N. DARABIHA
Samedi 29 Mai 2010, 11h00 – 12h30
Généralités sur la cinétique de combustion
1
Nasser DARABIHA
http://www.em2c.ecp.fr
Téléphone : +33 1 41 13 10 72
Télécopie : +33 1 47 02 80 35
Laboratoire d'Energétique Moléculaireet Macroscopique, Combustion
ECOLECENTRALEPARIS
2
ChemicalKinetics
Thermodynamics
FluidMechanics
Transport
Phenomena
Combustion
Multi-phase flo
w
3
• Production and destruction of pollutants• Ignition,• Extinction,• Flame Structure,• ……..
Why Chemical Kinetics?
4
Temperature Collisions Reaction
Reacting mixture
5
CONTENTS
- Some Definitions- Thermodynamics - Chemical kinetics Arrhenius Law Reaction rate Chemkin- Reaction mechanism H2/O2 combustion
- 0-D formulation Auto-ignition PSR
- Pollutants in atmosphere
6
Mixing of N species inside a volume ν
nk moles of species k , n = nkk=1
N
Σ
!
Xk=nk
nMole fraction
!
Yk=m
k
mMass fraction
1 mole = 6,02252*1023 molecules
mass : m = mk = nk Mkk=1
N
Σk=1
N
Σ
!
Yk= X
k
Mk
MMixture Molar mass M = Xk Mk
!
=m
nk=1
N
Σ
7
!
Yk=m
k
m="k
"Mass fraction
!
"k=m
k
VDensity
!
"k
=nkM
k
V= C
kM
k
!
" = "k
k=1
N
#
!
Ck=nk
VConcentration
!
Ck
k=1
N
" =n
V#1
8
mass stoichiometric coefficient
stoichiometric coefficient
F + S O + γ’N2 Products
νFF + νO O + γN2 Products
Stoichiometric reaction
Enough oxidant O to burn all fuel F
9
Stoichiometric reactions:
CnHm + (n+m/4) (O2 + γ N2) nCO2 +(m/2) H2O + (n+m/4) γ N2
CH4 + 2 (O2 + γ N2) CO2 +2 H2O + 2 γ N2
C3H8 + 5 (O2 + γ N2) 3CO2 +4 H2O + 5 γ N2
H2 + 1/2 (O2 + γ N2) H2O + 1/2 γ N2
10
Any reaction ... :
CH4 + S’( O2 + γ N2) Products
Stoichiometric reaction:
CH4 + S (O2 + γ N2) Products
XCH4
XO2
XCH4
XO2( )
Stoich
φ =Mixture
equivalence ratio
!
=
1
" S
1
S
=S
" S
Mixture equivalence ratio
11
Any reaction ... :
CH4 + S (O2 + γ N2) Products
!
S
" S
CH4 + S (O2 + γ N2) Productsφ
Φ > 1 Rich mixture
Φ < 1 Lean mixture
Φ = 1 Stoichiometric mixture
12
Any reaction ... :
CH4 + S’ (O2 + γ N2) Products
YCH4
YO2
YCH4
YO2( )
Stoich
φ =Mixture
equivalence ratio
!
=
MCH 4
" S MO2
MCH 4
S MO2
=S
" S
φ CH4 + S (O2 + γ N2) Products
13
Air Factor
F = 1 / φ
Any reaction ... :
CH4 + S (O2 + γ N2) Productsφ
CH4 + F . S (O2 + γ N2) Products
14
Air volume (excess)
e < 0 Rich mixture
e > 0 Lean mixture
e = 0 Stoichiometric mixture
e = F - 1 = (1 −Φ)/Φ
CH4 + (1+ e ) . S (O2 + γ N2) Products
15
CONTENTS
- Some Definitions- Thermodynamics - Chemical kinetics Arrhenius Law Reaction rate Chemkin- Reaction mechanism H2/O2 combustion
- 0-D formulation Auto-ignition PSR
- Pollutants in atmosphere
16
ThermodynamicsFirst principle(no variation of kinetic and potential enery) :
dU = dQ + dWdW = -PdV
H=U + PV
dUdQ
dW
Constant volume reaction:dU = dQ
Constant pressure reaction:
dU = dQ -PdV
dH=dU + VdP + PdV= dQ + dW + PdV
dH = dQ
0
17
Thermodynamics
dUdQ
dWAdiabatic Constant volumereaction:
dU = 0
Adiabatic Constant pressurereaction:
dH = 0
0
18
Thermodynamics
Species enthalpy
Mixture enthalpy
chemical sensible
19
Thermodynamics
Mixture enthalpy :
chemical sensible
with
20
YCO2
T
initial state : 1
final state: 2
Initial state - final state
If time is infinite Final state = Equilibrium
P1T1Y1k
P2T2Y2k
Gibbs energy: G = H - TS
Equilibrium is reached when G is minimal (S is maximal)
21
At the equilibrium state, burnt gases are composed of:
Thermodynamic Equilibrium
CH4 + 2(O2 + 3,76 N2) Final equil. products
CO2 , H2O , N2 , CO, CH2 , CHi , H, H2 , OH , , O , O2 ,.,.,…. , NO , NO2 ,.,.,….
