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.