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Formation of Secondary Organic Aerosol
And
At
John H. SeinfeldCalifornia Institute of Technology
EPA STAR September 21, 2010
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Ambient Organic Aerosol
•About 5 years ago: Higher SOA Levels in the atmosphere than models predicted.Finding “missing SOA”
is no longer the issue. We need to understand the complete life cycle of the VOC/SOA system.
–Laboratory chamber experiments do not account fully for ambient aging;functionalization vs. fragmentation, gas‐phase aging vs. OH surface reactions uncertain
–
Gas‐phase oxidation chemistry is critical in establishing path to SOA
formation; role of NOx
key
– Role of aerosol acidity, aqueous‐phase uptake and reaction of small molecules like glyoxal, and heterogeneous chemistry in overall SOA formation not yet fullyunderstood
– Models now have the machinery to represent SOA formation and aging but at present the parameterization of aging awaits experimental input
• We have learned in the last 5 years:
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Fate of Peroxy (RO2 ) Radicals Determines Product Volatility
Pankow and Asher (2007)
Changes to vapor pressure of a VOC upon addition of common functional groups
Small VOCs - NOx yields high-volatility fragmentation products (e.g., alkyl radical + small carbonyls)NOx yields lower-volatility products (e.g., hydroperoxides) Higher SOA Yields
Large VOCs - NOx yields low-volatility products (large organic nitrates + isom. products) Higher SOA YieldsNOx yields higher-volatility products
Level of NOx (= NO + NO2 ) Determines Fate of RO2 Radicals:
≤
C10
C15
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Comparison of Atmospheric AMS Components
• SV‐OOA has larger variability in 43/org; Increasing photochemical age collapses variability,OOA components become increasingly similar to each other• 44/org ratios of the LV‐OOA components similar to those from HULIS collected in filtersamples • Increasing photochemical age: increasing O/C, 44/43 ratio
Ng et al., ACP
(2010)
Photoche
mical age
O/C ratio
0.30
0.25
0.20
0.15
0.10
0.05
0.00
44/o
rg
0.200.150.100.050.0043/org
OOA LV-OOA SV-OOA
HULIS Fulvic
HULIS, Fulvic data:E. Dinar, Y. Rudich
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SOA Formation from Isoprene
Isoprene (C5H8)
Monoterpenes (C10H16) Other Methane
~500 ~130 ~562 ~93 ~510
Biogenic VOCsAnthropogenic
VOCs
Estimated Global VOC and Methane Emissions (TgC yr-1)
Guenther et al. [1999]
• Laboratory Experiments Confirm Isoprene as SOA Source
2-methylglyceric (2-MG) acid
diastereoisomeric 2-methyltetrols
[Claeys et al., 2004; Edney et al., 2005]
• Previously Assumed Not to be Significant Source of SOA Despite High Emissions:
OH
OHOH
HO
OHOH
O
HO
Detection in Ambient Aerosol Indicated Isoprene as Likely Source
MW 120
MW 136
[Kroll et al., 2006;Surratt et al, 2006]
• Isoprene Estimated to Contribute Significantly to Global SOA Burden
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Role of NOx
in Isoprene Oxidation
• Peroxy radical chemistry: RO2
+HO2
vs RO2
+NO
Kroll et al., ES&T
(2006)Less volatile products
More volatile products
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OH-initiated Oxidation of Isoprene: First-Generation Products
Measured Yields under High-NOXMVK 32-44%
MACR 22-28%3-methylfuran <2-5%
Hydroxynitrate (and isomers) 4-14%C5 -hydroxycarbonyl 15-19%
O
OONO2
OH/O2/NO2
methacryloylperoxynitrate (MPAN)
OH
HO
OHO OHepoxydiols of isoprene
(IEPOX)IEPOX only recentlydiscoveredPaulot et al., Science (2009)
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Formation of Isoprene Low-NOx SOA Constituents and Oligomers
Products Characterized by GC/ITMS with prior derivatization:
These Compounds are Ambient Tracers for SOA Formation from Isoprene
Under Low-NOx Conditions
Key to the Low-NOxMechanism
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Increased Aerosol Acidity Reveals Role of IEPOX in Low-NOx SOA Formation
• Photooxidation of 49 and 40 ppb of isoprene in presence of acidified or neutral sulfate seed aerosol, respectively.
• SOA mass yield increased from 1.3% for the neutral case to 28.6% for the acidic case.
• RO2 + HO2 reactions dominate.
• 9 ppb to 0.6 ppb of IEPOX was measured in the neutral and acidic cases, respectively; no other differences observed for other isoprene oxidation products
• Mixing ratios of other gas-phase products: ROOH ~ 12 ppb, and diols, hydroxycarbonyls, and tetrols all < 0.8 ppb
• All particle-phase products substantially increase inpresence of acidic seed; for example, 2-methyltetrols increased from 0.1 μg m-3 for the neutral case to 5.1 μg m-3 for the acidic case.
• Particle-phase IEPOX could be due to un-reacted IEPOXthat partitioned favorably into wet aerosol.
