California Air Resources Board

Air Monitoring Technology

Ambient air monitoring utilizes a variety of continually evolving technologies to measure ambient air pollutant concentrations. A suite of air monitoring technologies are available to measure criteria air pollutants and toxic air contaminants. Descriptions of air monitoring applications are provided along with applicable measurement technologies (Applications Table), and their relative availability/cost (Technologies Table). This website will be updated as new information becomes available. Additional community science resources are available within the Community Air Monitoring Toolbox. If there is anything specific you would like to see represented here, please let us know at cam@arb.ca.gov .

The mention of trade names or products does not constitute endorsement or recommendation for use by CARB.

Measurement Application

The table below outlines a variety of monitoring applications, pollutants, and measurement technologies. Each technology in the table may require additional sample preparation and separation techniques, and sample collection platforms are not included.

Air Monitoring Applications

Source Attribution

Source attribution is an analysis technique used to characterize sources contributing to ambient air pollution in communities. The goal of source attribution is to identify how much an emissions source or source category contributes to the overall pollutant concentrations in around a community. This information can be used to inform emissions reduction strategies for reducing the overall exposure burden in communities. Source attribution can be performed with several different methodologies including emission inventories, source-modeling, and receptor-modeling approaches. The air monitoring data required for source attribution methods can vary from method to method. Some methods require meteorological measurements to supplement air monitoring data whereas others require more detailed speciated information. For example, receptor-oriented source attribution generally requires detailed "speciated" information, or concentrations of individual compounds, to produce meaningful results. The collection of detailed speciated air monitoring data requires robust, analytical methods that are more complex than other monitoring methods. It is important to select appropriate measurement technologies to support source attribution methods specific to the needs of each program. More information can be found in the Online Resource Center on the Community Air Protection website.

Health Research

Air monitoring data can be used to inform public health research. Air monitoring can provide public health researchers information on the public's exposure to health-relevant air pollutants such as criteria pollutants and toxic air contaminants. The objective and scope of the health research will often dictate what type of air monitoring needs to be conducted. For example, personal exposure research may require portable, real-time monitoring equipment, such as air sensors, that can be carried by an individual throughout their day. Other health research studies may require detailed information on the concentrations of toxic air contaminants that may be present at difficult to detect concentrations in a community.

Provide Air Quality Information

A key application of air monitoring data is to provide accurate and timely air quality information. This data application can vary from regulatory monitoring network expansion (US EPA) to simply providing general air quality information to the public. This monitoring can utilize technologies ranging from air sensors to laboratory-grade or regulatory grade instrumentation depending on the desired objective. Typically, this monitoring focuses on criteria pollutants but can include air toxics monitoring in areas of interest.  Broadly this style of monitoring can provide a quantitative understanding of air quality (i.e. detecting if and how much of a pollutant is present in the environment), but may not necessarily require the same level of data quality as source attribution or enforcement compliance monitoring.

Enforcement Actions

Monitoring data can be a useful tool to help guide enforcement actions by acting as a screening tool for compliance testing. Compliance testing is directed by strict regulatory requirements and/or tools that are often difficult or expensive to operate. However, common air montioring technologies such as air sensor networks or mobile monitoring can be used to target compliance testing or inspections for more efficient enforcement.

Hotspot Identification

A "hotspot" can be defined as a location within a community where a given pollutant is significantly greater than that of the surrounding area. These "hotspots" are most often centered at or near emission sources, and may only be present for a short period of time. These factors require monitoring to have high temporal, spatial, and quantitative resolution. Hotspot identification can use technologies ranging from mobile monitoring, satellite remote sensing, dense networks of air sensors, or fixed monitoring sites. Information obtained from hotspot identification can inform where and when to deploy more robust monitoring methods or when to initiate follow-up analyses such as source attribution.

 Air Monitoring Technologies

All technologies, methods, techniques, and equipment listed below are subject to a wide range of variables that can influence measurement quality. These can include, but are not limited to, equipment siting, correct operation, proper and timely calibration, and maintenance per manufacturer recommendations. A monitoring plan that outlines all appropriate operating procedures should be developed prior to implementing a measurement technology (guidance available here).

