Roadside measurement of N₂O and CH₄ emissions from vehicles in Hong Kong

Methane (CH₄) and nitrous oxide (N₂O) are powerful greenhouse gases (GHG) and provide an opportunity beyond CO₂ for climate mitigation. In addition, CH₄ acts as a precursor for tropospheric ozone formation. Addressing these gases allows for simultaneous action against climate change and ozone-related environmental issues. However, emissions of these gases from on-road transport sectors are not well-characterised. This study conducts roadside monitoring at four sites in Hong Kong, measuring roadside concentrations of CH₄, N₂O and CO₂ in exhaust plumes by Middle-Infrared Laser Absorption Spectroscopy at 1 Hz. A camera captured images of passing vehicles and subsequently machine learning determined vehicle type. Emission Factors (EFs) could be estimated for individual vehicles by extracting these from isolated peaks in the concentration profiles. The EFs were determined from the concentration ratio with CO₂ from 858 plumes emitted by 667 vehicles. Results indicate that double-decker buses exhibit significantly high EF_N2O (median 248 mg km^-1) while trucks, which though lower also exhibit relatively high EF_N2O values (median 24 mg km^-1). No significant differences in EF_N2O were observed between Euro 5 and Euro 6 buses. Mini-buses fuelled by LPG have the highest EF_CH4, with a median of 213 mg km^-1, while LPG-fuelled taxis with smaller engines also show notable values (median 156 mg km^-1). Although the transport sector is the second largest emitter of GHGs in Hong Kong (∼20 % of total CO₂ equivalents), only a small fraction (<8 %) arises as CH₄ and N₂O, with variations among vehicle groups. The measured vehicles generally comply with the emerging, albeit lenient, Euro 7 standards for non-CO₂ GHG emissions. The real-world measurement method proposed in this study has the potential to enhance on-road vehicle emissions assessments, particularly for non-CO₂ emissions. This approach can serve as a valuable tool for future evaluations of real driving emissions (RDE) of non-CO₂ GHGs.