Figure 5.24 shows a distribution of stratospheric aerosol surface area as a function of altitude from 18 to 30 km inferred from satellite measurements. The surface area units used in Figure 5.24 are cm2 cm"3, and typical values of the stratospheric surface area at, say, 18 km altitude are about 0.8 x 10~8 cm2 cm-3. This is equivalent to 0.8pm2 cm3. As a useful rule of thumb, stratospheric aerosol surface area in the lower stratosphere ranges between 0.5 and 1.0 pm2 cm-3.

Figure 5.25 depicts the complete NO, N02, N03, N205, HN03 system (top). During daytime, photolysis of N03 is so rapid that [N03] Q* 0, and HN03 is formed only by the gas-phase OH + N02 + M —> HN03 + M reaction. At night, the system shifts to that shown in the bottom panel, in which HN03 forms entirely by the heterogenous path involving stratospheric sulfate aerosol. HN03 has a relatively long lifetime (~ 10 days) so that the HN03 formed during daylight remains at night, to be augmented by that formed heterogeneously at night.

At night, the N03 concentration achieves a steady state given by

At night:

Formation Hn03



At night:


FIGURE 5.25 Stratospheric NO,, system. Lower panel shows only the paths that operate at night.


FIGURE 5.25 Stratospheric NO,, system. Lower panel shows only the paths that operate at night.

and N205 also attains a steady state by

Because of its relatively long lifetime, HN03 achieves a steady-state concentration over both day and night so that total production and loss are in balance:

Wno2+m[OH][N02][M] + 2fcn2o5+h2o(s)[N205] = Whno3[OH][HN03] +7hno3 [HNO3]

During daytime when [N2C>5] = 0



As altitude increases, [M] decreases, and the [N02]/[HN03] ratio increases. At night, /hno, = 0 and [OH] = 0 so HNO3 builds up. However, over a full daily cycle HN03 achieves a steady state given by



The "bottleneck" reaction in the heterogeneous formation of HNO3 is that of NO; and 03. As altitude increases, temperature increases and the rate of this reaction increases; the faster this reaction, the more aerosol surface area is needed to effectively compete for the increased amount of N2Os. The conversion of N02 to HNO3 at mid latitude saturates for aerosol loadings close to typical background levels of 0.5 ptrrcm-3 near 20 km; at 30km, saturation occurs at aerosol loadings of about 3 fim2 cm-3. However, from Figure 5.24 we note that aerosol surface area concentration decreases with increasing altitude, so the amount of NO* present, as opposed to NOy, actually increases with altitude because the hydrolysis of N205 becomes aerosol surface area-limited.

Returning to Figure 5.9, we can now explain the observed altitude variation of HN03 and NOi. The stratospheric aerosol layer resides between about 20 and 24 km, and it is in this region that heterogeneous formation of HNOj is most effective. At altitudes above the stratospheric aerosol layer, the daytime formation of HN03 by OH + N02 + M runs out of [M] as altitude increases; for this reason the HN03 concentration drops off with altitude, and the [N02j/jHN03] ratio increases in accord with (5.36). The conversion of a reservoir species N205 into a relatively stable species HN03 serves to remove N02 from the active catalytic NO* system, reducing the effectiveness of 03 destruction by the NO* cycle. The reduction of N02, on the other hand, decreases the formation of C!0N02, allowing more CIO to accumulate than in the absence of the N20s + H20(s) reaction. More CIO means that the effectiveness of the CIO* cycles is increased. Although photolysis of HN03, with the H atom derived from aerosol H20, provides an additional source of HO* to the system, this is not predicted to have a major effect on the HO* cycles. Effectively, the hydrolysis of N1O5 on stratospheric aerosol moves N02 from C10N02 with a 1-day lifetime to HN03 with a 10-day lifetime, leading to substantially lower NO* concentrations. As a result, lower stratospheric 03 becomes more sensitive to halogen and HO* chemistry and less sensitive to the addition of NO*.

"I Effect of NiOs Hydrolysis on the Chemistry of the Lower Stratosphere

N03 forms from reaction 1 and is consumed by reactions 2 and 3. During daytime, photolysis of N03 is very efficient; jNo, from both channels 3a and 3b ts about 0.3 s giving a mean daytime lifetime of NO, of 3 s. At night, reaction 2 is the only removal pathway for N03. The reaction rate coefficient of reaction 2 from Table B.2 has k) = 2.2 x 10-30(r/300)~3'9 and k^ = 1.5 x 10" 12(r/300)-°'7cm3 molecule 's At 20km, T = 220K, [M] = 2 x I0IS molecules cm"3, and theTroe formula gives kj — 1.2 x 10 12 cm3 molecule s"1. Assuming that N02 is present at 1 ppb, the mean lifetime of N03 against reaction 2 is 400 s. In summary, the lifetime of a molecule of N03 at 20 km increases from 3 s in daytime to about 7 min at night.

Each N03 molecule formed at night is converted to N2Os in about 7 min, so over the course of a 12-h night, every time reaction 1 occurs, a molecule of N203

n02 + 03 —> n03 + 02 n03 + n02 + m —* n2o5 + m n03 + hv no2 + o a no + 02

(reaction 1) (reaction 2) (reaction 3a) (reaction 3b)

is formed, with consumption of two N02 molecules. NO* is converted to N205 at the following rate:

¿[NO*] _ ^2A-i[03][N02] ki = 1.2 x 10"13 exp(-2450/F)

At T - 220 K, ki = 1.7 x 10~1B cm3 molecule"1 s~!. The lifetime of NO* against reaction 1 is

With [03] = 5 x 1012 molecules cm-3, Tno., = 5.9 x 104s. The fractional decay of NO* is given by exp(—//tnoJ- Over a single 12-h night (4.3 x 104s), 52% of the NO* is converted to N205.

N205 is hydrolyzed on stratospheric aerosol to produce nitric acid:

The conversion can be represented as a pseudo-first-order reaction with rate coefficient (5.31)

4 \nmx2oJ

which depends on the stratospheric aerosol surface area Ap and the reactive uptake coefficient y. Previously, we noted that a representative value for y for reaction 4 is 0,06. Values of kA for Ap = lx 10~Bcm2cirr3 and 10 x 10"8cm2cm_3 at T = 220 K are

The lifetime of N2O5 against reaction 4 is just if^o, = 1 /Î4.

What fraction of NO* is converted to HNO3 over the nighttime? N2O5 is gradually produced throughout the night at a rate determined by reaction I, As N205 is produced, N2O5 itself is gradually converted to HN03 by reaction 4. The system is the classic reactions in series: A—+ B —>C. In this case, A = N02t B = N2O5, and C = HNO3. (Here, two molecules of C are produced in the B —» C reaction.) To determine the amount of C formed over the night starting from a given concentration of A at sundown, one must integrate the rate equations:

dt Two,

dt tN3o,

The factor of A in the rate equation for [B] reflects the fact that each molecule of B formed requires the consumption of two molecules of A. The fractional conversion of an initial concentration of A, [A]« into C is 2[C]/[A]0, where the factor of 2 accounts for the fact that two molecules of HNO3 are formed in reaction 4, The solutions for [A], [B], and [C] are

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