Exploring the link between ozone pollution and stratospheric intrusion under the influence of tropical cyclone Ampil

Tropical cyclones (TCs) are synoptic-scale systems with a diameter of up to 2000 km, and may persist for several days to weeks. They can significantly affect the atmospheric conditions and ozone (O₃) concentrations in coastal areas. They also induce stratospheric intrusion (SI, the injection of stratospheric O₃ into the troposphere) by changing the height of the tropopause. Focusing on the Yangtze River Delta (YRD) region, a highly urbanized coastal area with severe O₃ pollution, we systematically analyze the characteristics of O₃ pollution and SIs as well as their connections under the influence of tropical cyclone Ampil. Based on surface observations, 120-h backward trajectories and ERA5 reanalysis meteorological data, the urban O₃ pollution affected by tropical cyclone Ampil mainly resulted from active photochemical reactions inside the boundary layer and poor diffusion conditions characterized by weak winds. SIs induced by tropical cyclone Ampil were important for the upper and middle troposphere, where O₃ concentration could increase up to 180 ppbv. But they hardly reached the ground over the YRD. Therefore, there was no direct connection between O₃ pollution and SIs. However, the location of SIs can predict urban O₃ pollution. SIs moved with tropical cyclone Ampil but appeared on the northwest side of Ampil, usually 500-1000 km away from tropical cyclone Ampil center. At 200 hPa, they corresponded to a high-PV (>2 PVU) air mass rich in O₃ (>200 ppbv). Below this stratospheric PV signature, urban O₃ concentration was usually high.

Anomalous surface O3 changes in North China Plain during the northwestward movement of a landing typhoon

As high impact weather in a large scale, typhoon movement from the northwest Pacific into inland regions influencing ambient O₃ changes is unclear, especially in North China Plain (NCP). A landing Typhoon Ampil during July 17-24, 2018 was studied herein to characterize the surface O₃ anomalies during its movement over NCP. Landing typhoons present large negative O₃ anomalies at the center of the typhoon and positive O₃ anomalies 600-1700 km away from the center. During the northwest movement of Typhoon Ampil to the NCP, the area and magnitude of both positive and negative O₃ anomalies shrank, particularly in the western and northern periphery, where the typical diurnal change of O₃ dissipated with nocturnal O₃ enhancement in the NCP. The spatiotemporal patterns of surface O₃ anomalies in the NCP were induced significantly during various stages of typhoon movement with a stable structure in the atmospheric boundary layer, strong solar radiation on sunny days, and stratosphere-to-troposphere transport (STT) in the typhoon periphery, depending on the changing intensity, distance, and orientation of the typhoon center. Among them, the STT played a considerable role and contributed 32% to the positive anomalies of surface O₃ in the NCP. Under the influence of westerly jets and high pressure at mid-latitudes on the typhoon movement, strong wind convergences in the upper troposphere were formed intensifying the downdraft of O₃-rich stratospheric air to the boundary layer in the NCP with an asymmetrical distribution of surface positive O₃ anomalies over the periphery of typhoon. This study could improve our understanding of regional ozone changes with meteorological influences.

An instrumented station for the survey of ozone and climate change in the southern tropics

The assessment of changes induced by human activities on Earth atmospheric composition and thus on global climate requires a long-term and regular survey of the stratospheric and tropospheric atmospheric layers. The objective of this paper is to describe the atmospheric observations performed continuously at Reunion Island (55.5 degrees east, 20.8 degrees south) for 15 years. The various instruments contributing to the systematic observations are described as well as the measured parameters, the accuracy and the database. The LiDAR systems give profiles of temperature, aerosols and ozone in the troposphere and stratosphere, probes give profiles of temperature, ozone and relative humidity, radiometers and spectrometers give stratospheric and tropospheric integrated columns of a variety of atmospheric trace gases. Data are included in international networks, and used for satellite validation. Moreover, some scientific activities for which this station offers exceptional opportunities are highlighted, especially air mass exchanges nearby dynamical barriers: (1) On the vertical scale through the tropical tropopause layer (stratosphere-troposphere exchange). (2) On the quasi-horizontal scale across the southern subtropical barrier separating the tropical stratospheric reservoir from mid- and high latitudes.

Description and evaluation of a tropospheric ozone lidar implemented on an existing lidar in the southern subtropics

Rayleigh-Mie lidar measurements of stratospheric temperature and aerosol profiles have been carried out at Reunion Island (southern tropics) since 1993. Since June 1998, an operational extension of the system is permitting additional measurements of tropospheric ozone to be made by differential absorption lidar. The emission wavelengths (289 and 316 nm) are obtained by stimulated Raman shifting of the fourth harmonic of a Nd:YAG laser in a high-pressure deuterium cell. A mosaic of four parabolic mirrors collects the backscattered signal, and the transmission is processed by the multiple fiber collector method. The altitude range of ozone profiles obtained with this system is 3¿17 km. Technical details of this lidar system working in the southern tropics, comparisons of ozone lidar profiles with radiosondes, and scientific perspectives are presented. The significant lack of tropospheric ozone measurements in the tropical and equatorial regions, the particular scientific interest in these regions, and the altitude range of the ozone measurements to 16¿17 km make this lidar supplement useful and its adaptation technically conceivable at many Rayleigh-Mie lidar stations.