Surratt et al., PNAS (2010)
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(-)CIMS Time Traces Further Demonstrate Role of IEPOX
Dark Reactive Uptake of2,3‐epoxy‐1,4‐butanediol
(BEPOX) Standard
Butadiene Photooxidation underLow‐NOx
Conditions
Isoprene Photooxidation under
Low‐NOx
Conditions
neutral seed
acidic seed BEPOX(acidic seed)
ROOH
(acidic seed)
IEPOX (acidic seed)
IEPOX (neutral seed)
ROOH (acidic seed)ROOH (neutral seed)
• Synthesized BEPOX standard, which is butadiene derivative of IEPOX, shows exact same reactive uptake behavior as that found for BEPOX produced from butadiene photooxidation and IEPOX produced from isoprene photooxidation in presence of acid seed
• Conclusion: Significant loss of epoxydiols (i.e., BEPOX and IEPOX) to highly
acidic seed aerosolresults from acid‐catalyzed particle‐phase reactions
• All of the detected particle‐phase constituents in BEPOX or butadiene SOA are exact analoguesof isoprene low‐NOx
SOA (i.e., differ by a mass of 14 amu, which corresponds to CH2
group)
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Methacrolein photooxidation
• HONO + hv OH + NO• CH3
ONO (no net NO production):– CH3
ONO CH3
O + NO– CH3
O + O2
HO2
+ HCHO– HO2
+ NO OH + NO2
• Not an effect of enhanced HNO3
CH3
ONO
HONOadding NO2
adding NO
Total amount of SOA trends with NO2 /NO ratio
Chan et al., ACP (2010)
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Chan et al., ACP (2010)
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Summary of NOx
on Isoprene SOA
OH/O2 RO2
HO2low-NOx SOA
NOO
OH/O2R'O2
HO2no aerosol
no aerosol
MPAN high-NOx SOA
NO
NO2
NOx
vs HO2
NO2
vs NO
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Other unsaturated aldehydes
acrolein crotonaldehyde
Chan et al., ACP (2010)
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SOA formation from aldehydes/alcohols
Chan et al., ACP (2010)
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Chan et al., ACP (2010)
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Recent work (CMU) indicates there is a large potential for low‐volatility
organic compounds ( C* < 106 μg/m3 ) to form organic aerosol
SVOC
POA+ Ox
O-SVOCSOASOA
IVOC
+ Ox
O-IVOC
log10 (C* [μg/m3])
Station fire from Roof Lab, Caltech 2009
SVOCs: Semivolatile
Organic Compounds
(C* < 104)
IVOCs: Intermediate Volatility Organic Compounds (104 < C* < 106 μg/m3)
Low-Volatility Organics
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Kautzman et al., J. Phys. Chem. (2010)
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Kautzman et al., J. Phys. Chem. (2010)
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SOA from biomass burning:
A case for studying methoxyphenols
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Emissions/Importance of MeOPhOHs
Schauer et al. (2001)
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0 50 100 150 200 2500
20
40
60
80
100
120
140
Guaiacol High NOX Growth Curve
Guaiacol (g m-3)
M
0 (
g m
-3)
02/0502/0302/0503/023/283/3104/064/94/134/154/22
O:C = 0.9OH
OCH3
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0 50 100 150 200 250 3000
20
40
60
80
100
120
140
Guaiacol (g m-3)
M
0 (
g m
-3)
Guaiacol Low-NOx Growth Curve
1/292/022/042/06
O:C = 0.9OHO
CH3
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Laboratory SOA
0.30
0.25
0.20
0.15
0.10
0.05
0.00
44/o
rg
0.40.30.20.10.043/org
OZONOLYSIS Caltech ap (acid seed) Caltech ap (nonacid seed) Harvard ap (Shilling et al., 2008) Caltech biogenics
PHOTOOXIDATION
PSI ap (Alfarra et al., 2006) Caltech ap (HONO) Caltech ap (H2O2) Caltech mxylene (NO/NO2) Caltech mxylene (HONO) Caltech mxylene (H2O2) Caltech toluene (HONO) Caltech toluene (H2O2) Caltech benzene PSI 1,3,5_TMB (Alfarra et al., 2006) Caltech biogenics (NOx/propene) Caltech longifolene (HONO) Caltech longifolene (H2O2+NO) Caltech longifolene (H2O2) LBNL diesel Caltech isoprene (HONO) CMU n_heptadecane (Presto et al., 2009) CMU wood smoke (Grieshop et al., 2009)
•
Data from chamber experiments fall into the same space as ambient data; 44/org of most of the chamber data
are < 0.1, indicating that chamber SOA is not as oxidized as ambient aerosol Ng et al., ACP
(2010)]
0.30
0.25
0.20
0.15
0.10
0.05
0.00
44/o
rg
0.40.30.20.10.043/org
isoprene (H2O2) isoprene (H2O2+NO)
Data from Sean Kessler,
Jesse Kroll
erythritol
44.0544.0043.95
CO2 C2H4O
500
400
300
200
100
0
Diff
, Hz
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The Future
• Seek to bridge the gap between detailed molecular
mechanisms of VOC oxidation leading to SOA and the
ultimate ambient state that is characterized by O:C, H:C,
volatility, polarity.
• Laboratory chamber experiments are the essential source of
information on SOA formation; need to devise ways to extend
the chemical lifetime of such experiments (within the
confines of inevitable wall loss) to simulate ambient aging.
• Atmospheric models need to be able to track the sources of
SOA; the challenge is to retain this identification in view of
the evolution of SOA towards its highly oxidized state.