Krish Vijayaraghavan,Rochelle Balmori, Shu-Yun Chen,

Prakash Karamchandani and Christian SeigneurAER, San Ramon, CA

Justin T. Walters and John J. JansenSouthern Company, Birmingham, AL

Eladio M. KnippingEPRI, Palo Alto, CA

CMAS Conference, Oct 1-4, 2007Chapel Hill, NC

Modeling of Atmospheric Nitrogen Deposition to the Escambia Bay and Watershed

2

Overview

• Objective

– Estimate impact of NOx and SO2 controls at the Crist power plant on nitrogen deposition in Escambia Bay and its watershed in Florida/Southern Alabama

• Tools

– Three versions of CMAQ v. 4.5.1

3

Escambia Bay and Watershed

Escambia Bay

Escambia BayWatershed

4

Air Quality Models

• CMAQ

– CMAQ v. 4.5.1 with SOA modifications by VISTAS

– CBM-IV for gas-phase chemistry

– AERO4 aerosol module

• Heterogeneous nitrate formation in the PM phase only

• Includes sea salt emissions but does not account for coarse nitrate formation due to sea-salt/HNO3 interactions

• CMAQ-MADRID

– Based on CMAQ 4.5.1 and also utilizes CBM-IV

– Aerosols: Model of Aerosol Dynamics, Reaction, Ionization, and Dissolution

– Heterogeneous nitrate formation in aqueous and PM phase

– Comprehensive sea-salt/HNO3 chemistry (fine and coarse size ranges)

• CMAQ-MADRID-APT

– Builds upon CMAQ-MADRID

– Advance Plume Treatment (APT) for 40 power plants, including Plant Crist

5

Plume Chemistry & Dispersion Relevance to Nitrogen Chemistry

Early Plume Dispersion

NO/NO2/O3 chemistry

1

2

Mid-range Plume Dispersion

Reduced VOC/NOx/O3 chemistrySlow PM formation from OH and NO3/N2O5 chemistry

Long-range Plume Dispersion

3

Full VOC/NOx/O3 chemistryPM and O3 formation

Negligible PM formation

(NO3

-, SO4

=)

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ALGA Modeling Domain

2002 reference year

• 12 km horizontal grid resolution

• 19 layers up to 15 km altitude

• 40 power plants, including Plant Crist, with APT

• Inputs from VISTAS/GEPD

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Emission Reductions at Plant Crist

• Anticipated emission reductions due to installation of FGD and SCR/SNCR

Emissions in base case (tpy)

Change in emissions (tpy)

Relative change in emissions (%)

NOx 10900 -8600 -79%

SO2 37600 -35700 -95%

NH3 0 25 --

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Nitrogen Species

Nitrogen Grouping Modeled Species or Sub-groups

NOx NO + NO2

Organic NOz (O-NOz) PAN + NTR

Gas Inorganic NOz (Gas I-NOz) NO3 + N2O5 + HNO3 + HONO + PNA

Gas NHx NH3

PM NHx (NH4_1 + NH4_2) or (ANH4I + ANH4J)*

PM I-NOz (NO3_1 + NO3_2) or (ANO3I + ANO3J)*

I-NOz Gas I-NOz + PM I-NOz

Gas NOy NOx + Gas I-NOz + O-NOz

PM NOy PM I-NOz * Coarse mode not present for CMAQ

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Spatial Distribution of Total Nitrogen Deposition

Change in annual dry + wet deposition flux due to controls on Plant Crist

CMAQ CMAQ-MADRID CMAQ-MADRID-APT

APT: Less oxidation of NOx to HNO3 => Less dry deposition near the plant

Maximum reduction in deposition flux

0.68 kg/ha 0.85 kg/ha 0.42 kg/ha

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Dry Deposition of Nitrogen over the Escambia Bay Watershed (tpy)

• Highest contributor: Gas I-NOz (mostly HNO3)

