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Frequently Asked Questions about Air Emissions Monitoring

What is monitoring?

Monitoring is a general term for the on-going collection and use of measurement data or other information for assessing performance against a standard or status with respect to a specific requirement. With regard to EPA's air quality regulatory requirements, monitoring includes: 1) ambient air quality monitoring, and 2) stationary source emissions monitoring. Ambient air quality monitoring refers to collecting and measuring samples of ambient air to evaluate the status of the air pollutants in the atmosphere as compared to clean air standards and historical information. Stationary source emissions monitoring refers to collecting and using measurement data (or other information) at individual stationary sources of emissions (i.e., facilities, manufacturing plants, processes, etc.) to assess the level of air pollutant emissions from a process, to assess the performance of an emissions control device, or to verify work practices.

For example, under 40 CFR part 63, subpart A (General Provision), monitoring is defined as the "collection and use of measurement data or other information to control the operation of a process or pollution control device or to verify a work practice standard relative to assuring compliance with applicable requirements."

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Why is monitoring so important?

Ambient air monitoring evaluates the status of the atmosphere and provides air quality information to regulators, scientists, industry, and the public. Ambient air quality monitoring is required to determine whether a geographical region or area is meeting the National Ambient Air Quality Standards (NAAQS) for criteria pollutants. In addition to determining areas where air quality standards are not being achieved (non-attainment areas), the monitoring data are used to assess trends in air quality, and to assess the impact of pollution generated by various activities. The emissions from stationary sources (and other types of sources, e.g., mobile sources such as automobiles) directly impact the ambient air quality of a region and the pollutant levels detected by ambient monitoring.

Stationary source emissions monitoring provides data and information from a regulated stationary source (facility) to demonstrate compliance with certain regulatory requirements in Federal or State rules and/or in an operating permit, as well as provides useful information to the facility operator about the performance of the process and air pollution control device so that corrective action can be taken, if necessary. Most monitoring that stationary sources must conduct is related to specific regulations resulting from the Clean Air Act (CAA).

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What pollutants are typically monitored?

Pollutants that are monitored may include criteria pollutants and toxic air pollutants. There are six criteria pollutants that the Clean Air Act has outlined as being most important to examine: Carbon Monoxide (CO), Particulate Matter (PM), Sulfur Dioxide (SO2), Nitrogen Dioxide (NO2), Ozone (O3), and Lead (Pb). Toxic air pollutants, also referred to as Hazardous Air Pollutants (HAPs), are known or suspected to cause serious health problems. There are currently 188 HAPs that are regulated under EPA's federal National Emission Standard for Hazardous Air Pollutants (NESHAP) program. Individual local and State air pollution programs also often define what specific pollutants are considered to be toxic air pollutants.

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How is ambient air monitoring accomplished?

For ambient air quality monitoring, a network of monitoring stations are strategically located to measure regional air quality. These stations collect air on a periodic basis using either integrated or continuous sampling. In integrated sampling, a sample is collected over a period of time and then sent to a lab for analysis. In continuous sampling, a specified sample volume is collected and immediately analyzed using specific monitors that operate continuously at the monitoring site.

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How is stationary source emissions monitoring accomplished?

In general, stationary source emissions monitoring is composed of four elements, including (1) indicator(s) of performance, (2) measurement technique, (3) monitoring frequency, and (4) averaging time.

An indicator of performance is the parameter you measure or observe for demonstrating proper operation of the air pollution control measures or compliance with the applicable emissions limitation or standard. Indicators of performance may include direct emissions measurements, surrogate emissions measurements (including opacity), operational parametric measurements that correspond to process or control device (and capture system) efficiencies or emission rates, and recorded findings of inspections of work practice activities, material tracking, or design characteristics.

The measurement technique refers to the means by which you gather and record information of or about the indicators of performance. The components of the measurement technique include the detector type, location and installation specifications, inspection procedures, and quality assurance and quality control measures. Examples of measurement techniques include continuous emission monitoring systems (CEMS), continuous opacity monitoring systems (COMS), continuous parametric monitoring systems (CPMS), and manual inspections that include making records of process conditions or work practices.

The monitoring frequency refers to the number of times you obtain and record monitoring data over a specified time interval. Examples of monitoring frequencies include at least four points equally spaced for each hour for CEMS or CPMS, at least every 10 seconds for COMS, or at least once per operating day (or week, month, etc.) for CPMS, work practice, or design inspections.

The averaging period refers to the period over which you average and use data to verify proper operation of the pollution control approach or compliance with the emissions limitation or standard. Examples of averaging time include a 3-hour average in units of the emissions limitation, a 30-day rolling average emissions value, a daily average of control device operational parametric range (e.g., operating temperature), and an instantaneous alarm.

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What is a stationary source emissions monitoring device?

