Dynamic Link-Level On-Road Transportation Emission Inventories in Hong Kong for Environmental Assessment and Roadside Environment Exploration

Abstract

Effective management of air quality requires a comprehensive understanding of on-road traffic air pollutant emission patterns, particularly in urban environments where capturing these dynamics poses significant challenges. This study addresses this gap by developing a novel approach that integrates traffic congestion index from Google Maps, a traffic density model, and local emission factors to create a dynamic on-road traffic emission inventory for Hong Kong. This inventory encompasses a wide range of pollutants, including NO_x, CO, NMVOC, CO₂, PM_2.5, PM₁₀, and CH₄, as well as over 80 speciated VOCs from both exhaust and evaporative emissions.

Our findings indicate that exhaust emissions on weekdays can increase by up to 50% compared to Sundays, primarily due to heightened traffic congestion, with significant emission rates observed on highways during weekday peak hours. Public light buses, goods vehicles, and taxis emerged as the largest contributors to NMVOC, NO_x, and CO emissions, respectively. Control policy testing revealed that transitioning franchised buses to electric buses could substantially reduce NO_x and PM emissions, while replacing liquefied petroleum gas-fueled public light buses with electric alternatives effectively manages NMVOC emissions.

Additionally, phasing out older private cars proved beneficial in reducing CO emissions.

Furthermore, we dynamically quantified the emissions of speciated VOCs from both tailpipe and evaporative emissions. employing a real-time model to assess street-scale evaporative emissions and their health and environmental impacts in urban areas for the first time. Notably, evaporative emissions of non-methane organic gases were found to be comparable to those from tailpipe sources, with aromatic compounds from evaporative sources being notably abundant. Multi-effects evaluation underscores the necessity to regulate aromatics emissions from evaporative sources and alkenes emissions from exhaust, while highlighting stricter controls on intermediate volatile organic compounds (IVOCs) from diesel vehicles. Although the average health index (HI) across the 18 districts remained below 1, indicating no chronic inhalation non-carcinogenic risks, certain locations exhibited high risks (HI > 1). Carcinogenic risks were observed in Wan Chai, Central & Western, Kowloon City, Kwun Tong, Sham Shui Po, and Yau Tsim Mong districts, primarily driven by benzene, 1,3-butadiene emissions. While evaporative emissions release more toxic aromatics, exhaust emissions still warrant attention. The vehicle types that contribute most significantly to health risks were identified. Vehicle electrification scenarios indicated that simply transitioning to electric vehicles may not effectively mitigate incremental health risks; instead eliminating the largest toxic species contributor would be more effective.

Through an extensive observation of roadside VOC concentrations across diverse urban sites, we identified distinct VOC profiles at different sites. The identified contributors to health risks at roadsides, emphasizing the need for stringent policies to mitigate harmful pollutants as well. The comparative analysis of model simulations and observational data highlights the necessity for further refinement of chemical reaction mechanisms to enhance model simulation accuracy.

In summary, this research provides a high-resolution, dynamic on-road emission inventory for Hong Kong, offering valuable insights into urban air pollution dynamics and informing effective policymaking aimed at improving air quality and public health.

Date of Award	2025
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Dasa GU (Supervisor)