Synergistic Effects of Local Photochemistry and Regional Transport on Ozone Formation in Hong Kong

The Guangdong-Hong Kong-Macao Greater Bay Area (GBA), particularly Hong Kong, faces severe challenges regarding ozone (O₃) pollution. As volatile organic compounds (VOCs) are primary precursors driving near-surface O₃ formation, accurately assessing their contribution is essential for developing effective synergistic control strategies. In this study, high-resolution online observations of O₃-sensitive VOCs were conducted at a coastal site in Hong Kong using Proton-Transfer-Reaction Mass Spectrometry (PTR-MS). We investigated emission characteristics, photochemical transformations, and the evolution of VOCs during regional transport.

Results indicate that Oxygenated VOCs (OVOCs) consistently exhibited higher concentrations during daytime. Methanol was the most abundant species (average 3.73 ppb), while concentrations of isoprene and methyl ethyl ketone (MEK) exceeded levels previously reported in coastal regions. Crucially, the Empirical Kinetic Modeling Approach (EKMA) confirmed a nonlinear relationship between O₃, nitrogen oxides (NO_X), and VOCs. The photochemical regime shifted from VOC-limited in the morning to a transition regime in the afternoon. Notably, by accounting for chemical loss, the calculated Photochemical Initial Concentration (PIC-VOC) was found to be 8.2 ppb higher than the observed concentration (OBS-VOC). This discrepancy highlights that neglecting photochemical consumption significantly leads to an underestimation of the local Ozone Formation Potential (OFP).

Source apportionment via Positive Matrix Factorization (PMF) revealed that the site was significantly influenced by urban plumes transported from the GBA (contributing 63.7%) and oceanic emissions (13.5%). During three identified high-O₃ episodes (with a maximum peak of 382.65 µg/m³), backward trajectory analysis attributed the pollution to long-range transport (52%), short-range transport (28%), and local sources (20%). These findings demonstrate that elevated ozone levels in Hong Kong result from the synergistic effects of local photochemical production and regional pollutant transport, providing a critical scientific basis for refining regional air quality assessments.