The Net Effect

The net change of O, following a volcanic eruption is determined by the combination of the three effects discussed in this chapter: the increase in the rate of heterogeneous reactions, changes in the photolysis frequencies, and changes in the circulation of the stratosphere.

The dominant effect is the increase in the rate of heterogeneous reactions. Models [207-209] suggest that this effect by itself led to significant decreases of O, after the eruption of Mount Pinatubo (as well as after El Chichón [195]). The effect is most pronounced at mid- and high latitudes during the winter and early spring. Here the enhancement of N205 hydrolysis is most effective because of the long nights found there.

Additionally, in regions of cold temperatures (high latitude, wintertime, lower stratosphere), the heterogeneous chlorine reactions (reactions (6.4) to (6.7)) become important due to the enhanced SAD. These reactions convert the relatively unreac-tive CI,, reservoir species HC1 and CIO NO, into more easily photolyzcd species such as CI, and HOC1, thereby increasing CI,.

Models predict that the heterogeneous-chemistry perturbation alone reduced lower stratospheric O, by 20-40% at mid- and high latitudes during winter and early spring, leading to a -10% decrease in column Ov. At other times of year, extratropical column decreases were a few percent. In the tropics, the heterogeneous-chemistry perturbation had little effect, consistent with the general lack of importance of chemical loss for Or in this region. Predicted changes in tropical column O, from the heterogeneous-chemistry perturbation were a percent or less throughout the year.

The models predict that the effect from changes in the photolysis frequencies is small. The maximum effect occurs in the tropics, where changes in the photolysis frequencies decreased column 0, by at most a percent or so.

Finally, the models predict that changes in the strength of the stratospheric circulation reduced column ozone in the tropics by a few Dobson units, i.e. a few percent. This low-latitude decrease is accompanicd by an increase in extratropical O, of about the same magnitude.

Column O, measurements do indeed show substantial decreases in column ozone (of the order of 5-10%) over large regions of the globe following the eruption of Mount Pinatubo (Figure 6.6) [25]. The largest losses were observed in northern hemisphere middle and high latitudes during winter—spring of each year (largest in 1992-1993), over southern hemisphere high latitudes in spring 1993, and episodically over the tropics during 1991-1993. In general, these changes are consistent with model simulations of the post-Mount Pinatubo stratosphere.

Year

Figure 6.6 Globally integrated (65°S-N) and area-weighted TOMS column ozone anomaly. The anomaly is calculated as differences from a seasonally varying pre-Pinatubo average calculated over the 4 years 1987—1990. The dotted lines marked "HI Chichón" and "Pinatubo" show the times of the eruptions. QBO effects have been removed. (After Randel et al. |25], Figure 15.)

Year

Figure 6.6 Globally integrated (65°S-N) and area-weighted TOMS column ozone anomaly. The anomaly is calculated as differences from a seasonally varying pre-Pinatubo average calculated over the 4 years 1987—1990. The dotted lines marked "HI Chichón" and "Pinatubo" show the times of the eruptions. QBO effects have been removed. (After Randel et al. |25], Figure 15.)

1. Would a volcano erupting at high latitude have as significant global effect on stratospheric chemistry as one erupting near the equator?

2. Because ./HlONO, > ¿"BfONO,+H^„ the rate of BrONO, hydrolysis scales linearly with aerosol surface area density. How would it scale if 7BrONo, <! ^HBrONo,~n,o''

3. In steady state the abundance of HQ is proportional to the abundance of Q atoms. Derive the ratio [C1]/([C1,| + [ClONO,]) in terms of rate constants, photolysis frequencies, and the abundances of other trace species. Assume steady state applies. How do you expect HQ to change if NO, changes (you can assume the ratio ¡NOl/lNO,J does not change)?

4. It turns out that the reactive uptake coefficients for reactions (6.4)-(6.7) increase with increasing water vapor abundance. If water vapor were to increase (but other geophysical parameters remained the same), would that make a Pinatubo-like eruption have a greater or lesser impact on stratospheric O,? Explain your answer.

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