. if O, behaved like a passive tracer. This calculation shows

i i i I i i i i I i i i i I i i i i I i i i i I i i i i I i i i i I i i i i I

Figure 7.22 Arctic vortex-averaged daily O, loss per day versus day of the year. Values were calculated as the sums of the rates of the ClO-CIO and ClO-BrO catalytic cycles, using L'ARS MLS GO measurements (version 4) and assuming [BrO] = 12 pptv. This sum is adjusted upward by 5% to account for other cycles, such as CIO + O. (Adapted from Wu and Dessler [2621.)

i i i I i i i i I i i i i I i i i i I i i i i I i i i i I i i i i I i i i i I

40 45 50 55 60 65 70 75 Day of year

Figure 7.22 Arctic vortex-averaged daily O, loss per day versus day of the year. Values were calculated as the sums of the rates of the ClO-CIO and ClO-BrO catalytic cycles, using L'ARS MLS GO measurements (version 4) and assuming [BrO] = 12 pptv. This sum is adjusted upward by 5% to account for other cycles, such as CIO + O. (Adapted from Wu and Dessler [2621.)

Figure 7.23 Time series of O, abundance on Ihree potential temperature surfaces. Each point is one day of LIARS MLS O, data (version 4, 205 GHz) averaged over the Arctic polar vortex. The boundary of the Antarctic polar vortex is determined from the method of Nash et a!. [219] using UKMO PV.

Day No. of 1993

Figure 7.23 Time series of O, abundance on Ihree potential temperature surfaces. Each point is one day of LIARS MLS O, data (version 4, 205 GHz) averaged over the Arctic polar vortex. The boundary of the Antarctic polar vortex is determined from the method of Nash et a!. [219] using UKMO PV.

3/3/94 Date

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