Determination of Carbon Monoxide Content in the Atmosphere: Testing Methods, Standards, and Applicat

The determination of carbon monoxide (CO) content in the atmosphere is a core task of ambient air quality monitoring, occupational hygiene, and indoor environmental assessment. Because CO is colorless, odorless, and highly toxic, accurate measurement using validated techniques such as non-dispersive infrared spectrophotometry and gas chromatography is essential for regulatory compliance and public health protection. This article explains the sampling strategies, analytical methods, applicable standards, quality control requirements, and typical applications of atmospheric CO determination.

Technical Overview

Carbon monoxide (CO) is a colorless, odorless, and tasteless gas produced by the incomplete combustion of carbon-containing fuels such as gasoline, natural gas, coal, oil, wood, and biomass. In the atmosphere, CO originates mainly from motor vehicle exhaust, industrial combustion processes, residential heating, biomass burning, and certain chemical production operations. Although CO has a lifetime of weeks to months in the free atmosphere, where it is gradually oxidized to carbon dioxide, its concentration near emission sources can reach levels that pose a significant health risk.

The toxicity of CO arises from its strong affinity for hemoglobin, with which it forms carboxyhemoglobin (COHb) roughly 200 to 250 times more readily than oxygen. This reduces the oxygen-carrying capacity of the blood and causes tissue hypoxia. Symptoms of CO poisoning range from headache, dizziness, and fatigue at low exposure levels to loss of consciousness and death at high concentrations. For this reason, CO is classified as a criteria pollutant and is subject to strict limit values in ambient, indoor, and occupational air quality regulations worldwide.

• WHO air quality guideline: 4 mg/m³ as a 24-hour mean concentration, with additional short-term guidance values.
• GB 3095 ambient air quality standard: 4 mg/m³ (24-hour average) and 10 mg/m³ (1-hour average).
• US EPA NAAQS: 9 ppm (8-hour average) and 35 ppm (1-hour average), not to be exceeded more than once per year.
• Typical occupational exposure limits: 20–50 ppm as an 8-hour time-weighted average, depending on jurisdiction.

Reliable determination of atmospheric CO content therefore requires validated sampling procedures, high-sensitivity analytical instrumentation, rigorous calibration with certified reference gases, and comprehensive quality assurance. Accredited third-party laboratories and official monitoring stations routinely perform these measurements in accordance with international and national standard methods.

Testing Samples

Ambient (Outdoor) Air

Samples are collected at fixed air quality monitoring stations in urban, suburban, industrial, and traffic-adjacent locations. Continuous in-situ analyzers draw ambient air through inert sampling lines, and concentrations are recorded as 1-hour and 24-hour averages for comparison against ambient air quality standards. Data from these stations support air quality index reporting, trend analysis, and regulatory compliance assessment.

Indoor Air

Indoor environments such as residences, offices, schools, underground parking structures, shopping malls, and public transport stations are tested to evaluate ventilation effectiveness and identify combustion-related sources such as gas stoves, water heaters, boilers, and improperly vented heating appliances. Indoor measurements are often conducted both with and without combustion equipment operating to characterize source contributions.

Workplace Atmospheres

Occupational environments including boiler rooms, foundries, refineries, steel mills, warehouses with forklift traffic, vehicle repair workshops, and road tunnels are monitored to verify that worker exposure remains below occupational exposure limits. Measurements may include short-term (15-minute) peak values and full-shift 8-hour time-weighted averages obtained with portable instruments or personal sampling devices.

Confined Spaces and Special Environments

Tanks, manholes, sewers, silos, ship holds, and other confined spaces are tested for CO prior to and during entry, because accumulation of combustion gases in poorly ventilated volumes can be rapidly lethal. Pre-entry screening is typically performed with portable multi-gas detectors equipped with electrochemical CO sensors.

Testing Items

The primary testing item is the concentration of carbon monoxide in air, reported either as a mass concentration (mg/m³) or a volume fraction (ppm). Supporting measurements and quality-control items are typically reported alongside the concentration result to establish data validity.

• CO mass concentration (mg/m³) at reference conditions (0°C, 101.325 kPa or 25°C, 101.325 kPa, as specified by the applicable standard).
• CO volume fraction (ppm); conversion: 1 ppm = 1.25 mg/m³ at 0°C and 1.145 mg/m³ at 25°C, based on the molar mass of CO (28.01 g/mol).
• 1-hour average and 24-hour average concentrations for ambient compliance evaluation.
• 8-hour time-weighted average (TWA) and 15-minute short-term exposure concentrations for occupational assessment.
• Carboxyhemoglobin (COHb) estimation modeling in exposure assessments, where required.
• Analyzer performance data: linearity, zero drift, span drift, detection limit, and measurement uncertainty.

Testing Methods

Non-Dispersive Infrared Spectrophotometry (NDIR)

NDIR is the reference method for ambient CO monitoring and the most widely used automated technique. CO molecules absorb infrared radiation in the characteristic wavelength band near 4.6–4.7 µm. The analyzer measures the attenuation of an IR beam passing through a multi-pass sample cell and relates absorbance to CO concentration via the Beer–Lambert law. Modern instruments incorporate gas filter correlation (GFC) technology, in which a rotating cell filled with high-concentration CO and a neutral gas alternately modulates the beam, allowing selective identification of CO absorption and effective compensation for interferences from water vapor, carbon dioxide, and hydrocarbons. NDIR analyzers offer detection limits of approximately 0.05–0.1 ppm, excellent long-term stability, and suitability for unattended continuous operation. Relevant standards include GB 9801, ISO 4224, EN 14626, and ASTM D3162 for workplace atmospheres.

