INTRODUCTION
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
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Ambient Air Quality Monitoring
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Stationary Source Emissions Monitoring
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National Ambient Air Quality Standards (NAAQS)
[40 CFR parts 50, 51, 53, and 58] |
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New Source Performance Standards (NSPS)
[40 CFR part 60]
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National Emission Standards for Hazardous Air Pollutants (NESHAP)
[40 CFR parts 61 and 63]
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Compliance Assurance Monitoring (CAM)
[40 CFR part 64]
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Title V-Permits (title V Operating permits)
[40 CFR parts 70 and 71]
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Title - IV Acid Deposition Control (Acid Rain)
[40 CFR part 75] |
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AMBIENT
AIR QUALITY MONITORING
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.
STATIONARY
SOURCE EMISSIONS MONITORING
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.
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Per subpart A (General Provisions)
of part 63, Title 40, Chapter I of the Code of Federal Regulations:
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"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:
- 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.
- 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.
- 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.
- 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.
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CONTINUOUS
MONITORING SYSTEMS
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.
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