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 (·NO 3 )

The OH radical: main tropospheric oxidant Primary source: O 3 + hv  O 2 + O( 1 D) (1) ‏ O( 1 D) + M  O + M (2) ‏ O( 1 D) + H 2 O  2OH (3) ‏ Sink: oxidation of reduced species CO + OH  CO 2 + H Major OH sinks CH 4 + OH  CH 3 + H 2 O HCFC + OH  H 2 O +‏… GLOBAL MEAN [OH] = 1.2x10 6 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( 1 D) h ν HONO HCHO H 2 O NO O 3 NO h ν HNO 3 OH HO 2 H 2 O 2 + O 2 NO 2 HO 2 CH 4 HCHO H 2 SO 2 RO 2 H 2 SO 4 RO 2 + O 2 VOC CO NMHC O 3 RO 2 (Boy et al., ACP, 2005)

Formation of HO 2 Radical, examples HCHO + hv  ·H + HCO ·H+O 2 +M  ·HO 2 +M HCHO + hv  H 2 + CO CO + ·OH  CO 2 + ·H ·H+O 2 +M  ·HO 2 +M

Reaction of HO 2 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 HO 2 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 • SO 2 + ·OH + M  ·HOSO 2 + M • ·HOSO 2 + O 2  ·HO 2 + SO 3 • H 2 O + SO 3 + M  H 2 SO 4 + 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 O 3 + hv  O 2 + O( 1 D) (1) ‏ O( 1 D) + M  O + M (2) ‏ O( 1 D) + H 2 O  2OH (3) ‏

Tropospheric ozone (O 3 ): Global budget GEOS-CHEM model budget terms, Tg O 3 yr -1 Chem prod in 4920 Chem loss in 4230 O 2 troposphere troposphere h  Transport from 475 Deposition 1165 O 3 stratosphere STRATOSPHERE 8-18 km TROPOSPHERE h  NO 2 NO O 3 h  , H 2 O H 2 O OH HO 2 H 2 O 2 Deposition CO, VOC D.J. Jacob

Depending on the NO/HO x ratio Ozone production: CO + ·OH → ·H + CO 2 ·H + O 2 + M → ·HO2 + M (b) HO 2 + NO → OH + NO 2 NO2 + hν (λ < 420 nm) → NO + O O + O2 → O3 Ozone destruction: CO + OH → H + CO 2 H + O 2 + M → HO 2 + M (a) HO 2 + O 3 → 2O 2 + OH Ra/Rb = Ka[HO2][O3] / Kb[HO2][NO] =2.5E-4 * [O3]/[NO] Break- even concentration ≈ 2.5 10 -4 (20 ppb O 3 ≈ 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 – NO 3 The third important oxidant is the nitrate radical NO 2 + O 3 →‏NO 3 + O 2 and is in equilibrium with N 2 O 5 according to NO 2 + NO 3 +‏M‏↔‏N 2 O 5 + M During daytime it photolyze rapidly by two reactions NO 3 + hν (λ‏<‏700‏nm)‏→‏NO‏+‏O 2 NO 3 + hν (λ‏<‏580‏nm)‏→‏NO 2 + O Typically NO 3 mixing ratios: daytime‏≈‏few‏ ppts night‏≈‏up‏to‏several‏hundreds‏of‏ ppt

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 oxidant. • Also we will use a fixed concentration of alpha pinene. In next lectures you will learn how to implement emissions in your model.

Methane oxidation mechanism Starting the methane oxidation is the reaction with OH: CH 4 +‏OH‏→‏CH 3 + H 2 O The methyl radical reacts rapidly adds O 2 : CH 3 + O 2 →‏CH 3 O 2 The methylperoxy radical is considered part of the HO x -family and its main sinks are the reaction with HO 2 and NO. In the overall following reaction chain the C(-IV) atom in CH 4 is successively oxidized to C(+IV) in CO 2 . Consider high NO x -atmosphere: CH 4 + 10 O 2 -> CO 2 + H 2 O + 5 O 3 + 2 OH Consider now a low NO x -situation: CH 4 + 3 OH + 2 O 2 -> CO 2 + 3 H 2 O + HO 2 Ozone production and radical conversion or source is only reached in a high NO x -atmosphere. This results show the critical role of NO x for maintaining O 3 and OH in the troposphere.

Oxidation of methane k CH4+OH = 9.65x10 -20 ∙T CH 4 + OH ∙exp( -1082/T) cm 3 Molek. CH 3 + H 2 O (+O 2 ) +RO 2 +HO 2 NO NO 2 +NO 2 0.67 HCHO + CH 3 OH CH 3 OOH CH 3 O 2 CH 3 OONO 2 k CH3O2+HO2 = 3.80x10 -13 ∙exp(780/T) cm 3 Molek. -1 s -1 k CH3O2+RO2 = 1.82x10 -13 ∙exp(416/T) cm 3 Molek. -1 s -1 k CH3O2+NO = 3.00x10 -12 ∙exp(280/T) cm 3 Molek. -1 s -1 HCHO