Mathematically, the equilibrium compositionis obtained by minimizing G
22
Equilibrium burnt gases temperatureadiabatic constant pressure
Adiabatic:
!
" # kAk
k=1
N
$ % " " # kAk
k=1
N
$
initial state : 1 final state: 2
The only unknown
!
Y1khk
k=1
N
" (T1,P) = Y
2khk
k=1
N
" (T2,P)
: H = cst
23
Equilibrium burnt gases temperatureadiabatic constant pressure
!
Y1k hk
k=1
N
" (T0,P) + Y
1k c pk
k=1
N
" ( # T ,P)T0
T1
$ d # T =
!
Y2k hk
k=1
N
" (T0,P) + Y
2k c pk
k=1
N
" ( # T ,P)T0
T2
$ d # T
!
cp2( " T )
24
Heat released by thecombustion
Burnt gases temperatureconstant pressure
Unknown
!
cp2( " T )T0
T2
# d " T $ cp1( " T )T0
T1
# d " T =
!
(Y1k"Y
2k) h
k
k=1
N
# (T0,P)
25
CO
CO2!
cp1(T,P) = Y1k c pk
k=1
N
" (T,P)
26
Assume all cpk = cst = cp and not temperature dependent
Burnt gases temperatureconstant pressure
UnknownHeat released by the combustion
!
cp (T2 "T1) = (Y1k "Y2k ) hk
k=1
N
# (T0)
27
Calorific value
Calorific value at constant pressure :
!
(PC)p = (Y1k "Y2k ) hk
k=1
N
# (T0, p
0)
Calorific value at constant volume:
!
(PC)p = h1(T0, p
0) " h
2(T0, p
0)
!
(PC)v =U1(T0, p
0) "U
2(T0, p
0)
It is the quantity of heat that can theoretically be released per unit mass of fuel
28
Calorific value
Calorific value: The quantity of heat released by thecomplete combustion, per unit mass of a fuel, the vaporproduced by the combustion of the gas being assumedto remain as a vapour.
High calorific value: The amount of heat released bycomplete combustion, per unit mass of a fuel, the vaporproduced by the combustion of the gas being assumedto be completely condensed and its latent heat released.
29
CONTENTS
- Some Definitions- Thermodynamics - Chemical kinetics Arrhenius Law Reaction rate Chemkin- Reaction mechanism H2/O2 combustion
- 0-D formulation Auto-ignition PSR
- Pollutants in atmosphere
30
Temperature Collisions Reaction
Reacting mixture
31
Collision frequency
Motion molecular velocity
Volume through which the moleculesweeps in 1s
For one molecule (if all othermolecules are motionless)
Kuo, 2005
32
Collision frequency
Kuo, 2005
33
Collision frequency
But all collisions do not lead to reaction
A + A P
A + B P
2A + 3B P2 3
2 3
34
Arrhenius Law
Svante Arrhenius (1859-1927)
Only molecules with E > Ea will react ...Ea =Activation Energy
collision frequencysteric factor (depends on theorientation of the collidingmolecules)
Boltzmann factor
with RR!
dCB
dt= " fc P e
"Ea
RT
35
Order of reaction
Overall order of reaction :
!
" # kAk
k=1
N
$ % " " # kAk
k=1
N
$
!
m = " # k
k=1
N
$!
dCM k
dt= ( " " # k $ " # k ) k f (CM k
)" # k
k=1
N
%
36
First order reactions! Valid only forelementary reactions
37
Second order reactions
AB
38
Third order reactions
--> Becomes a 2nd orderreaction if CM is cst
Third order reactions
39
Consecutive reactions
k1
k2
40
Parallel reactions
Twin reactions
!