* $ = < $2,000; $$ = $2,000 - $25,000; $$$ = $25,000 - $50,000; $$$ = $50,000 - $100,000; $$$$+ = > $100,000

^**  Estimated number of manufacturers (mfrs): M = < 20 mfrs;  MM = 20 - 50 mfrs; MMM = 50 - 100 mfrs; MMMM = 100+ mfrs.

^***  Amount of expertise for operation: 1 = Minimal level of expertise needed for operation, "plug and play" operation; 2 = Moderate expertise level, will require moderate training for all operators/users; 3: High expertise level, requires extensive training for operation, use, and analysis

Spectroscopic Methods

Spectroscopy is an analytical method to quantify the concentration of a compound by measuring the interaction of a target compound with "light". Light, is passed through a sample and the amount of absorbed radiation is measured. "Light" refers to wavelengths of energy in the electromagnetic spectrum - including radio waves, microwaves, infrared (IR), visible light, ultraviolet (UV), x-rays, and gamma rays. The amount of absorbed radiation, or attenuation, is directly proportional to a given compound. Electromagnetic radiation includes radio waves, microwaves, infrared (IR), visible light, ultraviolet (UV), x-rays, and gamma rays.

Figure 1: Diagram of the electromagnetic spectrum outlining the electromagnetic radiation used in spectroscopic methods and their relative energies (Source: NASA, "Imagine the Universe").

Spectroscopic measurements are performed in a wide variety of mediums (solid, liquid, or gas) and can measure a wide variety of compounds. The wavelength of radiation most strongly absorbed by a given compound is dependent on the unique atomic structure, and is utilized to accurately quantify and identify the species. For example, ultraviolet radiation is commonly used to quantify ambient levels of ozone (example). Compound detection and quantification by spectroscopy is generally the last step in a given method, and is often preceded or coupled with several sample collection and isolation steps (e.g. chromatography, filtration).

Mass Spectrometry

Mass spectrometry (MS) is a powerful analytical technique, capable of measuring chemical compounds in simple and complex mixtures (example). MS ionizes samples and sorts the ions based on their mass-to-charge ratios. The sorted mass-to-charge ratios provide molecular and/or elemental characteristics of samples, which can be used for sample identification and quantification. The major components of a mass spectrometer include an ion source, an ion analyzer, and a detector. The ion source converts analytes into ions. The ion analyzer selects and transfers ions of interest to the detector based on their mass-to-charge ratios. The detector measures the ion signals, which can be used to calculate the amount of analytes introduced into the ion source.  

More than one technique can be used for creating ions, ion selection and detection, respectively. Examples of these components are provided below.

These techniques can be combined to achieve different analysis objectives. For example, mass spectrometers utilizing electron ionization, quadrupole mass analyzers, and electron multipliers have been widely used for measuring organic compounds, such as BTEX (benzene, toluene, ethylbenzene, and xylene) and PAHs (polyaromatic hydrocarbons).

Figure 2: Schematic of electron ionization gas chromatography (GC) mass spectrometry. Samples separated by the GC enter the ionization chamber (left box), where a charge is applied. The individual ions are then detected within the quadrapole rods based on their mass-to-charge ratio prior to entering the detector. (Source: Wittmann, Christoph. (2007). Fluxome analysis using GC-MS. Microbial cell factories. 6. 6. 10.1186/1475-2859-6-6).

Mass spectrometry can be coupled with other techniques to better characterize the physical and chemical properties of atmospheric pollutants. For example, the combination of mass spectrometry and gas chromatography is capable of identifying thousands of organic species. Therefore, mass spectrometry can be a valuable tool for air pollution modeling, source attribution, and health research.

Chromatography Separation

Chromatography is an analytical technique commonly used for separating a mixture of chemical substances into its individual components. Chromatography separates individual compounds by dissolving samples into a "mobile phase" and then passing them through a "stationary" phase. Individual components travel at different speeds through the chromatography system based on their chemical interactions between the mobile and stationary phases, which causes them to separate before the detection system.

There are many types of chromatography. The most common form of chromatography in air monitoring is column chromatography due to its ability to speciate air samples, which are typically complex mixtures. Various combinations of mobile and stationary phases may be selected depending on the compounds of interest.

Figure 3: Sample diagram of gas chromatography. In this example, a mixed species within the mobile phase is passed through a stationary phase, resulting in separated gas species (source).