Gas Chromatography with Flame Ionization Detection (GC-FID) and Catalytic Methanizer

In this method, air samples are separated on a chromatographic column, and CO is passed through a nickel catalyst (methanizer) where it is quantitatively converted to methane before detection by the FID. Because the FID responds sensitively to methane, this technique achieves detection limits in the low parts-per-billion range, making it ideal for background atmospheric research, clean-air baseline studies, and laboratory-based verification of field instrument readings. Samples may be introduced from gas-tight syringes, sampling bags, or pressurized canisters.

Electrochemical Sensor Method

Portable and fixed instruments using controlled-potential electrochemical cells are widely applied for occupational surveys, indoor air diagnostics, confined-space entry screening, and personal exposure monitoring. CO diffuses through a membrane to a sensing electrode where it is electrochemically oxidized, generating a current proportional to concentration. Typical resolution is 0.5–1 ppm with a measurement range extending to thousands of ppm. Electrochemical instruments are compact, inexpensive, and highly specific, but sensors require periodic bump testing, annual calibration, and replacement every two to four years. This approach is recognized in national occupational hygiene standard methods for workplace CO determination.

Short-Term Detector Tubes and Colorimetric Methods

Colorimetric detector tubes draw a defined air volume through a reagent-filled glass tube; CO reacts with the indicating chemical and produces a stain length proportional to concentration. Although less precise than instrumental methods, detector tubes provide rapid on-site screening results within minutes and are covered by standards such as EN 1231. They are commonly used for preliminary surveys, emergency response, and verification measurements in industrial hygiene practice.

Advanced Optical Techniques

Emerging high-precision methods include tunable diode laser absorption spectroscopy (TDLAS), cavity ring-down spectroscopy (CRDS), and quantum cascade laser (QCL)-based analyzers. These techniques exploit the same fundamental absorption lines of CO but achieve sub-ppb precision and near-zero drift, making them suitable for research networks, calibration laboratories, and trace-level atmospheric studies.

Sampling, Calibration, and Quality Assurance

Regardless of the analytical technique, data quality depends on disciplined sampling and calibration practice. Key requirements include:

• Sampling lines and probe materials made of inert substances such as PTFE, FEP, or stainless steel, kept as short as practical, with particulate filters installed upstream of the analyzer.
• Leak checks of the sampling train and verification of sample flow rate against a calibrated flow meter.
• Multi-point calibration (at least four points including zero) using CO reference gases certified and traceable to national metrology institutes, spanning the expected measurement range.
• Routine zero and span checks, automated daily internal calibration in continuous monitors, and correction of zero/span drift exceeding method tolerances.
• Interference management through gas filter correlation, sample drying (e.g., Nafion permeation dryers), or software compensation for water vapor and CO2 cross-sensitivity.
• Participation in inter-laboratory comparisons and independent audits to maintain accreditation for the testing scope.
• Full documentation of calibration certificates, instrument logs, and raw data to ensure traceability and defensibility of reported results.

Applications

• Ambient air quality monitoring networks and regulatory compliance reporting against national and international CO limit values.
• Occupational health and safety programs in manufacturing, metallurgy, petrochemical, mining, and transportation sectors.
• Indoor air quality investigations in residential, commercial, and public buildings, including source identification for combustion appliances.
• Road tunnel, underground garage, and vehicle emission hotspot studies supporting ventilation design and traffic management.
• Industrial process and combustion efficiency evaluation for boilers, furnaces, and incinerators, including stack emission testing under methods such as US EPA Method 10.
• Emergency response, fire-related smoke exposure assessment, and post-incident clearance verification.
• Atmospheric research, including background CO trend monitoring and support for climate and air chemistry studies.

FAQ

What is the most accurate method for determining CO in the atmosphere?

Gas filter correlation NDIR spectrophotometry is the internationally recognized reference method for continuous ambient CO monitoring, combining low detection limits (approximately 0.05–0.1 ppm), high selectivity, and long-term stability. For ultra-trace measurements at the parts-per-billion level, gas chromatography with a methanizer and FID offers superior sensitivity.

How are ppm and mg/m³ converted for carbon monoxide?

Using the molar mass of CO (28.01 g/mol), 1 ppm equals 1.25 mg/m³ at 0°C and 101.325 kPa, or 1.145 mg/m³ at 25°C and 101.325 kPa. The reference conditions specified in the applicable standard method must be stated with the result to avoid misinterpretation.

What interferences affect CO measurement and how are they controlled?

Water vapor and carbon dioxide are the principal spectral interferents in infrared methods, while certain hydrocarbons and hydrogen can affect electrochemical sensors. Interference is controlled through gas filter correlation optics, sample conditioning such as permeation drying, cross-interference correction algorithms, and sensor designs with selective filter membranes.

How often should CO monitoring instruments be calibrated?

Continuous ambient analyzers should undergo zero and span verification at least daily using automated internal checks, with full multi-point calibration performed after maintenance, transport, or when drift tolerances are exceeded. Portable and personal instruments require bump testing before each day of use and full calibration at least annually, or more frequently per manufacturer recommendations and applicable occupational standards.

Can electrochemical sensor readings be used for regulatory compliance?

Electrochemical instruments are widely accepted for occupational exposure screening and indoor air assessment when calibrated with certified reference gases and operated per validated procedures. For ambient air compliance reporting, reference-equivalent methods such as GFC-NDIR operated under the applicable standard are generally required.

How long does a typical CO determination take?

Instrumental readings are essentially real-time, with stable results available within a few minutes of purging and stabilization. Regulatory ambient evaluations, however, are based on statistics over defined averaging periods, typically 1-hour and 24-hour averages, while occupational assessments require measurements over the full work shift to obtain a valid 8-hour time-weighted average.