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Technical Support Document (TSD) for the Response to the September 

2010 Section 126 Petition from New Jersey Regarding Sulfur Dioxide 

(SO2) Emissions from the Portland Generating Station

Docket ID No. EPA-HQ-OAR-2011-0081

Trajectory Analysis of High SO2 Episodes at the 

Chester, New Jersey Monitor

U.S Environmental Protection Agency

March 2011
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ADDITIONAL SUPPORTING INFORMATION

Trajectory Analysis of High SO2 Episodes at the Chester, NJ Monitor

A. Summary of trajectory analysis submitted by NJDEP

The New Jersey Department of Environmental Protection (NJDEP) submitted an analysis 

using air parcel trajectories on days with high concentrations to support its assertion that the 

Portland Plant causes violations of the 1-hour SO2 national ambient air quality standard 

(NAAQS) in New Jersey1. NJDEP examined air trajectories from two episodes with high SO2

concentrations recorded at the nearby Chester ambient air monitor in 2008 and 2009. The 

Chester monitor (34-027-3001) is located in Morris County, New Jersey, approximately thirtyfour kilometers from the Portland Plant. The Chester monitor is part of New Jersey’s regular 

State and Local Air Monitoring Station network and was situated to monitor the exposure of the 

general population of the area to SO2 and was not designed to monitor the specific impact of the 

Portland Plant or any other impact of any particular emissions source.

B. HYSPLIT Model selection

NJDEP used the Air Resources Laboratory’s HYbrid Single-Particle Lagrangian

Integrated Trajectory (HYSPLIT) model2as a tool to help determine if high concentrations of 

SO2 are coming from the Portland Plant. HYSPLIT is a complete system for computing both 

simple air parcel trajectories and complex dispersion and deposition simulations. The model 

calculation method is a hybrid between the Lagrangian approach, which uses a moving frame of 

reference as the air parcels move from their initial location, and the Eulerian approach, which

uses a fixed three-dimensional grid as a frame of reference. In the model, advection and 

diffusion calculations are made in a Lagrangian framework following the transport of the air 

parcel, while pollutant concentrations are calculated on a fixed grid. 

In this case, NJDEP used the Lagrangian features of the model to determine the direction 

and speed of the air. When NJDEP applied the HYSPLIT model, it used a meteorological data 

set with a grid size of 12 kilometers, which is the finest resolution data set available for use with 

the model. The HYSPLIT model’s grid size is less than the distance between the Portland Plant 

and the Chester monitor, so the model is using a data set that is more likely to be an appropriate 

tool for this analysis. EPA agrees that using the HYSPLIT model as NJDEP applied it provides 

useful information on the direction and speed of air parcels in the atmosphere. 

C. NJDEP’s Review of Monitoring Data and Nearby Emission Sources
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NJDEP reviewed air monitoring data for days when high concentrations occurred at its 

Chester, NJ monitor located 34 kilometers east-southeast of the Portland Power Plant. The 

monitoring data from the Chester, NJ monitor have been fully quality controlled, validated and 

certified by NJDEP, as required by EPA. The data are available in EPA’s Air Quality System 

(AQS) database.

During the years 2008 and 2009, the highest 1-hour SO2 concentrations occurred in two 

episodes, July 17-18, 2008 and December 7, 2009. During the July 2008 and the December 2009 

episodes, the Portland reported that the Plant was operating at 85 and 55 percent of capacity, 

respectively. NJDEP’s review of emission inventories from its emissions database and 

Pennsylvania’s PADEP eFACTS database show that yearly emissions from Portland are 1000 

times greater than any other sources between the Chester monitor and the Portland Plant. In the 

area around surrounding the Portland Plant, the Portland Plant’s yearly emissions are 1000 times 

greater than any other source in the area, except for the Martins Creek Power Plant. Hourly 

continuous emissions monitoring systems (CEMS) data from EPA’s Clean Air Markets database 

(see http://www.epa.gov/airmarkets/) show that the Martins Creek Power Plant was operating at 

very low levels during these two episodes, with actual emissions recorded well under the level of 

emissions measured at the Portland Plant. 

