Atmospheric Carbon Monoxide Testing Procedure: Sampling, Analysis, and Quality Control Protocol

This atmospheric carbon monoxide (CO) inspection procedure defines the standardized technical requirements for sampling and determining CO concentrations in ambient, indoor, and workplace air. Based on internationally recognized standards such as ISO 4224, EN 14626, GB 9801, and GB 3095, the protocol covers non-dispersive infrared spectrometry, gas filter correlation, electrochemical detection, calibration, and quality control. It is intended for environmental monitoring agencies, accredited third-party laboratories, and industrial hygiene professionals seeking accurate and traceable CO measurements.

This article establishes a complete inspection procedure for carbon monoxide in atmospheric environments, covering sample types, testing items, instrumental methods, calibration, quality assurance, and data reporting in accordance with ISO, EN, GB, ASTM, and JIS standards.

Technical Overview

Carbon monoxide is a colorless, odorless, and tasteless gas generated by the incomplete combustion of carbon-containing fuels such as coal, natural oil, gasoline, natural gas, and biomass. Because CO binds to hemoglobin with an affinity roughly 200 to 250 times greater than that of oxygen, even short-term exposure to elevated concentrations impairs oxygen transport in the blood and can be lethal. Principal emission sources include motor vehicle exhaust, industrial combustion, residential heating, biomass burning, and tobacco smoke. A systematic testing procedure is therefore essential for verifying compliance with ambient air quality standards such as GB 3095 and for protecting both public health and occupational safety.

This procedure specifies the technical requirements for monitoring site selection, sample collection, instrumental analysis, calibration, quality control, and result reporting. It applies to both manual short-term or time-weighted measurements and continuous automatic monitoring performed by environmental agencies, accredited third-party laboratories, and industrial hygiene teams.

Testing Samples

Ambient (Outdoor) Air

Samples are collected at fixed monitoring stations representative of urban, suburban, industrial, or background locations. Inlet height is typically 3 to 15 meters above ground, positioned away from direct emission sources, tall buildings, and heavy vegetation, in accordance with national monitoring network siting specifications such as HJ 664 and HJ 654.

Indoor and Public Place Air

Samples are taken in residences, offices, hotels, shopping malls, and other enclosed public spaces, with the sampling probe placed in the breathing zone at 1.2 to 1.5 meters above floor level and at least 0.5 to 1 meter away from walls, doors, windows, and ventilation outlets. Reference requirements follow GB/T 18204.2.

Workplace Air

Samples are collected in the operator breathing zone to assess occupational exposure against time-weighted average (TWA) and short-term exposure limit (STEL) values defined in GBZ 2.1. Both personal sampling and fixed-point area monitoring are acceptable, following the direct-reading approach described in ASTM D3162.

Stationary Source and Confined Space Samples

Flue gas from boilers, furnaces, and incinerators is sampled with heated probes and particulate filters, while confined spaces such as tunnels, underground garages, and pipelines are screened at multiple heights due to potential stratification of CO.

Bag and Cylinder Samples for Laboratory Analysis

When on-site analysis is not feasible, air is transferred into inert fluoropolymer or aluminum-lined gas sampling bags, filled with a non-contaminating pump, and analyzed in the laboratory preferably within 24 to 72 hours to minimize loss and contamination.

Testing Items

The core deliverables of the inspection are listed below:

• Instantaneous (real-time) CO concentration, expressed in mg/m³ or ppm (v/v)
• 1-hour average CO concentration
• 8-hour and 24-hour rolling or calendar averages
• Time-weighted average (TWA) concentration for occupational exposure assessment
• Short-term (15-minute or STEL) peak concentration comparison values
• Instrument performance verification: detection limit, linearity, response time, zero drift, and span drift
• Comparison of measured values against applicable limit values in GB 3095, GBZ 2.1, or client-specified criteria

Testing Methods

Non-Dispersive Infrared Spectrometry (NDIR)

CO selectively absorbs infrared radiation in the 4.6 to 4.7 micrometer band. The analyzer measures the differential attenuation between a sample cell and a reference cell, and the absorption signal is converted to concentration via Beer-Lambert calibration. NDIR is the reference method specified in ISO 4224, GB 9801, and EN 14626, with a typical detection limit of 0.05 ppm or lower and a measurement range covering ambient to source-level concentrations.

Gas Filter Correlation (GFC) NDIR

A rotating gas cell filled with high-purity CO alternately provides a correlated and a reference beam path. Because only true CO absorption correlates between the two states, cross-sensitivity to water vapor, carbon dioxide, and hydrocarbons is strongly suppressed. GFC-NDIR is widely designated as a reference or equivalent method in automated national monitoring networks and is covered by instrument standards such as JIS B 7959.

Electrochemical Sensor Method

Portable direct-reading instruments oxidize CO at a sensing electrode, generating a current proportional to concentration. They are suitable for spot checks, indoor air surveys, confined-space entry screening, and workplace short-term monitoring per ASTM D3162. Cross-sensitivity, sensor aging, and temperature effects must be checked regularly against certified gas standards.