A + Bk1" # " C
!
A + Bk2" # " D
!
dCC
dt= k
1CACB
!
dCD
dt= k
2CACB
!
"CC
CD
=k1
k2
41
Parallel reactions
Competitive reactions
!
A + Bk1" # " C
!
A + Ek2" # " D
!
dCC
dt= k
1CACB
!
dCD
dt= k
2CACE
!
" lnCB0
CB0
#CC
=k1
k2
lnCE0
CE0
#CD
42
Opposing (or reverse) reactions
!
" # k Ak
k=1
N
$k f% & %
kb' % %
" " # k Ak
k=1
N
$
43
Opposing (or reverse) reactions
But complex mechanisms: N species, I reactions :
!
" # k,i Ak
k=1
N
$k f% & %
kb' % %
" " # k,i Ak
k=1
N
$ for i =1,.....,I
Each reaction i is characterized by a rate of progress qi
44
Reaction rate of species k in the ith reaction is:
Reaction rate of species k for the overall mechanism is:
General expression of reaction rates
.
.
45
as
Molar reaction rate:
General expression of reaction rates
Mass reaction rate
!
dCk
dt=
•
"k
mole
m3s
#
$ %
&
' (
!
dCkMk
dt= Mk
•
"kkgm3s
#
$ %
&
' (
!
CkM
k= "
k= " Y
k
!
d"Yk
dt= M
k
•
#k
46
Then:
General expression of reaction rates
!
Mk
•
"k
= 0k=1
N
#
!
d"Yk
dt= M
k
•
#k
!
d"Yk
dtk=1
N
# =k=1
N
# Mk
•
$k
Conservationof mass
47
CHEMKINKinetics Scheme
CHEMKINInterpreter
LINK file
ThermodynamicsData Base
Set of CHEMKINRoutines :
CKXTYCKYTXCKXTCCKCPYCKWXPCKWYP
.
.
User’s program (x. : equil, ….)
48
CONTENTS
- Some Definitions- Thermodynamics - Chemical kinetics Arrhenius Law Reaction rate Chemkin- Reaction mechanism H2/O2 combustion
- 0-D formulation Auto-ignition PSR
- Pollutants in atmosphere
49
DETAILED CHEMISTRY
CH4 + 2O2 CO2 +2 H2O
H2 + 1/2 O2 H2O
Even at the equilibrium state, burnt gases are composed of:
CH4 + 2 O2 Products
CO2 , H2O , CO, CH2 , CHi , H, H2 , OH , , O , O2 ,.,.,….,.,.,….
50
Chain reactions• Series of competitives, consecutives and opposing reactions• Apparition of intermediate species• Importance of free radicals
Radicals: highly reactive molecules or atoms
Chain-initiation reaction (A2 has alower Ea than B2)
!
A2
+ M" 2A + M
Chain-carrying reactions:
radical -> radical (fast propagation)
!
A + B2" AB + B
!
B + A2" AB + A
!
A + AB" A2
+ B
!
B + AB" B2
+ A
Chain terminating reaction
!
2A + M" A2
+ M
!
2B + M" B2
+ M
51
H2 / O2 combustion
52
DETAILED CHEMISTRY(H2 - O2 combustion)
INITIATION : free - radical production
Temperature Collisions
O2 + M 2 O + M
H2 + M 2 H + M
M denotes all other species as third body (O2, H2O, …)Delivering its energy to O2 and H2 helping them to dissociate
Chain-Initiatingreactions
53
Chain-branching reaction : Radicals are more produced thanconsumed
O + H2 H + OH
H + O2 O + OH
DETAILED CHEMISTRY
Very fast reactions
54
Example :
A 1 cm3 container with n =1019 moleculesand fc = 108 collisions / s
1 free radical per cm3
★ Chain-carrying: time for all molecules to react = 1019/108 = 30,000 years
55
Chain reactions
Time to react = 64 / 108 ≈ 1 µs
★ Chain-branching:
➡ 1 + 2 + 22 + … + 2L= (2L+1-1)/(2-1) = 1019 molecules after L=64 generations
Extremely fast reaction ....
