Port activities significantly alter local atmospheric chemistry, yet the nonlinear coupling mechanisms between nitrogen oxides (NOx) and ozone (O3) in these complex environments remain underexplored. In this study, MF-DFA and MF-DCCA were applied to explore the coupling dynamics between NOx and O3 in Hong Kong’s Kwai Chung port and Tap Mun non-port areas. Results indicate that while both pollutants exhibit multifractality, O3 shows stronger persistence and scale-invariant complexity than NOx (e.g., in the port area, spectral width Δα = 0.61 for O3 vs. 0.40 for NOx). Crucially, the non-port area demonstrates significantly stronger and more stable cross-correlations (with the cross-correlation Hurst exponent hxy(2) = 0.85 and hxy(q) ranging from 0.80 to 0.99) compared to the port area (hxy(2) = 0.60, hxy (q) ranging from 0.54 to 0.74). The weaker coupling in the port zone is attributed to the fact that intermittent factors such as ship emissions have disrupted the long-term memory of the system. The connections in non-port areas are stronger and more stable because they are less affected by local emissions and chemical processes. The cross-correlation exhibited obvious seasonal dependence, with the strongest multifractal intensity in summer (cross-multifractal Δα reaching up to 0.81) and the weakest in winter under the modulation of photochemical and meteorological conditions. These findings reveal that port-side pollution coupling is structurally more fragile and heterogeneous than the stable regional background, suggesting that effective air quality management requires strategies accounting for these cross-scale nonlinear dynamics.
Zhao et al. (Sun,) studied this question.