Gas Chromatography with Methanizer and FID

CO is chromatographically separated, catalytically reduced to methane in a methanizer, and quantified by flame ionization detection. This technique offers excellent precision and low detection limits and is frequently used as a laboratory reference or transfer-standard method for validating network analyzers and calibration gases.

Tunable Diode Laser Absorption Spectroscopy (TDLAS)

Laser-based methods probe individual CO rotational-vibrational lines with high spectral resolution, providing fast response, minimal interference, and open-path measurement capability for fenceline monitoring, tunnels, and research applications.

Mercury Replacement Method (Legacy)

In this historical manual technique, CO reduces heated mercuric oxide, and the liberated mercury vapor is measured photometrically. Due to mercury handling hazards and limited automation, the method has largely been superseded by optical techniques and is retained only for reference or comparison purposes in older protocols.

Sampling Requirements

• Use inert sample lines of PTFE or FEP, kept as short as practicable, with a PTFE particulate filter (2 to 5 micrometers) at the inlet
• Prevent moisture condensation using Nafion or permeation dryers or by maintaining lines above the dew point
• Verify sampler flow rate with a calibrated dry gas meter or electronic flow meter; deviation must not exceed 2 percent
• For continuous analyzers, record 1-minute averages and compute hourly means; a valid hourly value requires at least 45 valid minutes of data
• Avoid sampling during instrument stabilization, calibration, or zero/span operations, and log all anomalies

Quality Assurance and Calibration

• Use zero gas (CO-free air) and span gases certified and traceable to national metrology institutes, with certificate uncertainty typically within 1 percent
• Perform multipoint calibration with at least five concentration levels spanning the working range; linearity correlation coefficient r² must be 0.999 or greater, with each point within 2 percent of its certified value
• Conduct zero and span checks daily at automated stations and before and after each field survey; apply drift correction if drift is within 10 percent, otherwise repeat calibration and affected measurements
• Evaluate interference from water vapor, CO₂, and hydrocarbons and apply compensation or drying as required by ISO 4224 and EN 14626
• Collect parallel or duplicate samples; relative deviation between duplicates should not exceed 10 percent
• Correct results to reference conditions of temperature and pressure and document the complete calibration and audit trail per HJ 818 operational quality control requirements

Data Processing and Reporting

Convert instrument readings using the relationship: concentration in mg/m³ equals ppm value multiplied by 28.01 and divided by 24.45 at 25 °C and 101.325 kPa, or divided by 22.41 at 0 °C and 101.325 kPa. Compare validated averages with applicable criteria; for example, GB 3095-2012 specifies a 24-hour limit of 4 mg/m³ and a 1-hour limit of 10 mg/m³ for CO. The final report shall identify the sampling location and period, the analytical method and standard, instrument identification, calibration and QC records, averages with associated measurement uncertainty, and a clear conformity statement.

Applications

• Urban and regional air quality monitoring networks and air quality index reporting
• Compliance assessment against ambient air quality standards and environmental impact acceptance
• Occupational health surveys and industrial hygiene exposure assessment
• Indoor air quality evaluation for residences, schools, and public places
• Tunnels, parking garages, and transportation facility ventilation control
• Combustion efficiency diagnostics and stationary source emission testing
• Atmospheric chemistry research, emission inventory validation, and calibration gas verification

FAQ

What is the permissible concentration of CO in ambient air?

Under GB 3095-2012, the 24-hour average limit is 4 mg/m³ and the 1-hour average limit is 10 mg/m³. For workplaces, GBZ 2.1 specifies a PC-TWA of 20 mg/m³ and a PC-STEL of 30 mg/m³, which are markedly higher because occupational limits apply to healthy adult workers over defined exposure durations.

How do you convert ppm of CO to mg/m³?

At 25 °C and 101.325 kPa, multiply the ppm value by 1.145 to obtain mg/m³; at 0 °C and 101.325 kPa, multiply by 1.25. Always state the reference temperature and pressure used in the conversion.

Why is NDIR the preferred method for continuous ambient CO monitoring?

NDIR provides continuous, maintenance-friendly, and highly stable measurements with detection limits well below ambient concentration levels, and it is formally specified as the reference principle in ISO 4224, GB 9801, and EN 14626. The gas filter correlation variant further suppresses water vapor and CO₂ interference, making it the standard choice for automated networks.

What are the main interferences in CO measurement?

Water vapor and carbon dioxide exhibit partial spectral overlap with the CO absorption band, while certain electrochemical sensors show cross-sensitivity to hydrogen and reducing gases. Interferences are controlled through gas filter correlation optics, sample drying, chromatographic separation, or sensor cross-sensitivity verification with certified gases.

How often must CO analyzers be calibrated?

Automated monitoring stations typically perform daily automatic zero and span checks with full multipoint calibration at least quarterly or following major maintenance. Portable field instruments must be zeroed and span-checked immediately before and after each measurement campaign, and any drift exceeding 10 percent invalidates the intervening data unless a valid drift correction is applied.