★ If 1% of the reactions are chain-branching:➡ t ≈ 40 µs
Assume chain-branching reactions
1 radical --> 2 radicals
56
Chain-carrying : Radicals and final products
OH + H2 H2O + H
O + H2O 2 OH
DETAILED CHEMISTRY
57
Chain-terminating steps : recombination mechanism
H + H + M H2 + M
O + O + M O2 + M
H + O + M OH + M
H + OH + M H2O + M
DETAILED CHEMISTRY
58
H2 + O2 2 OHH2 + OH H2O + HO + OH H + O2O + H2 OH + HH + O2 +M HO2 + MOH + HO2 H2O + O2H + HO2 2 OHO + HO2 O2 + OHOH + OH O + H2OH + H + M H2 + MH + H + H2 H2 + H2H + H + H2O H2 + H2OH + OH + M H2O + M….….
COMBUSTION of H2 / O2
>10 species>40 reactions
59
C2H4
C2H5
C2H6
CH4
CH3
CH2O
HCO
CO
C2H3
C2H2
CH2
CH
CO, CO2
C3...
C3...
COMBUSTION of CH4 / O2
60
OXYDATION of CO
CO + OH CO2 + H
CO + O2 CO2 + O
O + H2O 2 OH
H + O2 OH + O
CO + HO2 CO2 + OH
Chain-terminating steps:
Main OH production reactions:
At the flame front:
61
CONTENTS
- Some Definitions- Thermodynamics - Chemical kinetics Arrhenius Law Reaction rate Chemkin- Reaction mechanism H2/O2 combustion
- 0-D formulation Auto-ignition PSR
- Pollutants in atmosphere
62
Application of spontaneous combustion
• Hypersonic combustion (SCRAMJET)• Diesel engines
To be avoided in :
• Safety characteristic length of fuel tanks Semenov theory
• Spark ignition engines (avoid pinking)
63
Adiabatic, constant pressure
Spontaneous combustion
64
Initial conditions :
Auto-ignition of a reactive mixture Adiabatic, constant pressure
65
Auto-ignition of a reactive mixture
T0 > Ti
τi
Auto-ignitingdelay time
66
Auto-ignition delay time
τi decreases exponentially with T0
τi changes as P01-n
Influence of equivalence ratio and dilutant (YN2)
H2
C2H2
COKerosene
CH4
T °C1100 1000 900 800 700 600
τi ms
40
10
1
0.5
67
Spontaneous combustion
• T = ambient: metastable state, reaction rate isalmost null
• When T increases:• If T > Ti Beginning of exothermic oxidationreaction:
production of enough radicals to ignite
• If T < Ti Heat released is not sufficient toincrease the temperature, because endothermicreactions absorb the heat to crack the fuel:
not enough radicals are produced to ignite
68
Container with constant volume withisothermal wall (Tw) and a wall surface S
Spontaneous combustion
V T
YF YO
STw
Container balance energy equation ….
Heat losses Q
69
!
d"VYk
dt=V M
k#
k
•
!
d"VU
dt= # K S (T #T
w)
!
U =k=1
N
" YkU
k
!
(" VYk
dUk
dt+U
k" V
dYk
dt)
k=1
N
# = $ K S (T $Tw)
!
(" VYk
dUk
dt+U
kVM
k#
k
•
)k=1
N
$ = % K S (T %Tw)
Spontaneous combustion
70
!
" V cv
dT
dt= # (V M
kU
k$
k
•
)k=1
N
% # K S (T #Tw)
Spontaneous combustion
!
" V cvdT
dt=V #F
•
Qv0 $ K S (T $Tw )
Production Heat loss
Fuel consumption rate modeling:
71
Auto-ignition conditions
Semenov theory
Production > Heat loss
!
"F
•
Qv0
# KS
V(T $T
w)
!
y1T( )
!
y2T( )
72
y(T)
T
Y1(T)
Production(chemical)
!
"F
•
Qv0
73
T
Heat loss
Tw
Y2 (T)Y (T)
!
