Photochemistry of Smog and
Secondary Aerosol formation
Junfeng Liu
College of Urban and Environmental Sciences, Peking University
Sino-France Winter School
February 13-16, Beijing, China
Outline
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Introduction
Mechanism of ozone formation
Secondary aerosol formation
Gas and liquid-phase production of
SOA
Introduction
• Smog is a mixture of Smoke and Fog
• Smog type:
-London or the Classical Smog
-Los Angeles or The Photochemical Smog
London Smog
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Caused by huge amounts of coal
burning in December 1952
It is a sulfurous smog, a mixture
of fog, soot, SO2, PM, ...
100,000 were ill; 4000 people
died prematurely of respiratory
problems, followed by additional
8000 deaths
Some incidents of deaths associated
with London type smog
Los Angeles Smog
• New kind of smog
• First observed
in Los Angeles
• Primary source Velicle emissions
•High ozone level
•Low visibility (haze)
Los Angeles
Ozone concentrations over Los
Angeles and Boston
Interactions among air pollution emissions,
photochemistry, radiation, and climate & health impacts
Indirect effect
Scattering
Lapse rate
warm
Droplet
evaporation
cool
CCN
Climate change
Cloud chemistry
Absorbing
PA
O3
SA
SO42-
Photochemistry
Acid Rain
SO2
VOC
NOx
O3.
Health effects
O3.
PM2.5
Mechanism of photochemical smog
• Ozone formation
• Secondary aerosol formation
Null cycle of NOx and O3
hv
NO2
O3 + NO Æ NO2 + O2
NO2 + hv Æ NO + O
NO
O
O + O2 + M Æ O3 + M
O2
NOx
O3
Production of OH radical
hv
NO2
Dry:
O3 + hv Æ O(1D) + O2
NO
O
O(1D) + M Æ O(3P) + M
O(3P) + O2 Æ O3
O2
O2
O3
hv
Humid:
O(1D) + H2O Æ 2 OH
(<320nm)
O3P
M
O1D
H2O
2OH
Ozone accumulation
(Case of CO)
hv
NO2
O3
NO
O
NO
Prerequisites for high ozone:
• NOx: catalysts
• Photo: energy
• CO:
fuel
• H2O: OH precursor
O2
O2
O3
hv UV
H + O2 Æ HO2
CO2
O3P
M
CO + OH Æ H + CO2
HO2
O1D
H 2O
OH
OH
CO
NOx, CO, HC
Ozone accumulation
(Case of CH4)
O3
hv
NO
NO2
NO
CH3O2
O
O2
O2
O2
O2
O3
hv
OH
CH3
HCHO
H2O
O3P
M
CH3O
O1D
H 2O
OH
OH
hv
CHO
HO2
CO
CH4
NO
NO2
O3
O2
Ozone accumulation
(Case of HC)
O3
hv
NO
NO2
NO
RO2
O
O3
hv
R
.
.
O1D
H 2O
OH
OH
H
O2
'
RC
O
O2
'
RCHO
H 2O
O3P
M
'
RCO
O2
O2
O2
.
HO2
R-H
NO
NO2
O3
.
Secondary aerosol formation
Secondary Aerosol Formation
(Nitrate)
hv
O3
NO2
NO
NO
O
O2
O2
HO2
H 2O 2
O3
hv
hv
CO2
O3P
M
low NOx/HC
Low NOx
O1D
H2O
CO
OH
OH
no cloud
NO2
High NOx
NH3
HNO3
Nitrate
Secondary Aerosol Formation
(sulfate)
hv
O3
NO2
NO
SO2
Low NOx
NO
O
O2
HO2
O3
hv
O2
O3P
M
H2 O2
O2
O1D
H2O
SO3
O3
H2O
H2SO4
gas phase
SO2
OH
OH
SO2 + OH Æ HOSO2
NO2
High NOx
cloud
H2 SO4
aqueous phase
HNO3
NH3
HOSO2 + O2 Æ SO3 + HO2
Nitrate
Secondary Aerosol Formation
(oxidation of organics)
HCHO (formaldhyde)
H2C=O + hv Æ HC=O + H
H + O2 Æ HO2
HC=O + OH Æ CO + HO2
CH2=CH2 (ethene)
CH2=CH2 + OH Æ OHCH2-CH2
OHCH2-CH2 + O2 Æ OHCH2CH2O2
OHCH2CH2O2 + NO Æ NO2 + OHCH2CH2O
OHCH2CH2O Æ H2C=O + ·CH2OH
O2 + ·CH2OH -> H2C=O + HO2
These reactions produce a host of radicals which “fuel” the smog reaction process
Alkene + OH
First OH radicals attack the electron rich
double bond of an alkene
Oxygen then add on the hydroxy radical
forming a peroxy-hydroxy radical
the peroxy-hydroxy radical radical can
oxidize NO to NO2 ,just like HO2 can
Further reaction takes place resulting in
carbonyls and HO2 which now undergo
further reaction; the process then
proceeds…
Alkene + O3
O=CH
Aromatic
Reactions
CH3
CH3
OH
+ H2O
O
+ HO2
benzaldehyde
o-cresol
NO
NO2
CH3
+O2
CH2.
CH3
OH
OH
OH
*
H
.
toluene
CH3
OH
.O
+
O2
CH3
NO2
+O2
H
CH3
OH
.
O
H
H
NO
H
H
O
oxygen bridge
radical
rearrangement
OH
H
O.