Nitrogen Species

Grouping

CMAQ base

MADRID base

APT base

CMAQ control-

base

MADRID control-

base

APT

control-base

NOx 695 797 793 -20 -27 -17

O – NOz 613 651 653 -2 -2 -3

Gas I-NOz 3251 3491 3469 -55 -68 -63

PM2.5 NO3 9 8 8 0 0.1 0.1

PM10-2.5 NO3 -- 45 47 -- 3 1

PM2.5 NH4 85 72 72 -2 -1 -0.4

PM10-2.5 NH4 -- 15 15 -- 0 0

NH3 950 928 943 25 22 16

TOTAL 5602 6007 6000 -55 -73 -66

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Ammonia Dis-benefit due to SO2 Controls

• SO2 controls => Less PM ammonium sulfate => More gas NH3

• Dry deposition velocity of gas NH3 > PM NH4+

• Increase in NH3 deposition >> Decrease in PM NH4+ deposition

Outcome• Planned controls result in an increase in the NHx component of nitrogen

deposition.

Caveat• Downward revision of the NH3 dry deposition velocities will decrease the

dis-benefit.

Difference between models• APT has less dis-benefit because of less sulfate formation in the plume.

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Sea-salt Dis-benefit due to NOx/SO2 Controls

• MADRID and APT account for coarse NaNO3 formation from sea-salt/HNO3

• SO2 controls => Less fine and coarse sodium sulfate• Less coarse sodium sulfate => More coarse sodium nitrate

Outcome• Planned controls result in a small increase in coarse nitrate deposition.

Caveats• There is enough HNO3 so coarse nitrate formation is not affected by NOx

controls.• Dis-benefit will be lower if more fine NaCl in sea-salt emissions• CMAQ will also exhibit sea-salt dis-benefit if it accounts for coarse NaNO3.

Difference between models• APT has less dis-benefit than gridded models because of less sulfate/nitrate

formation.

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Wet Deposition of Nitrogen over the Escambia Bay Watershed (tpy)

• Models can not distinguish between gas and PM wet deposition

• Largest contributors: I-NOz followed by NHx

Nitrogen Species

Grouping

CMAQ base

MADRID base

APT base

CMAQ control-

base

MADRID control-base

APT

control-base

NOx 0.9 0.0 0.0 0.0 0.0 0.0

O – NOz 1.1 0.4 0.4 0.0 0.0 0.0

I – NOz 1804 1368 1434 -31 -23 -33

NHx 1610 1288 1315 -4 -3 -6

TOTAL 3416 2656 2750 -35 -26 -39

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Conclusions

• Three versions of CMAQ were used to estimate the decrease in atmospheric nitrogen deposition in Escambia Bay/watershed due to NOx and SO2 emissions controls at the nearby Crist power plant.

• Differences in results between the three models are due to differences in model formulation and configuration

• CMAQ-MADRID has more comprehensive heterogeneous nitrate and coarse sea-salt nitrate chemistry than CMAQ.

• CMAQ-MADRID-APT includes plume-in-grid treatment of large point sources (here, 40 large power plants including Crist).

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Conclusions

• Gaseous inorganic NOz (mostly HNO3) has the largest contribution (~60%) to nitrogen dry deposition in all three models.

• Inorganic NOz (gaseous + particulate) is the largest contributor (~52%) to wet deposition with a slightly lower contribution from NHx.

• NOx emission controls result in reductions in nitrogen deposition but SO2 controls result in an increase in nitrogen deposition due to an “ammonia dis-benefit”. NH3 dry deposition velocities in CMAQ need to be investigated further.

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Conclusions

• Over Escambia Bay and its watershed, CMAQ, MADRID and APT predict:

– total N deposition reductions of 91, 100, and 106 tons/yr, respectively.

– maximum reductions in gridded deposition fluxes of 0.68, 0.85, and 0.42 kg/ha/yr, respectively.

• APT simulates less dry deposition of HNO3 and PM sulfate near Plant Crist than CMAQ and MADRID due to its correct treatment of plume dispersion and chemistry. It is important to use a plume-in-grid treatment of emissions from large elevated point sources so that nitrogen deposition can be correctly simulated.

• Air quality modeling results were subsequently used in a watershed modeling study to estimate net nitrogen loading to Escambia Bay.