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PM Model Performance Goals and Criteria
James W. BoylanGeorgia Department of Natural Resources
- VISTAS
National RPO Modeling MeetingDenver, CO
May 26, 2004
Outline
• Standard Bias and Error Calculations
• Proposed PM Model Performance Goals and Criteria
• Evaluation of Eight PM Modeling Studies Using Proposed Goals and Criteria
• Discussion: Should EPA recommend PM Model Performance Goals and Criteria in PM Modeling Guidance Document?
PM Model Evaluations
• Air Quality Modeling and Ambient Measurements are two different ways to estimate actual ambient concentrations of pollutants in the atmosphere
• Both modeling and measurements have some degree of uncertainty– Measurements should not be considered the
absolute truth– Large differences between monitoring networks
due to sampling and analysis techniques
• Normalized bias and error calculations should not be normalized by observations, but rather the average of the model and the observations.
Performance Metrics Equation
Mean Bias (g/m3)
Mean Error (g/m3)
Mean Normalized Bias (%) (-100% to +) Mean Normalized Error (%) (0% to +)
Normalized Mean Bias (%) (-100% to +) Normalized Mean Error (%) (0% to +)
Mean Fractional Bias (%) (-200% to +200%) Mean Fractional Error (%) (0% to +200%)
N
iom CC
NMB
1
1
N
i mo
om
CC
CC
NMFE
1
2
1
N
i mo
om
CCCC
NMFB
1
2
1
N
io
N
iom
C
CCNME
1
1
N
io
N
iom
C
CCNMB
1
1
N
i o
om
C
CC
NMNE
1
1
N
i o
om
C
CC
NMNB
1
1
N
iom CC
NME
1
1
Performance Metrics
• Mean Normalized Bias and Error – Usually associated with observation-based minimum
threshold• Some components of PM can be very small making it
difficult to set a reasonable minimum threshold value without excluding a majority of the data points
– Without a minimum threshold, very large normalized biases and errors can result when observations are close to zero even though the absolute biases and errors are very small
• A few data points can dominate the metric
– Overestimations are weighted more than equivalent underestimations
– Assumes observations are absolute truth
Performance Metrics
• Normalized Mean Bias and Error – Biased towards overestimations– Assumes observations are absolute truth
• Mean Fractional Bias and Error– Bounds maximum bias and error – Symmetric: gives equal weight to underestimations
and overestimations– Normalized by average of observation and model
Example Calculations
• Mean Normalized Bias and Error– Most biased and least useful of the three metrics
• Normalized Mean Bias and Error • Mean Fractional Bias and Error
– Least biased and most useful of the three metrics
Model g/m3
)
Obs. g/m3
)
MBg/m3)
NMB (%)
MNB (%)
MFB (%)
ME g/m3
)
NME (%)
MNE (%)
MFE (%)
0.05 1.0 -0.95 -95 -180.95 +0.95 +95 +180.95
1.0 0.05 +0.95 +1900 +180.95 +0.95 +1900 +180.95
1.0 0.01 +0.99 +9900 +196.04 +0.99 +9900 +196.04
0.04 0.05 -0.01 -20 -22.22 +0.01 +20 +22.22
0.5225 0.2775 +0.245 +88.3 +2921.3 +43.5 0.725 261.3 2978.8 145.0
PM Goals and Criteria• Performance Goals: Level of accuracy that
is considered to be close to the best a model can be expected to achieve.
• Performance Criteria: Level of accuracy that is considered to be acceptable for regulatory applications.
