The core of online monitoring of greenhouse gases is to capture the characteristic signals of gas molecules in real time and convert them into quantifiable concentration data, thereby achieving continuous monitoring of greenhouse gases such as carbon dioxide (CO ₂), methane (CH ₄), nitrous oxide (N ₂ O), etc. Its working principle revolves around the physical or chemical properties of gas molecules, and the differences between different monitoring technologies are mainly reflected in the signal capture methods.
Optical absorption method: capturing the "fingerprint" spectrum of gas
Principle core: Utilizing the selective absorption characteristics of greenhouse gas molecules towards specific wavelengths of light.
Each gas molecule has a unique absorption spectrum (like a "molecular fingerprint"), for example, CO ₂ strongly absorbs infrared light with a wavelength of 4.26 μ m, while CH ₄ is sensitive to infrared light with wavelengths of 3.31 μ m and 7.66 μ m. Online monitoring equipment emits light of a specific wavelength through a light source. When the light passes through a gas containing greenhouse gases, some of the light is absorbed by the gas, and the degree of attenuation of the light intensity is proportional to the gas concentration (following Lambert Beer's law).
By detecting changes in light intensity, the gas concentration can be inferred. Among these methods, non dispersive infrared spectroscopy (NDIR) is widely used for online monitoring of fixed pollution sources (such as factory chimneys) and ambient air due to its simple structure and fast response; Fourier transform infrared spectroscopy (FTIR) can simultaneously monitor multiple gases and is suitable for complex working conditions.
Laser spectroscopy: precise concentration measurement with high specificity of laser
Principle core: Using laser as the light source and utilizing the absorption lines of gas molecules for high-sensitivity detection.
Laser has the characteristics of strong monochromaticity and adjustable wavelength, which can accurately target specific absorption lines of greenhouse gas molecules (such as adjusting the laser wavelength to the absorption peak of CH ₄). When the laser passes through the gas to be measured, the gas molecules absorb the laser energy, resulting in a decrease in laser intensity, which is related to the gas concentration.
Common laser spectroscopy techniques include tunable semiconductor laser absorption spectroscopy (TDLAS) and cavity enhanced laser absorption spectroscopy (CELAS). This type of method has strong anti-interference ability and low detection limit (up to ppb level), suitable for online monitoring of trace impurities in low concentration greenhouse gases (such as CH ₄ in ambient air) or high-purity gases.
Gas chromatography: detecting mixed gases after separation
Principle core: First, separate the different components in the mixed gas, and then detect their concentrations one by one.
The gas to be tested enters the chromatographic column through the sampling system, and the fixed phase (such as adsorbent) inside the column will produce different adsorption desorption effects on different greenhouse gas molecules, resulting in different retention times of each component in the chromatographic column, thus achieving separation. The separated gases enter the detector in sequence (such as flame ionization detector FID for detecting CH ₄, and thermal conductivity detector TCD for detecting CO ₂). The detector generates an electrical signal based on the reaction between the gas and a specific reagent (such as the ion flow generated by the combustion of CH ₄ in FID), and the signal strength is proportional to the concentration.
Gas chromatography has high accuracy and can analyze multiple gases simultaneously, but its response speed is slow (usually every few minutes), making it suitable for scenarios that require high accuracy but do not require high-frequency monitoring, such as greenhouse gas inspections at the boundaries of industrial parks.
Mass spectrometry: Identification of gases by ion mass charge ratio
Principle core: Ionize gas molecules into ions, distinguish different gases based on the mass charge ratio (m/z) of ions, and calculate the concentration.
After entering the ion source, the gas to be tested is ionized (such as electron bombardment ionization), and the generated ions are deflected in an electric or magnetic field. Different masses of ions have different deflection trajectories, ultimately forming different signal peaks on the detector. The gas species can be identified by the position of the peak (corresponding to m/z) (such as CO ₂ with m/z of 44 and CH ₄ with 16), and the intensity of the peak reflects the concentration.
Mass spectrometry has fast response, can simultaneously detect multiple gases, and does not require complex pretreatment. However, it has high equipment costs and complex maintenance, and is often used in laboratories or special industrial scenarios (such as real-time tracking of greenhouse gases in chemical reaction processes).
Summary: Applicable scenarios of different principles
The various technologies for online monitoring of greenhouse gases essentially achieve concentration measurement by identifying the unique physical and chemical properties of gas molecules. Optical absorption method and laser spectroscopy method have the advantages of fast and real-time, and dominate online continuous monitoring; Gas chromatography wins with high precision and is suitable for periodic detection; Mass spectrometry plays a role in complex component analysis. These technologies together constitute the core means of monitoring greenhouse gas emissions in fields such as petrochemicals, electricity, and environmental protection, providing data support for emission reduction decisions.