Hawaii WEP/AOI Analysis

January 26, 2021

An Area of Influence (AOI) and Weighted Emissions Potential (WEP) analysis was performed for Hawaii to identify the anthropogenic sources of emissions within and nearby the state that had the potential to contribute the most to visibility impairment on the volcano-adjusted Most Impaired Days (VADJ MID) from 2014 to 2018. Both Class I Areas (CIA) in the state were included: Halaekala NP and Hawaii Volcanoes NP. The analysis methods and available data products are described below.

Data and Methods

IMPROVE Data for Most Impaired Days

The MID (impairment group 90) at each IMPROVE site during 2014 to 2018 were downloaded from the TSS Ambient Data Analysis - Administrative Express Tools webpage (EPA, 2018; 2020a,b) using the “M.I.D. (RHR3) and Clearest Days (RHR2) only” tool in the Raw Data download section. The MID used in the analysis include volcanic adjustments (VADJ) performed by the EPA. The VADJ MID used the same screening method to eliminate high sulfate days that are assumed to be due to volcano emissions as used to eliminate high fire and dust days in the standard IMPROVE MID. The VADJ datasets for Haleakala (HALE1) and Hawaii Volcanoes (HAVO1) IMPROVE sites were used for the analysis of Halaekala NP and Hawaii Volcanoes NP, respectively. Note that the HALE1 monitor stopped operating in 2012, therefore the analysis is based on the “hybrid” HALE1_RHTS for which the EPA were determined the VADJ MID using combined data from both the HALE1 and Haleakala Crater (HACR1) monitors. The HACR1 monitor location was used as the initiation point for the back-trajectories as it was the active monitor in the 2014-2018 analysis period.

The extinction due to ammonium sulfate (Amm_SO4), ammonium nitrate (Amm_NO3), organic aerosol (OA) and elemental carbon (EC) mass on each MID was extracted for use in the extinction weighted residence time analysis.

HYSPLIT Back Trajectory Modeling

The Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model (Stein et al., 2015; Rolph et al., 2017) was used to calculate 72-hour back trajectories arriving on each of the MID at four times per day (6:00, 12:00, 18:00, 24:00 local standard time) and at four heights above the ground (100 m, 200 m, 500 m and 1,000 m). The archived NAM hybrid sigma-pressure gridded (NAMS) meteorological data for Hawaii was downloaded for 2014 to 2018 from the NOAA Air Resources Laboratory FTP server for use in HYSPLIT model. The NAMS data is output hourly and covers Hawaii at 2.5 km resolution. Note that there was a change in the Hawaii NAMS domain effective March 21, 2017 which is detailed in the NAMS readme document.

Analysis Domain

The analysis was performed using the 3-kilometer (km) domain of EPA’s Regional Haze Modeling platform for Hawaii aggregated to 9-km resolution. Trajectory endpoints located outside the EPA 3-km domain were dropped from the analysis. The analysis domain and IMPROVE sites are shown in Figure 1.

Figure 1. Hawaii IMPROVE monitors and the EPA 3-kilometer modeling domain used for the AOI and WEP analysis

Emissions

The WEP analysis was performed using both gridded emissions from the EPA’s 2016 Hawaii modeling platform and 2017 and 2028 facility-level emissions data provided by Hawaii’s Clean Air Branch (CAB). CAB provided point source emissions of NOx and SO2 from the 2017 NEI along with projected 2028 emission reductions resulting from unit shutdowns. The 2028 emission reductions were applied to the 2017 NEI Data to get 2028 facility-level emissions.