• It has been suggested that we need different performance goals and criteria for:– Different Species– Different Seasons– Different Parts of the Country– 20% Haziest and 20% Cleanest Days
• Answer: performance goals and criteria that vary as a function of concentration
PM Modeling Studies Used for Performance Benchmarks
• SAMI (GT)– July 1995 (URM/IMPROVE/variable grid)– July 1991 (URM /IMPROVE /variable grid)– May 1995 (URM /IMPROVE /variable grid)– May 1993 (URM /IMPROVE /variable grid)– March 1993 (URM /IMPROVE /variable grid)– February 1994 (URM /IMPROVE /variable grid)
• VISTAS (UCR/AG/Environ)– July 1999 (CMAQ/IMPROVE/36 km)– July 1999 (CMAQ /IMPROVE/12 km)– July 2001 (CMAQ /IMPROVE/36 km)– July 2001 (CMAQ /IMPROVE/12 km)– January 2002 (CMAQ /IMPROVE/36 km)– January 2002 (CMAQ /IMPROVE/12 km)
PM Modeling Studies Used for Performance Benchmarks
• WRAP 309 (UCR/CEP/Environ)– January 1996 (CMAQ/IMPROVE/36 km)– February 1996 (CMAQ/IMPROVE/36 km)– March 1996 (CMAQ/IMPROVE/36 km)– April 1996 (CMAQ/IMPROVE/36 km)– May 1996 (CMAQ/IMPROVE/36 km)– June 1996 (CMAQ/IMPROVE/36 km)– July 1996 (CMAQ/IMPROVE/36 km)– August 1996 (CMAQ/IMPROVE/36 km)– September 1996 (CMAQ/IMPROVE/36 km)– October 1996 (CMAQ/IMPROVE/36 km)– November 1996 (CMAQ/IMPROVE/36 km)– December 1996 (CMAQ/IMPROVE/36 km)
PM Modeling Studies Used for Performance Benchmarks
• WRAP 308 (UCR/CEP/Environ)– Summer 2002 (CMAQ/IMPROVE/36 km/WRAP)– Summer 2002 (CMAQ/IMPROVE/36 km/US)– Winter 2002 (CMAQ/IMPROVE/36 km/WRAP)– Winter 2002 (CMAQ/IMPROVE/36 km/US)
• EPA (Clear Skies)– Fall 1996 (REMSAD/IMPROVE/36 km)– Spring 1996 (REMSAD/IMPROVE/36 km)– Summer 1996 (REMSAD/IMPROVE/36 km)– Winter 1996 (REMSAD/IMPROVE/36 km)
PM Modeling Studies Used for Performance Benchmarks
• MANE-VU (GT)– July 2001 (CMAQ/IMPROVE/36 km)– July 2001 (CMAQ/SEARCH/36 km)– January 2002 (CMAQ/IMPROVE/36 km)– January 2002 (CMAQ/ SEARCH /36 km)
• Midwest RPO– August 1999 (CMAQ/IMPROVE/36 km)– August 1999 (CAMx/IMPROVE/36 km)– August 1999 (REMSAD/IMPROVE/36 km)– January 2000 (CMAQ/IMPROVE/36 km)– January 2000(CAMx/IMPROVE/36 km)– January 2000(REMSAD/IMPROVE/36 km)
PM Modeling Studies Used for Performance Benchmarks
• EPRI (AER/TVA/Environ)– July 1999 (CMAQ/IMPROVE/32 km)– July 1999 (CMAQ/IMPROVE/8 km)– July 1999 (MADRID/IMPROVE/32 km)– July 1999 (MADRID /IMPROVE/8 km)– July 1999 (CAMx/IMPROVE/32 km)
Mean Fractional Error
Combined Modeling Studies
0.00
50.00
100.00
150.00
200.00
0.0 4.0 8.0 12.0 16.0 20.0 24.0 28.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Err
or
Sulfate
Nitrate
Ammonium
Ammonium Bi
Organics
EC
Soils
PM2.5
PM10
CM
Combined Modeling Studies
-200.00
-150.00
-100.00
-50.00
0.00
50.00
100.00
150.00
200.00
0.0 4.0 8.0 12.0 16.0 20.0 24.0 28.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Bia
s
Sulfate
Nitrate
Ammonium
Ammonium Bi
Organics
EC
Soils
PM2.5
PM10
CM
Mean Fractional Bias
Proposed PM Goals and Criteria
• Based on MFE and MFB calculations• Vary as a function of species
concentrations– Goals: MFE +50% and MFB ±30%– Criteria: MFE +75% and MFB ±60%– Less abundant species should have less stringent
performance goals and criteria
• Continuous functions with the features of:– Asymptotically approaching proposed goals and
criteria when the mean of the observed and modeled concentrations are greater than 2.5 g/m3
– Approaching +200% MFE and ±200% MFB when the mean of the observed and modeled concentrations are extremely small
Proposed Goals and Criteria
• Proposed PM Performance Goals
• Proposed PM Performance Criteria
301703/5.0
)(5.0
mg
CC mo
eMFB
501503/75.0
)(5.0
mg
CC mo
eMFE
751253/75.0
)(5.0
mg
CC mo
eMFE
601403/5.0
)(5.0
mg
CC mo
eMFB
MFE Goals and Criteria
0
