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### WEIGHTED EMISSIONS POTENTIAL ANALYSIS **Introduction** The Weighted Emissions Potential analy sis (WEP) was developed as a screening tool for states to decide which source regions have the potential to contribute to haze form ation at specific Class I areas , b ased on both the 2002 and 2018 emissions inventories
. This method does not produce highl y accurate results because, unlike the air qualit y model and associated PSAT anal ys is, it does not account for chemistry a nd removal processes. In stead, 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 necessaril y imply t he area contributed significantl y 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 anal ys is. **Emissions Data Inputs** The emissions data used wer e the annual, 36km grid SMOKE - processed, model
- read y emissions inventories provided by t he WRAP Regional Modeling Center (RMC)
. The anal ys is was performed for nine ( 9 ) pollutants (maps were generated for a ll 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 an d preserved through the anal ys is:

 Biogenic  Natural fire  Point  Area  WRAP oil and gas  Off
- shore  On - road mobile  Off
- road mobile  Road dust  Fugitive dust  Windblown dust  Anthropogenic fires2

**Residence Time Inputs** The back trajectory

r esidence times were provide d by t he WRAP Causes of Haze Assessment (COHA)
. The COHA project used NOAA’s HYSPLIT model to generate eight (8) back trajectories dail y for each WRAP Class I area for the entire five
- year baseline period (2 000 - 04). The major model parameters selected for this analy sis are presented in Table 1. F rom these individual trajectories, residence time fields were generated for one - degree latitude by o ne degree longitude grid cells
. R esidence time analysis compute s the amount of time (e.g., number of hours) or percent of time an air parcel is in a horizontal grid cell. P lotted 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 gi ven Class I area. The residence time fields for the 20% worst and best IM PROVE - monitored extinction day s were selected for the WEP anal ys is to highlight the potential emissions sources during those specific periods. Table 1 Ba ck Trajectory

Model Parameters Selected for WEP Analy sis Model Parameter Value Trajectory

duration 192 hours (8 day s) 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 analy sis consist ed of weighting the annual gridded emissions (by p ollutant and source category ) by

t he worst and best extinction day s residence times for t he five
- year baseline period. To account for deposition alon g the trajectories, the result wa s further weighted by a o ne - over distance factor, measured as the distance in km between the centroid of each emissions grid cell and the centroid of the grid cel l containing the Class I area monitoring site under investigation. ( The “home” grid cell of the monitoring site wa s wei ghted by o ne fourth of the 36km grid cell distance, or one - over - 9km, to avoid a large response in that grid cell.) T he resulting weight ed emissions field wa s normalized by the highest grid cell to ease interpretation. An example series of maps illustrating the WEP analy sis 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 day s at a Class I area, and the resulting weighted emissions map. Both the 2002 and 2018 cases are presented. I nterpretation of the results should focus on which grid cells (or larger regions) have significant po ten tial 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 a nd region is presented in Figure 2. I t is important to note that these charts show normal ized values with no direct connection to original emissions values. I nterpretation of the results should focus3

on the relative contributions by e ach source category

a nd region, and the changes between 2002 and 2018. **Caveats** The WEP is not a rigorous, st and
- alone anal ys is , but a simple, straightforward use of existing data. A s such, there are several caveats to keep in mind when using WEP results as part of a comprehensive weight of evidence anal ys is:

 This analy sis does not take into account an y emissio ns chemistry
.  While actual emissions may vary c onsiderabl y throughout the ye ar, this analy sis pairs up annual emissions data with 20% worst/best extinction day s residence times

– this is likely m ost problemati c 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 re quires an understanding of actual emissions values and their relative contribution to haze at a given Class I area.4

2002 NOx Emissions (TPY) Residence Time Field , Worst 20% Monitored Day s (2000
- 2005) 2002 NOx Emissions Weighted b y Residence Ti me and One - Over - Distance

2018 NOx Emissions (TPY) Residence Time Field, Worst 20% Monitored Day s (2000
- 2005) 2018 NOx Emissions Weighted b y Residence Time and One
- Over - Distance Figure 1. E xample series of maps for WEP analysis at Bridger Wildern ess, WY. F rom left to right: s ingle
- ye ar annual emissions density map; five
- ye ar residence time map; emissions weighted by residence time, by

one - over - distance, and normalized to the highest grid cell. Top row presents 2002 results, bottom row presents 2018 results.5 **Sources and A reas of Potential Nitrogen Oxide Emissions Influence** 

**2000-2004 Baseline for Bridger Wilderness, WY**

**20% Worst Visibility**

**Day s** 

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 Pac ific Offshore CENRAP

Eastern US Mexico Canada**P e** **r c** **e n** **t o** **f T** **o t** **a l** **D i** **s t** **a n** **c e** **W e** **i g** **h t** **e d** **E m** **i s** **x R** **e s** **T i** **m e**  Biogenic Natural Fire Point Area WR AP Area O&G Off-Shore On-Road Mobile Off-Road Mobile Road Dust Fugitive Dust WB Dust Anthro Fire

**Sources and A reas of Potential Nitrogen Oxide Emissions Influence** 

**2018 Projections for Bridger Wilderness, WY**

**20% Worst Visibility**

**Day s** 

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 Pac ific Offshore CENRAP

Eastern US Mexico Canada**P e** **r c** **e n** **t o** **f T** **o t** **a l** **D i** **s t** **a n** **c e** **W e** **i g** **h t** **e d** **E m** **i s** **x R** **e s** **T i** **m e**  Biogenic Natural Fire Point Area WR AP Area O&G Off-Shore On-Road Mobile Off-Road Mobile Road Dust Fugitive Dust WB Dust Anthro Fire

Figure 2. E xample source category b ar charts based on WEP anal ys is at Bridger Wilderness, WY. Top chart presents 2002 results, bottom chart presents 2018 results.