Atmospheric chemistry II Oxidizing atmosphere The main oxidants in - - PowerPoint PPT Presentation

Atmospheric chemistry II

Oxidizing atmosphere

• The main oxidants in the atmosphere are

– OH radical (·OH) – Ozone – Nitrate Radical (·NO3)

The OH radical: main tropospheric oxidant

O3 + hvO2 + O(1D) (1)‏ O(1D) + M O + M (2)‏ O(1D) + H2O 2OH (3)‏ Primary source: Sink: oxidation of reduced species CO + OH CO2 + H CH4 + OH CH3 + H2O HCFC + OH H2O +‏… Major OH sinks GLOBAL MEAN [OH] = 1.2x106 molecules cm-3 and a lifetime of about 100 ms Earths surface, 30º N

D.J. Jacob

Tropospheric OH production takes place in a narrow UV window (300-320 nm)‏

30 equinox midday Solar spectrum

D.J. Jacob

O(1D) RO2 HONO HCHO OH HO2 H2O2 + O2 HNO3 H2SO4 RO2 + O2

RO2

NO hν NO2

SO2

NO O3

CH4 HCHO H2 CO NMHC O3 HO2

hν

VOC

H2O

(Boy et al., ACP, 2005)

Formation of HO2 Radical, examples

HCHO + hv ·H + HCO ·H+O2 +M·HO2+M HCHO + hv  H2 + CO CO + ·OH CO2 + ·H ·H+O2 +M·HO2+M

Reaction of HO2 To be included

• OH + O3 = HO2 :

1.70D-12*EXP(-940/TEMP) ;

• OH + H2 = HO2 :

7.70D-12*EXP(-2100/TEMP) ;

• OH + CO = HO2 : 1.30D-13;
• OH + H2O2 = HO2 : 2.90D-12*EXP(-160/TEMP) ;
• HO2NO2 = HO2 + NO2 : 5.2D-6*EXP(19900/TEMP) ;
• NO3 + H2O2 = HNO3 + HO2 : 4.1D-16 ;
• HCHO = CO + HO2 + HO2 :

J(9);

– Cross section:3.60 D-20 cm2 – Quantum yield:0.9

Reaction of HO2 To be included

• HO2 + O3 = OH :

2.03D-16*((TEMP/300)**4.57)* EXP(693/TEMP) ;

• OH + HO2 = dummy :

4.80D-11*EXP(250/TEMP) ;

• HO2 + HO2 = H2O2 : 2.20D-13*EXP(600/TEMP);
• HO2 + NO = OH + NO2 :

3.60D-12*EXP(270/TEMP) ;

• HO2 + NO2 = HO2NO2 : 1.4D-13 ;
• HO2 + NO3 = OH + NO2 : 4.00D-12 ;

Reactions of OH to be included

• All HO2 including OH
• O1D = OH + OH :

2.20D-10*H2O ;

• H2O2 = OH + OH : J(3) ;

– Cross section:33.04D-20 cm2 – Quantum yield:0.49

• HONO = OH + NO : J(7);

– Cross section:5.19D-19 cm2 – Quantum yield:0.40

• HNO3 = OH + NO2 : J(8);

– Cross section:4.29D-18 cm2 – Quantum yield:0.88

Reactions of OH to be included

• OH + NO = HONO : 1.50D-11*EXP(TEMP/300)**-0.5 ;
• OH + NO2 = HNO3 : 2.4S-11*(TEMP/300)**-1.7 ;
• OH + NO3 = HO2 + NO2 : 2.00D-11 ;
• OH + HO2NO2 = NO2 : 1.90D-12*EXP(270/TEMP) ;
• OH + HONO = NO2 : 2.50D-12*EXP(-260/TEMP) ;
• OH + HNO3 = NO3 : 7.20D-15*EXP(785/TEMP);

Formation of Sulfuric acid

• SO2 + ·OH + M ·HOSO2 + M
• ·HOSO2 + O2  ·HO2 + SO3
• H2O + SO3+ M  H2SO4 + M

Reactions to be included

• 1. O + SO2 = SO3 : 4.00D-32*EXP(-1000/TEMP)*M ;
• 2. OH + SO2 = HSO3 : 1.5D-12*(TEMP/300) ;
• 3. HSO3 = HO2 + SO3 : 1.30D-12*EXP(-330/TEMP)*O2 ;
• 4. SO3 = H2SO4 : 5.D-15*H2O ;

How d[SO3]/dt should look like?

