Roadside remote sensing (RS) is utilized by Hong Kong’s Environmental Protection Department to tackle urban air quality issues from on-road vehicles. This technology scans numerous vehicles without disrupting traffic. Currently, horizontal remote sensing (HRS) devices are limited to single-lane applications, while vertical remote sensing (VRS) can scan multiple lanes but faces installation and maintenance challenges.

A diagonal remote sensing (DRS) system modifies conventional RS by elevating and tilting the light source to scan exhaust plumes diagonally, reducing maintenance needs. This study evaluates a DRS prototype’s performance in detecting emissions from a diesel truck, comparing it with HRS and VRS. The systems measured tailpipe emissions under various conditions, revealing average NOx to CO2 ratios of 44% (HRS), 32% (VRS), and 36% (DRS) compared to portable emission measurement systems (PEMS).

Linear regression analyses indicated the strongest correlation between HRS and PEMS, followed by VRS and DRS. The average NOx signal magnitudes were 0.52 (HRS), 0.61 (VRS), and 0.21 (DRS). The findings suggest that DRS performance is comparable to HRS and VRS, potentially enhancing urban air quality control efforts.

Acknowledgement

This work was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. UGC/IDS(C)25/E02/19).