D. Episode I: July 17-18, 2008

The 1-hour SO2 concentrations measured during Episode I were the highest three hourly 

concentrations recorded at the Chester monitor during 2008-2009. Data in EPA’s Air Quality

System show that during three consecutive hours beginning at 10 pm local time on the 17th, 

hourly SO2 concentrations ranged from 85 parts per billion (ppb) to 53 ppb. The first two hours 

of the episode exceeded the 75 ppb 1-hour SO2 NAAQS. NJDEP included plots of these data 

showing a rapid rise at the beginning of the episode and drop off at the end of the episode of SO2

values; values were at or near zero ppb except during the air quality episode. 

NJDEP used the HYSPLIT model to calculate forward trajectories from the Portland 

Plant at a height representative of the plume emitted from its stacks (i.e., 100 meter plume rise 

added to the 121 meter stack height), and backward trajectories at a height representative of the 

Chester monitor (i.e., 10 meters above ground) during the time periods associated with high SO2

concentrations at the Chester monitor on July 17, 2008. New Jersey submitted forward and back 

trajectories for a 6 hour period (to allow for the transport of the plume from the stack to the 

Chester monitor) based on wind speeds of 4.5 meters per second (m/s) at 221 m above ground 

level (near plume height) to 2.7 m/s at 10 m above ground level (near the monitor’s height) 

during the episode. 

E. Episode II: December 7, 2009
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The 1-hour SO2 concentrations measured during Episode II included four of the twelve 

highest hourly concentrations in the Air Quality System for the Chester monitor during 2008-

2009. During four consecutive hours beginning at 1am local time on the 7th, hourly SO2

concentrations ranged from 28 to 33 ppb. Hourly SO2 during this episode rose and fell more

gradually; concentrations were less than 10 ppb on either side of the seven-hour episode. 

NJDEP used the HYSPLIT model to calculate forward trajectories from the Portland 

Plant at a height representative of the plume emitted from its stacks (i.e. 100 meter plume rise 

added to the 121 meter stack height), and backward trajectories at a height representative of the 

Chester monitor (i.e. 10 meters above ground) during the time periods associated with high SO2

concentrations at the Chester monitor on December 7, 2009. NJDEP submitted forward and 

back trajectories for a 3 hour period (to allow for the transport of the plume from the stack to the 

Chester monitor) based on wind speeds of 11.2 m/s at 221 m above ground level (near plume 

height) to 3 m/s at 10 m above ground level (near the monitor’s height) during the episode. 

F. Possible Sources Affecting the Readings Observed at Chester

At the time of these two episodes, emissions from the Portland Plant were vastly greater 

than other sources between Chester and the Portland Plant or around the Portland Plant. The 

only other facility in the vicinity showing potentially significant amounts of SO2 is the Martins 

Creek Power Plant, however, it was operating at levels well less than the Portland Plant, during 

the two episodes3. 

The following tables show the hourly emissions from Portland Units 1 and 2, and from 

Martins Creek Units 3 and 4 during the two air quality episodes. 

Table 1 Hourly Sulfur Dioxide Emissions on July 17, 2008

Hour (EDT) Portland Unit 1

(lbs/hr)

Portland Unit 2

(lbs/hr)

Martins Creek Units 3 & 4

(lbs/hr)

15 5,024 7,574 35

16 5,041 7,536 42

17 5,051 7,436 33

18 5,013 7,569 21
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19 5,003 7,556 16

20 4,969 7,580 16

21 4,913 7,532 9

22 4,986 7,548 0

23 4,475 6,792 0

Table 2 Hourly Sulfur Dioxide Emissions on December 6 and 7, 2009

Hour (EDT) Portland Unit 1

(lbs/hr)

Portland Unit 2

(lbs/hr)

Martins Creek Units 3 & 4

(lbs/hr)

Dec 6-18 3,503 5,326 0

19 3,554 5,818 0

20 3,493 5,819 0

21 3,352 5,611 0

22 2,830 4,612 0

23 2,856 4,535 0

Dec 7-00 3,192 4,617 0

01 3,305 4,653 0

02 3,423 4,532 0

The forward trajectories provided by NJDEP show that the SO2 emissions from the Portland 

