Document ID: EPA-R08-OAR-2011-0851-0013
Agency: epa
Document Type: Supporting & Related Material
Title: 
Posted Date: 2012-04-20T04:00Z

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   HYPERLINK
"http://pah.cert.ucr.edu/aqm/308/"  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

Residence Time Inputs

The back trajectory residence times were provided by the WRAP  
HYPERLINK "http://coha.dri.edu/index.html"  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 one-degree 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-over-distance 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 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)

2018 NOx Emissions (TPY)	Residence Time Field, Worst 20% Monitored Days
(2000-2005)	2018 NOx Emissions Weighted by Residence Time and
One-Over-Distance

Figure 1.  Example series of maps for WEP analysis at Bridger
Wilderness, WY.  From left to right:  single-year annual emissions
density map; five-year 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.

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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