Common column chromatography techniques used in analyzing atmospheric pollutants include:

• Gas chromatography (example)
• Liquid chromatography (example)
• Ion chromatography (example)

Column chromatography techniques can be combined with mass spectrometry to provide more detailed information of chemical composition of atmospheric pollutants for a variety of applications, such as health research, source attribution, and hotspot identification.

Gravimetric Methods

Gravimetric analysis involves monitoring the change in weight resulting from the accumulation of matter on a substrate such as a filter. In air monitoring, gravimetric analysis is used to measure particulate matter concentrations. PM mass concentrations are calculated by measuring a filter's weight before and after sample collection and then dividing the difference by the total volume of air sampled through the filter. Class I and Class II Federal Reference Methods (FRM) or Federal Equivalent Methods (FEM) particulate mass measurement requires gravimetric analysis. These methods provide particulate mass for two size bins, PM_2.5 (particulate matter smaller than 2.5 μm) and PM₁₀ (particulate matter smaller than 10 μm), at the highest data quality available (example PM_2.5 mass system). The US EPA's chemical speciation network (CSN) and the Interagency Monitoring of Protected Visual Environments (IMPROVE) network are two examples of particulate matter FEM/FRM measurement platforms. FRM/FEM collect PM at 24 hour time intervals, and require detailed, precise sampling handing steps. Other gravimetric techniques, such as collection on a tapered element oscillating microbalance (TEOM), provide PM mass information an hourly timescales, but still require regular replacement of the surface on which mass accumulates. Therefore, due to logistical concerns, gravimetric measurements are generally more useful for long-term monitoring applications such as health research and compliance rather than time sensitive applications.

Particle Counting

Particles in air have the ability to scatter light. Optical particle counters (OPC) measure the amount of light scattered by particles to estimate the number and size of particles present in air. OPCs typically use a laser and detectors to capture light scattered at a particular angle from the beam. The wavelength of laser light sources results in complex light scattering patterns (Mie scattering) dependent not only on particle size (e.g. PM_2.5 and PM₁₀), but shape and refractive index. OPC instruments are calibrated to air samples with known particle sizes and concentrations. Measurement of smaller particles requires a more intense light source and a more sensitive detector; therefore, OPC instruments typically focus on a particular size range or include multiple detectors. 

Figure 4: Schematic of an optical particle counter sensor (source: AQ-SPEC).

The cost of OPCs ranges from tens of dollars to thousands of dollars depending on the particle size range and precision of the instrument. Examples range from the low-cost optical counters to laboratory grade condensation particle counters (CPC). Due to this variability, OPCs can be used on a variety of platforms and for several applications. Ground based OPCs can monitor changes in PM to provide information for source attribution and health research. Low cost sensor networks typically employ inexpensive OPCs and extensive networks are proving to be useful tools for hotspot identification and exploratory monitoring. Intensive aircraft campaigns for exploratory monitoring and hotspot identification often carry more expensive models to sample the ambient air mass and provide real-time particle number and size distribution information.

Conductivity Measurements

Conductivity measurement technologies operate by quantifying the changing flow (e.g. resistance or impedance) of electricity between two electrodes. This change in electrical characteristics correlates to the concentration of a compound. Traditional examples of conductivity measurements are thermal conductivity and electrical conductivity detectors. Thermal conductivity technology operates by measuring the changing resistance across a circuit relative to a reference circuit due to the increased concentration of a gaseous compound (example). Electrical conductivity technology operates by monitoring the change in electrical impedance due to increased ionic strength between two electrodes. Conductivity measurements are often coupled with ion chromatography to separate ionic compounds (e.g. nitrate, sulfate) within a sample based on their electrical charge prior to quantification using a conductivity detector.

Conductivity measurements can also take the form of electrochemical and metal oxide sensors. Electrochemical sensors utilize changes in electrochemical potential to detect and measure gas concentrations. In an electrochemical sensor, the target gas diffuses across a porous membrane into a cell containing an electrolyte and sensing electrodes. When the target gas comes into contact with the electrolyte, a change in electrochemical potential occurs between the electrodes causing electrons to flow. The sensor interprets the current signal, which can then be correlated to the concentration of a specific pollutant. For some electrochemical sensors, interference or cross sensitivities may occur as the electrolyte may respond to multiple gas species (example). In metal oxide sensors, a semiconductor metal oxide placed on a substrate is heated to allow target gas to interact (example). This interaction results in electron generation at the surface of the semiconductor metal oxide. The sensor then detects the change of resistance of the metal oxide, like electrochemical sensors. These sensors often exhibit cross sensitivity for some gases.