KS
V(T "T
w)
74
y
T
y1 y2
Tw
Case 1: T0 high,
Auto-ignition
75
y
T
y1 y2
Tw
If T < Tc, The temperature increases up to T=Tc
Tc
Case 2: two curves are tangent
If T > Tc, Auto-ignition
dT/dt =0 (unstable)T = Tc + ε , Ignition
c
76
y
T
y1
y2
TwIf T < Ta , the temperature increases up to Ta and remains stable
a
b
Case 3:
If Ta < T < Tb , the temperature decreases to Ta and remains stable
If T > Tb, Auto-ignition
77
Varying K or (S/V)
The critical temperatureTc is not an intrinsicmixture property.
y
TTw
K(S/V)
Indeed, it is functionof K and (S/V)
S / V = 1 / LIf L goes up, S/V decreases and it Increasesthe risk of auto-ignition
If the system is well insulated, K decreases,and it increases the risk of auto-ignition
78
Varying P
There exists a critical pressure Pc above whichthere is always mixture auto-ignition.
w
79
Determination of ignition limits (Pi, Ti)y
T
y1 y2
Tw Ti
C
At the critical point C:
!
Y1
="F
•
Qv0
!
Y2
= KS
V(T "T
w)
80
One shows that:
81
Equivalence ratio
Equivalence ratio
Determination of ignition limits (Pi, Ti)
Ignition
Ignition
Lean limit Rich limit
Lean limit Rich limit
no combustion
no combustion
82
no combustion
Determination of ignition limits (Pi, Ti)
Ignition
83
Hydrocarbons ignition limits
84
Influence of T0 on ignition (C7H16-air)
P0 =30 bars, Φ=0.5
Time (s)
Tem
pera
ture
(K)
85
Influence of T0 on ignition (C7H16-air)
P0 =30 bars, Φ=0.5
Igni
tion
dela
y (s
)
Cold flamePrincipal ignition
Initial temperature (K)
86
CONTENTS
- Some Definitions- Thermodynamics - Chemical kinetics Arrhenius Law Reaction rate Chemkin- Reaction mechanism H2/O2 combustion
- 0-D formulation Auto-ignition PSR
- Pollutants in atmosphere
Perfectly stirred reactor (PSR)
Feed at the inlet at the state: X0
Homogeneous combustion
Burnt gases at the outlet
INLET OUTLET
, Yk0 , hk0, T0
Volume V
Yk , hk, T
Yk , hk, T
!
m
•
τ = Residence time
88
!
" VdY
k
dt= m
•
Yk0# m
•
Yk
+$kM
kV k =1,...,N
.
IN OUT PRODUCTION.
Species mass balance equation :
.
89
with
!
" Vdh
dt= m
•
Yk0hk0k=1
N
# $ m•
Ykhkk=1
N
# $Qh
!
h = Yk
k=1
N
" hk
!
dh
dt= Y
k
dhk
dt
"
# $
%
& '
k=1
N
( + hk
dYk
dt
"
# $
%
& '
k=1
N
(
!
Cp = Ykk=1
N
" Cpk
!
dhk
dt= Cpk
dT
dt
!
dh
dt= Cp
dT
dt+ hk
k=1
N
"dYk
dt
Enthalpy balance:
90
!
" V Cp
dT
dt= m
•
Yk0hk0k=1
N
# $ m•
Ykhkk=1
N
# $Qh
$ hkk=1
N
# " VdYk
dt
%
& '
(
) *
!
" VdY
k
dt= m
•
Yk0# m
•
Yk
+$k
•
MkV
!
" V Cp
dT
dt= m
•
Yk0hk0k=1
N
# $m•
Ykhkk=1
N
# $Qh
$ m•
Yk0hkk=1
N
# + m•
Ykhkk=1
N
# + $ V hkk=1
N
# %•
k Mk
&
' (
)
* +
Enthalpy balance:
91
Balance Equations
!
" VdY
k
dt= m
•
Yk0# m
•
Yk
+$k
•
MkV k =1,...,N
!
" V Cp
dT
dt= m
•
Yk0hk0k=1
N
# $ m•
Yk0hkk=1
N
# $Qh +V $ hkk=1
N
# %•
k Mk
&
' (
)
* +
Heat release rate
!
h
•
92
ATTENTION
!
" V Cp
dT
dt= m
•
Yk0hk0k=1
N
# $ m•
Ykhkk=1
N
# $Qh + h•
!
" Vdh
dt= m
•
Yk0hk0k=1
N
# $ m•
Ykhkk=1
N
# $Qh
!
