WEIGHTED EMISSIONS POTENTIAL ANALYSIS 

Introduction 

The Weighted Emissions Potential analysis (WEP) was developed as a screening tool for states 

to decide which source regions have the potential to contribute to haze formation at specific 

Class I areas, based on both the 2002 and 2018 emissions inventories. This method does not 

produce highly accurate results because, unlike the air quality model and associated PSAT 

analysis, it does not account for chemistry and removal processes. Instead, it relies on an 

integration of gridded emissions data, back trajectory residence time data, a one-over-distance 

factor to approximate deposition, and a normalization of the final results. Residence time over 

an area is indicative of general flow patterns, but does not necessarily imply the area contributed 

significantly to haze at a given receptor. Therefore, users are cautioned to view the WEP as one 

piece of a larger, more comprehensive weight of evidence analysis. 

Emissions Data Inputs 

The emissions data used were the annual, 36km grid SMOKE-processed, model-ready emissions 

inventories provided by the WRAP Regional Modeling Center (RMC). The analysis was 

performed for nine (9) pollutants (maps were generated for all but the last three): 

 Sulfur oxides 

 Nitrogen oxides 

 Organic carbon 

 Elemental carbon 

 Fine particulate matter 

 Coarse particulate matter 

 Ammonia 

 Volatile organic carbon 

 Carbon monoxide. 

The following source categories for each pollutant were identified and preserved through the 

analysis: 

 Biogenic 

 Natural fire 

 Point 

 Area 

 WRAP oil and gas 

 Off-shore 

 On-road mobile 

 Off-road mobile 

 Road dust 

 Fugitive dust 

 Windblown dust 

 Anthropogenic fires 

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Residence Time Inputs 

The back trajectory residence times were provided by the WRAP Causes of Haze Assessment 

(COHA). The COHA project used NOAA’s HYSPLIT model to generate eight (8) back 

trajectories daily for each WRAP Class I area for the entire five-year baseline period (2000-04). 

The major model parameters selected for this analysis are presented in Table 1. From these 

individual trajectories, residence time fields were generated for one-degree latitude by onedegree longitude grid cells. Residence time analysis computes the amount of time (e.g., number 

of hours) or percent of time an air parcel is in a horizontal grid cell. Plotted on a map, residence 

time is shown as percent of total hours in each grid cell across the domain, thus allowing an 

interpretation of general air flow patterns for a given Class I area. The residence time fields for 

the 20% worst and best IMPROVE-monitored extinction days were selected for the WEP 

analysis to highlight the potential emissions sources during those specific periods. 

Table 1 

Back Trajectory Model Parameters Selected for WEP Analysis 

Model Parameter Value 

Trajectory duration 192 hours (8 days) backward in time 

Top of model domain 14,000 meters 

Vertical motion option used model data 

Receptor height 500 meters 

Meteorological Field EDAS and FNL (location dependent) 

Integration of Emissions and Residence Time Data 

The WEP analysis consisted of weighting the annual gridded emissions (by pollutant and source 

category) by the worst and best extinction days residence times for the five-year baseline period. 

To account for deposition along the trajectories, the result was further weighted by a one-overdistance factor, measured as the distance in km between the centroid of each emissions grid cell 

and the centroid of the grid cell containing the Class I area monitoring site under investigation. 

(The “home” grid cell of the monitoring site was weighted by one fourth of the 36km grid cell 

distance, or one-over-9km, to avoid a large response in that grid cell.) The resulting weighted 

emissions field was normalized by the highest grid cell to ease interpretation. 

An example series of maps illustrating the WEP analysis is presented in Figure 1. This example 

shows the annual emissions for NOx across the domain, the specific residence time pattern for 

the 20% worst monitored days at a Class I area, and the resulting weighted emissions map. Both 

the 2002 and 2018 cases are presented. Interpretation of the results should focus on which grid 

cells (or larger regions) have significant potential to affect the Class I area, and on changes 

between 2002 and 2018. 

An example of associated bar charts showing the estimated contribution by source category and 

region is presented in Figure 2. It is important to note that these charts show normalized values 

with no direct connection to original emissions values. Interpretation of the results should focus 

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on the relative contributions by each source category and region, and the changes between 2002 

and 2018. 

Caveats 

The WEP is not a rigorous, stand-alone analysis, but a simple, straightforward use of existing 

data. As such, there are several caveats to keep in mind when using WEP results as part of a 

comprehensive weight of evidence analysis: 

 This analysis does not take into account any emissions chemistry. 

 While actual emissions may vary considerably throughout the year, this analysis pairs 

up annual emissions data with 20% worst/best extinction days residence times – this 

is likely most problematic for carbon and dust emissions, which can be highly 

episodic. 

 Coarse particle and some fine particle dust emissions tend not to be transported long 

distances due to their large mass. 

 The WEP results are unitless numbers, normalized to the largest-valued grid cell. 

Effective use of these results requires an understanding of actual emissions values and 

their relative contribution to haze at a given Class I area. 
2002 NOx Emissions (TPY) Residence Time Field, Worst 20% Monitored 

Days (2000-2005) 

2002 NOx Emiss

Time and 

2018 NOx Emissions (TPY) Residence Time Field, Worst 20% Monitored 

Days (2000-2005) 

2018 NOx Emiss

Time and 

Figure 1. Example series of maps for WEP analysis at Bridger Wilderness, WY. From left to right: single-year 

map; five-year residence time map; emissions weighted by residence time, by one-over-distance, and normalized to row presents 2002 results, bottom row presents 2018 results. 

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Sources and Areas of Potential Nitrogen Oxide Emissions Influence

2000-2004 Baseline for Bridger Wilderness, WY

20% Worst Visibility Days

4.8

12.6

0.8

22.1

3.4 4.3

0.5 0.2

4.6

0.1

19.9

3.7

15.1

3.2

1.3 0.3 0.7

2.3

0

10

20

30

40

Arizona

California

Colorado

Idaho

Montana

Nevada

New Mexico

North Dakota

Oregon

South Dakota

Utah

Washington

Wyoming

Pacific Offshore

CENRAP

Eastern US

Mexico

Canada

Percent of Total Distance Weighted Emis x ResTime

Biogenic Natural Fire Point Area WRAP Area O&G Off-Shore 

On-Road Mobile Off-Road Mobile Road Dust Fugitive Dust WB Dust Anthro Fire 

Sources and Areas of Potential Nitrogen Oxide Emissions Influence

2018 Projections for Bridger Wilderness, WY

20% Worst Visibility Days

3.2

7.7

0.6

20.6

3.1 3.7

0.6 0.2

3.3

0.1

13.9

2.2

33.1

2.7

1.0 0.2 0.7

3.2

0

10

20

30

40

Arizona

California

Colorado

Idaho

Montana

Nevada

New Mexico

North Dakota

Oregon

South Dakota

Utah

Washington

Wyoming

Pacific Offshore

CENRAP

Eastern US

Mexico

Canada

Percent of Total Distance Weighted Emis x ResTime

Biogenic Natural Fire Point Area WRAP Area O&G Off-Shore 

On-Road Mobile Off-Road Mobile Road Dust Fugitive Dust WB Dust Anthro Fire 

Figure 2. Example source category bar charts based on WEP analysis at Bridger Wilderness, 

WY. Top chart presents 2002 results, bottom chart presents 2018 results. 

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