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Facilities that produce air emissions can benefit by proposing limits on their own operations to avoid certain types of regulatory requirements. You might accept limits to stay under emission thresholds that require federal Title V/Part 70 permits. Or you may want to avoid being subject to federal New Source Review/Prevention of Significant Deterioration (PSD) regulations. If yours is already a major PSD facility, you may restrict changes to your facility to avoid the challenges of a PSD "major" modification review.

Limits like these are referred to synthetic minor limits, which are incorporated into permits. Their advantages include less complex permitting processes and simpler monitoring or modeling requirements.

However, before proposing synthetic minor limits for your facility, make sure you can live with them! It would require a major permit amendment to increase your limits later on.

Limit requirements

Acceptable synthetic minor limits must include:

Emission limits by themselves are not adequate. For example, "do not exceed 100 tons per year" in a permit is too vague to be enforceable. Examples of acceptable operational or production restrictions:

  • Limiting your equipment's hours of operation
  • Limiting material or fuel usage
  • Combined restrictions, such limiting materials and hours, and meeting a specific pollution-control efficiency

If you accept hours-of-operation limits, your potential hourly emissions still must be calculated using the maximum design capacity allowed by the permit; an hours-of-operation limit may be more restrictive than process limits.

Time periods for limits

EPA's preferred daily limits may be cumbersome, particularly for seasonable or unpredictable operations. A longer time period, such as a straight monthly limit may be acceptable, if it can be justified.

You can propose annual limits and, every month, calculate the total – or the average – over the previous 12 months (called a 12-month rolling sum, or a 12-month rolling average). The rolling limits allow for flexibility by accounting for operational high and lows, in cases with no short-term concerns.

Emission units that are subject to federal New Source Performance Standards often require a 30-day rolling period. Some complex facilities need a 365-day rolling average. If you propose a rolling limit for a new unit, you must provide another way to show compliance in the first 12 months of operation, such as a formula or a temporary monthly limit. When requesting new limits on an existing source, you can use existing operating data for the first 12 months.

For a limit on a pollutant with an ambient air quality standard, you may need to show modeled compliance with the standard. If the form of the standard is a 1-, 3-, or 24-hour average, propose your limits using a shorter averaging period, to restrict your short-term emissions for use in modeling compliance. Twelve-month rolling limits don't restrict your short-term potential to emit, only the annual average emissions.

Margin below threshold

Proposed synthetic minor limits must provide an adequate margin of error below the threshold the permit-holder is trying to avoid. The proposed limit must account for the uncertain outcome of the proposed compliance method and any emissions that may be left out of the ongoing compliance tracking (e.g., insignificant activities). Facilities that propose a limit to avoid a threshold and hope to rely on general or non site-specific emissions factors typically receive limits with a margin of 10%. If you are willing to collect ongoing site-specific data (e.g., continuous emissions monitoring, analyze contents of raw materials, etc.), a narrower margin might be appropriate.


Synthetic minor permits require records to demonstrate compliance with the limits. In the permit application, you must propose a compliance method. Typically it involves a combination of performance testing, monitoring, daily recordkeeping, and monthly calculations. Your level of monitoring should account for:

  • The likelihood of violating the limit
  • Whether controls are needed to meet the limit
  • Variability of emissions
  • Type of data available
  • Feasibility of monitoring methods
  • Type of monitoring on similar units

Every year, you must submit certification that you are complying with the limits to the MPCA.

Examples of synthetic permit limits

Spray painting facility

The potential emissions for a spray-painting operation with no control for volatile organic compounds (VOC) made the facility subject to both Part 70 and New Source Review (NSR)/Prevention of Significant Deterioration (PSD) regulations. The facility's past actual VOC emissions were less than 100 tons per year, so they accepted permit limits on the amount of material used per month, to keep actual VOC emissions below 100 tons per year. The facility is now considered a synthetic minor source under NSR/PSD. (With the limit in this example, the company could still be a major source for hazardous air pollutants, and still a major source under Part 70.)



Volatile organic compounds (VOCs)

VOC-containing material use shall not exceed 12,900 pounds per month, calculated each calendar month by the 5th of the month.

12,900 lbs. per month/2000 lbs. = 6.45 tons per month
6.45 tons per month x 12 months = 77.4 tons per year)

Proposed compliance method

Daily records of materials used using flow meters and usage logs and monthly calculations as detailed above

New diesel generator

When a facility considered installing a new diesel generator, they found that its unrestricted potential emissions would classify the installation as a major modification under NSR/PSD. The proposed synthetic minor limit keeps potential nitrogen oxide emissions from the generator to less than 100 tons per year, and PSD review is avoided.



Nitrogen oxides (NOx)

Diesel generator use: Not to exceed 300 hours of operation in any 12 month period

Compliance with the limit shall be based upon a monthly, 12-month rolling sum. For the first 12 months of operation, compliance shall be based upon the following formula:
Hn < 27.27 + 22.73n *
Hn = the total number of hours of operation through month n
n = number of months of unit operation

Proposed compliance method

Install hour meter on the generator, record hours daily, calculate hours operated monthly as detailed above

* Facility allotted 50 hours for first month, and 250 hours for the next 11 months. 250/11 = 22.73 hours. The first term of the formula is then (50 - 22.73) = 27.27.

Limiting fuel use

A facility has boilers that use several different fuels; the boilers' total capacity is about 70,000 pounds of steam per hour (total rated heat input is 100 MMBtu/hr). The facility's actual emissions have not exceeded major NSR/PSD thresholds, but the potential emissions (if all boilers operated at full capacity all the time) have. The facility limits their potential emissions by limiting facility-wide fuel and avoids being classified as a major source.



All criteria pollutants

The types and total amounts of each fuel consumed in any 12 month period shall not exceed:

  • Natural gas/propane: 102 million cubic feet
  • Distillate oil (not to exceed 0.5% sulfur by weight): 720,000 gallons

Proposed compliance method

Track fuel usage daily using meters and usage logs, calculate total usage monthly as detailed above; fuel supplier certification with each shipment re: sulfur content

Grain unloading operation

A new grain unloading operation will be installed at an existing major facility. The potential to emit of the modification, without limits, would be a major modification under NSR. The facility proposes to limit the new operation by totally enclosing the unloading area, using emission controls, and limiting actual throughput. Based on emissions factors and the proposed control efficiencies, particulate matter smaller than 10 microns is the limiting pollutant.



Particulate matter (PM), particulate matter smaller than 2.5 microns (PM2.5) and 10 microns (PM10)

Grain throughput limited to 6,500,000 lbs/yr on a 12-month rolling sum basis

Install and operate a total enclosure to capture emissions during grain unloading and install and operate a fabric filter connected to the total enclosure with a control efficiency of 99% for PM and 93% for PM10 and PM2.5

(Calculation: 6,500,000 lbs/year x 0.059 lb of PM10/lb of grain x (1-.93)*ton/2000 lb = 13.5 tons of PM10 (threshold of 15 tons per year)

Proposed compliance method

  • Track grain throughput daily and calculate the 12-month rolling sum throughput each month by the 10th of the month
  • Propose to do one-time stack test to measure PM10 and PM2.5 emissions to confirm the emissions factor used to derive the throughput limit
  • Keep doors and other functional openings into the grain unloading area closed during grain unloading operations; maintain signage in area around doors instructing staff to keep doors closed
  • Monitor the fabric filter daily by pressure drop and checking for visible emissions; proposed fabric filter pressure drop range of 2-5 inches of water column