+O2
H
H
OH
H
?
ring
cleavage
radical
O
H
+ HO2
+ HO2
O +
H
butenedial
H
CH3
methylglyoxal
Isoprene
oxidation
Formation of secondary
organic aerosols
Motivations (conti…)
Jimenez et al. [2009] Science
Formation of Sulfate and SOA
Atmospheric chemistry
O
O
Cloud
Chemistry
glyoxal
SO2
H2O2
NOx
OH
SO2
O
Evaporation
O
isoprene
Methylglyoxal
SO4
SOA
O3
Toluene
α-pinene
m-xylene
Nucleation
Condensation
Sulfuric acid
Carboxylic acids
Gas-phase production
Sulfate production:
SO2 + OH + M
SO4 + M
ΔSOA
Y=
ΔVOC
Y
SOA yield is defined as:
Two-Product model:
⎛ Kom,1M o ⎞
⎛ Kom, 2 M o ⎞
⎜
⎟
⎜
⎟
+
α
Y = α1 ⎜
2
⎟
⎜ 1+ K M ⎟
om, 2
o ⎠
⎝ 1 + Kom,1M o ⎠
⎝
M0 is the concentration of total organic aerosol;
α1/2 is mass based stoichiometric coefficients;
Kom,1/2 is equilibrium partition coefficients;
Odum et al. [1996]
M0 (ug/m3)
Mechanism of aqueous-phase SOA production
toluene
isoprene
α-pinene
+
(OH,O3,NO3)
O
O
glyoxal
HO
Gas to liquid
diffusion
OH
hydroxyacetone
Uptake at
droplet surface
O
O
O
glyoxal
O
O
methylglyoxal
OH
hydroxyacetone
Aqueous phase
diffusion
O
HO
O
O
methylglyoxal
glycolaldhyde
O
HO
OH
glycolic acid
OH
O
HO
O
OH
glycolic acid
O
OH
glyoxylic acid
O
O
O
pyruvic acid
O
O
pyruvic acid
O
OH
acetic acid
HO
OH
oxalic acid
O
HCOOH
formic acid
CO2
O
CH2(OH)2
formaldehydehydrated
O
OH
glyoxylic acid
O
OH
acetic acid
HO
OH
oxalic acid
CO2
HCHO
formaldehyde
HCOOH
formic acid
Aqueous phase
reactions
OH
O
O
gas phase
reactions
O
O
glycolaldhyde
gas
Cloud droplet
evaporation
Aerosol
Aerosol formed through droplet
evaporation: a mixture of sulfate,
carboxylic acids, and oligomers
Coupled gas- and aqueous phase chemistry in
GFDL coupled chemistry-climate model AM3
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Gas-phase chemistry is being updated from MOZART-2 to MOZART-4
chemistry mechanisms [Horowitz et al. 2003; Emmons et al. 2009]
Aqueous chemistry is based on an optimized mechanism for sulfur
chemistry, merged with the updated cloud SOA chemistry [Jacob 1986;
Pandis and Seinfeld 1989; Ervens et al. 2004,2008; Tan et al. 2009]
Coupled mechanism includes: ~100 gas phase species, ~50 aqueous
phase species, and totally ~370 reactions
Physical and chemical processes include:
– Gas-liquid mass transfer
– Aqueous-phase ionic equilibrium
– Cloud hydrogen ion concentrations (or PH value) prediction: based on
electroneutrality equation
Cloud droplet size: currently assumed to be10um
Cloud droplet lifetime: model time step (30min) or 10min
Could fraction: coupled chemistry is solved only for the cloudy area.
Cloud entrainment: mixing of outside air into the cloud is neglected.
Cloud evaporation: SOA and sulfate are formed when cloud evaporates
Global in-cloud production of SO4 and SOA (Tg/yr)
SO4
Gas-phase production
22.3 (Tg SO4/yr)
Liquid-phase production
92.6 (Tg SO4/yr)
SOA
Gas-phase production
20 - 30 (Tg/yr)
Liquid-phase production
22 - 30 (Tg/yr)
Radiative Flux Perturbation due to IC-SOA (watts/m2)
（with IC-SOA – without ）
RF change
(SW+LW)
DJF
MAM
JJA
SON
Average
TOA
0.12
-0.14
-0.22
0.12
-0.03
Surface
0.05
-0.25
-0.18
-0.14
-0.13
Atmos. Abs.
0.07
0.11
-0.04
0.26
0.10
Measurements in Beijing City
End
END
Gas-phase SO4 and SOA production
SO4: SO2+OH Æ SO4
22.3 (Tg SO4/yr)
SOA: two-product model parameterization
30.3 (Tg/yr)
Emissions
(Tg/yr)
SOA
Production
(Tg/yr)
Average
Yield (%)
Heald et al.
[2008]
Henze et al.
[2008]
Lack et al.
[2004]
Isoprene
565.6
8.8*
1.6
19.2
14.4
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Terpene
143.7
16.2
11.3
3.7
10.8
22.7
Aromatics
34.3
5.2
15.1
1.4
3.5
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Higher
Alkanes
78.9
0.13
0.2
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Total
822.5
30.3
24.3
30.3
22.7
Precursors
*NOx dependent SOA production from isoprene, based on Carlton et al. [2009]
The SOA production is assumed to be approximately 15% of Terpene emissions in the original AM3
configuration with adjustments on temperature [Donner et al., 2011].