Modeling for PM2.5 Attainment 

Demonstrations in EPA Region 10

(WA, ID, OR, & AK)

An Assortment of Proposed Models

from CMAQ to Linear Rollback

Robert Kotchenruther, Ph.D.

EPA Region 10

June, 2011
Motivation for this presentation

• In the recent past, Region 10 has had relatively few NAAs. 

• But now, with the strengthening of the PM2.5 standard in 2006, we 

currently we have five PM2.5 nonattainment areas.

 All are based on violations of the daily 24-hr 35 ug/m3 standard (no 

violations of the 2006 annual standard).

 Excluding summertime exceptional events (wildfire, dust), all exceedances

are in the wintertime and related to air stagnation events.

 Many have significant contributions from primary PM2.5 emissions

• For attainment demonstration models, our states and locals have 

proposed using a wide range of tools from modified forms of 

WRF/CMAQ to more simplified models (e.g., linear rollback). 

• One goal of this presentation is to inform other Feds, states, and locals 

about the various modeling approaches being attempted in Region 10 

by our states and locals.

• A second goal is to explain EPA R10’s approach and how we’re working 

with our states to developing attainment demonstration models in each 

nonattainment area. 
What does EPA Guidance and Regulations say 

about attainment demonstration models for the 

PM2.5 NAAQS? 

Both the OAQPS 2007 modeling guidance and the PM2.5 modeling 

sections of Appendix W say similar things:

• If you need to control the formation of secondary PM2.5 for your 

attainment demonstration, states should use a photochemical 

grid model (CMAQ/CAMx).

• However, if the PM2.5 problem can be addressed solely by 

controlling primary PM2.5, a more simplified modeling approach 

or mix of modeling approaches could be appropriate.

(caveat: state needs to show that any potential increases in secondary 

PM are not likely to cause a failure to reach attainment)

So, both offer at least the possibility that models simpler than regional 

scale CMAQ/CAMx could be appropriate for PM2.5 attainment 

demonstrations.
What is EPA Region 10’s approach to the 

question of what form of attainment 

demonstration model is appropriate?

For areas with significant secondary PM2.5 impacts or complexity:

the state should attempt photochemical modeling (CMAQ) as a starting 

point for the attainment demonstration.

But there are caveats:

• All our NAA’s are dominated by wintertime, air stagnation based 

exceedances, which are very difficult for photochemical models to 

accurately simulate.

• So, while CMAQ may be the starting point, states need to have fallback 

options if the photochemical model performance proves inadequate to 

demonstrate attainment with confidence.
EPA Region 10’s approach (cont.)

For areas with significant primary PM2.5 impacts:

states and locals may attempt the attainment demonstration with a more 

simplified modeling approach (i.e., simpler than CMAQ/CAMx)

And there are caveats here too:

• If using a simplified approach, the state will need to make conservative 

assumptions regarding the impacts of such things as:

 background PM2.5

 secondary PM2.5

 inhomogeneity of sources in the NAA (point, area, & mobile) 

• and these conservative assumptions may cause the state to be unable 

to make a successful attainment demonstration. If so, then the state 

may need to layering in added complexity using more sophisticated 

modeling tools to better constrain uncertainties. (e.g., chemical box 

modeling, point source modeling, etc.). 

• Also, this may mean that the final form of the attainment 

demonstration model is not the same as initially proposed ... it may 

evolve during the SIP development process.
Where are R10’s PM2.5 NAA’s?

