Appalachian Basin

DEP to Clean Air Council: Compressor Station Emissions Model Doesn’t Reflect Actual Data

The Pennsylvania Department of Environmental Protection (DEP) recently responded to a report put out by the Clean Air Council (CAC) of Philadelphia, which had alleged high emissions from the Barto compressor station in Penn Township, Lycoming County. Real data, however, show emission levels far below federal requirements.

In January, the Heinz-funded and Philadelphia-based Clean Air Council released a modeling report of projected emissions from the Barto compressor station, located about three miles up the road from my house. The model compared one hour of NO2 emissions with the one hour National Ambient Air Quality Standard (NAAQS). According to the CAC, which has been traveling from Philadelphia on a regular basis to oppose compressor stations across the Marcellus, the projections are way over the federal standard. DEP responded to this accusation with a letter explaining in detail the reality of the emissions level at the Barto station and others in the region: levels continue to be well below the NAAQS and there is continued monitoring to ensure this remains the case.

DEP is in the process of completing an analysis of the modeling done by CAC, but they are awaiting access from CAC to the data figures used in the modeling. In the meantime, they sent a letter to the organization explaining some basic flaws in the project and informing them the state will conduct week-long monitoring at the Barto site as a part of their analysis. You can read the entire DEP letter here, but below are some key excerpts from it (emphasis added):

DEP is surprised by the results of this modeling analysis,  because existing ambient NO2 monitors located in the vicinity of the Barto Compressor Station and other compressor stations in the Commonwealth have not measured any exceedances of the 1-hour NO2 NAAQS.


Barto Compressor Station in Penn Township

Vincent Brisini — the DEP official who penned the letter — goes on to explain the current monitoring, including the only exception areas not in compliance with the NAAQS: urban Philadelphia, where the CAC is located, and Pittsburgh.

With the exception of the large urban areas of Pittsburgh and Philadelphia, DEP is monitoring NO2 concentrations that demonstrate compliance with the NAAQS standard for NO2 at every other monitoring site within the Commonwealth. It is noteworthy that the NO2 monitor site in Tioga County has four compressor stations located within five miles of the site and these facilities have combined NOx emissions over 100 TPY (expressed as NO2) and that monitor is measuring concentrations well below the NO2 NAAQS.

Additional the NO2 monitor sited as part of DEP’s long-term Marcellus study is also measuring concentrations well below the NO2 NAAQS. While the Houston Processing Plant is one of the very few natural gas facilities in the state that utilizes electric compressor engines, the facility reported 47 tons of NOx emissions (expressed as NO2) for 2012. Actual NO2 monitoring data collected near other compressor facilities in the state does not support the report modeled results.

DEP then identifies some preliminary issues with the report modeled by CAC, which could be the cause of such a large discrepancy between the CAC projections and the reality of what’s occurring with air quality in near the Barto station.

The Model Used in the Analysis is Outdated and Overestimates

The CAC NO2 analysis utilizes the U.S. Environmental Protection Agency (EPA)/American Meteorological Society Regulatory Model (AERMOD), version 12060 (AERMOD 12060). DEP believes the updated version AERMOD 12345, which was released December 17, 2012, is a more refined version. AERMOD 12345 enables the use of revised meteorological data.

EPA acknowledges that using the older version of AERMOD likely overestimates the NO2 concentrations at very low wind speeds. It should be recognized the AERMOD 12060 exhibits concentrations which may not reflect actual concentrations due to limitations in the ability to model these very low concentrations and very short duration averaging times accurately.

However, regardless of the version used the model results likely over predict ambient NO2 concentrations.

The Site Location Used is Inaccurate

Based on the data contained in the report, DEP believes the CAC has modeled the plant at a location approximately 1 to 2 miles north of its actual location. This will have a significant impact on the modeling results.

Stack Parameter Values Used are Inaccurate

DEP is concerned that the CAC analysis used inaccurate stack parameter data related to stack height, diameter, temperature and exit velocity.

DEP believes that the use of inaccurate parameters such as low exit velocities will impact the modeled momentum of the plume exhausted from the stack. As you are aware , momentum is important in determining how much lift a plume achieves in the model. Higher momentum will lead to higher plume height. That situation will result in the plume impacting terrain at different elevations that are currently represented by modeling results.

