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where V is the thermal velocity of the reactant molecule, A is the aerosol SAD, and 7 is the reactive uptake coefficient (the fraction of collisions between X and the particle that result in a reaction).

The increase in the SAD following an eruption leads to an increase in the rate constants of the heterogeneous reactions. As we will show, this in turn leads to changes in the partitioning of the chemical families relative to the nonvolcanic state. If these changes include changes in the abundances of 0,-destroying radicals, then changes in the abundance of Ot may result. Finally, because the increase in SAD due to a volcano occurs primarily in the lower stratosphere, the major impact of heterogeneous chemistry occurs there.

N20-i hydrolysis The most important heterogeneous reaction in the stratosphere is the hydrolysis of N2Os. During the night, NX)., is formed in the reaction between

NO, and NO, (reaction (4.41)). This N-,0.: is subsequently converted to UNO, via hydrolysis (reaction (4.48)) [101]:

The increase in the aerosol SAD after volcanic eruptions increases the rate constant ^'n,o,««iu»í.i> which, in general, increases the rate at which N305 is hydrolyzed and HNO, is formed. Because the rate at which HNO, is converted back to NO, is unchanged, the abundance of NO, relative to HNO-, decreases [116].

Figure 6.2 shows the [NO,]/[NOr] ratio in the lower stratosphere as a function of the SAD in three different seasons. Consistent with our previous discussion, this figure shows that an increase in the SAD is generally accompanied by a decrease in the abundance of NO,, Additionally, the abundance of NO, (relative to NO,) increases from winter to equinox to summer. This occurs because of the decreasing length of night (during which NO, is converted to HNO, via N20, hydrolysis) and increasing photolysis rates of the NO, reservoirs HNO,, N.O,, and CI0N02 (whose photolysis produces NO,).

Interestingly, changing the aerosol SAD has the largest effect on the [NO,]/[NOv] ratio at low SADs. Above ~5 (irrr cm ', changes in the SAD have little effect on the NO,/NO,, ratio [1 16,200]. Note that the background SAD in the lower stratosphere, ~1 pirr cm is in the region where the [NOJ/[NO„] ratio shows a large sensitivity to changes in the SAD.

The small sensitivity to changes in the aerosol SAD seen in Figure 6.2 when the

Aerosol Surface Area Density

Figure 6.2 Daytime-averaged steady-state lower-stratospheric [NO,|/|NO,[ ratio versus the aerosol SAD (pm~ cm ') from a model. Model run for 45°N, 220 K, and 50 hPa.

Aerosol Surface Area Density

Figure 6.2 Daytime-averaged steady-state lower-stratospheric [NO,|/|NO,[ ratio versus the aerosol SAD (pm~ cm ') from a model. Model run for 45°N, 220 K, and 50 hPa.

surface area is greater than ~5 jim2 cm ' is often referred to as "saturation"—i.e. the atmosphere is saturated with respect to N,Os hydrolysis, so further increases in surface area have no effect on the NO, abundance. Saturation occurs when the lifetime of N20, with respect to hydrolysis (1/H:0^"N.)() +) is much shorter than the length of the night. When this is true, every N,Os formed during the night is hydrolyzed to form HNO,. In this case, the rate at which NO, is converted to UNO, through hydrolysis is limited by the rate at which N,(X is formed, and not by the rate of N20, hydrolysis. Additional SAD does not increase the rate of N205 formation, and therefore does not affect the rate of conversion of NO, to HNO,.

There is a subtle point here. The addition of aerosol SAD always increases the rate constant for the heterogeneous reaction. However, the rate of the reaction is the rate constant times the concentration of the reactant. For N20, hydrolysis, once saturation is reached, further increases in the rate constant are accompanied by decreases in the concentration of N,0,, so that the product of these is unchanged.

BrONG2 hydrolysis Another hydrolysis reaction is the hydrolysis of BrONO, (reaction (4.87)):

There are two fundamental differences between the hydrolysis of N,05 and that of BrONO,. First, because formation of N,05 occurs only at night, the effectiveness of the hydrolysis of N,0< increases as the length of the night increases. BrONO,, on the other hand, is formed during the day. In air exposed to sunlight, the following cycle converts NO, and H,0 to HNO, and OH:

After sunset, photolysis ceases, bringing the cycle to a halt. As a result, the effectiveness of this cycle increases with the length of day.

Second, BrONO, hydrolysis cycle does not saturate with increasing aerosol abundance [ 102], Assuming that BrONO, is in the photochemical steady state, we can write its abundance as

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