Long Range Transport 

Assessments for NAAQS and 

PSD Increment

Tyler Fox

EPA/OAQPS

11th Modeling Conference

Research Triangle Park, NC

August 12, 2015
Background

• The 2003 Guideline recommended CALPUFF 

as the preferred model for long-range transport 

(i.e., source receptor distances of 50 km to 

several hundred km) for primary criteria 

pollutants.

– Largely applied to address PSD increment.

• CALPUFF also considered on case-by-case 

basis as an alternative model subject to 

approval under Section 3.2 for near-field 

applications where complex winds or terrain 

warranted use of a puff model. 2
Long-Range Transport Assessments: Section 4

• EPA is proposing to remove CALPUFF as a preferred 

model for long-range transport and recommending its 

use as a screening technique along with other 

Lagrangian models for addressing PSD increment 

beyond 50 km from a new or modifying source.

• For NAAQS demonstrations: Near-field modeling is 

sufficient to address whether a source will cause of 

contribute to a NAAQS violation so EPA does not 

consider a LRT assessment beyond 50 km necessary for 

inert pollutants.

– CALPUFF or other Lagrangian models still available for use in 

near-field as an alternative model subject to approval under 

Section 3.2 3
Screening Approach for PSD Increment: Section 4

• EPA recognizes that LRT assessments may be necessary in 

limited situations for PSD increment so recommending 

screening approach.

• First step: Based on near-field application of appropriate 

model, determine significance of ambient impact at or about 

50 km from new/modifying source. 

• Second step: In consultation with EPA Regional Office, 

determine appropriate screening approach using CALPUFF 

or other Lagrangian model to determine significance of 

impacts at specific Class I areas of concern.

• For very limited situations where a cumulative impact 

assessment is necessary, then selection and use of an 

alternative model by applicant with appropriate approval 

under Section 3.2.2(e)

4
Addressing secondary pollutant impacts: Section 5

• Provides flexibility to the user community in 

estimating single-source secondary pollutant impacts 

with more appropriate modeling techniques, such as 

photochemical models, that address science issues 

of models like CALPUFF (see next slide).

• Based on IWAQM Phase 3 reports & published 

literature, EPA has placed emphasis on using 

chemical transport models or techniques that reflect 

state of science atmospheric chemistry.

• Preamble discusses future considerations for visibility 

modeling with full chemistry photochemical models in 

regulatory applications consistent with proposed 

changes for O3 and secondary PM2.5.

5
CALPUFF Modeling System has Several 

Limitations and Issues (USEPA, 2009)

• Chemical conversion algorithm in the regulatory 

versions is almost three decades old and inconsistent 

with our current knowledge on secondary PM2.5 

formation chemistry

• Recent chemistry algorithm updates still do not contain 

photochemical reactions, which are important to 

simulate secondary PM formation

• CALPUFF does not estimate ozone formation from 

single emissions sources

• CALPUFF sensitivity modeling using alternative 

CALMET meteorological inputs found wide variations 

in model estimates.

6
EPA Concerns about Management & 

Maintenance of CALPUFF

• Interactions between EPA and the model developer have been 

complicated by the changes in ownership and the related 

uncertainties of the development process as evident during the 

update to the VISTAS version of CALPUFF (summarized by EPA 

presentations at the 9th Modeling Conference).

• There have been a number of updates to the CALPUFF modeling 

system both in the EPA approved series 5 versions and in the 

model developer’s series 6 versions with a lag in the ability for EPA 

to adequately understand, review and approve changes largely due 

to the lack of an open development process.

• EPA believes that regulatory application of this modeling system 

has been unduly complicated by these changes in ownership and 

the related uncertainties of the development process for this 

modeling system. 7
Regional Haze Program

• In 2005, EPA issued guidelines for implementing 

BART requirements under Regional Haze Rule and 

recommended the use of CALPUFF for single-source 

assessments despite the lack of full evaluation for 

secondary pollutant formation.

• States could use alternative approaches, including 

photochemical model, if done in consultation with 

EPA Regional Office.

• Proposed changes do not affect the EPA’s 

recommendation in the 2005 BART Guidelines to use 

CALPUFF in the BART determination process. 8
Summary

• EPA is proposing to remove CALPUFF as a preferred model 

for long-range transport and recommending its use as a 

screening technique along with other Lagrangian models for 

addressing PSD increment beyond 50 km from a new or 

modifying source.

• For NAAQS demonstrations, EPA does not consider a LRT 

assessment necessary beyond 50 km for inert pollutants.

• No change in ability to use CALPUFF in the near-field as an 

alternative model if appropriately approved under Section 

3.2.

• Flexibility to the user community in estimating single-source 

secondary pollutant impacts with emphasis on chemical 

transport models that include Lagrangian puff models and 

Eulerian grid (e.g. photochemical transport) models. 9