Building Downwash is Not Considered in Report

The potential effect of building downwash does not appear to be considered in the CAC analysis. DEP believes this effect must be taken into account because building downwash can lead to higher impacts being modeled on the facility property. As you are aware, these onsite locations are not considered ambient air and therefore should not be considered in the modeling analysis.

Terrain Data Not Explained

Because terrain is going to play an important factor in determining the maximum concentration location with sources, like Barto, that have low stack heights, DEP needs to evaluate the terrain based receptor data that were used in the CAC modeling exercise.

Exclusion of Areas Not Within the Ambient Air

The CAC analysis does not appear to have excluded areas in the model that do not fall under the definition of “ambient air” under 40 CFR 50.1(e). Ambient air is defined as the “portion of the atmosphere, external to buildings, to which the public has access.” The highest impacts could be onsite rather than in the ambient air. Only ambient air concentrations should be considered within the modeling analysis.

Surface Meteorological Data Does Not Reflect Actual Conditions

The use of the Williamsport-Lycoming County Airport meteorological data may not provide the meteorological measurements that accurately reflect meteorological conditions in the vicinity of the Barto station. If that is the situation it would significantly affect the modeled ambient air quality conditions.

DEP believes the proximity of the higher terrain to the north and south of the meteorological tower at the airport may cause two features to affect the representativeness of the data for use in modeling the Barto station. The topography could result in the winds being constrained to more a westerly to easterly direction than is actually experienced at the Barto Compressor Station. The second issues is that the winds are lighter due to more friction because of the higher terrain that is experienced at the Barto station.

Meteorological Data Processing Used Older Model

The CAC analysis utilized meteorological data that was preprocessed using the older version of AERMOD.

DEP believes strongly that the pervious version of AERMOD significantly overestimates concentrations at very low wind speeds. Moreover, the newer version of the model may also have overestimation issues related to wind speed. Use of the correct meteorological data is critical to final modeled ambient air concentrations.

Background NO2 Concentration Data Was from State College Near Interstate 80 – Inappropriate for a Rural Compressor Station

The CAC analysis used data from DEP’s State College NO2 monitor to determine the background NO2 concentration.

The State College NO2 monitor in an area where Interstate 80 is directly to the north and State College is directly to the south. DEP believes there is a likelihood of very strong influences from mobile source emissions at the State College NO2 monitor due to this location. Consequently, those ambient NO2 data would not be indicative of background conditions in the proximity of the Barto Compressor Station.

The NO2 Design Value Provides an Unrealistic Background Concentration

The CAC analysis utilized the NO2 design value of 34.7 parts per billion as a conservative “1st-Tier” monitored background concentration. The utilization of this figure provides an unrealistic background concentration because it is 34.7 percent of the 1-hour NAAQS.

Any modeling analysis should take into account the daily and seasonal variation in NO2 concentrations in order to provide a more accurate estimate NO2 hourly and seasonal variability. The use of the design values will overestimate the contribution to the background of the modeling analysis.

Model Inputs for the Plume Volume Molar Ratio Method Need Review

The approriateness of the model inputs for the Plume Volume Molar Ratio Method should be reviewed, because the NO2/NOx in-stack ratio was identified in the CAC report, but the ozone data was not mentioned in the report.

DEP recognizes the use of ozone data as a model input is important. The use of Plume Volume Molar Ration Method within AERMOD would provide for a better conversion between NO and ozone as it converts to NO2 and O2. As is shown with the results of the modeling report, the use of Plume Volume Molar Ratio Method within AERMOD effectely lower the NO2 concentrations within the modeling area.

To put all of that in layman’s terms, the CAC used an outdated study and did not take into account known issues with the model they used. The model used inaccurate data, background concentrations, and locations, and it significantly overestimated emissions levels at the station.

Few people who haven’t made it their career to understand air quality will realize the flaws of this study, and so the CAC now has a packaged model that sounds scary to the average person. It’s certainly useful if you’re trying to stop all natural gas development and the associated infrastructure it requires, but it’s not the greatest if you’re interested in, you know, accuracy.

Personally, I live by this compressor station and am thrilled to learn the levels put out by CAC were greatly exaggerated, as is much of what they’ve shared about other facilities. And I’m grateful the DEP took the time to lay out all of the issues with it in such a comprehensive fashion. After all, for those of us who actually live here, it’s important to be as educated as possible on what’s occurring around us. It’s also useful to separate fact from fear mongering, and not waste valuable time worrying about non-issues. I hope the CAC will update its model to reflect this information, but I won’t hold my breath.


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