APPENDIX C 

AERMOD/CALPUFF COMPARISON 
During the early stages of this project, a review of eligible dispersion models was 

conducted to inform the selection of a model for use in conducting source-receptor 

analyses. Because Sierra anticipated modeling only directly emitted PM10, the review 

was limited to the dispersion models that are “Preferred/Recommended” by the U.S. 

Environmental Protection Agency (EPA),1 they include: 

AERMOD Modeling System - A steady-state plume model that incorporates air 

dispersion based on planetary boundary layer turbulence structure and scaling 

concepts, including treatment of both surface and elevated sources, and both simple 

and complex terrain. 

CALPUFF Modeling System - A non-steady-state puff dispersion model that 

simulates the effects of time- and space-varying meteorological conditions on 

pollution transport, transformation, and removal. CALPUFF can be applied for longrange transport and for complex terrain. 

Other Models - Other dispersion models including BLP, CALINE3, 

CAL3QHC/CAL3QHCR, CTDMPLUS, and OCD. 

Since the Other Models category includes models designed to address unique modeling 

problems (e.g., industrial source plume rise and downwash, roadway intersections, 

complex terrain, etc.) the choice was between AERMOD and CALPUFF. The initial 

focus was on AERMOD as it was the choice of the Air Quality Planning Team and EPA 

guidance states that it is the “best state-of-the-practice Gaussian plume dispersion 

model.” During the course of the analysis, however, it became clear that AERMOD had 

difficulty representing concentrations under low wind stagnant conditions, particularly 

during night time hours. A review of EPA guidance2

 indicated that CALPUFF has 

advantages over plume models like AERMOD, in dealing with calm winds and stagnant 

conditions. Therefore, a comparison study was conducted between CALPUFF and 

AERMOD. Key differences between the models are as follows: 

AERMOD is a steady-state model which assumes that a plume disperses in the horizontal 

and vertical directions resulting in Gaussian (i.e., bell shaped) concentration distributions. 

It does not track the contribution or carryover of plumes from previous hours. 

Consequently, each hour a plume is dispersed in the direction of that hour’s meteorology 

in a straight-line trajectory. Given the estimates of particle residence time and settling 

velocity developed in this study it is expected that carryover from previous hours can be a 

significant contributor to concentrations during low wind conditions. Since AERMOD 

does not account for this effect it could explain why the model performs relatively poorly 

under certain conditions. 

 

CALPUFF is a multi-layer, multi-species, non steady-state Lagrangian puff dispersion 

model. Dispersion is simulated for discrete “puffs” of species emitted from modeled 

sources. The puffs are tracked until they have left the modeling domain while calculating 

dispersion, transformation and removal along the way. Since this process can take hours 

 

1

 http://www.epa.gov/scram001/dispersion_prefrec.htm 

2

 Joseph S. Scire, David G. Strimaitis, and Robert J. Yarmartino, 2000. A User’s Guide for the CALPUFF 

Dispersion model. 

C-1
under low wind conditions, an important effect of non steady-state dispersion is that the 

puff can change direction with changing winds, allowing a curved trajectory. The ability 

to track this change would appear to give CALPUFF an advantage relative to AERMOD 

in replicating concentrations under low wind conditions. 

Since there are two versions of CALPUFF available, full CALPUFF and CALPUFF-Lite, 

a decision had to be made on which version to employ in the comparison. Because the 

mechanisms described above are fully utilized in both versions of the model and the 

meteorological data required for full CALPUFF was not directly available, a decision 

was made to use CALPUFF-Lite in the comparison. An additional consideration was that 

most of the sources in the Salt River have a low release height and the modeling domain 

is relatively small therefore the vertical and spatial meteorology advantages of full 

CALPUFF would not significantly impact the accuracy of the estimates. 

The principal limitation of CALPUFF (both full and Lite) is the maximum number of 

volume sources that can be represented is 200. In comparison, the number of volume 

sources represented in AERMOD Salt River modeling domain was more than an order of 

magnitude higher. Since, roadway emissions were found to be in close proximity and 

significant contributors to each of the Durango Complex and West 43rd monitoring sites, 

the analysis was limited to a comparison between road source impacts. Because roughly 

3,000 volume sources are needed to represent the primary road network in the Salt River 

modeling domain, the comparison was further limited to the Durango Complex. The full 

domain of primary roads was used in the inventory for AERMOD (3,168 volume 

sources) and the roads surrounding the Durango Complex up to the limit of 200 volume 

sources was represented in CALPUFF-Lite (198 volume sources). 

A comparison of the predicted hourly concentrations from the two models is presented in 

Figure C-1 for December 5th and 6th, 2006. It shows that both models successfully 

predicted the rise in morning concentrations but significantly underestimated night time 

and early morning concentrations. Due to the lack of discernable differences between the 

results, the level of effort that would be required to reconfigure CALPUFF (either full or 

Lite) to represent additional sources and the time/resource constraints required to 

complete the study, a decision was made to proceed with AERMOD to complete the 

analysis. 

C-2
Figure C-1 

Comparison of AERMOD and CAPUFF-Lite Diurnal Distribution of Predicted and 

Measured Concentrations for Durango Complex 

December 5, 2006 

Time

 0:00 4:00 8:00 12:00 16:00 20:00 0:00

PM10 (ug/m3

)

0

100

200

300

400

500

600

700

0

100

200

300

400

500

600

700

PM10 24 hr average: 115 ug/m3

Calpuff Traffic impacts

AERMOD traffic impacts

December 6, 2006 

Time

 0:00 4:00 8:00 12:00 16:00 20:00 0:00

PM10 (ug/m3

)

0

100

200

300

400

500

600

0

100

200

300

400

500

600

PM10 24 hr Average: 170 ug/m3

Calpuff traffic impacts

AERMOD traffic impacts

C-3