Marine vessels are indispensable to the global economy, transporting over 80% of goods worldwide. However, their emissions, including sulfur oxides (SOₓ), nitrogen oxides (NOₓ), particulate matter, and volatile organic compounds (VOCs), pose a growing threat to the marine and coastal atmospheric environment, especially in busy shipping channels and major port cities. Effective regulation and mitigation of these emissions necessitate efficient and accurate monitoring techniques.
Current optical imaging remote sensing methods, while vital, often fall short due to insufficient detection accuracy and inadequate spatiotemporal resolution. Techniques like satellite and airborne remote sensing are limited by resolution and cloud cover, while portable systems are costly and cannot evaluate plume diffusion. Other spectral imaging methods such as UV/IR cameras and FTIR imaging have their own limitations in quantifying specific pollutants or mobile imaging.
In a new paper published in Light: Science & Applications, a team of scientists, led by Professor Cheng Liu from the University of Science and Technology of China, and co-workers have developed a fast-hyperspectral imaging remote sensing technique. This system achieves precise imaging and quantification of nitrogen dioxide (NO₂) and sulfur dioxide (SO₂) emissions from marine vessels.
The innovative instrument features a coaxial design of three cameras (hyperspectral, visible, and multiwavelength filters) and a high-precision spectrometer temperature control system maintaining 20∘C±0.5∘C to reduce noise. A key advancement is the plume categorization method based on O₄ variations, allowing for tailored Air Mass Factor (AMF) calculations for aerosol-present and aerosol-absent plumes, significantly improving accuracy. Furthermore, multiwavelength filters combined with spectral analysis enable precise plume outline identification and detailed observation of trace gas distribution. The system can complete a plume scan in under 4 minutes with a spatial resolution of less than 0.5m × 0.5m.
These scientists summarize the operational principle of their technique:
"We developed a system that integrates a telescope with visible, multichannel UV, and hyperspectral cameras, a 2D scanning system, and a temperature-controlled spectrometer. The instrument performs continuous 'S'-shaped scanning to cover the target area. Vertical Column Densities (VCDs) are calculated from Differential Slant Column Densities (DSCDs) and AMF. AMF accuracy is enhanced by categorizing plumes based on O₄ DSCD variations to determine aerosol presence and applying specific retrieval algorithms. For precise plume outline and concentration, a plume reconstruction scheme uses differential absorption intensities from filter cameras to create high-resolution distribution weights, correcting the hyperspectral camera's initial measurements."
The study successfully quantified NO₂ and SO₂ emissions from a large ocean cargo ship and an offshore small cargo ship in Qingdao. For instance, maximum NO₂ and SO₂ concentrations from a large ocean cargo ship were 0.124 and 0.425 mg m⁻³, respectively. The technique also captured variations in emissions as ships approached port, likely due to changes in fuel quality and engine power.
"This hyperspectral imaging technique provides a new avenue for atmospheric environment monitoring and the establishment of dynamic, measurement-driven emission inventories, overcoming the timeliness issues of current methods," the scientists forecast. "While challenges like developing comprehensive absorption cross-section libraries and enabling nighttime and greenhouse gas imaging remain, this work is a significant step towards better pollution control and a healthier marine environment." The team also proposed a future-proof concept for nighttime hyperspectral imaging using an active multiwavelength LED source combined with a UAV-mounted reflector and high-precision tracking.
