NoO [O

The table below shows [N0]/[N02] and Tno, as a function of altitude.

"Assumes jwo2 = 0.015 s-1 (independent of altitude), 03 mixing ratio = 50 ppb, and [OH] = 106 molecules cm .

The value j'no2 is essentially constant with altitude in the troposphere. For the calculation, the ozone mixing ratio can be taken as essentially uniform vertically, but [03] decreases with altitude because of the decrease of the number concentration of air. The ratio [N0]/[N02] < 1 at the surface, increasing to about 12 at 10 km. Two factors contribute to this increase. First, &no+o3 decreases as temperature decreases, slowing down the return of NO to N02. The second factor is the decrease of [03] with altitude, also serving to slow down the rate of the NO + 03 reaction. The lifetime of NO* increases from between 1 and 2 days at the surface to about 2 weeks in the upper troposphere. The relatively short lifetime at the surface is a result of the fact that most of the NO* is in the form of N02 at the surface, and the OH + N02 reaction dominates the lifetime of NO*. In the upper troposphere, the opposite condition holds; with most of the NO* in the form of NO, the net removal of NO* by OH + N02 is slowed considerably.

6.5.2 Nighttime Behavior

At night, N02 does not photolyze, and, as a result, the chemistry of the NO* family is entirely different from that during daytime. Any NO present at night reacts rapidly with 03 (¿no+o3 = 1.9 x 10~14 cm3 molecule"1 s 1 at 298 K)3; as a result, almost all NO* at night is converted to N02. We recall from Chapter 5 that N02 reacts with 03 to produce the nitrate (N03) radical:

N02 + 03 N03 + 02 ki = 1.2 x 10"13 exp(-2450/T) cm3 molecule"1 s"1

(reaction 1)

Reaction 1 is the only direct source of the N03 radical in the atmosphere. During daytime, N03 radicals photolyze rapidly via two paths

N03 + hv{X < 700 nm) —> NO + 02 N03 + hv(X < 580 nm) —> N02 + O

with a noontime lifetime of ~ 5 s, and react with NO

sufficiently rapidly that NO and N03 cannot coexist at mixing ratios of a few parts per trillion (ppt) or higher. For typical daytime conditions of [N02] = 40ppb, [03] = 50ppb, and [NO] = 40 ppb, the maximum N03 mixing ratio will be less than 1 ppt. At nighttime, however, when NO concentrations drop near zero, the N03 mixing ratio can reach 100 ppt or more. At night, the nitrate radical reacts with N02 to produce N2O5

and N2Os itself can thermally decompose back to N02 and N03:

3In Section 3.1 the lifetime of NO at 298 K in the presence of 50 ppb 03 was estimated to be 42 s.

Reactions 2 and 3 establish an equilibrium on a timescale of only a few minutes:

The equilibrium constant K2? = 3.0 x 10"27 exp (10990/7) cm3 molecule-1 (Sander et al. 2003). The value of K2,3 at different altitudes in the troposphere is

0 288 1.1 xlO"10

As temperature decreases and N02 levels increase, the equilibrium is shifted more and more to the right.

N03 mixing ratios at night in urban plumes have been observed to reach values of a few hundred ppt, and values up to 40 ppt are common in more remote regimes. N2O5 mixing ratios of up to 3 ppb have been observed near Boulder, Colorado (Brown et al. 2003a) and up to 200 ppt in the area of San Francisco Bay (Wood et al. 2004).

Whereas gas-phase reactions of N2O5 are quite slow, we have already seen the importance in the stratosphere of the heterogeneous (particle-phase) hydrolysis of N2O5:

Together with the OH + NO2 reaction, reaction 4 is one of the major paths for removal of NO, from the atmosphere.

Because NO3 and N2O5 are related through reactions 2 and 3, it is useful to introduce a chemical family N03 = NO3 + N2O5. The dynamics of the NO3 family are depicted in Figure 6.6. Following the same analysis as in the NO, lifetime, we find that the lifetime of the NO3 family at night is tno: — tn2o5


The lifetime of an N2O5 molecule is that against reaction with H20(s):


FIGURE 6.6 Chemical family NO, = N03 + N205.


FIGURE 6.6 Chemical family NO, = N03 + N205.

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At typical boundary-layer conditions, £4 = 3 x 10"4 s" ', so tN2o5 = 0.9 h. The N03/N205 ratio is obtained from the equilibrium constant ^2,3:


[N2O5] £2,3 [NO2] Combining the last three relations, we obtain

At mixing ratios of N02 > 1 ppb, most of the NO3 is in the form of N205, and tno; — tN2o; • For example, at an N02 mixing ratio of 2 ppb, we obtain z, km

288 256 228

For a more complete analysis of timescales in the N03/N205 system, we refer the reader to Brown et al. (2003b).

