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Uma Shankar1 and Prakash Bhave2
Sixth Annual CMAS ConferenceOctober 1-3, 2007
1 UNC Institute for the Environment 2 Atmospheric Modeling Division, NOAA (in
partnership with EPA-NERL)
Box Model Tests of Two Mass Transfer Methods for Volatile Aerosol Species
in CMAQ
Overview• Treatment of Coarse PM in CMAQ• Mass Transfer Theory• Approach: Box Model Development• Results
Fine-particle Equilibrium Fully Dynamic Approaches (4 schemes)
• Next Steps
NO3-
NH4+
SO4=
H2O
POA
SOAa
SOAb
EC
OtherSVOCs
HNO3
NH3
H2O
Sea Salt
Soil, Other
COARSE MODE2 FINE MODES
Coarse-Mode Chemistry in CMAQ
H2SO4
Prior to CMAQv4.5:• Coarse mode is inert.• Fine mode species equilibrate
instantaneously w/ inorganic gases
NO3-
NH4+
SO4=
Na+
Cl-
H2O
POA
SOAa
SOAb
EC
OtherSVOCs
HNO3
NH3
H2O
SO42-
Na+
Cl-
Soil, Other
COARSE MODE2 FINE MODES
Coarse-Mode Chemistry in CMAQ
H2SO4
HCl
CMAQv4.6 (current) treatment:• Coarse mode is inert.• New species shown in RED.• Fine mode species equilibrate
instantaneously w/ inorganic gases
NO3-
NH4+
SO4=
Na+
Cl-
H2O
POA
SOAa
SOAb
EC
OtherSVOCs
HNO3
NH3
H2O
NO3-
NH4+
SO42-
Na+
Cl-
H2O
Soil, Other
COARSE MODE2 FINE MODES
Coarse-Mode Chemistry in CMAQ
H2SO4
HCl
Next CMAQ release:
• Coarse mode will interact with inorganic gases
• New species and interactions are shown in RED
Time Scale for Mass TransferDp = 0.2 μm
Dp = 3.0 μm
Reference: Z. Meng & J.H. Seinfeld, Atmos. Environ., 30:2889-2900 (1996).
Coarse PM takes ~10h to reach equilibrium with surrounding gases, so
instantaneous equilibrium approach is not applicable.
“Dynamic” approach needed for gas-particle mass transfer
Mass Transfer Rate, J
p
sjvpj
DKn
ccNDKn
DJ
2
21
12
where
Size-dependent term proportional to surface area
for small particles, proportional to diameter for
large particles
Composition-dependent term concentration at the particle’s surface (cs) is determined by
gas/particle equilibriumpositive gradient condensationnegative gradient evaporation
Most implementations of dynamic mass transfer to date have been done in sectional models (e.g., PMCAMx, CMAQ-MADRID).
One exception: Modal Aerosol Module in Polyphemus (Sartelet et al., 2006).
Approach• Adapt aerosol code from CMAQ v4.6 to develop a
stand-alone box model for aerosol microphysics• Extend the box model to treat gas-particle
transfer with all 3 modes dynamically• Add some simplifying assumptions to maintain
computational efficiency• Resulting module will be implemented in next
release of CMAQ.
Approach• Adapt aerosol code from CMAQ v4.6 to develop a
stand-alone box model for aerosol microphysics• Extend the box model to treat gas-particle
transfer with all 3 modes dynamically• Add some simplifying assumptions to maintain
computational efficiency• Resulting module will be implemented in next
release of CMAQ.
• Test case. Mimics the transport of a marine air mass into a polluted urban area such as Los Angeles
Box-Model Test Conditions
0
4
8
12
0 6 12 18 24 30 36
Hour
Em
iss
ion
s (m g
m-3
h-1
)
H2SO4 × 10
HNO3
NH3 × 0.1
Reference: Pilinis et al., Aerosol Sci. Technol., 32:482-502 (2000).