50
100
150
200
0.0 5.0 10.0 15.0 20.0 25.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Err
or
Goal
Criteria
Zone 1
Zone 3
Zone 2
MFB Goals and Criteria
-200
-150
-100
-50
0
50
100
150
200
0.0 5.0 10.0 15.0 20.0 25.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Bia
s
(+) Goal
(-) Goal
(+) Criteria
(-) CriteriaZone 1
Zone 3
Zone 3
Zone 2
Zone 2
Model Performance Zones
• Zone I – Good Model Performance– Level I Diagnostic Evaluation (Minimal)
• Zone II– Average Model Performance– Level II Diagnostic Evaluation (Standard)
• Zone III– Poor Model Performance– Level III Diagnostic Evaluation (Extended)
and Sensitivity Testing
Combined Modeling Studies
0.00
50.00
100.00
150.00
200.00
0.0 4.0 8.0 12.0 16.0 20.0 24.0 28.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Err
or
Sulfate
Nitrate
Ammonium
Ammonium Bi
Organics
EC
Soils
PM2.5
PM10
CM
Goal
Criteria
Mean Fractional Error
Combined Modeling Studies
-200.00
-150.00
-100.00
-50.00
0.00
50.00
100.00
150.00
200.00
0.0 4.0 8.0 12.0 16.0 20.0 24.0 28.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Bia
s
Sulfate
Nitrate
Ammonium
Ammonium Bi
Organics
EC
Soils
PM2.5
PM10
CM
(+) Goal
(-) Goal
(+) Criteria
(-) Criteria
Mean Fractional Bias
Combined Modeling Studies
0.00
50.00
100.00
150.00
200.00
0.0 2.0 4.0 6.0 8.0 10.0 12.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Err
or
Sulfate
Goal
Criteria
Sulfate Mean Fractional Error
Combined Modeling Studies
-200.00
-150.00
-100.00
-50.00
0.00
50.00
100.00
150.00
200.00
0.0 2.0 4.0 6.0 8.0 10.0 12.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Bia
s
Sulfate
(+) Goal
(-) Goal
(+) Criteria
(-) Criteria
Sulfate Mean Fractional Bias
Combined Modeling Studies
0.00
50.00
100.00
150.00
200.00
0.0 1.0 2.0 3.0 4.0 5.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Err
or
Nitrate
Goal
Criteria
Nitrate Mean Fractional Error
Combined Modeling Studies
-200.00
-150.00
-100.00
-50.00
0.00
50.00
100.00
150.00
200.00
0.0 1.0 2.0 3.0 4.0 5.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Bia
s
Nitrate
(+) Goal
(-) Goal
(+) Criteria
(-) Criteria
Nitrate Mean Fractional Bias
Combined Modeling Studies
0.00
50.00
100.00
150.00
200.00
0.0 1.0 2.0 3.0 4.0 5.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Err
or
Ammonium
Ammonium Bi
Goal
Criteria
Ammonium Mean Fractional Error
Combined Modeling Studies
-200.00
-150.00
-100.00
-50.00
0.00
50.00
100.00
150.00
200.00
0.0 1.0 2.0 3.0 4.0 5.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Bia
s
Ammonium
Ammonium Bi
(+) Goal
(-) Goal
(+) Criteria
(-) Criteria
Ammonium Mean Fractional Bias
Combined Modeling Studies
0.00
50.00
100.00
150.00
200.00
0.0 1.0 2.0 3.0 4.0 5.0 6.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Err
or
Organics
Goal
Criteria
Organics Mean Fractional Error
Combined Modeling Studies
-200.00
-150.00
-100.00
-50.00
0.00
50.00
100.00
150.00
200.00
0.0 1.0 2.0 3.0 4.0 5.0 6.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Bia
s
Organics
(+) Goal
(-) Goal
(+) Criteria
(-) Criteria
Organics Mean Fractional Bias
Combined Modeling Studies
0.00
50.00
100.00
150.00
200.00
0.0 1.0 2.0 3.0 4.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Err
or
EC
Goal
Criteria
EC Mean Fractional Error
Combined Modeling Studies
-200.00
-150.00
-100.00
-50.00
0.00
50.00
100.00
150.00
200.00
0.0 1.0 2.0 3.0 4.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Bia
s
EC
(+) Goal
(-) Goal
(+) Criteria
(-) Criteria
EC Mean Fractional Bias
Combined Modeling Studies
0.00
50.00
100.00
150.00
200.00
0.0 1.0 2.0 3.0 4.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Err
or
Soils
Goal
Criteria
Soils Mean Fractional Error
Combined Modeling Studies
-200.00
-150.00
