Modeling Dynamic Partitioning of Semi-volatile Organic Gases to Size-Distributed Aerosols
Rahul A. Zaveri Richard C. Easter
Pacific Northwest National Laboratory
1
International Workshop on Air Quality Forecasting Research November 29, 2011 Potomac, MD, USA
2
Motivation
SOA formation processes
SOA modeling challenges
MOSAIC aerosol modeling framework
Gas-particle partitioning of organic gases (new)
Sample results
Future Directions
Outline
3 Battelle Proprietary
Dust
Phytoplankton
Dimethylsulfide
H2SO4, MSA
Oxidation Soot
Sea-salt
Direct Radiative Forcing
Indirect Radiative Forcing
Terp
enes
Oxi
datio
n
Fore
st F
ires
Nucleation
Hydrocarbons, NOx, SO2, NH3, POA
H2SO4, HNO3, Organic aerosol
Activation
Resuspension
chemistry
Need to efficiently and reliably model aerosol size, number, mass, composition, and their climate related properties at urban to global scales
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Up to 90% of submicron aerosol mass is composed of organics (Kanakidou et al., 2005; Zhang et al., 2007)
SOA formation is quite rapid (within a few hours) during daytime (Volkamer et al., 2006; Kleinman et al., 2007; de Gouw et al., 2008)
SOA from oxidation of SVOCs from diesel exhaust may help explain some of the missing organic aerosol mass in models (Robinson et al., 2007)
Observed rapid growth of newly formed particles (via homogeneous nucleation) is thought to be by SOA condensation (Kuang et al., 2008)
Anthropogenic and biogenic SOA precursors may interact to enhance the overall SOA yield (Weber et al., 2007)
Particle-phase reactions of absorbed VOCs within inorganic particles can form SOA (Jang et al., 2003; Kroll et al., 2005; Liggio et al., 2005)
Accretion reactions, including aldol condensation, acid dehydration, and gem-diol condensation can transform VOCs into oligomeric compounds (Gao et al., 2004; Jang et al., 2003; Kalberer et al., 2004)
SOA Formation Processes
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Gas-particle partitioning processes are still poorly understood at a fundamental level
How do we handle the complexities in the gas-phase VOC chemistry?
Should we use Raoult’s Law or some sort of reactive uptake formulation as driving force for gas-particle mass transfer?
Should we use Henry’s Law if the organics are dissolved in the aqueous phase?
Are the organic particles liquid or solid? Virtanen et al. (2011) and Vaden et al. (2011) suggest that SOA particles are solid.
How do we treat organic-inorganic interactions and the associated phase transitions?
How do we treat particle-phase reactions? What are the time scales?
What are the anthropogenic-biogenic interactions? How do we reliably represent them in models?
SOA Modeling Challenges
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General Problem Solving Approach
Develop a comprehensive aerosol model framework that includes all the processes that we think are (or might be) important
Evaluate the roles of specific processes using appropriate laboratory and field observations
Simplify, parameterize, and optimize the process model as much as possible to increase computational efficiency and decrease memory requirements
MOSAIC Aerosol Module
Model for Simulating Aerosol Interactions and Chemistry (Zaveri et al., 2008) Comprehensive aerosol module for air quality and climate modeling Flexible framework for coupling various gas and aerosol processes Robust, accurate, and highly efficient custom numerical solvers for several processes Suitable for 3-D regional and global models Implementation in:
Weather Research and Forecasting Model (WRF-Chem) – done Global model: Community Atmosphere Model (CAM5) – in progress EPA’s CMAQ – planned
Thermodynamics & Mass Transfer Treatments in MOSAIC
H2SO4
HNO3 HCl
Solid Salts
Aqueous Phase
OM, BC
NH3
H2O
OH-
NO3-
Cl-
SO4-
Na+ NH4
+ H+
Ca2+ Particle Phase
Thermodynamic equilibrium within the particle phase (depends on composition and RH)
Kinetic mass transfer between the gas and size-distributed particles (1 to 10,000 nm)
