Multi-scale impacts of Indochina biomass burnings on tropospheric ozone in coastal South China: Insights from long-term (2000–2024) observations

Biomass burning is an important source of tropospheric ozone (O₃). This study explored the impacts of Indochina springtime (March–April) biomass burnings on the variability and trend of tropospheric O₃ in coastal South China using long-term (2000–2024) ozonesondes in Hong Kong and satellite fire retrievals in the Indochina Peninsula (ICP), complemented with EAC4 reanalysis data. We find that the lower-free-tropospheric O₃ (LFTO₃) concentrations in Hong Kong are significantly correlated with the Indochina biomass burnings, particularly with the two-day-ago biomass burnings in northern Laos (r = 0.57, p < 0.01). While Indochina biomass burning contributes more than 30 ppbv enhancements in LFTO₃ concentrations over coastal South China, their impacts on surface O₃ concentrations are insignificant. During the study period, there is a significant increasing trend in springtime LFTO₃ concentrations in Hong Kong (0.37 ppbv/year), despite decreasing quantity and intension of Indochina biomass burnings. This long-term LFTO₃ increasing trend is mainly driven by the eastward migration of Indochina biomass burnings (mainly due to the increase in biomass burnings in the central ICP region), which reduces transport distance to Hong Kong by ∼300 km and thereby improves the transport efficiency, ultimately contributing ∼90 % of the long-term LFTO₃ increase in Hong Kong. These findings advance understanding of Indochina biomass burning transport impacts on multi-scale tropospheric O₃ variability in coastal South China.