Infrared Determination of Carbon Monoxide in Ambient Atmosphere: Principles, Methods, and Applicatio

Infrared determination of carbon monoxide (CO) in the ambient atmosphere is a widely adopted analytical technique that delivers rapid, sensitive, and continuous measurement of this toxic combustion-derived pollutant. This article provides a comprehensive technical overview of non-dispersive infrared (NDIR) and gas filter correlation (GFC) methodologies, relevant national and international standards, sample types, testing procedures, quality control requirements, and typical applications in environmental monitoring. It serves as a practical reference for environmental engineers, laboratory analysts, air quality professionals, and facility managers seeking reliable CO measurement solutions.

Technical Overview

Carbon monoxide is a colorless, odorless, and tasteless gas produced primarily by the incomplete combustion of carbon-containing fuels. Major atmospheric sources include motor vehicle exhaust, industrial combustion processes, residential heating, biomass burning, and certain chemical manufacturing operations. Because CO readily binds to hemoglobin in human blood to form carboxyhemoglobin, reducing the oxygen-carrying capacity of the blood, accurate measurement of ambient CO concentrations is essential for public health protection and regulatory compliance.

Infrared (IR) determination is the most commonly used analytical approach for measuring atmospheric CO. The technique exploits the fact that CO molecules strongly absorb infrared radiation in their fundamental vibration-rotation band centered at approximately 4.6–4.7 micrometers. By measuring the attenuation of infrared energy passing through a sample cell, the concentration of CO can be quantified with high sensitivity and excellent selectivity. Compared with alternative methods such as gas chromatography with reduction detection or electrochemical sensors, infrared analyzers offer faster response times, continuous monitoring capability, minimal reagent consumption, and stable long-term operation, making them the instrument of choice for automated air quality monitoring networks worldwide.

The key standardized methods include GB 9801 (Air quality — Determination of carbon monoxide — Non-dispersive infrared spectrophotometry) in China, ISO 4224 (Ambient air — Measurement of the mass concentration of carbon monoxide — Gas filter correlation infrared spectrometry method) internationally, and corresponding reference methods published by environmental protection agencies such as the United States EPA automated reference methods for CO. These standards define instrument performance criteria, calibration requirements, and data quality objectives that accredited third-party laboratories follow when reporting defensible measurement results.

Testing Samples

Ambient (Outdoor) Air

Ambient air samples are collected from urban, suburban, industrial, and roadside monitoring stations. Air is drawn continuously through a particulate filter and a moisture-removal device (such as a permeation dryer or Nafion tube) into the infrared analyzer. Typical sampling heights are 1.5 to 15 meters above ground, depending on the monitoring objective, and samples may be averaged over 1-hour or 8-hour reporting intervals to match air quality limit values.

Indoor Air

Indoor air samples from residences, offices, schools, underground parking garages, and commercial kitchens are tested to verify compliance with indoor air quality guidelines. Sampling points are typically positioned at breathing height (0.5–1.5 m) and away from windows, doors, and direct combustion sources unless source-proximate assessment is required. The applicable guideline values are commonly set at 10 mg/m³ for an 8-hour average under standards such as GB/T 18883.

Workplace and Industrial Process Air

Occupational exposure assessments require measurement of CO near combustion installations, boiler rooms, smelting operations, and confined workspaces. Grab samples may be collected in gas-tight bags or evacuated canisters for subsequent laboratory analysis, or portable infrared analyzers may be used for direct-reading field measurements. Results are compared against occupational exposure limits such as time-weighted averages and short-term exposure limits.

Vehicle Exhaust and Source Emissions

Although primarily regulated as a source emission parameter, CO in diluted or raw exhaust streams is frequently measured with infrared analyzers in engine test cells, inspection and maintenance programs, and tunnel ventilation studies, providing source data that support atmospheric dispersion modeling and inventory development.

Testing Items

The core testing item is the mass concentration of carbon monoxide in air, expressed in milligrams per cubic meter (mg/m³) at reference conditions (typically 25 °C and 101.325 kPa, or standard conditions of 0 °C and 101.325 kPa as specified by the applicable standard), or as a volume fraction in parts per million (ppm). Depending on the monitoring program, reported parameters include:

• Real-time CO concentration (1-minute to 1-hour averages)
• 1-hour maximum and 8-hour rolling average concentrations for ambient air compliance
• 24-hour and annual statistics for trend analysis and network reporting
• Detection limit, method detection limit, and measurement uncertainty statements
• Parallel determination results and relative deviation for quality assurance documentation

Ambient air quality standards such as GB 3095 typically specify limit values of 10 mg/m³ for a 1-hour average and 4 mg/m³ for a 24-hour average for CO in residential and mixed-use zones, and infrared measurement results are evaluated against these thresholds.

Testing Methods

Non-Dispersive Infrared (NDIR) Spectrophotometry

The non-dispersive infrared method, specified in GB 9801, is the classical technique for atmospheric CO determination. An infrared broadband source emits radiation through a measuring cell of fixed path length (commonly 100 mm to 1 m) through which the sample gas flows continuously. CO molecules in the cell absorb energy at their characteristic wavelengths. The radiation is then detected either by a gas-selective detector filled with CO (which responds only to the CO absorption band) or by solid-state detectors fitted with narrow-band optical filters centered near 4.6 micrometers. A parallel reference beam through a CO-free path compensates for source intensity drift. The difference in detector signals is proportional to the CO concentration according to the Beer–Lambert law. The method achieves a typical lower detection limit of about 0.1–0.3 mg/m³ with a linear working range extending to 62.5 mg/m³ or higher, suitable for all routine ambient monitoring scenarios.

