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. NOx

Figure 6.4 Schematic of the contributions of the various catalytic cycles to the rate O, loss as a function of NO, abundance. The solid vertical lines represent the situations in the lower and mid-stratosphere. (Modified from Wennberg et al. [74J. Figure 7.)

SAD is near background amounts, even small increases in the SAD above background can lead to important changes in the chemistry of the stratospheric O,. As a result, an energetic volcanic eruption like Mount Pinatubo can perturb the heterogeneous chemistry of the stratosphere over almost the entire globe. And the chemistry will remain perturbed until the SAD is back to near background, which takes several years.

Finally, it should be pointed out that the response of the atmosphere to Mount Pinatubo was possibly unique in the history of the Earth. It has been argued that for stratospheric total chlorine abundances consistent with only natural sources—about 0.6 ppbv—the decrease in O, loss caused by the decrease in NO, would have dominated increases in HO, and CI, [210]. As a result, O, would have increased throughout the stratosphere after a Pinatubo-like eruption.

6.2.2 Effects due to changes in photolysis rates

In addition to modifying heterogeneous chemistry, aerosols also absorb and scatter ultraviolet, visible, and infrared radiation. Thus, a change in the number, composition, or sizes of aerosol particles can modify the radiation field of the atmosphere |211,212], This can lead to changes in the photolysis frequencies of molecules in the stratosphere, leading to changes in the chemical composition of the stratosphere. For Mount Pinatubo, the greatest impact was in the tropics, where the SO, was injected, and immediately after the eruption, before the cloud had a chance to disperse. After about a year, the aerosol cloud was sufficiently dispersed that it had a negligible effect on the photolysis frequencies.

In the tropical lower and mid-stratosphere, abundances of NO,,, Clv, and Brv are low. Consequently, the most important photolysis frequencies are J(h and ./„ , the dissociation frequencies of O, (reaction (3.1)) and O, (reaction (3.4)), respectively. After the eruption, /,,., decreased by a few tens of percent in the lower stratosphere as a result of direct and diffusive beam attenuation due to the aerosol cloud |213). This slowed the production of O,, which tended to decrease the abundance of lower-and mid-stratospheric O,.

./0,, the photolysis frequency of O,, on the other hand, increased by several percent in the lower stratosphere. This increase occurs because the peak O, absorption occurs at wavelengths longer than -250 nm, where multiple scattering plays an important role in distributing the solar radiation. At these wavelengths the increase in scattered radiation due to the aerosol particles dominates the attenuation of the direct beam by the aerosol particles [213]. The increase in ,/0 increases the abundance of O atoms, which increases the rate of destruction of O,. This in turn tends to decrease the abundance of O,.

Thus, changes in ,/0) and Ja both tend to decrease O, in the lower and mid-stratosphere.

6.2.3 Changes in the circulation of the atmosphere

Another consequence of the perturbation of the radiation field by aerosols can be seen in Figure 6.5. This figure shows clearly that the aerosols from the Mount Pinatubo and El Chichón eruptions caused a rise in the temperature of the lower stratosphere [214,215] (Figure 6.5). This warming is collocated with the volcanic cloud, so for both El Chichón and Mount Pinatubo most of the wanning took place at low latitudes.

This heating resulted in a strengthening of the Brewer-Dobson circulation— enhanced rising motion in the tropics and enhanced down welling at mid- and high latitudes [209,214-216]. As discussed in the last chapter, the circulation of the stratosphere plays an important role in determining the distribution of stratospheric O, Changes in the circulation can therefore be expected to effect the distribution of stratospheric Ot.

In the tropical lower stratosphere, the air has low abundances of NO,,, Clr, and Brv, so chemical loss of O, is negligible compared to production and transport (see Figure 5.14). Horizontal transport can be neglected because of a combination of weak horizontal mass transport into the tropics [182,184] combined with the small horizontal gradient of O, It should be noted that for some species, such as N?0 or F11,

Year

Figure 6.5 Globally integrated (65°S-N) and area-weighted temperature anomaly in the MSLJ channel 4 data, which provide a measure of the weighted mean temperature in the 150-50 hPa layer. The anomaly is calculated as differences from a seasonally varying pre-Pinatubo average calculated over the 4 years 1987-1990. The dotted lines marked "El Chichón" and "Pinatubo" show the times of the eruptions. Quasi-biennial oscillation effects have been removed. (Adapted from Randel et al. [25j, Figure 15, but using updated MSU data W. J. Randel, F. Wu, personal communication. 1998.) Tick marks represent January 1 of each year.

Year

Figure 6.5 Globally integrated (65°S-N) and area-weighted temperature anomaly in the MSLJ channel 4 data, which provide a measure of the weighted mean temperature in the 150-50 hPa layer. The anomaly is calculated as differences from a seasonally varying pre-Pinatubo average calculated over the 4 years 1987-1990. The dotted lines marked "El Chichón" and "Pinatubo" show the times of the eruptions. Quasi-biennial oscillation effects have been removed. (Adapted from Randel et al. [25j, Figure 15, but using updated MSU data W. J. Randel, F. Wu, personal communication. 1998.) Tick marks represent January 1 of each year.

the gradient between the tropics and mid-latitudes is large enough that horizontal transport cannot be neglected 1182].

Based on this, O, in the tropical lower stratosphere (below -24 km altitude) can be modeled simply. We can write the continuity equation for O, in this region [217] as

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