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Long-term ozone lidar routine measurements are sparse in China. In this study, we used one year of continuous ozone lidar measurements to systematically investigate the effects of synoptic patterns on the vertical structure of ozone during high ozone episodes in Hong Kong. The height of the near-surface ozone layer was identified and compared with that of the mixing layer. The results showed that Hong Kong was greatly affected by ozone transport from the Pearl River Delta region when a tropical cyclone ("C" pattern) or a trough ("T" pattern) existed. When a high-pressure center was located to the north ("H" pattern) or when both a high-pressure system and a tropical cyclone existed ("CH" pattern), Hong Kong was affected by ozone transport from southeastern or southern China. Substantial amounts of ozone were transported within a layer near the ground with an average height of 0.83, 0.75, 0.94, and 0.85 km under the C, H, CH, and T synoptic patterns, respectively. Variation in the near-surface ozone layer height was consistent with that of the mixing layer height. Weak vertical mixing resulted in a shallow near-surface ozone layer under the C and H synoptic patterns. Due to the combined effect of near-surface transport and suppressed vertical mixing, the ozone concentrations were extremely high near the ground but rapidly declined with height under the C synoptic pattern. Our investigations of the vertical structure of ozone contribute to filling the knowledge gap between synoptic patterns and the evolution of high surface ozone episodes. • One-year lidar measurements of ozone vertical structure were investigated. • Ozone transport and vertical mixing were characterized under four synoptic patterns. • Substantial amounts of ozone were transported in a near-surface layer below 1 km. • The near-surface ozone layer height was largely determined by vertical mixing. • Ozone concentrations rapidly declined with height under the cyclone-related pattern. [ABSTRACT FROM AUTHOR]

As a highly urbanized city in East Asia, Hong Kong has faced severe photochemical ozone (O 3) pollution for years. A comprehensive understanding of pollution characteristics is crucial for effective mitigation of the O 3 problem. This study systematically investigated the multi-year trends of surface O 3 over the last decade using measurements from 11 selected Air Quality Monitoring Stations (AQMS). The analysis reveals upward trends in the annual average and daily maximum 8-h average (DMA8) O 3 from 2011 to 2022, with rates of 0.34 ppb year−1 (p < 0.05) and 0.22 ppb year−1 (p = 0.19), respectively. Though upward trends of DMA8 O 3 were observed at urban stations (0.10–0.88 ppb year−1), the significant decreases in NO 2 levels at all stations (−0.18 to −1.75 ppb year−1) led to a decline of total oxidants (O x , O 3 +NO 2). In addition to the daytime O 3 peak increase, a clear nighttime O 3 peak was present with a significant increasing trend, which was associated with the regional transport of O 3 -rich plumes and weakened NO titration effect. The relative contributions of local production and regional transport were investigated, and results indicated that local control efforts have predictably alleviated the locally produced O x , while the regional background O x still contributes significantly but appears to decline. Furthermore, an observation-based model was employed to examine the evolution of O 3 -precursors relationships. The results indicate that O 3 formation was still VOC-limited in Hong Kong, and the suppressive effect of NO x on O 3 formation has decreased in recent years. In terms of abatement strategies, apart from aromatics, we emphasize the significance of carbonyl compounds due to their high reactivity to O 3 production. • A comprehensive analysis presents the long-term upward trend of surface O 3 in Hong Kong. • Elevated nighttime O 3 peaks in Hong Kong due to the regional transport of O 3 -rich plumes with reduced NO titration effect. • Strict emission control measures have alleviated local O x production and ceased the degradation of regional background O x. • The O 3 formation regime remains VOC sensitive, with high reactivity contribution from aromatics and carbonyl compounds. [ABSTRACT FROM AUTHOR]

