Info

FIGURE 4.17 Primary quantum yield for O formation from N02 photolysis, as correlated by Demerjian et al. (1980).

about 420 nm dissociation does not occur. As noted earlier, the bond energy between O and NO, about 300 kJ mol-1, corresponds to the energy contained in wavelengths near 400 nm. At longer wavelengths, there is insufficient energy to promote bond cleavage. The point at which dissociation fails to occur is not sharp because the individual molecules of N02 do not possess a precise amount of ground-state energy prior to absorption. The gradual transition area (370-420 nm) corresponds to a variation in ground-state energy of about 40 kJ mol-1. This transition curve can be shifted slightly to longer wavelengths by increasing the temperature and therefore increasing the ground-state energy of the system. Table 4.5 gives tabulated values of the N02 absorption cross section and quantum yield at 273 and 298 K, and Table 4.6 illustrates the calculation of the photolysis rate /no, for noon, July 1, at 40°N.

Figure 4.18 shows the N02 photodissociation rate coefficient, , as a function of altitude. The majority of N02 photolysis occurs for X > 300 nm. Since in this region of the

TABLE 4.5 Absorption Cross Section and Quantum Yield for N02 Photolysis at 273 and 298 K

Absorption Cross Section

TABLE 4.5 Absorption Cross Section and Quantum Yield for N02 Photolysis at 273 and 298 K

Absorption Cross Section

From:

To:

At 273 K

At 298K

X, nm

X, nm

X, nm

1020ct, cm2

1020<j,cm2

cb

273.97

277.78

275.875

5.03

5.05

1

277.78

281.69

279.735

5.88

5.9

1

281.69

285.71

283.7

7

6.99

1

285.71

289.85

287.78

8.15

8.14

0.999

289.85

294.12

291.985

9.72

9.71

0.9986

294.12

298.51

296.315

11.54

11.5

0.998

298.51

303.03

300.77

13.44

13.4

0.997

303.03

307.69

305.36

15.89

15.8

0.996

307.69

312.5

310.095

18.67

18.5

0.995

312.5

317.5

315

21.53

21.4

0.994

317.5

322.5

320

24.77

24.6

0.993

322.5

327.5

325

28.07

27.8

0.992

327.5

332.5

330

31.33

31

0.991

332.5

337.5

335

34.25

33.9

0.99

337.5

342.5

340

37.98

37.6

0.989

342.5

347.5

345

40.65

40.2

0.988

347.5

352.5

350

43.13

42.8

0.987

352.5

357.5

355

47.17

46.7

0.986

357.5

362.5

360

48.33

48.1

0.984

362.5

367.5

365

51.66

51.3

0.983

367.5

372.5

370

53.15

52.9

0.981

372.5

377.5

375

55.08

54.9

0.979

377.5

382.5

380

56.44

56.2

0.9743

382.5

387.5

385

57.57

57.3

0.969

387.5

392.5

390

59.27

59

0.96

392.5

397.5

395

58.45

58.3

0.92

397.5

402.5

400

60.21

60.1

0.695

402.5

407.5

405

57.81

57.7

0.3575

407.5

412.5

410

59.99

59.7

0.138

412.5

417.5

415

56.51

56.5

0.0603

417.5

422.5

420

58.12

57.8

0.0188

TABLE 4.6 Calculation of the Photolysis Rate of N02 at Ground Level, July 1, Noon, 40° N, 298 K

From:

To:

Ia,

1019CTNO2,

X, nm

X, nm

photons cm 2 s 1

cm2

4)

Íno2, S 1

295

300

3.14 X 1012

1.22

0.9976

3.83 x ÎO-7

300

305

3.35 x 1013

1.43

0.9966

4.78 x ÎO"6

305

310

1.24 x 1014

1.74

0.9954

2.16 x 10~5

310

315

2.87 x 1014

2.02

0.9944

5.77 x ÎO"5

315

320

4.02 x 1014

2.33

0.9934

9.31 x ÎO"5

320

325

5.08 x 1014

2.65

0.9924

1.34 x 10~4

325

330

7.34 x 1014

2.97

0.9914

2.16 x 10"4

330

335

7.79 x 1014

3.28

0.9904

2.53 x 10"4

335

340

7.72 x 1014

3.61

0.9894

2.76 x 10"4

340

345

8.33 x 1014

3.91

0.9884

3.22 x 10"4

345

350

8.32 x 1014

4.18

0.9874

3.43 x 10~4

350

355

9.45 x 1014

4.51

0.9864

4.21 x 10"4

355

360

8.71 x 1014

4.75

0.9848

4.07 x 10~4

360

365

9.65 x 1014

5.00

0.9834

4.75 x 10"4

365

370

1.19 x 1015

5.23

0.9818

6.09 x 10~4

370

375

1.07 x 1015

5.41

0.9798

5.68 x 10"4

375

380

1.20 x 1015

5.57

0.9762

6.51 x 10~4

380

385

9.91 x 1014

5.69

0.9711

5.48 x 10-4

385

390

1.09 x 1015

5.84

0.9636

6.13 x 10"4

390

395

1.13 x 1015

5.86

0.9360

6.18 x 10~4

395

400

1.36 x 1015

5.93

0.7850

6.34 x 10"4

400

405

1.64 x 1015

5.86

0.4925

4.74 x 10~4

405

410

1.84 x 1015

5.89

0.2258

2.45 x 10"4

410

415

1.94 x 1015

5.78

0.0914

1.03 x 10"4

415

420

1.97 x 1015

5.73

0.0354

4.00 x 10"5

420

422.5

9.69 x 1014

5.78

0.0188

1.05 x 10-5

Total 7no2 = :

8.14 x 10~3 s~'

"Actinic flux from Finlayson-Pitts and Pitts (1986).

"Actinic flux from Finlayson-Pitts and Pitts (1986).

i 30

i 30

. • ■ ' ' 1 ' ' ■ ' j| ' ' ' ■

1 ' ' ' 1 .

i

_ /

1 /' SZA = 0°->

: / /

■ ../ .. 1 1 ... /

1 , . . , ■

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