Gas Filter Correlation (GFC) Infrared Spectrometry

Gas filter correlation infrared spectrometry, standardized in ISO 4224, is an advanced refinement of the NDIR principle designed to suppress cross-sensitivity to water vapor and carbon dioxide. A rotating wheel alternately inserts two gas-filled cuvettes into the optical path: one containing nitrogen (the reference or broadband position) and one containing a high concentration of CO (the measuring or correlation position). When the CO-filled filter is in the beam, radiation at CO absorption wavelengths is strongly attenuated before reaching the sample cell, so only interfering absorption by other species is measured; when the nitrogen filter is in place, absorption by both CO and interferents is measured. The ratio of the two signals isolates the CO-specific absorption with high selectivity. GFC analyzers routinely achieve detection limits below 0.05 mg/m³ (approximately 40 ppb) and are designated as equivalent or automated reference methods in many national monitoring networks.

Fourier Transform Infrared (FTIR) Spectrometry

FTIR analyzers acquire the full mid-infrared spectrum of the sample and quantify CO (and multiple co-emitted species simultaneously) using multivariate classical least-squares analysis against certified reference spectra. FTIR is particularly valuable in research applications, source characterization, and complex matrices where several infrared-active gases must be measured concurrently from a single sample.

Calibration and Quality Control

Rigorous calibration and quality assurance underpin all infrared CO measurements and are mandatory elements of the referenced standards. Key procedures include:

• Zero calibration with zero air or high-purity nitrogen certified to contain no detectable CO
• Span calibration with certified CO standard gases traceable to national metrology institutes, typically at 80–90 percent of the measurement range
• Multipoint calibration verification covering the expected ambient concentration range, with acceptance criteria for linearity (correlation coefficient typically ≥ 0.999)
• Removal or control of interferences from water vapor and carbon dioxide using dryers, optical correction, or the GFC technique
• Leak checks of the sampling manifold, verification of sample flow rate, and confirmation of sample cell pressure and temperature stability
• Periodic performance audits, including precision checks (relative deviation of parallel determinations, commonly ≤ 5 percent) and drift assessments before and after sampling campaigns

Accredited third-party laboratories conduct these determinations within documented quality management systems, ensuring that reported concentrations are traceable, reproducible, and legally defensible.

Applications

Infrared CO determination supports a broad spectrum of regulatory, industrial, and scientific applications:

• National and municipal ambient air quality monitoring networks, providing continuous CO data for public reporting and attainment assessment
• Indoor air quality investigations in residential, commercial, and public buildings, particularly following combustion appliance installation or suspected CO leakage incidents
• Occupational hygiene surveys at steel plants, refineries, boiler facilities, mines, and tunnel construction sites
• Environmental impact assessments and post-project verification for transportation corridors, parking structures, and industrial facilities
• Traffic-related air pollution studies, roadside monitoring, and vehicle emission inspection programs
• Research on atmospheric chemistry, urban airshed modeling, and validation of satellite and low-cost sensor CO measurements
• Emergency response and forensic investigations following fires, explosions, or suspected poisoning events

FAQ

Why is the infrared method preferred for atmospheric CO measurement?

The infrared method combines high selectivity for CO with rapid response, continuous unattended operation, low maintenance, and absence of wet chemicals. These attributes make it ideally suited to automated monitoring stations that must report hourly data year-round with minimal operator intervention, whereas manual methods are more labor-intensive and provide only snapshot results.

What is the typical detection range and accuracy of infrared CO analyzers?

Conventional NDIR analyzers offer working ranges of approximately 0–62.5 mg/m³ with detection limits near 0.1–0.3 mg/m³, while gas filter correlation instruments achieve detection limits below 0.05 mg/m³. Measurement uncertainty, when the instrumentation is properly calibrated with traceable standards, is typically within ±2 percent of the reading or ±2 percent of full scale, comfortably satisfying data quality objectives for compliance monitoring.

Do water vapor and carbon dioxide interfere with the measurement?

Both species absorb in regions of the infrared spectrum that can overlap the CO absorption band. Interferences are managed by drying the sample gas with permeation or condensation dryers, by applying optical filtering, and most effectively by using the gas filter correlation technique, which mathematically discriminates between the narrow CO absorption lines and the broad interfering features. Properly designed systems reduce combined interference effects to less than the instrument detection limit at typical ambient CO2 and humidity levels.

How are the analyzers calibrated in the field?

Field calibration involves introducing zero gas to set the baseline and certified span gas (typically around 80–90 percent of the operating range) to establish the sensitivity, followed by a mid-level check standard to verify linearity. Calibration gas cylinders must be certified and traceable to national metrology standards, and calibration records form part of the laboratory quality documentation for each monitoring campaign.

What sample conditioning is required before analysis?

The sample is passed through a particulate filter (commonly 2–5 micrometer pore size) to protect the optical cell, and water is removed by a permeation dryer or refrigerated condenser to prevent condensation and minimize humidity interference. Flow controllers maintain a constant sample flow, and the cell temperature is stabilized by the instrument thermostat to ensure measurement stability.

Can the same technique be used for source-level CO measurements?

Yes. Infrared CO analyzers with appropriate range extension or sample dilution are routinely applied to stack gas, vehicle exhaust, and tunnel measurements. When high-concentration samples are analyzed, dilution systems calibrated with traceable gases keep the sample within the linear range of the detector and prevent saturation of the absorption signal.