长株潭城市群PM_(2.5)多尺度演化的EEMD和多重分形分析

Multiscale ensemble empirical mode decomposition and multifractal approach of PM_(2.5) evolution in Changsha-Zhuzhou-Xiangtan Urban Agglomeration

为解析长株潭地区PM_(2.5)演化的多尺度特征,阐释其演化的主要动力机制,提出了一种集合经验模态分解(EEMD)和多重分形消除趋势波动分析(MFDFA)的新模型,研究了该区域2015年1月1日至2019年12月31日PM_(2.5)浓度的动力演化。利用EEMD方法获得了各城市PM_(2.5)的高频模态以及趋势项,趋势项结果表明PM_(2.5)浓度呈下降趋势,而PM_(2.5)的高频模态反映了PM_(2.5)浓度波动的非线性特征。进一步采用MFDFA方法对其高频累加模态进行分析,研究表明PM_(2.5)高频分量存在较强的多重分形特征。此外,还利用相位随机替代法和随机重构法研究了其多重分形的主要来源,结果表明PM_(2.5)浓度波动在不同时间尺度内的长期持续作用是造成高浓度PM_(2.5)污染涌现的主要动力因素。最后,讨论了气象条件对其高频分量多重分形强度的影响,结果发现,相对于其他季节,冬季PM_(2.5)高频模态的多重分形强度更强。分析表明,尽管该区域通过大气污染行动计划已取得积极的污染控制效果,但在冬季,污染物演化的长期持续动力机制对PM_(2.5)高频模态的演化发挥着更加主导的控制作用,不同时间尺度上PM_(2.5)非线性长期持续动力机制导致冬季仍有高浓度PM_(2.5)涌现的风险,甚至出现更为严重的污染。本研究结果对于区域PM_(2.5)多时间尺度演化动力特征的研究以及大气污染预测预警机制的建立具有重要意义。

To analyze the multi-scale characteristics of PM_(2.5) in Changsha-Zhuzhou-Xiangtan region,China,and explain the main dynamic mechanism of PM_(2.5) evolution,a novel ensemble empirical mode decomposition and multifractal detrended fluctuation analysis(EEMD-MFDFA)model is proposed,and then the dynamic evolution of PM_(2.5) hourly average concentrations in Xiangtan,Changsha and Zhuzhou from January 1,2015 to December 31,2019 is studied.Through EEMD,the high-frequency modes and trend terms of PM_(2.5) are obtained for the three cities.The results of the trend term show a decreasing trend of PM_(2.5) concentrations,and the high-frequency mode of PM_(2.5) reflects the nonlinear characteristics of PM_(2.5) concentration fluctuation.Furthermore,MFDFA method is used to analyze the high-frequency cumulative mode of PM_(2.5).The results indicate that the high-frequency mode of PM_(2.5) has strong multifractal characteristics.In addition,the main sources of multifractal characteristics are studied by using shuffling procedure and phase randomization.The results indicate that the long-term persistence of PM_(2.5) concentration fluctuation in different time scales is the main dynamic factor for the emergence of high concentration PM_(2.5).Finally,the influence of meteorological conditions on the multifractal strength of high-frequency modes of PM_(2.5) is discussed.The results show that the multifractal strength of PM_(2.5) in winter was stronger than that in other seasons.The analysis shows that although the air pollution in Changsha-Zhuzhou-Xiangtan Urban Agglomeration has been effectively controlled through the air pollution action plan,in winter,the long-term persistence mechanism of air pollution plays a more dominant role in controlling the PM_(2.5) evolutions,and the non-linear long-term persistent mechanism of PM_(2.5) at different time scales can lead to the risk of emergence of high concentration PM_(2.5) in winter,and even more serious air pollution events.The results have great significance for studying the dynamic characteristics of regional PM_(2.5) multi-scale evolution and forecasting haze.