Ozone Monitor

[Carmen Kalua]

An ozone monitor is electronic equipment that monitors for ozone concentrations in the air. The instrument may be used to monitor ozone values for industrial applications or to determine the amount of ambient ozone at ground level and determine whether these values violate National Ambient Air Quality Standards (NAAQS). Different types of ozone monitoring methods have been used throughout the decades, the two most notable and common methods being the Federal Reference Method and the Federal Equivalent Method.

The Federal Reference Method (FRM) was the original method of measuring ozone concentration in the air, being used throughout the United States around the 1970s and 1980s. It uses what is known as gas-phase ethylene-chemiluminescence or ET-CL.[1] The ozone content is measured based on the reaction when the air around the monitor reacts with the ethylene reactant gas within the monitor. As of 2015, the EPA added an additional format to the FRM using nitric oxide chemiluminescence or NO-CL. It functions in a very similar manner to that of the ET-CL format except it uses nitric oxide instead of ethylene gas.[2] The FRM has, for the most part, been phased due water vapor causing skewed results and has been replaced with the Federal Equivalent Method which uses ultraviolet absorption. However, the FRM it still used occasionally as the Federal Equivalent Method can be skewed by concentration of other pollutants in higher quantities such as mercury, sulfur dioxide, carbon dioxide, VOCs, and others.[2]

The Federal Equivalent Method (FEM) relies on the use of ultraviolet Absorption, more accurately, the ozone molecule absorbs ultraviolet radiation.[2] Most ozone monitors utilized in regulatory applications use ultraviolet absorption to accurately quantify ozone levels. An ozone monitor of this type operates by pulling an air sample from the atmosphere into the machine with an air pump.[3] During one cycle, the ozone monitor will take one air sample through the air inlet, and scrub the ozone from the air; for the next cycle, an air sample bypasses the scrubber and the ozone value calculated. The solenoid valve is electronically activated to shift the air flow either through the scrubber or to bypass it on a timed sequence. The difference between the two sampled values determines the actual ozone value at that time. The monitor may also have options to account for air pressure and air temperature to calculate the value of ozone.

The concentration of ozone is determined using the Beer-Lambert Law that basically says that the absorption of light is proportional to the concentration. For ozone, a 254 nanometer wavelength of light created by a mercury lamp is shined through a specific length of tubing with reflective mirrors. A photodiode at the other end of the tube detects the changes of brightness from the light.

The onboard electronics process the values obtained and display the value on the screen and can also output an electrical signal in volts or a 4-20 mA current that can be read by an electronic data logger. Other options for output are RS232 serial port or ethernet or internal data storage on flash memory.

CASTNET measures rural, ground-level ozone at more than 85 locations throughout the United States. The hourly data are used by the Agency and external stakeholders to answer important scientific and policy questions. Examples of how the CASTNET ozone data are used include:

CASTNET ozone data fill spatial gaps in the nation's air quality networks by providing measurements in rural locations where monitoring is often sparse. These data are used to determine if an area meets or exceeds the NAAQS, offering rural and tribal communities with information to make informed decisions about policies that will protect people and the environment. The figure below shows the most recent 3-year design value for CASTNET sites that met the data completeness requirements and the W126 values which provide an estimate of whether ozone exposure has likely caused damage to vegetation.

The map contains two layers that can be turned on and off in the legend (top left corner). Only one ozone layer will be visible at a time. The circles represent the CASTNET 2019-2021 three-year average of the 4th highest 8-hour daily maximum ozone concentration (ppb). The diamonds represent the cumulative W126 exposure index (ppm-hours) from 2021. Ozone concentrations at individual site locations are calculated from ambient measurements. The pop-ups at the individual site locations include an arrow in the top right corner to see more information about the location depending on which layers are selected.

All on-site transfer standards are certified Level III, meaning they have been calibrated by a Level II standard. The Level II transfer standards are used to calibrate the on-site ozone transfer standards twice per year. The Level II transfer standards are calibrated once per year at NIST or at one of the EPA regional laboratories by a Standard Reference Photometer (SRP), otherwise known as a Level I standard.

