大气圈地球化学及其环境效益.

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大气圈地球化学及其环境效益

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

Oxidants in the atmosphere OH 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)

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.

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

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.

Oxidants in the atmosphere OH measurement: 14C as the tracer Source: 1) Manning et al., Nature, 436, 2005, 1001-1004 2) Chemical & Engineering News, Aug 22, 2005, pp 9

Oxidants in the atmosphere OH measurement: 14C as the tracer Source: 1) Manning et al., Nature, 436, 2005, 1001-1004 2) Chemical & Engineering News, Aug 22, 2005, pp 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.

Oxidants in the atmosphere Global Budget of methane

Oxidants in the atmosphere Global Budget of ozone

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

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.

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

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

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.

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

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

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

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)

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.