-100.00
-50.00
0.00
50.00
100.00
150.00
200.00
0.0 1.0 2.0 3.0 4.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Bia
s
Soils
(+) Goal
(-) Goal
(+) Criteria
(-) Criteria
Soils Mean Fractional Bias
Combined Modeling Studies
0.00
50.00
100.00
150.00
200.00
0.0 5.0 10.0 15.0 20.0 25.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Err
or
PM2.5
Goal
Criteria
PM2.5 Mean Fractional Error
Combined Modeling Studies
-200.00
-150.00
-100.00
-50.00
0.00
50.00
100.00
150.00
200.00
0.0 5.0 10.0 15.0 20.0 25.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Bia
s
PM2.5
(+) Goal
(-) Goal
(+) Criteria
(-) Criteria
PM2.5 Mean Fractional Bias
Combined Modeling Studies
0.00
50.00
100.00
150.00
200.00
0.0 5.0 10.0 15.0 20.0 25.0 30.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Err
or
PM10
Goal
Criteria
PM10 Mean Fractional Error
Combined Modeling Studies
-200.00
-150.00
-100.00
-50.00
0.00
50.00
100.00
150.00
200.00
0.0 5.0 10.0 15.0 20.0 25.0 30.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Bia
s
PM10
(+) Goal
(-) Goal
(+) Criteria
(-) Criteria
PM10 Mean Fractional Bias
Combined Modeling Studies
0.00
50.00
100.00
150.00
200.00
0.0 2.0 4.0 6.0 8.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Err
or
CM
Goal
Criteria
CM Mean Fractional Error
Combined Modeling Studies
-200.00
-150.00
-100.00
-50.00
0.00
50.00
100.00
150.00
200.00
0.0 2.0 4.0 6.0 8.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Bia
s
CM
(+) Goal
(-) Goal
(+) Criteria
(-) Criteria
CM Mean Fractional Bias
SAMI: May 1995
0.00
50.00
100.00
150.00
200.00
0.0 5.0 10.0 15.0 20.0 25.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Err
or
Sulfate
Nitrate
Ammonium
Ammonium Bi
Organics
EC
Soils
PM2.5
PM10
CM
Goal
Criteria
SAMI Mean Fractional Error
SAMI: May 1995
-200.00
-150.00
-100.00
-50.00
0.00
50.00
100.00
150.00
200.00
0.0 5.0 10.0 15.0 20.0 25.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Bia
s
Sulfate
Nitrate
Ammonium
Ammonium Bi
Organics
EC
Soils
PM2.5
PM10
CM
(+) Goal
(-) Goal
(+) Criteria
(-) Criteria
SAMI Mean Fractional Bias
SAMI: 6 Episodes
0.00
50.00
100.00
150.00
200.00
0.0 5.0 10.0 15.0 20.0 25.0 30.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Err
or
Sulfate
Nitrate
Ammonium
Ammonium Bi
Organics
EC
Soils
PM2.5
PM10
CM
Goal
Criteria
SAMI Mean Fractional Error
SAMI: 6 Episodes
-200.00
-150.00
-100.00
-50.00
0.00
50.00
100.00
150.00
200.00
0.0 5.0 10.0 15.0 20.0 25.0 30.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Bia
s
Sulfate
Nitrate
Ammonium
Ammonium Bi
Organics
EC
Soils
PM2.5
PM10
CM
(+) Goal
(-) Goal
(+) Criteria
(-) Criteria
SAMI Mean Fractional Bias
EPA Mean Fractional Error
EPA: 4 Seasons (1996)
0.00
50.00
100.00
150.00
200.00
0.0 2.0 4.0 6.0 8.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Err
or
Sulfate
Nitrate
Organics
EC
Soils
PM2.5
Goal
Criteria
EPA Mean Fractional Bias
EPA: 4 Seasons (1996)
-200.00
-150.00
-100.00
-50.00
0.00
50.00
100.00
150.00
200.00
0.0 2.0 4.0 6.0 8.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Bia
s
Sulfate
Nitrate
Organics
EC
Soils
PM2.5
(+) Goal
(-) Goal
(+) Criteria
(-) Criteria
VISTAS: 3 Episodes (2 Grids)
0
50
100
150
200
0.0 5.0 10.0 15.0 20.0 25.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Err
or
Sulfate
Nitrate
Ammonium
Organics
EC
Soils
PM2.5
PM10
CM
Goal
Criteria
VISTAS Mean Fractional Error
VISTAS: 3 Episodes (2 Grids)
-200
-150
-100
-50
0
50
100
150
200
0.0 5.0 10.0 15.0 20.0 25.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Bia
s
Sulfate
Nitrate
Ammonium
Organics
EC
Soils
PM2.5
PM10
CM
(+) Goal
(-) Goal
(+) Criteria
(-) Criteria
VISTAS Mean Fractional Bias
MANE-VU Mean Fractional Error
MANE-VU: 2 Episodes (IMPROVE + SEARCH)
0.00