Figure 5: Schematic of electrochemical sensor (Reference: Szulczynski G. and Gebicki, J, Environments, 2017, 4, 21).

Ionization Methods

A compound is considered ionized if it has lost or gained an electron, giving it a net positive or negative charge, respectively. This "ionization" occurs by applying a high energy source to the compound, such as from a flame in a flame ionization detector (FID - example) or radiation (e.g. ultraviolet light) in a photoionization detector (PID - example). The ionized compounds are then collected utilizing an electrode or electrometer.

Ionization methods can be operated as stand-alone instrumentation or coupled with separation methods. Gas phase ionization methods for air monitoring generally measure carbon based volatile organic compounds (VOCs). Stand-alone gas phase ionization methods result in "bulk" measurements of compound classes, such total VOC concentration, whereas methods coupled with separation methods, such as chromatography, provide compound specific concentrations. For example, an ionization technique such as an FID or PID could be coupled with chromatography to measure the concentration of BTEX (benzene, toluene, ethylbenzene, and xylene) in ambient air samples.

Figure 6: Simple schematics of flame ionization detector (A; source: Cambustion, LLC) and photoionization detector (B; source: Zimmer et al., J. Sens. Sens. Syst., 4, 151-157, 2015).

Air Monitoring Approaches

Federal Reference and Equivalent Methods

These air monitoring methods specify equipment and procedures to monitor criteria air pollutants that meet regulatory requirements as prescribed in the Code of Federal Regulations. Data from these methods are used for determining attainment or non-attainment of national and State ambient air quality standards, supporting public information services, forecasting expected high pollution events, and supporting the development of emissions reduction programs.

The latest List of Designated Reference and Equivalent Methods are available from the US EPA.

Figure 7: Examples of regulatory (FRM/FEM) particulate monitoring stations operated by the California Air Resources Board.

Air Toxics Monitoring

Most air monitoring methods for toxics involve collecting air samples in the field and returning them to the laboratory for subsequent analysis. Data from these methods may take weeks or in some cases months after sampling to become available as these sophisticated methods often require labor-intensive analytical procedures. There are some methods that can analyze toxics at shorter time scales (e.g. hourly samples in the field), but the instruments are expensive and require significant siting and data infrastructure, as well additional time for data analysis. Air toxic monitoring data may be used to identify sources contributing to air toxic pollution and trends in the concentration of air toxics over time. Data can be used to support regulatory and enforcement actions (health research and exploratory monitoring applications) when collected following appropriate protocols.

Remote Sensing

Remote sensing is the application of a measurement technology at a distance from the target. Remote monitoring of pollution involves detection of radiation to indirectly estimate the constituents of the atmosphere through which the radiation passed (see Spectroscopy). Passive remote sensing systems detect incoming radiation originating from the sun or the planet, whereas active remote sensing systems emit radiation and detect the back scattered signal. Remote sensing can be useful for detecting particulate matter, gaseous criteria pollutants, and, to some extent, volatile organic compounds.

Remote sensing can involve deploying instrumentation aboard aircraft, on satellite-based platforms in orbit, or ground-based platforms. Each have benefits and trade-offs. Aircraft remote sensing covers a larger area than ground-based monitoring but has a limited sampling duration. Satellite-based platforms provide coverage over very large areas and can sample for long periods. However, this comes with the cost of lower spatial and temporal resolution and often larger measurement uncertainties (i.e. interference from other compounds in the atmosphere). Satellite-based remote sensing systems should be used in conjunction with surface measurements for calibration and evaluation. Remote sensing data is often used for exploratory monitoring and hotspot identification.

Figure 8: A CALIOP "curtain" showing backscatter from aerosol and cloud at different altitudes along the track of the CALIPSO satellite. The surface shows collocated MODIS instrument retrievals in the visible and infrared channels (fire hotspots indicate in red). Image credit: NASA

Mobile Monitoring

Mobile platforms collect environmental data while in motion, for example in a car, van, or aircraft. They utilize instrumentation that can quickly measure air pollutant concentrations and provide instantaneous snapshots of air pollutant concentrations at a specific location, time, and fine spatial resolution (i.e. across a city block). Many primary air pollutants can have widely variable concentration gradients within a very small area. For example, vehicle-related pollutants (e.g. black carbon or nitrogen oxides) can be found at greater concentrations near roadways, but decrease quickly a short distance away. Mobile monitoring measurements can supplement stationary air monitoring data by filling in information gaps between monitoring sites. Mobile monitoring also allows for rapid deployments and helps air monitoring professionals react quickly in response to emerging air quality issues.