The EPA’s gridded emissions for 2016 were aggregated into the following source sectors for the gridded WEP analysis:

• TOTAL_ANTHRO – All anthropogenic emissions
• PT_EGU – Electric generating unit emissions
• PT_NON-EGU – Point source emissions from industrial activities
• NON-POINT – Low-level area source emissions including non-point, agricultural, residential wood combustion, and fugitive dust emissions
• ON-ROAD – On-road mobile source emissions
• NON-ROAD – Off highway mobile source emissions including non-road, airport, commercial marine (C1, C2, and C3), and rail sources

For the MID analysis, EPA 2016 gridded emissions of nitrogen oxides (NOx), sulfur oxides (SOx), primary organic aerosol (POA), and primary elemental carbon (PEC) were used for the analysis of Amm_NO3, Amm_SO4, OC, and EC, respectively. The analysis of the facility-level WEP on the MID (RANK_POINT) used point source emissions of NOx and SO₂.

WEP/AOI Products

The WEP/AOI analysis products are provided for the 100 m and 1000 m trajectory heights and for a combined analysis in which data from all four trajectory heights are aggregated (All). The products include:

• Plots of residence time (RT), extinction weighted residence time (EWRT), and WEP for each CIA
• Plots of the gridded 2016 EPA emissions used in the WEP analysis for each of the source sectors described above (EMISSIONS)
• Excel spreadsheets of facility-level 2014 and 2017 precursor emissions and the corresponding WEP at each CIA (RANK_POINT)

The RT, EWRT, WEP, and EMISSIONS plots are provided at the following link using the directory structure shown below with HAVO1_VADJ as an example.

AOI, WEP, and Gridded Emissions Plots

Shapefiles of the RT, EWRT, and WEP results by source sector are provided at the link below (WEP_SHP). In each CIA subdirectory there is a zip file for each source sector containing the ESRI shapefile (.shp) and related files (.cpg, .dbf, .prj, .shx). The RT, EWRT, and WEP results data are available within the attribute tables of the shapefiles provided which can be accessed in ArcMap or ArcCatalog. The attribute tables can be exported to csv, then opened in Excel. Files are provided for the 100m and 1000m trajectory heights and for a combined analysis in which data from all four trajectory heights are aggregated (All). The directory structure for the WEP_SHP is shown below using the HAVO1_VADJ results aggregated across all trajectory heights as an example.

WEP/AOI Shapefiles

The facility-level RANK_POINT results for NOx and SO2 are provided at the link below. The directory structure is shown below and the files for HAVO1_VADJ are shown as an example. The results are provided in Microsoft Excel spreadsheets with the 2017 and 2028 results provided in separate tabs.

RANK_POINT Spreadsheets

Residence Time (RT):  The RT folder contains plots showing the AOI that back trajectories of air parcels traveling from a given location arrived at the IMPROVE monitor on the 2014-2018 MID (with volcanic adjustment). HYSPLIT 72-hour back trajectories are calculated to arrive on each of the MID four times a day (6:00, 12:00, 18:00, 24:00 local standard time) and at four heights above the ground (100 m, 200 m, 500 m and 1,000 m). Plots are provided for the 100m and 1000m heights and for a combined analysis in which data from all trajectory heights are aggregated (All). RT plots for Hawaii Volcano National Park (HAVO1_VADJ) are provided as an example in Figure 1.

Figure 2. Residence Time (RT) analysis for Hawaii Volcano National Park (HAVO1_VADJ) Class I area IMPROVE monitoring site and back trajectories that arrive at the site on the Most Impaired Days for each year 2014-2018 at 100 m (left), 1000 m (middle) and All (right) heights above ground.

Extinction Weighted Residence Time (EWRT):  EWRT are calculated by weighting the HYSPLIT trajectories by the monitored extinction at the IMPROVE site on each MID. Each Class I area EWRT folder contains 12 plots showing the extinction weighted for sulfate (Amm_SO4), nitrate (Amm_NO3), Organic Aerosol (OA) and Elemental Carbon (EC) at three heights (100 m, 1000 m, All) (12 = 3 x 4). Figure 2 shows the EWRT plots for sulfate and nitrate at HAVO1_VADJ using the aggregated trajectory height analysis (All) as an example.

