1. 大气圈地球化学及其环境效益

2. 6.3 Oxidizing Power
of the Troposphere
中国科学技术大学 环境地球化学概论

3. Oxidants in the atmosphereOH
Others (O, Cl, etc.)
Generation of OH radical in the atmosphere (mainly through photolysis reactions)
O3 + hv  O2 + OID (R1)
OID + M  O + M (R2)
OID + H2O  2OH (R3)

4. Oxidants in the atmosphere
Tropospheric production of OH
Right: Absorption cross-section of O3 at 273K.
Left: Solar actinic flux at sea level for 30℃ zenith angle and a typical O3 column overhead.

5. Oxidants in the atmosphere
Tropospheric production of OH
Right: Rate constant k1
Left: OID quantum yield at 273K.

6. Oxidants in the atmosphere
Lifetime of OH troposphere
ni : the number density of species i reacting with OH
ki : the corresponding rare constant of OH reaction with species i.
Question: Estimating lifetime of OH assuming CO and CH4 are the
major sinks of OH.

7. Oxidants in the atmosphere
OH measurement: 14C as the tracer
Source: 1) Manning et al., Nature, 436, 2005,
2) Chemical & Engineering News, Aug 22, 2005, pp 9

8. Oxidants in the atmosphere
OH measurement: 14C as the tracer
Source: 1) Manning et al., Nature, 436, 2005,
2) Chemical & Engineering News, Aug 22, 2005, pp 9

9. Oxidants in the atmosphere
Global Budget of CO
CO and CH4: major sinks for OH  control [OH]
 oxidizing power of the atmosphere
CO is not well-mixed in the troposphere:
low in rural areas and high in urban areas.

10. Oxidants in the atmosphere
Global Budget of methane

11. Oxidants in the atmosphere
Global Budget of ozone

12. Oxidants in the atmosphere
Global Budget of ozone
The abundance of troposphere O3 is largely controlled by chemical production and loss within the troposphere.

13. Oxidants in the atmosphere
Global Budget of nitrogen oxides
consider the budget of the NOX family as a whole, as in the stratosphere.
(most appropriate)
Rapid cycling of NO and NO2:
Human activity is a major source of NOX in the troposphere.

14. Oxidants in the atmosphere
Global Budget of nitrogen oxides
HNO3 is effectively scavenged by precipitation in the troposphere.

15. Oxidants in the atmosphere
Peroxyacelnitrate(PAN): CH3C(O)OONO2
Large range transportation of anthropogenic NOX to the global troposphere.

16. Oxidants in the atmosphere
Peroxyacelnitrate(PAN): CH3C(O)OONO2
Properties of PAN:
Sparingly soluble in water.
 not effectively removed by wet deposition
Its thermal decomposition is strongly dependent on temperature.
PAN heat CH3C(O)OO + NO2
295 K, τ(PAN) 1hr
250 K, τ(PAN) several months
PAN can be transported over long distances and decompose to release NOX far from its source.

17. Oxidants in the atmosphere
Peroxyacelnitrate(PAN): CH3C(O)OONO2

18. Oxidants in the atmosphere
Global distribution of CO, NOX, O3 and OH

19. Oxidants in the atmosphere
Global distribution of CO, NOX, O3 and OH

20. Oxidants in the atmosphere
Global distribution of CO, NOX, O3 and OH
Observations:
Higher in the tropics
reflecting higher water concentration
and UV.
Peak [OH] in the mid-troposphere
reflecting opposite vertical trends of
[H2O] and UV.
[OH] higher in N hemisphere than S hemisphere
 higher O3 and NOX in the N
hemisphere (more than compensating
the loss due to CO consumption)

21. Oxidants in the atmosphere
Global distribution of CO, NOX, O3 and OH
NOX has been enhanced by a factor of 2-8.
CO has been enhanced by a factor of 3-4.
O3 has increased in both N and S hemispheres.
OH is enhanced in most of the lower troposphere and is reduced in the upper troposphere and in the remote S hemisphere.