Custom numerical techniques have been developed to solve these equations efficiently and accurately
Reversible Gas-Particle Reactions
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Organic-inorganic interactions within the particle to determine water uptake and phase separation (partially implemented)
Size-distributed, dynamic mass transfer between gas and particles
Raoult’s Law in the absence of aqueous phase (implemented)
Reactive uptake that instantly converts VOC to non-volatile products (implemented)
Henry’s Law in the presence of aqueous phase (future)
Particle-phase reactions (future)
Gas-Particle Partitioning of Organics
Organics H2SO4
HNO3 HCl
Salts, BC, Org
Particle Phase
NH3
H2O
OH-
NO3-
Cl-
SO4-
Na+ NH4
+ H+
Ca2+
Gas Phase
Org
Oligomer
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Raoult’s Law vs. Reactive Uptake
HC + oxidant α1 G1 + α2 G2
dGi dt
xi = mole fraction
vapor pressure
A1 A2
= -ki (Gi – xiPi0)
Raoult’s Law Based Mass Transfer
HC + oxidant α1 G1 + α2 G2
dGi dt
A1 A2
= -kiGi
Reactive Uptake Mass Transfer
Sample Results
Idealized Case
Sacramento Urban Air Case
Idealized Case
Diameter, Dp (µm)
0.01 0.1 1
Num
ber C
once
ntra
tion
(cm
-3)
0
20
40
60
80
100
120
140t = 0 h (initial)
Initial aerosol composition: mass ratio OA/(NH4)2SO4 = 1
Idealized Case
Diameter, Dp (µm)
0.01 0.1 1
Num
ber C
once
ntra
tion
(cm
-3)
0
20
40
60
80
100
120
140 t = 0 (initial)t = 9 h (Raoult's Law)
Time (h)
0 2 4 6 8 10
SOA
Mas
s (µ
g m
-3)
6
8
10
12
14
16
Initial aerosol composition: mass ratio OA/(NH4)2SO4 = 1
Idealized Case
Diameter, Dp (µm)
0.01 0.1 1
Num
ber C
once
ntra
tion
(cm
-3)
0
20
40
60
80
100
120
140
t = 0 h (Initial)t = 9 h (Raoult's Law)t = 9 h (Reactive Uptake)
Time (h)
0 2 4 6 8 10
SOA
Mas
s (µ
g m
-3)
6
8
10
12
14
16
Initial aerosol composition: mass ratio OA/(NH4)2SO4 = 1
Sacramento June 6, 2010
Initial aerosol composition: mass ratio OA/(NH4)2SO4 = 10
Diameter, Dp (µm)
0.01 0.1
Num
ber C
once
ntra
tion
(cm
-3)
0
200
400
600
800
1000
1200 Observed at 9:00 amInitial (fitted to obs.)Observed at 11:00 am
Sacramento June 6, 2010
Initial aerosol composition: mass ratio OA/(NH4)2SO4 = 10
Diameter, Dp (µm)
0.01 0.1
Num
ber C
once
ntra
tion
(cm
-3)
0
200
400
600
800
1000
1200 Observed at 9:00 amInitial (fitted to obs.)Observed at 11:00 amModel at 11:00 am (Raoult's Law)
Sacramento June 6, 2010
Initial aerosol composition: mass ratio OA/(NH4)2SO4 = 10
Diameter, Dp (µm)
0.01 0.1
Num
ber C
once
ntra
tion
(cm
-3)
0
200
400
600
800
1000
1200 Observed at 9:00 amInitial (fitted to obs.)Observed at 11:00 amModel at 11:00 am (Raoult's Law)Model at 11:00 am (Reactive Uptake)
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Diameter, Dp (µm)
0.01 0.1
Num
ber C
once
ntra
tion
(cm
-3)
0
200
400
600
800
1000
1200 Observed at 9:00 amInitial (fitted to obs.)Observed at 11:00 amModel at 11:00 am (Raoult's Law)
Sacramento June 6, 2010 Aitken mode mass ratio OA/(NH4)2SO4 = 10
Accumulation mode mass ratio OA/(NH4)2SO4 = 0.01
Diameter, Dp (µm)
0.01 0.1
Num
ber C
once
ntra
tion
(cm
-3)
0
200
400
600
800
1000
1200 Observed at 9:00 amInitial (fitted to obs.)Observed at 11:00 amModel at 11:00 am (Raoult's Law)Model at 11:00 am (Reactive Uptake)
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Sacramento June 6, 2010 Aitken mode mass ratio OA/(NH4)2SO4 = 10
Accumulation mode mass ratio OA/(NH4)2SO4 = 0.01
Future Directions Perform additional constrained Lagrangian model analyses to test different SOA formation mechanisms
Use carefully designed chamber experiments to constrain and evaluate different formulations
Extend model analyses to mixtures of organic and inorganic species at different relative humidities
Implement and evaluate new SOA formulations in WRF-Chem using urban to regional field observations
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Thank you for your attention
Funding for this work was provided by Department of Energy (DOE) Atmospheric System Research (ASR) Program