6.6 OZONE BUDGET OF THE TROPOSPHERE AND ROLE OF NO* 6.6.1 Ozone Budget of the Troposphere

The tropospheric ozone budget can be calculated by global chemical transport models. Table 6.2 presents the tropospheric 03 budget of Wang et al. (1998). The budget is for the

TABLE 6.2 Global Budget for Tropospheric Ozone

Global Northern Hemisphere Southern Hemisphere

Sources, Tg 03 yr 1

In situ chemical production 4100 2620 1480

Transport from stratosphere 400 240 160

Total 4500 2860 1640

Sinks, Tg 03 yr1

In situ chemical loss 3680 2290 1390

Dry deposition 820 530 290

Total 4500 2820 1680

Interhemispheric transport Tg 03 yr~' 0 —40 40

Burden, Tg 03 310 180 130

Residence time, days 25 23 28

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air column extending up to 150 mbar. The annual budget amounts were computed for the entire odd-oxygen family O* to account for reservoir species that, on dissociation or reaction, effectively release 03 to the atmosphere. Since 03 accounts for over 95% of Ov, the budgets of O* and 03 are essentially equivalent. Chemical production dominates the source of tropospheric 03 (4100 Tg yr 1), as compared with 400 Tg yr 1 estimated for transport down from the stratosphere. The 03 sink is also dominated by chemical loss (3700 Tg yr"1). Dry deposition accounts for about 800 Tg yr"1 of loss. In situ chemical production of 03 results primarily from reactions of peroxy radicals with NO; in situ chemical loss of 03 results mainly from O('D) + H20, 03 + H02, and 03 + OH. In 03 production, about 70% of the peroxy radical + NO reactions are H02 + NO, about 20%, CH302 + NO, and the remainder larger peroxy radical + NO reactions. Ozone loss occurs roughly as O('D) + H20 ~ 40%; 03 + H02 ~ 40%; 03 + OH ~ 10%. The interhemispheric difference in the 03 burden is not as great as the difference in NO* emissions; the explanation is that the ozone production efficiency in the Southern Hemisphere is higher than that in the Northern Hemisphere, 46 mol 03/mol NO* in the SH versus 23 mol 03/mol NO* in the NH, as computed by Wang et al. (1998). The global mean residence time of a molecule of 03 is computed to be about 25 days.

In situ chemical formation and consumption dominate the balance of tropospheric ozone. The local concentration of NO is critical in determining whether the atmosphere in a particular region is either a source or a sink of 03. An approximate assessment of this can be made by considering the fate of the H02 radical. In the CO oxidation system it was seen that the rate of 03 production is equal to the rate of the H02 + NO reaction, (6.9). The competing reaction, H02 + 03—»OH + 202, leads to ozone loss. Thus, the ratio of the rates of these two reactions is indicative of whether a particular region of the atmosphere is one in which 03 is being produced or consumed:

For a given level of 03, the concentration of NO at which this ratio is unity can be called the breakeven concentration of NO; below the breakeven concentration, 03 is consumed and above it, 03 is produced. At 298 K, the ratio of the two rate coefficients is


In remote regions the 03 mixing ratio is about 20ppb. Thus, the breakeven NO mixing ratio in such regions is about 5 ppt. This amount of NO is roughly equivalent to 15-20ppt NO*.

In CH4 oxidation, 03 production occurs as a result of both H02 + NO and CH302 + NO reactions. A competition exists between NO and H02 for reaction with the CH302 radical; the former reaction leads to 03 production and the latter reaction to formation of CH3OOH. The CH302 + NO and CH302 + H02 rate coefficients are of roughly comparable magnitude (Table B.l). On the basis of approximate H02 radical concentrations, Logan et al. (1981) calculated that the reaction of CH302 with NO exceeds that with H02 for NO mixing ratios > 30 ppt. (Note that CH3OOH may serve as only a temporary sink for HO*.)

Because of 03 consumption by photolysis, the NO breakeven concentration at which net 03 production occurs is somewhat larger than the value based solely on the ratio of the H02 + 03 and H02 + NO reactions. The approximate crossover NO* concentration between 03 destruction and production is considered to be about 30 ppt. Ozone mixing ratios in the boundary layer over the remote Pacific Ocean are only about 5 to 6 ppb; NO* levels are about 10 ppt. Thus, this region of the atmosphere tends to be below the crossover point.

Local production and loss of 03 in the background troposphere can be estimated from

Po3 = sMh , no[H02] + kCHlo2 ■ NoiCH302] }[N0] lo} = ^0(.d)+h2o[0(1D)][H20] +^ho2+o3[H02] [03]

Figure 6.7 shows calculated, 24-hour-average production and loss rates for the free troposphere above Hawaii during the MLOPEX (Mauna Loa Photochemistry Experiment) as a function of NO* mixing ratio. The 03 loss rate is seen to be almost independent of NO*, at about 5 x 105 molecules cm-3 s_1. For an 03 mixing ratio of 40 ppb, this loss rate gives an 03 lifetime of 17 days. Data for upper tropospheric concentrations over Hawaii indicate that [NO*] is typically ~ 30 ppt, with midday [NO] at ~ 10 ppt (Ridley et al. 1992). From Figure 6.7 we see that at these levels 03 production and loss are just about in balance, with loss predicted to be slightly greater.

Ozone lifetimes in the troposphere vary significantly depending on altitude, latitude, and season. Lifetimes are shorter in the summer than in the winter as a result of the higher i c/3


T3 c c3

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