Mixing-layer-height-referenced ozone vertical distribution in the lower troposphere of Chinese megacities: Stratification, classification, meteorological, and photochemical mechanisms

Abstract. Traditional tropospheric ozone (O₃) climatology uses a simple average substantially smoothed stratification structure in individual O₃ profiles, limiting our ability to properly describe and understand how O₃ is vertically distributed at the interface between the mixing layer (ML) and free troposphere (FT). In this study, we collected 1,897 ozonesonde profiles from two Chinese megacities (Beijing and Hong Kong) over the period 2000–2022 to investigate climatological vertical heterogeneity of lower-tropospheric O₃ distribution with a mixing-layer-height-referenced (h-referenced) vertical coordinate system. The mixing layer height (h) was first estimated following an integral method that integrates the information of temperature, humidity, and cloud. After that, a so-called h-referenced vertical distribution of O₃ was determined by averaging all individual profiles expressed as a function of z/h rather than z (where z is altitude). We found that the vertical stratification of O₃ is distributed heterogeneously in the lower troposphere, with stronger vertical gradients at the surface layer and ML–FT interface. There are low vertical autocorrelations of O₃ between the ML and FT, but high autocorrelations within each of the two atmospheric compartments. These results suggest that the ML–FT interface acts as a geophysical “barrier” separating air masses of distinct O₃ loadings. This barrier effect varies with season and city, with an ML-to-FT detrainment barrier in summer (autumn) and an FT-to-ML entrainment barrier in other seasons in Beijing (Hong Kong). Based on Student’s t test, h-referenced O₃ profiles were further classified into three typical patterns: MLO₃-dominated, FTO₃-dominated, and uniform distribution. Although the FTO₃-dominated pattern occurs most frequently during the whole study period (69 % and 54 % of days in Beijing and Hong Kong, respectively), the MLO₃-dominated pattern prevails in the photochemical active season, accounting for 47 % of summer days in Beijing and 54 % of autumn days in Hong Kong. These occurrences of the MLO₃-dominated pattern are significantly more frequent than in previously reported results at northern mid-latitudes, indicating intensive photochemical MLO₃ production under the high-emission background of Chinese megacity. From FTO₃-dominated to MLO₃-dominated pattern, the O₃ precursor CH₂O (NO₂) experiences a substantial increase (decrease) in Beijing, but a slight increase (decrease) in Hong Kong. Vertically, the increment of CH₂O is larger in the upper ML and the decrement of NO₂ is larger in the lower ML. Such changes in O₃ precursors push O₃ production sensitivity away from the VOC-limited regime and facilitate high-efficiency production of O₃ via photochemical reactions, particularly in the upper ML.