Post Date: 30 June 2026

Emission Estimation of Halocarbons Using a Bayesian Inversion Model Based on Hong Kong Observations and Environmental Assessment

Abstract:

Halogenated hydrocarbons (halocarbons), including chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and hydrofluorocarbons (HFCs), remain central to atmospheric chemistry and environmental policy. Although ozone-depleting substances (ODS) have been phased down under the Montreal Protocol, their replacements are potent greenhouse gases and continue to affect regional air quality and human exposure. In East and Southeast Asia, rapid industrialization and widespread use of refrigerants and solvents make halocarbons important for both climate accounting and urban health assessment. Hong Kong, at the interface of southern China and the South China Sea, offers a valuable platform for studying ambient levels, source contributions, and regional emissions.

This thesis first examines intra-urban hazardous VOC exposure in Hong Kong through a two-year campaign (August 2020 to July 2022) at two urban and one suburban sites. Thirty-two species were measured and evaluated using ozone formation potential, secondary organic aerosol formation potential, and toxicity-based metrics, with Positive Matrix Factorization applied to quantify source contributions. Toluene dominated integrated health risks, and solvent usage was the leading source at all sites, although urban sites showed stronger vehicular influence and the suburban site greater combustion contributions. Cancer risk concerns were driven largely by benzene and short-lived halocarbons such as chloroform, many of which are not routinely regulated.

The second component evaluates FLEXPART and Bayesian inversion for regional halocarbon emission estimation using observations at HKUST Supersite. Particle sensitivity tests supported operational use of 40,000 particles per release, and source-receptor analysis over two years guided design of a nested inversion domain spanning southern China, the northern South China Sea, and mainland Southeast Asia. Prior scenario experiments for HFC-134a using uniform, EDGAR-based, and population-redistributed priors consistently identified the Pearl River Delta as the dominant upwind hotspot, while adjustments over the Yangtze River Delta and parts of Southeast Asia were more prior dependent.

Building on this framework, the thesis quantifies emissions of HFC-134a and HCFC-142b from 2022-2023 over southern China and Southeast Asia using Bayesian inversion model FlexInvert. The inversion resolves hotspots in the Yangtze River Delta, Pearl River Delta, eastern Sichuan, and selected Southeast Asian urban centres. A global perspective analysis shows southeastern China stands out in area-normalised intensity, while per capita HFC-134a emissions in the study region remain closer to the global mean. Notably, northern Southeast Asia emerges as a non-negligible regional source of HFC-134a, suggesting that the role of Southeast Asia in East Asian halocarbon emissions has been underestimated relative to southern China in previous assessments.

In summary, this thesis links halocarbon-related health risks, long-term observations, and observation-driven emission estimates into a coherent assessment of halocarbon pollution in and around Hong Kong. The findings support expanded monitoring, improved inventories, and risk-based management addressing both legacy and replacement compounds, and establish a basis for continued top-down assessment of regional mitigation progress under the Montreal Protocol and the Kigali Amendment.