Differential absorption LiDAR (DIAL) is an advanced laser remote sensing technique that can accurately measure the concentration and distribution of pollutants, greenhouse gases, and industrial emissions. This article will provide an overview of DIAL's application for emission detection, its notable benefits, recent advancements, and prospects for the future.
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The Industrial Revolution saw a significant shift towards burning fossil fuels like coal, oil, and gas, driving impressive advancements but also causing extensive environmental harm. Emissions from these activities can harm air quality and are a major contributor to global warming and climate change. Therefore, accurately monitoring these emissions is imperative for environmental protection and public well-being.
Traditional methods based on emission inventories can have limited accuracy, highlighting the need for an independent measurement-based approach capable of autonomously assessing and verifying emissions from localized point sources.
Differential absorption lidar (DIAL) has emerged as a valuable technique for measuring and detecting emissions with high accuracy. It relies on the backscattering of laser light by atmospheric molecules and aerosols through Rayleigh and Mie scattering.
Initially developed in 1966 for water vapor (H2O) measurements, DIAL has since been employed to measure various atmospheric gases, including ozone (O3) and numerous pollutant gases such as nitrogen dioxide (NO2), sulfur dioxide (SO2), ammonia (NH3), carbon dioxide (CO2) and carbon monoxide (CO). The Network for Detection of Atmospheric Climate Change (NDACC), an international initiative, incorporates a global network of DIAL ozone lidars.
How Does Differential Absorption LiDAR Enable Accurate Emission Measurement?
The DIAL technique relies on differences in the absorption of laser light by the target emission at carefully selected wavelengths. The instrument transmits laser pulses at two wavelengths - one coinciding with a strong absorption line of the targeted emission (on-resonance) and the other at a wavelength with minimal absorption (off-resonance).
As these two laser pulses propagate through the atmosphere, the on-resonance beam is preferentially absorbed by the target emission, while the off-resonance beam is minimally impacted. The backscattered light from both wavelengths is analyzed to detect the resulting difference in signal intensity, which reveals the gas concentration integrated along the laser beam path.
By measuring the difference in signal strength and the time-of-flight, DIAL systems acquire range-resolved measurements to build up concentration profiles and two- or three-dimensional concentration maps. Combining this spatial mapping data with meteorological data enables accurate quantification of emission rates from sources. In addition, conducting DIAL measurements in a vertical plane enables the creation of a concentration data plane, from which the emission flux can be calculated with knowledge of wind speed and direction.
The optimal DIAL configuration for measuring emission sources is at a distance of 100-200 meters with a clear line of sight in both upwind and downwind directions, ensuring high sensitivity and minimal interference.
Advantages and Disadvantages
DIAL directly quantifies emissions without relying on estimated combustion efficiency or operating conditions, allowing for the testing and validation of emission estimates. Its remote sensing capability is particularly valuable for hazardous flares, eliminating the need for direct access and enabling measurements in challenging-to-reach locations. In addition, it provides range-resolved, three-dimensional emissions mapping, allowing precise identification and quantification of emission sources.
However, it is typically employed for short-term assessments and may not capture rapid variations, and the physical size of DIAL facilities can limit access to smaller sites. While it can operate in various weather conditions, very light winds and heavy fog pose measurement challenges.
Recent Research and Development
QLM's Advanced Differential Absorption LiDAR Excels in Methane Detection Trials
In recent tests conducted at Colorado State University's Methane Emissions Technology Evaluation Center (METEC), UK-based startup QLM Technology has demonstrated the industry-leading performance of its lidar imaging system for methane emissions detection.
This system, called "Quantum Gas Lidar," combines differential absorption lidar with tunable diode laser absorption spectroscopy and time-correlated single-photon counting. It generates lidar-based images of greenhouse gases like methane, allowing for the rapid visualization and quantification of leaks in pipelines and gas storage facilities.
During the 81-day trial at the METEC's "Advancing Development of Emissions Detection" (ADED) project, QLM's methane monitoring solution successfully detected 264 methane leaks, ranging in size from 0.05 to 7 kg/h, identifying 1.39 tons (75%) of the total 1.85 tons of trial emissions at distances of 10-80 meters.
"We believe this performance is among the best ever demonstrated for any type of continuous methane monitoring technology and that the demonstrated performance will exceed the anticipated US EPA (Environmental Protection Agency) requirements for continuous monitoring solutions for fugitive emissions in the oil and natural gas industry," QLM Technology.
High Repetition Rate Mobile DIAL for Real-Time Quantification of Industrial Emissions
Although substantial progress has been made in ultraviolet and visible DIAL systems, research in the mid-infrared spectral region, particularly between 2.5 μm and 4 μm, has been relatively limited. However, this spectral region offers strong absorption characteristics for various atmospheric components and hazardous gases like CH4, NO2, SO2, and H2S, making it promising for gas detection.
A study published in the Journal of Cleaner Production introduced a high-repetition-rate mid-infrared DIAL system mounted on a mobile platform. It used a solid-state tunable optical parametric oscillator laser that emits pulses at 500 Hz, covering a wavelength range of 2.5 μm to 4 μm. The system includes real-time laser energy recording monitoring channels and employs convolution correction for laser wavelength selection to ensure precision.
The designed DIAL system demonstrated rapid real-time monitoring of gas plumes during field tests. It showed excellent sensitivity in measuring low NO2 and SO2 concentrations indicative of industrial/environmental emissions levels. In addition, the mobile nature and second-level temporal resolution make this DIAL ideal for early warning, leak detection, and emissions mapping around industrial facilities.
Differential absorption LiDAR, a versatile in situ gas measurement method, holds great potential for various applications in environmental monitoring, climate research, and industrial emissions management. With technology advancements, DIAL is expected to see increased adoption, ranging from focused field investigations to the establishment of permanent monitoring networks. Its integration with satellites promises global-scale monitoring to support emissions reporting and verification worldwide.
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References and Further Reading
Aaron Van Pelt. (2023). QLM Quantum Gas Lidar Leads Industry in Independent Trials. [Online]. Available at: https://qlmtec.com/news/post/?id=649b59e577ba2ec8a02b2d5a
Gong, Y., Bu, L., Yang, B., & Mustafa, F. (2020). High repetition rate mid-infrared differential absorption lidar for atmospheric pollution detection. Sensors, 20(8), 2211. https://doi.org/10.3390/s20082211
Ismail, S., & Browell, E. V. (2015). LIDAR| Differential Absorption Lidar. https://doi.org/10.1016/B978-0-12-382225-3.00204-8
Shi, T., Han, G., Ma, X., Zhang, M., Pei, Z., Xu, H., ... & Gong, W. (2020). An inversion method for estimating strong point carbon dioxide emissions using a differential absorption Lidar. Journal of Cleaner Production, 271, 122434. https://doi.org/10.1016/j.jclepro.2020.122434
Von Fabrizio Innocenti & Rod Robinson. (2022). Differential Absorption Lidar (DIAL) Measurements of Pollutants and Greenhouse Gases from Industrial Emissions. [Online]. https://www.ingenieur.de/fachmedien/gefahrstoffe/messverfahren/differential-absorption-lidar-dial-measurements-of-pollutants-and-greenhouse-gases-from-industrial-emissions/