Tacoma, WA

pop 200,000

DV (08-10) = 37 ug/m3

Oakridge, OR

pop 3,200

DV = 37 ug/m3

Klamath Falls, OR

pop 20,000

DV = 45 ug/m3

Cache Valley, UT/ID

pop 50,000 (Logan)

DV = 43 ug/m3

Fairbanks, AK

pop 35,000

DV = 52 ug/m3
Speciation Key

OMC (1.4)

EC

NO3

SO4

NH4

Other

Fairbanks, AK

DV = 52 ug/m3

Klamath Falls, OR

DV = 45 ug/m3

Oakridge, OR

DV = 37 ug/m3 Logan, UT

DV = 43 ug/m3

R10’s PM2.5 Chemical Speciation

• 2008-2010 Design Values (24hr 35ug standard)

• Speciation pies (06-10 winter data >25 ug/m3)

Tacoma, WA

DV = 37 ug/m3
Speciation Key

OMC (1.4)

EC

NO3

SO4

NH4

Other

Fairbanks, AK

DV = 52 ug/m3

Klamath Falls, OR

DV = 45 ug/m3

Oakridge, OR

DV = 37 ug/m3

Logan, UT

DV = 43 ug/m3

Tacoma, WA

DV = 37 ug/m3

Which Approach for Attainment Demonstration Modeling?

2 Areas 

starting 

with 

WRF/CMAQ

3 Areas 

starting 

with 

simplified 

approaches
Areas attempting simpler models: Tacoma, WA

Tacoma, WA

DV = 37 ug/m3 Speciation Key

OMC (1.4)

EC

NO3

SO4

NH4

Other

Airshed:

• A major urban area, population 200,000 [# wood burning devices = 65,000(!)]

• State and local believes wood burning devices are driving nonattainment 

• Relatively minor contribution from inorganic secondary PM (SO4, NO3)

• All PM2.5 exceedances are wintertime, air stagnation events

Potential complexities

• source mix – home heating, major highway, port of Tacoma, light industry. 

• possible airshed emissions 

inhomogeneities

• How to deal with 65,000 wood 

burning devices?

• are secondary organics 

important? 

Tacoma, WA NAA

FRM Monitor

Port of Tacoma

Major Highway 
Proposed form of attainment model for Tacoma, WA 

Tacoma, WA

DV = 37 ug/m3 Speciation Key

OMC (1.4)

EC

NO3

SO4

NH4

Other

• WA is proposing linear rollback of primary PM2.5 emissions

• Their rollback model will attempt to integrate 1 km gridded emissions, with 

emissions impacts distance weighted and tuned to the monitoring network.

• Monitoring network includes 2 speciation monitors and ~ 8 PM2.5 monitors.

• Modeling will include conservative assumptions for background and secondary 

PM2.5

NAA Boundary

PM2.5 Speciation

PM2.5 Mass

• No attainment modeling results yet 

(gridded emissions pending)

• Supplementary modeling includes 

PMF analysis on both speciation 

monitors.
More Details – Tacoma, WA – PMF Results Factor Wood Smoke OP rich Secondary Nitrate Fe rich Secondary Sulfate Gasoline Vehicles Sea Salt Fugitive Dust Aged Sea Salt SO4 rich Diesel I--

0

10

20

30

40

50

60

70 Percent Factor Contributions

Heating Season (Oct-Mar)

PM2.5 mass > 15 ug/m3

 Alexander Ave.

 South L St.

% Contribution

Port location

Violating FRM 

(Residential)
Other areas attempting simpler models:

Klamath Falls, OR & Oakridge, OR

Klamath Falls airshed and potential complexities:

• Smaller town, eastern edge of Cascade range (pop 20,000)

• Some exceedance days have ~20% nitrate

• Source mix – home heating, mobile, a few small point sources

• PMF analyses indicates ~70% PM2.5 from biomass burning.

• Current focus is on using rollback as the attainment 

demonstration model, making conservative assumptions about 

NO3.

