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

Traditional tropospheric ozone ( O 3 ) climatology uses a simple average substantially smoothed stratification structure in individual O 3 profiles, limiting our ability to properly describe and understand how O 3 is vertically distributed at the interface between the mixing layer (ML) and free troposphere (FT). In this study, we collected 1897 ozonesonde profiles from two Chinese megacities (Beijing and Hong Kong) over the period 2000-2022 to investigate the climatological vertical heterogeneity of the lower-tropospheric O 3 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 O 3 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 O 3 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 O 3 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 O 3 loadings. This barrier effect varies with season and city, with an ML-FT detrainment barrier in summer (autumn) and an FT-ML entrainment barrier in other seasons in Beijing (Hong Kong). Based on a Student's t test, daily h -referenced O 3 profiles were further classified into three typical patterns: MLO 3 -dominated, FTO 3 -dominated, and uniform distribution. Although the FTO 3 -dominated pattern occurs most frequently during the whole study period (69 % and 54 % of days in Beijing and Hong Kong, respectively), the MLO 3 -dominated pattern prevails in the photochemically active season, accounting for 47 % of summer days in Beijing and 54 % of autumn days in Hong Kong. These occurrences of the MLO 3 -dominated pattern are significantly more frequent than in previously reported results at northern mid-latitudes, indicating intensive photochemical MLO 3 production under the high-emission background of a Chinese megacity. From a FTO 3 -dominated to MLO 3 -dominated pattern, the O 3 precursor CH 2 O ( NO 2 ) experiences a substantial increase (decrease) in Beijing but a slight change in Hong Kong. Vertically, the increment of CH 2 O is larger in the upper ML, and the decrement of NO 2 is larger in the lower ML. Such vertical changes in O 3 precursors push O 3 production sensitivity away from the VOC-limited regime and facilitate high-efficiency production of O 3 via photochemical reactions, particularly in the upper ML of Beijing.