Every CASTNET ozone monitor within the network is audited once per year by an independent auditor who completes a Performance Evaluation (PE). The PE results are required to be submitted to AQS before annual data can be certified. A monitoring agency may perform an independent PE at a CASTNET site if they follow the procedures outlined in the Third Party Audit Guidelines(2 pp, 68 K, About PDF) document. In addition, each year 20% of the network participates in the National Performance Audit Program (NPAP). State, local, and tribal agencies participate in the NPAP to provide consistency in the data across all monitoring organizations.

Every three years the reporting agency is required to participate in a Technical Systems Audit (TSA) to verify the monitoring program complies with the established regulations in the Code of Federal Regulations (40 CFR Part 50, 53, and 58). In late 2012, the EPA CASTNET contractor, Wood, participated in the first CASTNET program TSA for regulatory ozone monitoring. ARS, Inc., the NPS and BLM-WSO CASTNET contractor, participated in a TSA at their facility in 2013. Every three years the program participates in a TSA conducted by an independent auditor. The final audit reports and contractor responses to the findings can be found on the Documents page.

The Ozone Monitoring Instrument (OMI) instrument can distinguish between aerosol types, such as smoke, dust, and sulfates, and measures cloud pressure and coverage, which provides data to derive tropospheric ozone.

OMI continues the TOMS record for total ozone and other atmospheric parameters related to ozone chemistry and climate. OMI measurements are highly synergistic with the other instruments on the Aura platform.

The OMI instrument employs hyperspectral imaging in a push-broom mode to observe solar backscatter radiation in the visible and ultraviolet. The hyperspectral capabilities improve the accuracy and precision of the total ozone amounts and also allow for accurate radiometric and wavelength self calibration over the long term.

OMI derives its heritage from NASA's Total Ozone Mapping Spectrometer (TOMS) instrument and the European Space Agency (ESA) Global Ozone Monitering Experiment (GOME) instrument (on the ERS-2 satellite). It can measure many more atmospheric constituents than TOMS and provides much better ground resolution than GOME (13 km x 25 km for OMI vs. 40 km x 320 km for GOME).

OMI is a key instrument on Aura for monitoring the recovery of the ozone layer in response to the phase out of chemicals, such as CFCs, agreed to by the nations of the world in the Montreal protocol and later modifications to it at Copenhagen and London.

OMI measures criteria pollutants such as O3, NO2, SO2, and aerosols. The US Environmental Protection Agency (EPA) has designated these atmospheric constituents as posing serious threats to human health and agricultural productivity. These measurements are made at near urban scale resolution and track industrial pollution and biomass burning.

The AirData Air Quality Monitors app is a mapping application available on the web and on mobile devices that displays monitor locations and monitor-specific information. It also allows the querying and downloading of data daily and annual summary data.

Gas-sensitive semiconductor technology enables accurate parts-per-billion (ppb) detection at a fraction of the cost of analyzer-based instruments. Employers, health and safety managers, and air quality professionals around the world use our best-in-class ozone sensor technology to protect worker health and achieve regulatory compliance.

Our portable ozone monitors are widely recognized and used for its proprietary best-in-class ozone technology that delivers highly selective ozone measurement at low concentrations (down to 1 ppb). They can be used both for indoor and outdoor air monitoring. They are ideal for:

An Aeroqual ozone sensor is recognized for having laboratory precision whilst still being affordable and easy to use in the field. Our range of ozone monitoring products spans portable and fixed instruments, as well as some OEM-ready sensor modules. These products are used all over the world in the laboratories and workplaces of blue chip companies like Samsung, Unilever, and Tesla Motors.

Ground-level ozone (O3) forms when volatile organic compounds (VOCs) react with nitrogen oxide emissions in the presence of sunlight. Ozone is harmful to humans when they breathe it in and to plants when they respire. Monitoring ozone concentrations in parks helps us to determine air quality trends, provide public health alerts, and assess compliance with national standards. It is federal law for the National Park Service (NPS) to protect air quality resources in parks.

Ozone data are collected continuously (1-minute averages) and reported in one-hour intervals. Because ozone is not stable, it must be measured onsite to accurately determine its concentration in the atmosphere. This also allows us to provide real-time ozone conditions to the public.

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