MPhil Thesis Presentation - Characterizing Long-term Trends and Formation Regimes of Surface Ozone in Hong Kong: Insights from a Localized Diagram Approach

Hong Kong has struggled with rising surface ozone (O3) pollution for decades, largely because the non-linear response of O3 formation to its major precursors—nitrogen oxides (NOx) and volatile organic compounds (VOCs)—frequently compromises the effectiveness of emission controls. To better understand this challenge, this study presents a localized Trans-EKMA diagram approach that, using only routine O3 and NOx monitoring data, diagnoses O3 formation regimes and tracks precursor emission trends. The method incorporates Hong Kong-specific inputs, including a 45-species VOC profile, 1990-2024 meteorological data, and the MCM v3.3.1 chemical mechanism.

From 2011 to 2025, annual mean surface O3 increased at a rate of 0.700 ppbv y-1, while Ox (O3 + NO2) declined, confirming that sustained NOx controls have weakened the NO titration effect and driven O3 upward. Over the 2000-2025 period, the four station types exhibit distinct evolutionary pathways: roadside stations record the largest NOx reductions (>50%), while new town stations exhibit the highest absolute O3 levels, attributable to regional transport. Notably, no station has crossed into the NOx-limited regime after 25 years of emission controls. Future projections reveal a non-linear trajectory: the daily photochemical O3 build-up (M1M1O3) peaks at 50.5 ppbv in 2027 before declining to 44.8 ppbv by 2030, when Hong Kong is projected to enter the NOx-limited regime for the first time. Yet this level remains substantially elevated relative to the 2025 baseline (30.9 ppbv), indicating that the regime shift does not guarantee immediate O3 relief.

These findings underscore the need for balanced VOC/NOx co-reduction, with a VOC/NOx ratio of at least 1:1 and ≥1.11:1 in roadside areas, alongside strengthened regional cooperation within the Guangdong-Hong Kong-Macau Greater Bay Area. The validated diagram approach provides a replicable, cost-effective template for other data-scarce urban regions to diagnose O3 regimes and evaluate precursor controls using readily available routine monitoring data.