While the crucial roles of ozone (O 3) 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 O 3 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 O 3 concentration, we performed a cross-sectional analysis to explore the total horizontal flux (THF) of O 3 across the PBL in Hong Kong. Throughout the entire study month, the THF of O 3 exhibited a predominant easterly component. However, during the O 3 pollution episodes, the THF of O 3 exhibited a predominant westerly component, indicating an increased regional transport from Greater Bay Area (GBA). The westerly winds between 240° and 300° contributed 61.2% to the total flux of O 3 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 O 3 pollution episodes, the wind with the peak O 3 flux shifted from westerly to northwesterly as the height increased in the PBL. The northwesterly THF of O 3 between 290° and 300° 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. • Total horizontal flux (THF) of O 3 across the PBL was explored using a wind LiDAR. • The PBL height was identified based on the vertical profile of wind speed shear. • The THF of O 3 exhibited a predominant westerly component during O 3 episodes. • Clockwise veering flux of O 3 were observed from the ground to the top of the PBL. • Our results enhance our understanding of the 3D pollutant transports in the GBA. [ABSTRACT FROM AUTHOR]

Urban fires in densely populated areas can burn large amounts of manmade structures and synthetic materials. Realworld chemical composition data of urban fire plumes are vital for evaluating health impacts; however, they are rarely available. Online monitoring of trace elements in ambient PM 2.5 at five sites throughout Hong Kong in 2021 provided unprecedented opportunities to observe the highly transient fire plumes. Sharp spikes were captured for bromine (up to 0.65 μg m−3), chlorine (7.0 μg m−3), and certain metals such as lead (0.97 μg m−3), zinc (8.3 μg m−3), and copper (0.32 μg m−3) during four urban fires, showing ∼10–6000 times higher concentrations than pre-fire periods. These elements showed decreased concentrations with increased distance from the fire location, but their concentration ratios remained relatively constant. During fire-influenced hours, these elements exhibited 3–101 times enrichment relative to iron, an element that had no discernible increase in concentration. Our results represent the first observation of the co-emissions of Br and certain metals from urban fires. Reactive bromine released from burning of the wide-spread brominated synthetic chemicals was suggested to facilitate the mobilization of toxic metals into the atmosphere. This mechanistic knowledge will help us understand the emissions and impacts of fires occurring in wildland urban interface zones. The work also calls attention to previously overlooked influence of urban fires-mobilized metals on human health and of bromine on atmospheric oxidizing capacity in megacities. [Display omitted] • Hourly element data in PM 2.5 at 5 sites in Hong Kong revealed impacts of urban fires. • Large co-emissions of Br, Pb, and other toxic metals from urban fires were observed. • Urban fire-impacted PM 2.5 had much higher enrichment of Pb–Zn–Cu–Br–Cl than hill fires. • Brominated synthetic chemicals likely facilitated mobilization of toxic metals during fires. [ABSTRACT FROM AUTHOR]

Vertical transmission of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) along a vertical column of flats has been documented in several outbreaks of coronavirus disease 2019 (COVID-19) in Guangdong and Hong Kong. We describe an outbreak in Luk Chuen House, involving two vertical columns of flats associated with an unusually connected two-stack drainage system, in which nine individuals from seven households were infected. The index case resided in Flat 812 (8th floor, Unit 12), two flats (813, 817) on its opposite side reported one case each (i.e., a horizontal sub-cluster). All other flats with infected residents were vertically associated, forming a vertical sub-cluster. We injected tracer gas (SF 6) into drainage stacks via toilet or balcony of Flat 812, monitored gas concentrations in roof vent, toilet, façade, and living room in four of the seven flats with infected residents and four flats with no infected residents. The measured gas concentration distributions agreed with the observed distribution of affected flats. Aerosols leaking into drainage stacks may generate the vertical sub-cluster, whereas airflow across the corridor probably caused the horizontal sub-cluster. Sequencing and phylogenetic analyses also revealed a common point-source. The findings provided additional evidence of probable roles of drainage systems in SARS-CoV-2 transmission. [Display omitted] • The vertical cluster of the outbreak is probably associated with drainage stacks. • Tracer gas measurement and phylogenetic analysis of SARS-CoV-2 genome sequences were applied. • The results of tracer gas monitoring agreed with the observed distribution of infected cases. • Leaked stack aerosols may have played the main role in the vertical cluster. • The chimney effect was the probable primary cause for the vertical spreading. [ABSTRACT FROM AUTHOR]