Extreme smog can have potentially harmful effects on human health, the economy and daily life. However, the average (mean) values do not provide strategically useful information on the hazard analysis and control of extreme smog. This article investigates China's smog extremes by applying extreme value analysis to hourly PM2.5 data from 2014 to 2016 obtained from monitoring stations across China. By fitting a generalized extreme value (GEV) distribution to exceedances over a station-specific extreme smog level at each monitoring location, all study stations are grouped into eight different categories based on the estimated mean and shape parameter values of fitted GEV distributions. The extreme features characterized by the mean of the fitted extreme value distribution, the maximum frequency and the tail index of extreme smog at each location are analysed. These features can provide useful information for central/local government to conduct differentiated treatments in cities within different categories and conduct similar prevention goals and control strategies among those cities belonging to the same category in a range of areas. Furthermore, hazardous hours, breaking probability and the 1-year return level of each station are demonstrated by category, based on which the future control and reduction targets of extreme smog are proposed for the cities of Beijing, Tianjin and Hebei as an example.

This paper is concerned with the statistical learning of the extreme smog (PM ${}_{2.5}$ ) dynamics of a vast region in China. Differently from classical extreme value modeling approaches, this paper develops a dynamic model of conditional, exponentiated Weibull distribution modeling and analysis of regional smog extremes, particularly for the worst scenarios observed in each day. To gain higher modeling efficiency, weather factors will be introduced in an enhanced model. The proposed model and the enhanced model are illustrated with temporal/spatial maxima of hourly PM ${}_{2.5}$ observations each day from smog monitoring stations located in the Beijing–Tianjin–Hebei geographical region between 2014 and 2019. The proposed model performs more precisely on fittings compared with other previous models dealing with maxima with autoregressive parameter dynamics, and provides relatively accurate prediction as well. The findings enhance the understanding of how severe extreme smog scenarios can be and provide useful information for the central/local government to conduct coordinated PM ${}_{2.5}$ control and treatment. For completeness, probabilistic properties of the proposed model were investigated. Statistical estimation based on the conditional maximum likelihood principle is established. To demonstrate the estimation and inference efficiency of studies, extensive simulations were also implemented.

Extreme haze was often observed at many locations in Beijing–Tianjin–Hebei region within several hours when they occurred, which is referred to as extreme co-movements and extreme dependence in statistics. This article applies tail quotient correlation coefficient to explore the temporal and spatial extreme dependence patterns of haze in this region. Hourly PM2.5 station-level data during 2014–2018 are used, and the results show that the tail quotient correlation coefficient between stations increases with month. Specifically, the simultaneous extreme dependence was strong in the fourth season, while the haze was severe. In the first season, while the haze was also severe, the extreme hazes only show strong co-movements with a time difference. These observations lead to the study of two special scenarios, that is, the concurrence/extreme dependence of the worst extreme haze and its lag effects. City clusters suffering simultaneous extreme haze or with certain time difference as well as the most frequently co-movement cities are identified. The extreme co-movements of these cities and the reasons for their occurrences have strong implications for improving the PM2.5 joint prevention and control in the Beijing–Tianjin–Hebei region. The importance of lag effects is also reflected in the precedence order of the extreme haze’s appearance. It is especially useful when setting the mechanism of the early warning system which can be triggered by the first appearance of extreme haze. The precedence orders also avail in investigating the transmission path of the haze, based on which more precise meteorological models can be made to benefit the haze forecasting of the region.

Ozone pollution in China has become increasingly severe in recent years. Considering the damage that extreme ozone pollution may cause and the fact that the occurrence of extreme ozone pollution among different locations may be related, this paper uses hourly ozone concentration data from national monitoring stations to investigate the co-movement of extreme ozone pollution in the Beijing-Tianjin-Hebei (BTH) Region. The extreme dependence analysis is adopted to assess such extreme co-movements between different cities. The co-occurrences of extreme ozone pollution at the same time or with certain time differences in the region are analyzed. City groups suffering simultaneous extreme pollution and those where the pollution occurs with certain time differences are identified under certain criteria. Furthermore, the order in which cities experience extreme ozone pollution is determined. With the publication of the New Three-year Action Plan for Winning the Blue Sky War, our results may be important for improving the joint early-warning and emergency response mechanism at city levels in the BTH Region.

Underwater pipelines play an important role in offshore oil and gas transmission, and underwater cables are vital for global interconnection communication. However, due to the harsh underwater environment, underwater pipelines and cables are easily damaged. In order to complete the inspection of underwater pipelines and cables, researchers have proposed many inspection platforms, detection and tracking methods to improve inspection efficiency and automation level. In this paper, the state-of-the-art techniques for underwater pipeline and cable external inspection are comprehensively reviewed. Firstly, underwater pipeline and cable inspection platforms are classified and introduced, and their advantages and disadvantages are discussed. Then, underwater pipeline and cable detection methods including acoustic method, passive optical method, active optical method and electromagnetic method are introduced, and their performance comparison are provided. Besides, underwater pipeline and cable tracking methods are analyzed. Finally, we discuss the key issues and future directions of underwater pipeline and cable inspection. This review will help researchers better understand the current research progress in underwater pipeline and cable inspection platforms, detection and tracking methods, which provides guidance for further research and deployment.