50.00
100.00
150.00
200.00
0.0 4.0 8.0 12.0 16.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Err
or
Sulfate
Nitrate
Ammonium
Organics
EC
PM2.5
Goal
Criteria
MANE-VU Mean Fractional Bias
MANE-VU: 2 Episodes (IMPROVE + SEARCH)
-200.00
-150.00
-100.00
-50.00
0.00
50.00
100.00
150.00
200.00
0.0 4.0 8.0 12.0 16.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Bia
s
Sulfate
Nitrate
Ammonium
Organics
EC
PM2.5
(+) Bias Goal
(-) Bias Goal
(+) Criteria
(-) Criteria
WRAP (309) Mean Fractional Error
WRAP: 12 Months (1996)
0
50
100
150
200
0.0 2.0 4.0 6.0 8.0 10.0 12.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Err
or
Sulfate
Nitrate
Ammonium
Organics
EC
Soils
PM2.5
PM10
CM
Goal
Criteria
WRAP (309) Mean Fractional Bias
WRAP: 12 Months (1996)
-200
-150
-100
-50
0
50
100
150
200
0.0 2.0 4.0 6.0 8.0 10.0 12.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Bia
s
Sulfate
Nitrate
Ammonium
Organics
EC
Soils
PM2.5
PM10
CM
(+) Goal
(-) Goal
(+) Criteria
(-) Criteria
WRAP (308) Mean Fractional Error
WRAP: 2 Seasons (WRAP & US Domains)
0.00
50.00
100.00
150.00
200.00
0.0 3.0 6.0 9.0 12.0 15.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Err
or
Sulfate
Nitrate
Ammonium
Organics
EC
Soils
PM2.5
PM10
CM
Goal
Criteria
WRAP (308) Mean Fractional Bias
WRAP: 2 Seasons (WRAP & US Domains)
-200.00
-150.00
-100.00
-50.00
0.00
50.00
100.00
150.00
200.00
0.0 3.0 6.0 9.0 12.0 15.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Bia
s
Sulfate
Nitrate
Ammonium
Organics
EC
Soils
PM2.5
PM10
CM
(+) Goal
(-) Goal
(+) Criteria
(-) Criteria
MRPO Mean Fractional Error
MRPO: 2 Episodes (CMAQ/CAMx/REMSAD)
0.00
50.00
100.00
150.00
200.00
0.0 2.0 4.0 6.0 8.0 10.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Err
or
Sulfate
Nitrate
Ammonium
Organics
EC
CM
Goal
Criteria
MRPO Mean Fractional Bias
MRPO: 2 Episodes (CMAQ/CAMx/REMSAD)
-200.00
-150.00
-100.00
-50.00
0.00
50.00
100.00
150.00
200.00
0.0 2.0 4.0 6.0 8.0 10.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Bia
s
Sulfate
Nitrate
Ammonium
Organics
EC
CM
(+) Bias Goal
(-) Bias Goal
(+) Criteria
(-) Criteria
EPRI Mean Fractional Error
EPRI: July 1999 (3 Models)
0
50
100
150
200
0.0 5.0 10.0 15.0 20.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Err
or
Sulfate
Nitrate
Ammonium
Organics
EC
Soils
PM2.5
PM10
CM
Goal
Criteria
EPRI Mean Fractional Bias
EPRI: July 1999 (3 Models)
-200
-150
-100
-500
50
100
150
200
0.0 5.0 10.0 15.0 20.0
Average Concentration (g/m3)
Me
an
Fra
cti
on
al
Bia
s
Sulfate
Nitrate
Ammonium
Organics
EC
Soils
PM2.5
PM10
CM
(+) Goal
(-) Goal
(+) Criteria
(-) Criteria
Concluding Remarks• Performance evaluation should be done on
an episode-by-episode basis or on a month-by-month basis for annual modeling.
• Recommended performance goals and criteria should be used to help identify areas that can be improved upon in future modeling – Failure to meet proposed performance criteria
should not necessarily prohibit the modeling from being used for regulatory purposes • Need to perform extended diagnostic evaluation
and sensitivity tests to address poor performance
Concluding Remarks (cont.)• As models mature, performance goals can be
made more restrictive by simply adjusting the coefficients in the performance goals and criteria equations (MFE and MFB)
• Performance goals and criteria for measurements with longer averaging times (e.g., weekly) should be more restrictive and those with a shorter averaging times (e.g., hourly) should be less restrictive.
• Discussion Questions:– Should EPA recommend PM model performance goals
and criteria in PM Modeling Guidance Document?– Is there a need for performance goals for gaseous
precursors and/or wet deposition species?