• Developed by Pandis et al.• 38-hour scenario to test
different gas-to-particle mass transfer schemes over a range of RH, particle acidity, and pollution concentrations.
• Used previously in development/testing of sectional aerosol models in CMAQ-MADRID and PM-CAMx
Large plumes of NH3 provide a realistic
challenge for dynamic-transfer module.
Box-Model Test Conditions• Initial conditions
NH3 0.3 μg m-3
HNO3 4.0 μg m-3
Marine particle distribution
• Convert to tri-modal distribution, for compatibility with CMAQ
Reference: J. Lu and F.M. Bowman, Aerosol Sci. Technol., 38:391-399 (2004).
Box-Model Test Results
• First, compare the fine particle equilibrium approach of CMAQ v4.6 with a “reference” model: a multi-component aerosol dynamics module (MADM) run with 10 sections
• Focus of comparisons is total PM concentrations of inorganic species predicted by different models as a function of time.
Box-Model Test Results
0
2
4
6
8
10
12
14
16
0 4 8 12 16 20 24 28 32 36
Hour of Simulation
Par
ticl
e-P
has
e S
ulf
ate
(mg
/m3)
ReferenceCMAQv4.6
Reference curve is from a state-of-the-science multi-component aerosol dynamics module (MADM) run with 10 sections.
Sulfate matches very well, because SO4
2- a non-volatile condensing species.
Box-Model Test ResultsCMAQv4.6 NH4
+ also matches reference very well.
Jim Kelly discovered an error in reference case past hour 30 and thus we excluded these data from subsequent comparisons.
0
2
4
6
8
10
12
14
16
18
20
0 4 8 12 16 20 24 28 32 36
Hour of Simulation
Par
ticl
e-P
has
e A
mm
on
ium
(m g
/m3)
ReferenceCMAQv4.6
Box-Model Test ResultsIn CMAQv4.6, nitrate is underpredicted throughout the simulation because During first 16 hours,
coarse-mode NaNO3 is not formed.
After NH3 is emitted on Hour 16, NH4NO3 formation is restricted to the fine modes.
0
10
20
30
40
50
60
0 4 8 12 16 20 24 28 32 36
Hour of Simulation
Par
ticl
e-P
has
e N
itra
te (m g
/m3)
ReferenceCMAQv4.6
Box-Model Test ResultsIn CMAQv4.6, Cl- is constant because Initial mass of Cl- is
entirely in coarse mode There is no coarse-
mode chemistry
In reference case In first 12 hours, Cl- in
coarse PM is gradually replaced by NO3
-.
On Hour 16, large NH3 plume leads to NH4Cl formation.
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0 4 8 12 16 20 24 28 32 36
Hour of Simulation
Par
ticl
e-P
has
e C
hlo
rid
e (m
g/m
3)
ReferenceCMAQv4.6
Box-Model Test Results• Next, we implemented a dynamic mass transfer
scheme with a uniform 10 s time step. Fluxes of volatile acids and NH3 are calculated
independently of each other – “uncoupled transfer” Call ISORROPIA in reverse mode w/ particle-phase
concentrations as input. Output is the equilibrium concentration, Cs, at particle surface.
• Focus on Hours 0 – 16, when marine aerosol is reacting gradually with HNO3, before encountering large NH3 emissions. Does the model capture the replacement of Cl- by NO3?
Box-Model Test Results
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0 4 8 12 16 20 24 28 32 36
Hour of Simulation
Par
ticl
e-P
has
e C
hlo
rid
e (m
g/m
3)
ReferenceCMAQv4.6Dynamic
In dynamic model, loss of Cl- from coarse
mode is captured quite accurately!
0
1
2
3
4
5
6
7
8
9
10
0 4 8 12 16 20 24 28 32 36
Par
ticl
e-P
has
e N
itra
te (m g
/m3 )
ReferenceCMAQv4.6Dynamic
In dynamic model, NaNO3 reaches the correct endpoint, but temporal evolution needs
further study.