= K1[O][SO2] + k3[HSO3]-k4[SO3]

Monoterpenes oxidation by OH radicals

Reaction included in the model

• APINENE + OH = APINAO2 :1.20D-11*EXP(444/TEMP) ;

Ozone in the troposphere

Greenhouse gas Toxic pollutant in surface air But it is the main precursor of OH and plays therefore a key role in maintaining the oxidizing power of the troposphere

O3 + hvO2 + O(1D) (1)‏ O(1D) + M O + M (2)‏ O(1D) + H2O 2OH (3)‏

Tropospheric ozone (O3): Global budget

O3

O2 h

O3 OH HO2

h, H2O Deposition

NO

H2O2 CO, VOC

NO2

h STRATOSPHERE TROPOSPHERE 8-18 km 1165

Deposition

475

Transport from stratosphere

4230

Chem loss in troposphere

4920

Chem prod in troposphere

GEOS-CHEM model budget terms, Tg O3 yr-1

D.J. Jacob

H2O

Depending on the NO/HOx ratio

Ozone production: CO + ·OH → ·H + CO2 ·H + O2 + M → ·HO2 + M (b) HO2 + NO → OH + NO2 NO2 + hν (λ < 420 nm) → NO + O O + O2 → O3 Ozone destruction: CO + OH → H + CO2 H + O2 + M → HO2 + M (a) HO2 + O3 → 2O2 + OH Ra/Rb = Ka[HO2][O3] / Kb[HO2][NO] =2.5E-4 * [O3]/[NO] Break-even concentration ≈ 2.5 10-4 (20 ppb O3 ≈ 5 ppt NO)

Reactions to be included in the model

• O = O3 : 5.60D-34*O2*N2*((TEMP/300)**-2.6) + 6.00D-

34*O2*O2*((TEMP/300)**-2.6) ;

• O3 = O1D : J(1) ;

– Cross section:1.137D-17 cm2 – Quantum yield:0.8

• O + O3 = dummy :

8.00D-12*EXP(-2060/TEMP) ;

• NO + O3 = NO2 :

1.40D-12*EXP(-1310/TEMP) ;

• NO2 + O3 = NO3 :

1.40D-13*EXP(-2470/TEMP) ;

• And all those involving O3 shown in OH and HO2 sections

Monoterpenes Oxidation by Ozone

• In a first step, ozone will be added to the

double bond forming a cyclic structure:

D-Limonene oxidation by Ozone

APINENE + O3 = 0.77 * OH + APINOOA : 1.01D-15*EXP(-732/TEMP) ;

The nitrate radical – NO3

The third important oxidant is the nitrate radical NO2 + O3 →‏NO3 + O2 and is in equilibrium with N2O5 according to NO2 + NO3 +‏M‏↔‏N2O5 + M During daytime it photolyze rapidly by two reactions NO3 + hν (λ‏<‏700‏nm)‏→‏NO‏+‏O2 NO3 + hν (λ‏<‏580‏nm)‏→‏NO2 + O Typically NO3 mixing ratios: daytime‏≈‏few‏ppts night‏≈‏up‏to‏several‏hundreds‏of‏ppt

Reactions to be include

• All above and:
• NO2 + O3 = NO3 : 1.40D-13*EXP(-2470/TEMP) ;
• NO + NO3 = NO2 + NO2 :

1.80D-11*EXP(110/TEMP) ;

• NO2 + NO3 = NO + NO2 : 4.50D-14*EXP(-1260/TEMP) ;
• NO2 + NO3 = N2O5 :

1.50D-12*(TEMP/300)**-0.7) ;

• N2O5 = NO2 + NO3 : 9.7D14*((TEMP/300)**-0.1)*EXP(-11080/TEMP) ;
• NO3 + H2O2 = HNO3 + HO2 : 4.1D-16 ;
• NO3 + NO3 = NO2 + NO2 : 8.5D-13*EXP(-2450./TEMP) ;

Reactions to be include

• NO3 = NO : J(5);

– Cross section: 4.58D-17 cm2 – Quantum yield:0.35

• NO3 = NO2 + O : J(6) ;

– Cross section:4.58D-17 cm2 – Quantum yield:0.60

Monoterpenes NO3 oxidation

APINENE + NO3 = NAPINAO2 : 1.19D-12*EXP(490/TEMP) ;

A word on organics

• Note that in our model we only will use

alpha-pinene, and only 1 oxidation path per

• xidant.
• Also we will use a fixed concentration of

alpha pinene. In next lectures you will learn how to implement emissions in your model.

Starting the methane oxidation is the reaction with OH: CH4 +‏OH‏→‏CH3 + H2O The methyl radical reacts rapidly adds O2: CH3 + O2 →‏CH3O2 The methylperoxy radical is considered part of the HOx-family and its main sinks are the reaction with HO2 and NO. In the overall following reaction chain the C(-IV) atom in CH4 is successively oxidized to C(+IV) in CO2. Consider high NOx-atmosphere: CH4 + 10 O2 -> CO2 + H2O + 5 O3 + 2 OH Consider now a low NOx-situation: CH4 + 3 OH + 2 O2 -> CO2 + 3 H2O + HO2 Ozone production and radical conversion or source is only reached in a high NOx-atmosphere. This results show the critical role of NOx for maintaining O3 and OH in the troposphere.

Methane oxidation mechanism

NO NO2

Oxidation of methane

CH4 + OH kCH4+OH = 9.65x10-20∙T ∙exp(-1082/T) cm3 Molek. CH3 + H2O (+O2) CH3O2 HCHO

CH3OOH

+HO2 +RO2

0.67 HCHO + CH3OH

+NO2

CH3OONO2 kCH3O2+HO2 = 3.80x10-13∙exp(780/T) cm3 Molek.-1 s-1 kCH3O2+RO2 = 1.82x10-13∙exp(416/T) cm3 Molek.-1 s-1 kCH3O2+NO = 3.00x10-12∙exp(280/T) cm3 Molek.-1 s-1