Mobile platforms can deploy a variety of instrumentation ranging from sensors, research-grade instrumentation (including spectroscopic, particle counting, chromatography, and mass spectrometry), and remote sensing devices. Mobile platforms configured with robust air monitoring technologies can act as mobile laboratories, providing on-site, high quality, analytical capabilities. Collected information can be used to identify persistent elevated pollutant concentrations, inform enforcement compliance testing, and indicate potential contributing sources within communities (hotspot identification or exploratory monitoring).  Mobile measurements may not be appropriate for situations in which the pollutant concentrations change significantly over time or emissions are expected to be intermittent as all mobile measurements are snapshots in time.

Figure 9: Examples of mobile monitoring platforms constructed by the California Air Resources Board.

Figure 10: Example of mobile monitoring measurement data utilized to identify local community black carbon "hotspots" within West Oakland (Source: Apte et al., Environmental Science and Technology, 2017, 51, 6999-7008).

Fence-line Monitoring

Fence-line monitoring is a monitoring strategy in which air quality is measured at the perimeter of a known emissions source. This type of monitoring may be used to help determine where and when leaks are occurring, at what rate emissions are leaving the source, and what chemicals are present in fugitive emissions (exploratory monitoring).

Depending on the air pollutant that is expected to be emitted, fence-line monitoring can use open path systems (e.g. an ultraviolet beam to measure the concentration of chemical plumes crossing its pathway via spectroscopy), point array systems (point array systems use a series of discrete air samplers, or sensors, at locations surrounding a facility), mobile monitors, and passive samplers (stationary sensor using sorbent materials with predetermined "thresholds" for elevated emissions). Fence-line systems used in conjunction with meteorological instruments can help to locate release sources and predict where the emissions will travel. Pollutants typically measured are gaseous organic species (e.g. volatile organic compounds) and inorganic gases typical of petroleum refinery operations (e.g. sulfides).

Figure 11: Schematic of fence-line air monitoring using open-path detection technologies.

Sensors and Sensor Networks

Air sensors measure air pollutants on a real-time or near real-time basis and are generally low in cost, highly portable, and can require less power and siting infrastructure than other air monitoring methods. Air sensors are available to measure a broad range of species, from gaseous criteria pollutants to particulate matter. Gaseous pollutant air sensors are often conductivity (electrochemical or metal oxide sensors), ion (small photoionization detectors). Small optical particle counters (OPCs) can be used to quantify particle count at various sizes (e.g PM₁₀ and PM_2.5). Currently, no low-cost (i.e., $2,000 or less) sensors meet federal reference or federal equivalent method requirements and many have not been robustly evaluated to determine the accuracy of their measurements; however, sensor technology is rapidly developing and their performance is expected to improve over time. The South Coast Air Quality Management District (SCAQMD) evaluates and shares sensor evaluation through their Air Quality Sensor Performance Evaluation Center (AQ-SPEC). This program evaluates the performance of sensors both in laboratory and field applications to help inform the general public of the performance of commercially available sensors. Similarly, the US EPA's air sensor toolbox provides guidelines on best practices in the selection, use, and data interpretation (Air Sensor Toolbox) of air sensors.

Sensors have the potential to provide local air quality data as part of coordinated, well-designed community-led air monitoring. The low entry cost for these sensors may allow the creation of larger networks than achievable in the past. Sensor networks can allow community-level assessment of air quality information, protect against single failing sensors, and provide opportunities for community members to become directly involved in air monitoring. The resulting data may be of sufficient quality to complement existing air monitoring networks and help understand spatial variability (exploratory monitoring), identify areas with relatively greater pollutant concentrations for further investigation with more robust technologies (hotspot identification), and evaluate personal exposure to air pollution (health research).

Figure 12: Examples of an air particulate matter sensor.

Glossary of Terms for Air Monitoring Species

A comprehensive glossary of air monitoring and air quality terms is available on the broader CARB website.