Horizontal flux of ozone in the planetary boundary layer in Hong Kong using wind LiDAR measurements

While the crucial roles of ozone (O3) transport in the planetary boundary layer (PBL) have been acknowledged for some time, there is currently limited knowledge about this aspect primarily due to the limited availability of measurements to determine the characteristics of the PBL. In this study, measurements from a wind Light Detection and Ranging (LiDAR) system were taken to monitor vertical profile of wind pattern at an urban site in Hong Kong in September 2022, a period when the city was frequently impacted by tropical cyclones and experienced severe O3 pollution levels. The PBL height was identified based on the vertical profile of wind speed shear. By combining information on the PBL height, vertical wind profile, and O3 concentration, we performed a cross-sectional analysis to explore the total horizontal flux (THF) of O3 across the PBL in Hong Kong. Throughout the entire study month, the THF of O3 exhibited a predominant easterly component. However, during the O3 pollution episodes, the THF of O3 exhibited a predominant westerly component, indicating an increased regional transport from Greater Bay Area (GBA). The westerly winds between 240 degrees and 300 degrees contributed 61.2% to the total flux of O3 in the PBL during these episodes. In addition, clockwise veering winds were observed from the ground to the top of the PBL, which can be attributed to the Ekman spiral. As a result, during the O3 pollution episodes, the wind with the peak O3 flux shifted from westerly to northwesterly as the height increased in the PBL. The northwesterly THF of O3 between 290 degrees and 300 degrees reached its peak at 600 m above ground level during these episodes. These findings enhance our understanding of the 3D pollutant transports for both long-term averages and short-term pollution episodes in the GBA and Hong Kong.