Tropospheric Ozone Variability Over Hong Kong Based on Recent 20 years (2000–2019) Ozonesonde Observation

The recent 20 years (2000–2019) ozonesonde profiles recorded at the Hong Kong Observatory are analyzed to study the Tropospheric Ozone (TropO₃) variability over Hong Kong, South China. The TropO₃ profiles are described without the constraints of traditional monthly averages; they are clustered based on a Self-Organizing Map (SOM) to explore the short-term variability, and separated into two 10-year periods to investigate the long-term variability based on pooled statistical analysis. The roles of precursor emissions, dynamic transport, and photochemical formation in multiple-timescale TropO₃ variability are further evaluated with the aid of meteorological and satellite data, surface observation, emission inventory, and backward trajectory analysis. The results suggest that TropO₃ over Hong Kong has a seasonal and vertical distribution featuring a widely recognized spring maximum. The SOM-based clustering analysis emphasizes a strong dependence of short-term variability in TropO₃ profile patterns on cross-latitude transport conditions, including rich-O₃ transport from midlatitude region and poor-O₃ transport from tropics. However, high O₃ levels throughout the troposphere (usually in early spring) are largely due to the combination of stratospheric intrusion and biomass burning. During the whole study period, the O₃ concentrations have increased substantially in the lower troposphere. Springtime wildfire-related emissions over the upwind Indochina Peninsula are responsible for the lower free-tropospheric O₃ increases. In contrast, the boundary-layer O₃ uptrends with a larger increasing rate near the surface are mostly attributed to the reduced NO titration effect due to the sharp reduction of NO_x emissions.