" V Cp
dT
dt= m
•
Yk0hk0k=1
N
# $ m•
Yk0hkk=1
N
# $Qh + h•
93
Application example
at t=0 , Yk(0)=Yk0 , T(0) = Ti
!
dYk
dt=1
"(Y
k0#Y
k) +
$k
•
Mk
%k =1,...,N
!
Cp
dT
dt=1
"Yk0hk0
k=1
N
# $1
"Yk0hk
k=1
N
# $Qh
%V+h•
%
T0 = 300 KH2/Air mixture
Inlet Volume V
Outlet
Pressure= 1 atm
Yk (t), hk (t), T(t)
94
Ti = 4000K
τ =0,01 s
Stoic. H2/AirT0 = 300 KPressure= 1 atm
95
Influence of τTi = 4000K
Time (s)
96
Influence of τTi = 4000K
Time (s)
97
Influence of τTi = 4000K
Residence time (s)
98
Influence of Tiτ =0,01 s
Time (s)
99
Influence of Tiτ =0,01 s
Time (s)
100
CONTENTS
- Some Definitions- Thermodynamics - Chemical kinetics Arrhenius Law Reaction rate Chemkin- Reaction mechanism H2/O2 combustion
- 0-D formulation Auto-ignition PSR
- Pollutants in atmosphere
101
• CO2, H2O • CO
• CH4• CHx• Soot
• NOx
• COV (combustion)• SOx• Cl…• Br….• …..• …..
(vegetations, combustion)
(Rice, Ruminants…., combustion)
(Lightning, combustion)
102
Impact of Pollutantson the atmosphere
Troposphere : Altitude < 10-15 km,T 220K
k2 very fast
k1 (hν)day
night! NO2 NO + O
O2 + O +M O3 + M
k1
k2
Production of O3 If steady state for NO:
!
O3
C =k1 NO
2C
k3 NOC
=> Fragile equilibrium
>
k3NO + O3 NO2 + O2
If NO decreases and NO2 increases : Increase of O3
104
Effect of CO
CO + OH CO2 + H
H + O2 + M OH2 + M
•Importance of OH :Initiation of theprocess
105
• If [O3] / [NO] < 5000
HO2 + NO NO2 + OH
Formation of O3
• If [O3] / [NO] > 5000
HO2 + O3 OH + 2 O2
Destruction of O3
Effect of CO
NO2 + hν NO + O (λ > 420 nm)O2+O+M O3 +M
Balance :CO + O3 CO2 + O2
Balance : CO +2 O2 CO2 + O3
106
Effect of CH4
CH4 + OH CH3 + H2O
CH3 + O2 + M CH3O2 + M
2 HO2 + 2 NO 2 NO2 + 2 OH
CH3O2 + NO CH3O + NO2
CH3O + O2 CH2O + HO2
CH2O + O2 HCO + HO2
HCO + OH CO + H2O3NO2 + hν 3NO + 3O (λ > 420 nm)3O2+3O+3M 3O3 +3MBalance :
CH4+ 6 O2 CO + 2 H2O + 3 O320% - 50% of COin atmosphere
107
NO - NO2 Photochemical Cycle
λ < 420nm
NO2NO O
O3
O2O2
• Day time– Solar radiation allows
formation of O– Formation of O3 (O + O2)– Produced NO reacts with O3
and forms O2 et NO2
– Establishment of equilibrium• Night
– no production of O– Consumption of O3 by NO
emitted by cars (unless thewind has moved O3 away)
108
NO - NO2 Photochemical Cyclein polluted atmosphere
λ < 420nm
NO2NO O
O3
O2O2
RO2+RH
• Day time– New mechanism of NO2 /
RH formation– No consumption of O3
– Continuous increase of O3
• Night– no production of O– Consumption of O3 by NO
emitted by cars (unless thewind has moved O3 away)
NO2 + OH HNO3
acid rain
Troposphere : < 10-15 km, T 220K
Stratosphere : > 10-15 km, T 270K
Destruction of O3 by X : NO, OH, Cl, Br, .....
X + O3 XO + O2
XO + O X + O2
O3 + O 2 O2
k1 (hν)O2 2 O
O + O2 O3
k1Ozone layer :
111
Direct Effects of NOx onatmospheric equilibrium
• NOx participate in the formation of ozonein the troposphere : Smog
• NOx participate in the destruction of theozone layer in the stratosphere