• State is working with Portland State University to conduct box 

modeling to evaluate SOA production to apportion primary vs. 

secondary OC. (box model uses latest GEOS Chem mechanism, 

which incorporates OC, SVOC, VOC volatility basis set concepts) 

Klamath Falls, OR

DV = 45 ug/m3

Oakridge, OR

DV = 37 ug/m3

Speciation Key

OMC (1.4)

EC

NO3

SO4

NH4

Other

Oakridge airshed and potential complexities:

• Small mountain town (pop 3,200)

• Virtually no inorganic secondary PM on exceedance days

• Focus is solely on reducing home heating sources (wood 

stove changeouts) 
Fairbanks, AK

DV = 52 ug/m3

Areas attempting WRF/CMAQ: Fairbanks, AK

Speciation Key

OMC (1.4)

EC

NO3

SO4

NH4

Other

Complexities:

• Significant sulfate (how oxidized in winter, dark, 0 to -40C?)

• Ice fog – heterogeneous chemistry?

• Monster inversions (delta-T, >40C/100m, e.g. -50C -> -10C)

• Multiple sources – home heating, mobile, EGU’s (coal) within city 

emitting at different altitudes in inversion 

• Using a modified form of WRF/CMAQ (no CMAQ results yet).
More Details - Fairbanks, AK

• expanded vertical resolution near 

surface 

(EPA & Fairbanks worked with Penn 

State to modify WRF with lowest 5 

levels at 4m, 8m, 12.5m, 23m, & 46m).

• Also required modification of nudging 

code and model/obs evaluation code.

• Significant improvement in met 

performance 

• Also requires emissions model 

modification to put some ‘surface’ 

emissions above the first layer.

• high horizontal resolution (4 & 1.3 km)

WRF/CMAQ modifications

T, U, V at 1⁄2 levels

2 m

6 m

10 m

WRF/CMAQ 

Vertical Levels
More Details - Fairbanks, AK

In addition to WRF/CMAQ (or as a fallback if performance is 

poor) Fairbanks is developing a Principal Component 

Analysis (PCA) based statistical model that could be used for 

attainment demonstrations. 

This PCA model uses: 

• meteorology and emissions as independent variables

• 24-hr PM2.5 as the dependant variable

• Thus, emissions can be modified with future year scenarios 

to predict 24-hr PM2.5, and develop an RRF

(the PCA model is essentially a sophisticated form of 

rollback)

Additional modeling - ‘fallback’ option
Logan, UT

DV = 43 ug/m3

Areas attempting CMAQ: Cache Valley (UT/ID) 

Speciation Key

OMC (1.4)

EC

NO3

SO4

NH4

Other

Complexities:

• NAA in 2 States, 2 EPA Regions

• Speciation dominated by sec. NH4NO3

• Sources dominated by mobile (NOx), 

agriculture (NH3)

• In CMAQ - difficulty capturing winter 

stagnation, multi-day PM buildup (see 

earlier Utah modeling presentation)

• Likely will use modified WRF/CMAQ with 

no vertical advection and 90% reduction 

in eddy diffusivities to capture NH4NO3 

buildup.

Idaho

Utah 
Thank you!

Questions?

Robert Kotchenruther

Kotchenruther.Robert@epa.gov

EPA Region 10

206-553-6218
From the 2007 modeling guidance:

“States should use a photochemical grid model to simulate the effects of strategies 

to reduce ... the secondary components of particulate matter (i.e., mass associated 

with SO4, NO3 and secondary OC).”

“Based on its conceptual description of a PM2.5 nonattainment problem, a State 

could conclude that the PM2.5 problem can be addressed solely by reducing primary 

components of measured PM2.5.”

“If this is the case, the State may not need to use a photochemical grid model in 

their attainment demonstration if it can present convincing qualitative arguments 

that an increase in the secondary components of PM will not cause [a failure to 

demonstrate attainment].”

From 40 CFR Part 51, Appendix W:

“Control agencies with jurisdiction over areas with secondary PM–2.5 problems are 

encouraged to use models which integrate chemical and physical processes (e.g., 

Models-3/CMAQ).”

“Primary components can be simulated using less resource-intensive techniques. 

Suitability of a modeling approach or mix of modeling approaches for a given 

application requires technical judgment.”