Monitors are devices designed to collect information and data from stationary sources. There are several measurement techniques that monitoring devices may use, including Continuous Emission Monitoring Systems (CEMS), Continuous Opacity Monitoring Systems (COMS), and Continuous Parametric Monitoring systems (CPMS), and some monitoring devices directly measure the pollutant of concern from a stationary source and some measure a surrogate for the pollutant of concern. An example of a monitoring device that directly measures the pollutant of concern is a Nitrogen Oxides (NOx) CEMS used to monitor the NOx concentration (emissions level) of the effluent from a process stack on a stationary source that must comply with a NOx emissions limit. An example of a monitoring device that measures a surrogate pollutant is a Carbon Monoxide (CO) CEMS used to monitor the CO concentration of the effluent from a stationary combustion source that must comply with a Volatile Organic Compound (VOC) limit. In this instance, CO concentration is used as a surrogate for VOC because CO is a product of incomplete combustion and elevated levels of CO indicate incomplete combustion. Examples of parametric monitoring devices include thermocouples for monitoring temperature and pressure transducers for monitoring pressure.

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Does the EPA require stationary source and emissions point monitoring?

Yes. The EPA requires monitoring of stationary sources and emissions points in Federal rules such as New source Performance Standards (NSPS), National emissions Standards for Hazardous Air Pollutants (NESHAP), the Compliance Assurance Monitoring (CAM) rule, the Clean Air Act title V operating permit program, etc. An NSPS or NESHAP rule will include specific monitoring requirements that an affected source will have to conduct; these rules often include monitoring options from which the affected source may choose, depending upon the emissions control technique employed. Monitoring, analysis, and reporting are required by title V in order to ensure compliance with the terms and conditions of the permit as well as any pollutant standards established by the Clean Air Act.

Also, States and local air quality agencies include monitoring requirements in their State Implementation Plans (SIP) to ensure that a stationary source is meeting the requirements of the rules that are part of their SIP. The SIP is the State's plan for ensuring that areas in the State will meet the National Ambient Air Quality Standards (NAAQS) for criteria pollutants. The monitoring requirements that are part of SIP rules also will be included in a facility's title V operating permit.

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What is Title V?

Title V is a title of the 1990 Clean Air Act Amendments (CAAA) that requires stationary (point) sources of emissions that are major sources (and some area sources) of criteria pollutants or HAP to have an operating permit. Examples of stationary sources that may be required to obtain a title V permit include chemical production facilities, hospitals, and electric utility facilities. Major sources are sometimes defined differently by each State and local region based on whether the region is in attainment or nonattainment with the National Ambient Air Quality Standards (NAAQS) for criteria pollutants. An example of a major source would be a stationary source that emits 10 tons per year or more of one Hazardous Air Pollutant (HAP) or 25 tons per year or more of multiple HAPs.

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What is Title V monitoring?

Monitoring is a term used in title V of the Clean Air Act of 1990 and the implementing regulations, 40 CFR parts 70 and 71, to encompass the data collection, inspection, data recording, and related activities that stationary sources conduct to assure compliance with permit terms and conditions. Monitoring and analysis are required by title V in order to ensure compliance with the terms and conditions of the permit as well as any pollutant standards set by the Clean Air Act.

In part, title V states that each permit shall contain the following requirements with respect to monitoring [40 CFR 70.6 (a)(3)]:
  1. all monitoring and analysis procedures or test methods required under applicable requirements, including part 64 (CAM), and
  2. where the applicable requirement does not require periodic testing or instrumental or noninstrumental monitoring (which may consist of recordkeeping designed to serve as monitoring), periodic monitoring sufficient to yield reliable data from the relevant time period that are representative of the source's compliance with the permit.

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What is the Compliance Assurance Monitoring (CAM) rule?

The CAM rule under 40 CFR part 64 requires monitoring for specific emissions units at a facility that is subject to the title V regulations. The CAM rule applies to a specific subset of emissions units at Clean Air Act title V facilities that meet the following requirements: (1) located at a major source that is required to obtain a title V permit (40 CFR part 70 or 71), (2) subject to an emission limit or standard for the applicable pollutant, (3) uses a control device to achieve compliance, (4) has potential precontrol emissions of the applicable pollutant from the unit that are at least at the major source level, and (5) is not otherwise exempt (i.e., units subject to New Source Performance Standards (NSPS) or National Emission Standard for Hazardous Air Pollutants (NESHAP) that were proposed after November 1990 are not subject to the CAM rule, units subject to Acid Rain requirements are not subject to the CAM rule, and a few other exemptions.) Basically, CAM monitoring is specific to large emissions units at title V facilities that use add-on control devices to achieve compliance with emissions limits.

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What is Compliance Assurance Monitoring (CAM) monitoring?

Monitoring under the CAM rule (40 CFR part 64) is conducted to determine that control devices, once installed, are properly operated and maintained in order to continue to achieve a level of control that complies with applicable requirements. The monitoring approach used must provide data for one or more indicators of performance of the control device, any associated capture system, and/or any processes significant to achieving compliance. Such indicators can include a measured or predicted emissions level; a pollution control device operating parameter; a process operating parameter; a recordkeeping item; a work practice activity; recorded findings of inspection and maintenance activities; or a combination of these types of indicators. Operating within the established range(s) of the indicator(s) of performance is designed to provide a reasonable assurance of compliance with applicable requirements. The operator must note and report any excursions from the established operating ranges for the indicators of performance; the operator has an obligation to take corrective action so that the cause of the excursion is corrected.

EPA's response to a number of Frequently Asked Questions specific to implementation of the CAM rule have been summarized and are provided.

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