Figure 3. Extinction Weighted Residence Time (EWRT) analysis for ammonium sulfate (Amm_SO4) and ammonium nitrate (Amm_NO3) at HAVO1_VADJ IMPROVE monitor for the Most Impaired Days during 2014-2018 aggregated across all trajectory heights.

Weighted Emissions Potential (WEP):  WEP is obtained by overlaying the EWRT results with the EPA 2016 emissions of light extinction precursors (e.g., NOx emissions for ammonium nitrate extinction) in each grid cell divided by the distance of the source to the IMPROVE monitor. The gridded WEP values for each source sector are then normalized by the sum of the WEP for the total anthropogenic emissions (TOTAL_ANTHRO) across all grid cells. Using the sum of the total anthropogenic values as a common denominator allows for the WEP results to be compared across source sectors. The WEP folder for the MID contains four subfolders corresponding to the precursor emissions for four major components of light extinction: EC (Elemental Carbon), NOx (Ammonium Nitrate), POA (Organic Aerosol) and SOx (Ammonium Sulfate). Each precursor species subfolder contains 18 plots in which the EWRT at three heights above ground are overlaid with 2016 emissions from 6 gridded Source Sectors (18 = 3 x 6). The source sectors are described in the Emissions section above, and example plots for HAVO1_VADJ are shown in Figure 3. The dark green and light green isopleths in the WEP plots correspond to the, respectively, 0.5 and 0.1 percent frequency from the corresponding EWRT.

Figure 4. Weighted Emissions Potential (WEP) analysis for ammonium nitrate extinction at the Hawaii Volcanoes National Park (HAVO1_VADJ) Class I area IMPROVE monitor on the Most Impaired Days during each year of 2014-2018 for NOx emissions from four Source Sectors: (1) total anthropogenic (top left), (2) Non-Road Mobile (top right), (3) EGU point source (bottom left) and (4) On-road mobile sources (bottom right). Results are aggregated across all trajectory heights.

As described above, shapefiles of the WEP results (along with RT and EWRT) for each CIA and source sector are also provided. The columns of the attribute tables of these files are described in Table 1 below.

Rank Point (RANK_POINT):  The RANK_POINT spreadsheets consist of facility level emissions for NOx or SO2 sources overlaid with the corresponding EWRT for Amm_NO3 or Amm_SO4, respectively, for 3 trajectory height scenarios (100 m, 1000 m and All). For each scenario there is a 2017 emissions scenario and a 2028 emissions scenario as separate tabs in the spreadsheet.

These results can be used to assess the potential contributions of specific facilities to visibility impairment at each Class I Area in various ways. We recommend sorting the facilities by the WEP metrics for Amm_NO3 extinction (WEP_NO3) and Amm_SO4 extinction (WEP_SO4). These metrics account the air parcel trajectories and extinction on the MID, facility precursor emissions, and facility distance from the IMPROVE monitor. The columns of the RANK_POINT sheets are described in Table 2, and the other metrics provided in the RANK_POINT files are described below.

The QoverD_NOX and QoverD_SO2 provide the simple emissions over distance metric (Q/D) using the facility-level NOx and SO2 emissions, respectively. While the Q/D metric can be used to screen the potential contributions of sources to visibility impairment at a given Class I area, it is simply based on the emission rate and distance from the IMPROVE monitor and does not account for the air parcel trajectories on the MID.

The EWRTxQ and WEP metrics for NOx and SO2 account for the air parcel trajectories on MID as they are calculated by multiplying the EWRT of the grid cell of the facility with the facility’s emission rate (Q) and Q/D, respectively.

Table 3 shows a subset of the results for NOx sources at HAVO_VADJ aggregated across all trajectory heights. The sources are ranked by [WEP_NO3] (last column) for ammonium nitrate, and the top 10 facilities whose NOx emissions potentially contribute to visibility impairment on the 2014-2018 IMPROVE MID (with volcanic adjustment) at HAVO_VADJ are shown.