Jump to main content.

Basic Information


Monitoring is a general term for 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 regards to EPA's air quality regulatory requirements, there are two basic types of monitoring with two different functions:

  • Ambient air quality monitoring collects and measures samples of ambient air pollutants to evaluate the status of the atmosphere as compared to clean air standards and historical information; and

  • Stationary source emissions monitoring collects and uses measurement data (or other information) at individual stationary sources of emissions (i.e., facilities, manufacturing plants, processes, emissions control device performance, or to verify work practices).
The emission levels of and monitoring conducted by 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 emissions levels detected by ambient monitoring. 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. (See the links below for EPA's websites for NAAQS and Ambient Monitoring.) The monitoring of focus here is stationary source emissions monitoring. Stationary source emissions monitoring is required to demonstrate that a source is meeting the requirements in Federal rules or in State rules that are part of a State Implementation Plan (SIP). Most monitoring that stationary sources must conduct is related to specific regulation resulting from the Clean Air Act (CAA).

The following diagram illustrates how each type of monitoring is related to the specific air-related programs in the CAA:

Clean Air Act

Ambient Air Quality Monitoring

Stationary Source Emissions Monitoring
National Ambient Air Quality Standards (NAAQS)
[40 CFR parts 50, 51, 53, and 58]
New Source Performance Standards (NSPS)
[40 CFR part 60]
National Emission Standards for Hazardous Air Pollutants (NESHAP)
[40 CFR parts 61 and 63]
Compliance Assurance Monitoring (CAM)
[40 CFR part 64]
Title V-Permits (title V Operating permits)
[40 CFR parts 70 and 71]
Title - IV Acid Deposition Control (Acid Rain)
[40 CFR part 75]

Top of page


EPA has numerous ambient air quality monitoring programs related to the NAAQS. The most basic is the Ambient Air Monitoring Program, which collects national air quality data on criteria pollutants: Carbon Monoxide (CO), Oxides of Nitrogen (NO2 and NO3), Ozone (O3), Lead (Pb), Particulate Matter (PM) - both particulates with aerodynamic diameters below 10 micrometers (PM-10) and particulates with aerodynamic diameters below 2.5 micrometers (PM-2.5), Sulfur Dioxide (SO2), and Volatile Organic Compounds (VOC). The program is carried out by EPA and State and local air pollution agencies with oversight and guidance provided by EPA. EPA has several other programs involving local, tribal, State, regional, and national air quality data. See the following link to go to the EPA's web page containing information and files on ambient air quality monitoring programs.

  Ambient Monitoring Technology Information Center (AMTIC)      

Top of page


The purposes of stationary source emissions monitoring are to provide: 1) data and information from a regulated stationary source (facility) to demonstrate compliance with certain regulatory requirements, and 2) performance information to the facility operator so that corrective action can be taken, if necessary. Applicable requirements can necessitate periodic or continuous monitoring related to permit terms or conditions (e.g., emission limits, work practice requirements, equipment design and operating requirements) that result from regulations.

The following definition shows how stationary source emissions monitoring relates to compliance under 40 CFR part 63 National Emission Standard for Hazardous Air Pollutants (NESHAP) of the CAA.

  Per subpart A (General Provisions) of part 63, Title 40, Chapter I of the Code of Federal Regulations:
  "Monitoring" means 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.

In general, stationary source emissions monitoring is composed of four elements, including: 1) indicator(s) of performance, 2) measurement techniques, 3) monitoring frequency, and 4) averaging time. These elements are explained as follows:
  1. Indicator(s) of performance - the parameter(s) measured or observed for demonstrating: (a) proper operation of the air pollution control measures, or (b) 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 inspection of work practice activities, material tracking, or design characteristics. An indicator range may be expressed as a single maximum or minimum value, a function of process variables (for example, within a range of pressure drops), a particular operational or work practice status (for example, a damper position, completion of a waste recovery task, materials tracking), or an interdependency between two or more than two variables.

  2. Measurement techniques - the means by which information from or about the indicators of performance is gathered and recorded. The components of a 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.

  3. Monitoring frequency - the number of times monitoring data are obtained and recorded 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.

  4. Averaging time - the period over which data are averaged and used 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, and an instantaneous alarm.

Top of page


There are different types of continuous monitoring systems (CMS), including:

  • Continuous emission monitoring systems (CEMS);
  • Continuous opacity monitoring systems (COMS), and;
  • Continuous parametric monitoring systems (CPMS).
A CEMS is an instrument that continuously measures actual emissions levels from a stationary source. The CEMS measures directly the pollutant of concern or measures a surrogate pollutant for the pollutant of concern. An example of direct measurement of the pollutant of concern is the use of a Nitrogen Oxides (NOx) CEMS 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 monitoring a surrogate pollutant is the use of a Carbon Monoxide (CO) CEMS to monitor the CO concentration of the effluent from a stationary combustion source that must comply with a Volatile Organic Compounds (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 (i.e., low CO concentrations indicate complete combustion of VOC compounds).

A COMS is an instrument that continuously measures opacity, which is a measure of the amount of light attenuated by particulate matter in effluent emissions. The percentage of visible light attenuated is defined as the opacity of the emissions. Transparent stack emissions that do not attenuate light will have a transmittance of 100 percent or an opacity of zero percent. Opaque stack emissions that attenuate all of the visible light will have a transmittance of zero percent or an opacity of 100 percent. Opacity often is used as an indicator of the degree of particulate matter emissions.

A CPMS, also called parametric monitoring, measures a parameter (or multiple parameters) that is a key indicator of system performance. The parameter is generally an operational parameter of the process or the air pollution control device (APCD) that is known to affect the emissions levels from the process or the control efficiency of the APCD. Examples of parametric monitoring include temperature, pressure, or flow rate monitoring.

A stationary source may be required in a specific rule to conduct one or more of these types of monitoring, or a source may have the option of selecting which type of monitoring is appropriate for its processes and air pollution control system. With any of these CMS, quality assurance/quality control (QA/QC) activities are an important aspect of the monitoring requirements. QA/QC activities assure that the CMS continues to collect valid data over time.

| Office of Air Quality Planning & Standards | Technology Transfer Network |
| Clearinghouse for Inventories & Emissions Factors |

Local Navigation

Jump to main content.