What happens in dynamic model after Hour 16?
0
5
10
15
20
25
0 4 8 12 16 20 24 28 32 36
Hour of Simulation
Par
ticl
e-P
has
e A
mm
on
ium
(m g
/m3 )
ReferenceCMAQv4.6Dynamic
Box-Model Test ResultsAfter encountering the NH3 plume on Hour 16, dynamic model becomes unstable.
Abrupt transition of coarse mode from acidic to alkaline, causes rapid NH3 evaporation, and the system never recovers...
So we investigated the use of special mass transfer schemes when particle composition approaches neutral pH
Treatment Near pH-Neutrality• 3 approaches in literature (all sectional models)
Sun & Wexler, Atmos. Environ. 1998“Coupled Transport” – Transfer acids and bases in equimolar quantities such that H+ remains stable near pH-neutrality.
Pilinis et al., Aerosol Sci. Technol. 2000Restrain the transfer of all volatile gases to allow only small changes in acidity during each time step.
Jacobson, Aerosol Sci. Technol. 2005Uncoupled dynamic transfer of acids followed by instantaneous equilibrium transfer of NH3.
Treatment Near pH-Neutrality• 3 approaches in literature (all sectional models)
Sun & Wexler, Atmos. Environ. 1998“Coupled Transport” – Transfer acids and bases in equimolar quantities such that H+ remains stable near pH-neutrality.
Pilinis et al., Aerosol Sci. Technol. 2000Restrain the transfer of all volatile gases to allow only small changes in acidity during each time step.
Jacobson, Aerosol Sci. Technol. 2005Uncoupled dynamic transfer of acids followed by instantaneous equilibrium transfer of NH3.
• Implement and test each scheme in box model.
Box-Model Test Results
If acids and base are both condensing or both evaporating, coupled transfer when near pH-neutral:Oscillatory behavior persists but trend improves substantially.Same as purple curve, but turned off transfer when flux gradients for acids and base had opposite signs: Periods of no transport exhibit step-like behavior in time series
Jacobson-like scheme:Reproduces magnitude of reference case, but some oscillations exist
Box-Model Test Results
Jacobson-like scheme:Best agreement with reference case
Box-Model Test Results
Jacobson-like scheme:Oscillations appear more pronounced due to scale of the plot. Under- prediction after hr 16 matches overprediction in NH4
+
Next Steps• Implement and test the Pilinis et al. mass transfer scheme
in our modal model• Develop a computationally-efficient solution for modal
model “Hybrid” scheme (fine particles at equilibrium w/ gas phase,
dynamic transfer of coarse particle mass) Tabulate Cs on coarse mode or treat as an irreversible
heterogeneous reaction (e.g., Hodzic et al., 2006)
• Benchmark our results against sectional implementation by Pilinis et al. against modal implementation by Sartelet et al. Compare size-resolved output to multiple reference cases
• Apply our fully-dynamic and computationally-efficient schemes in CMAQ simulations
• Incorporate into next year’s CMAQ release
Acknowledgements• Bill Benjey (EPA-ORD)• Frank Binkowski (UNC)• Frank Bowman (UND)• Adel Hanna (UNC)• Jim Kelly (EPA-ORD)• Bonyoung Koo (ENVIRON)• Spyros Pandis (CMU)• Christian Seigneur (AER)• Shaocai Yu (STC)
DisclaimerThe research presented here was performed under the Memorandum of Understanding
between the U.S. Environmental Protection Agency (EPA) and the U.S. Department of Commerce's National Oceanic and Atmospheric Administration (NOAA) and under agreement number DW13921548. This work constitutes a contribution to the NOAA Air Quality Program. Although it has been reviewed by EPA and NOAA and approved for publication, it does not necessarily reflect their policies or views.
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