image: Schematic diagram of ULOI events and their impacts on surface O3 concentration and secondary aerosol (SA) formation, spatial distribution of ULOI probability in China from December 2020 to February 2021.
Credit: ©Science China Press
The research team, led by Professors Yongchun Liu from Beijing University of Chemical Technology, along with Professor Jiannong Quan from the Chinese Meteorological Administration, and Professor Douglas R. Worsnop from the University of Helsinki, proposed a novel method for identifying upper-layer ozone intrusion (ULOIs) events by ranking observational ozone concentrations on the ground surface. The result shows that ULOI events substantially elevate surface ozone levels, intensify atmospheric oxidation capacity, and accelerate wintertime sulfate and secondary organic aerosol formation in the North China Plain.
Ozone is an important air pollutant on the ground, having adverse effects on human health and the ecosystem. Surface ozone usually shows obvious daily variation with a peak at noon, followed by a continuous decrease after sunset, due to photochemistry and evolution of mixing layer height.
However, anomalously nocturnal ozone enhancements are often observed, which cannot be explained by local photochemistry alone. This emphasizes the role of vertical transport or horizontal transport processes in the atmosphere, while it is a big challenge to identify such events and quantify their impacts on atmospheric chemistry based on ground surface observations.
In a new study, researchers proposed a simple but robust method to ULOI events based solely on surface ozone measurements before dawn. ULOI events can be identified by a threshold of ozone concentration, which is objectively determined according to a sudden increase in slope of the ozone concentration before dawn versus its ranking percentile. Unlike previous approaches that rely on isotopic analysis or complex chemical transport models, the new method ranks early-morning ozone concentrations to detect signatures of ozone transported downward from higher atmospheric layers, such as the residual layer, upper troposphere, or even the lower stratosphere.
Applying this method to nationwide observational data across China, the research team found that ULOI events are far more widespread than previously recognized. Depending on the region, these events occur during 22% to 74% of the analyzed periods, with particularly high frequencies in eastern and southern coastal areas. These regions are strongly influenced by meteorological systems such as sea–land breezes, low-level jets, and tropical cyclones, which promote vertical mixing and facilitate the downward transport of ozone-rich air.
The results show that ULOI events significantly elevate surface O3 concentrations. At night, O3 levels increase by approximately 13–43 ppbv, while daytime concentrations rise by 3–14 ppbv. These enhancements substantially strengthen atmospheric oxidation capacity and enhance the contribution of the O3 oxidation path to wintertime sulfate and secondary organic aerosol formation in the North China Plain.
This study highlights the critical role of interactions between different atmospheric layers in shaping surface air quality. It shows that physical processes have complicate influence on O3 pollution and secondary aerosol formation, underscoring the need to incorporate vertical transport and upper-layer influences into air pollution research and management strategies. As the observational network for air quality is developed worldwide, the results can be applied in other regions for evaluating the impacts of these events on surface air quality and the ecosystem.