The U.S. Bureau of Ocean Energy Management (BOEM) regulates E&P activity on the Outer Continental Shelf (OCS) of the Gulf of Mexico (GoM) west of 87.5°W longitude. If a proposed E&P plan has projected pollutant emissions greater than BOEM’s exemption threshold for the plan, an air quality review is required. BOEM has two levels of air quality review as part of its current air assessment procedure.

The first level is limited to only those pollutants emitted above BOEM’s thresholds for the plan’s location. It involves performing air dispersion modeling of a plan’s worst-case emissions to determine if the plan could cause or contribute to an exceedance of the air quality standards at the nearest shoreline. The second level adds a more complex analysis of the impact of the plan on the ability of a human to view scenery while visiting the Breton National Wildlife Refuge, a so-called “visibility” analysis.

Breton National Wildlife Refuge is a strip of barrier islands formed from remnants of the Mississippi River delta and the river’s historical westward course shift some 2,000 years ago. The refuge was established in 1904 and modified due to tides, wind erosion and tropical cyclones. Breton is managed by the U.S. Fish and Wildlife Service (FWS), which protects the refuge’s air quality, specifically, the clarity of the air.

The modeling necessary to evaluate whether a plan will impact the ability of a human to view scenery within Breton is relatively complex and requires the use of air dispersion models with advanced transport and chemistry algorithms along with detailed speciation of a plan’s emissions into numerous potential pollutants. A plan could be exempt from this level of analysis if it is located more than 200 km (124 miles) from Breton. However, most plans within 100 km (62 miles) are immediately sent to FWS for visibility impact evaluation, while plans between 100 km and 200 km from Breton undergo an initial impact screening procedure related to the amount of emissions and nearest plan distance to Breton (Figure 1).


This procedure has the potential and history of pulling in deepwater plans located in Mississippi Canyon and the western edge of De Soto Canyon for an evaluation of visibility impacts at Breton.

BOEM has released proposed rules that are more in line with current U.S. Environmental Protection Agency (EPA) evaluation methods. While the proposed rules narrow the number of plans being sent directly to FWS for visibility impact review to inside 50 km (31 miles), the agency may expand the scope of deepwater visibility impact reviews, exposing more plans located between 50 km and 300 km (186 miles) from Breton in Lloyd Ridge, Atwater Valley and Green Canyon regions to visibility evaluation requirements. This means protracted approval schedules that could involve negotiations with FWS for these deepwater plans.

Stricter benchmark

BOEM has defined certain shoreline ambient air quality concentrations below which a plan is considered to have no significant impact. To date, BOEM has defined these significance concentrations for several pollutants, including nitrogen dioxide (NO2). The annually averaged ambient concentration of NO2 at a shoreline point cannot exceed 1 μg/cu. m when a plan’s emissions are modeled during an air quality review.

If the modeled concentration exceeds this significance threshold, the plan must implement the best control technology available to reduce emissions as much as feasible. If BOEM further adds to the analysis of modeled concentrations exceeding the significance threshold by aligning with EPA’s analysis procedure, this could mean that a plan must consider the cumulative air quality impact of not only its emissions, but the emissions of the broader region upwind of the exceedance point along the vector to the plan location. This would require evaluating all emissions sources inside of a 90-degree sector centered on the plan location (Figure 2).


Records, plans and emission history of all sources of emissions inside that sector would be queried, inventoried and included in a full impact modeling analysis to compare with ambient air quality standards. A plan’s shoreline impacts rarely exceed the annual NO2 significance concentration threshold, and control requirements and this type of onerous multisource analysis can usually be avoided.

However, if BOEM moves forward with adopting EPA’s significance concentration thresholds as proposed, this would not only mean an annual significance concentration level to evaluate against but would further include a one-hour average concentration that would likely be exceeded by even a moderately sized plan. For instance, some deepwater plans show modeled concentrations at the shore of about 100 1 μg/cu. m (compared to the EPA significance level of 7.5). In other words, if BOEM adopts EPA’s one-hour NO2 benchmark, for example, more plans would be required to perform broader air quality impact modeling including emissions from nearby sources needing more schedule for data and documentation development and verification followed by review and approval.

If BOEM fully aligns with EPA on visibility impact evaluation procedures, air quality standards and benchmark concentrations, air quality reviews in the GoM will have broader scopes and become more complicated, affecting plan review schedules. BOEM’s proposed rules state that such an alignment will occur. A version of BOEM’s proposed rules have been waiting in the wings since April 2016, but modification of the proposed rules is under consideration. It is critical for E&P air quality teams to monitor and understand BOEM’s path to air quality rule alignment with EPA in the months ahead.

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