Plant arrive in the vicinity of the Chester monitor during high concentrations at the monitor. For 

example, winds from the Portland Plant at hour 15 (4 pm EDT) on July 17, 2008, end during the

three-hour episode of high SO2 concentrations at the Chester monitor. In addition, trajectories 

starting at hour 17 (5 pm EDT) on July 17, 2008 at Chester and going backward end up near the 

Portland Plant. The trajectories submitted by NJDEP visibly show, that high concentrations at 

Chester were associated with a source in the vicinity of the Portland Plant. Since the Portland 
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Plant is the only large emission sources in its vicinity, it is likely that emissions from the 

Portland Plant caused the high concentrations during these episodes. 

G. EPA analysis

NJDEP’s method of using HYSPLIT trajectory analyses during the two episodes of high 

concentrations of SO2 at the Chester NJ air monitoring station uses acceptable methods that 

support the conclusion that SO2 that was recorded at the Chester monitor was mainly due to 

emissions from the Portland Plant. The trajectories, run forward and backwards, show air

coming from the vicinity of the Portland Plant to the area near the Chester monitor. The Martins 

Creek Power Plant is the only facility in the area that has potential SO2 emissions on the order of 

the potential emissions of the Portland Plant. However, during these episodes, emissions from 

the Martins Creek Power Plant are less than one percent of the emissions of the Portland Plant. 

Thus, it is unlikely that any facility other than the Portland Plant caused these high 

concentrations. 

NJDEP’s analysis of monitoring data and wind trajectories provides supporting evidence 

that the Portland Plant has a noticeable influence over a wide area around the facility. For

reasons explained in the preamble to the proposed rule, EPA continues to rely primarily on 

modeling to determine whether downwind areas constitute nonattainment or maintenance 

receptors for purpose of analyzing a section 126 petition with respect to the 1-hour SO2 NAAQS. 

Nonetheless, elevated concentrations, of SO2 appear to come from the Portland Plant, observed 

at a monitor distant from the area of Portland Plant’s peak impact. This finding underscores the 

need for EPA to approve the petition and to establish limits and a compliance schedule to address 

these elevated concentrations of SO2 and is consistent with EPA’s finding that the Portland Plant 

contributes significantly to and interferes with maintenance of the 1-hour SO2 NAAQS in New 

Jersey. 

 

1

 Trajectory Analysis of High Sulfur Dioxide Episodes at the Chester, NJ Monitor. Bureau of Technical Services, 

Division of Air Quality, New Jersey Dept. of Environmental Protection. July 30, 2010. Submitted to USEPA as 

Exhibit 4 of the September 13, 2010 Supplement to New Jersey’s May 12, 2010 Petition Pursuant to Section 126 of 

the Clean Air Act , 42 U.S.C. § 7426 

2 Draxler, R.R. and Rolph, G.D., 2011. HYSPLIT (HYbrid Single-Particle Lagrangian Integrated Trajectory) Model 

access via NOAA ARL READY Website (http://ready.arl.noaa.gov/HYSPLIT.php). NOAA Air Resources 

Laboratory, Silver Spring, MD.

3

 SO2 emissions from the Portland Plant were around 5,000 and 7,500 lb/hr for units 1 and 2 respectively on July 

17th. The emissions for Martins Creek units 3and 4 were less than 42 pounds per hour during this time. During the 

episode of December 6-7, the Portland Plant units 1 and 2 emitted about 3,200 and 5,000 pounds per hour 

respectively. The Martin’s Creek units 3 and 4 emitted zero pounds per hour. NJDEP’s data from EPA’s Clean Air 

Markets data base. The Portland Plant’s potential to emit is 5,820 and 8,900 lb/hr for units 1 and 2 respectively as 

reported in NJDEP’s analysis. Martins Creek’s potential to emit is 5, 692 lb/hr for both units 3 and 4 as reported in 

its Title V operating permit.