Modeling Dynamic Partitioning of Semi-volatile Organic Gases to Size-Distributed Aerosols
Rahul A. ZaveriRichard C. Easter
Pacific Northwest National Laboratory
1
International Workshop on Air Quality Forecasting ResearchNovember 29, 2011Potomac, 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
OxidationSoot
Sea-salt
Direct RadiativeForcing
Indirect RadiativeForcing
Ter
pene
s
Oxi
dat
ion
For
est
Fire
s
Nucleation
Hydrocarbons,NOx, SO2, NH3, POA
Oxidation
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
4
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
5
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
6
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 TransferTreatments in MOSAIC
H2SO4
HNO3HCl
SolidSalts
Aqueous Phase
OM, BC
NH3
H2O
OH-
NO3-
Cl-
SO4-
Na+
NH4+H+
Ca2+ParticlePhase
Thermodynamic equilibriumThermodynamic equilibriumwithin the particle phase within the particle phase (depends on composition and RH)(depends on composition and RH)
Kinetic mass transfer between the gas and Kinetic mass transfer between the gas and size-distributed particles (1 to 10,000 nm)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
9
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 ofaqueous 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
OrganicsH2SO4
HNO3HCl
Salts, BC,Org
Particle Phase
NH3
H2O
OH-
NO3-
Cl-
SO4-
Na+
NH4+H+
Ca2+
Gas Phase
Org
Oligomer
10
Raoult’s Law vs. Reactive Uptake
HC + oxidant 1 G1 + 2 G2
dGi
dt
xi = mole fraction
vaporpressure
A1 A2
= -ki (Gi – xiPi0)
Raoult’s Law Based Mass Transfer
HC + oxidant 1 G1 + 2 G2
dGi
dt
A1 A2
= -kiGi
Reactive Uptake Mass Transfer
Sample Results
Idealized Case
Sacramento Urban Air Case
Idealized Case
Diameter, Dp (m)
0.01 0.1 1
Num
ber
Con
cent
ratio
n (c
m-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 (m)
0.01 0.1 1
Num
ber
Con
cent
ratio
n (c
m-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
SO
A M
ass
(g
m-3
)
6
8
10
12
14
16
Initial aerosol composition: mass ratio OA/(NH4)2SO4 = 1
Idealized Case
Diameter, Dp (m)
0.01 0.1 1
Num
ber
Con
cent
ratio
n (c
m-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
SO
A M
ass
(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 (m)
0.01 0.1
Num
ber
Con
cent
ratio
n (c
m-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 (m)
0.01 0.1
Num
ber
Con
cent
ratio
n (c
m-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 (m)
0.01 0.1
Num
ber
Con
cent
ratio
n (c
m-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)
18
Diameter, Dp (m)
0.01 0.1
Num
ber
Con
cent
ratio
n (c
m-3
)
0
200
400
600
800
1000
1200Observed 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 (m)
0.01 0.1
Num
ber
Con
cent
ratio
n (c
m-3
)
0
200
400
600
800
1000
1200Observed 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)
19
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
21
Thank you for your attention
Funding for this work was provided byDepartment of Energy (DOE) Atmospheric System Research (ASR) Program