MPhil Thesis Presentation - Integrated Monitoring and Source Characterization of Ambient Methane at a Coastal Urban Site in Hong Kong

Urban methane assessment in coastal environments requires explicit consideration of observational support, spatial representativeness, atmospheric transport, and the strength of source evidence. This thesis develops an integrated CH₄ observation and modelling framework at the HKUST coastal supersite in Hong Kong by combining retrievals from ground-based direct-sun Fourier transform infrared spectroscopy, Sentinel-5P/TROPOMI XCH₄ products, fixed-time near-surface CH₄ measurements, meteorological transport diagnostics, source-inventory data, and multimodal Earth-observation (EO) descriptors. The winter EM27/SUN campaign provided the ground-based column-observation component of the study. Across 24 archived campaign days, 45,312 raw interferograms yielded 3,748 valid XCH₄ retrievals on 20 days. Seventeen campaign days coincided with Sentinel-5P overpasses, but only 13 contained at least one valid TROPOMI pixel within 50 km of the site. At the selected 50 km collocation radius, strict overpass-centred daily pairing retained nine matched days and yielded a mean bias of −3.3 ppb, a root-mean-square error (RMSE) of 23.7 ppb, and a correlation coefficient of r = 0.77. These results highlight the extent to which limited sampling support and spatial representativeness constrain satellite–ground column comparisons under coastal winter conditions.

A leakage-controlled benchmark with strict temporal separation was subsequently constructed from 2,224 model-ready near-surface CH₄ observation slots collected at fixed local times between January 2024 and March 2026. On the independent test block, the final transport-aware model incorporating selected EO context achieved an RMSE of 29.00 ppbv, R² = 0.658, an area under the precision–recall curve (AUPRC) of 0.974, and an F1 score of 0.920. Independent evaluation using 28 whole-air canister samples collected after the benchmark definition and candidate-prioritization surface had been frozen showed that four of the five highest-ranked samples exceeded the predefined campaign-relative field-enhancement threshold, corresponding to Precision@5 = 0.80 and Recall@5 = 0.57.

Overall, this thesis demonstrates that coastal urban CH₄ assessment can be strengthened by integrating traceable XCH₄ column observations, representativeness-aware satellite comparison, transport-aware modelling, and independent field evidence. The resulting framework supports defensible candidate-source prioritization while keeping source interpretation commensurate with the strength and limitations of the available observational evidence.