Abstract:

The multiple scale characteristics of air pollution index change in Shanghai City,its trends and mutation features were examined via Wavelet analysis and MannKendall nonparametric test method based on the daily air pollution index sequence of PM10,SO2 and NO2 over a time period of recent 12 years.Firstly,continuous wavelet transform was employed to investigate the air pollution change,which revealed the multiple scale characteristics of the variance of air pollution sequences of PM10,SO2 and NO2.Wavelet variance analysis indicated that,the three air pollution sequences had different periodicity at medium and short time scales.The sequences had a medium scale period of around 1000 days and a short scale period of 300 days,while at small time scales,the sequences had primary peroids of around 300 days and secondary periods of around 30 days.The periodic analysis results at 300 days and 30 days time scales indicated that air pollution was heavier in summer and lighter in winter.Additionally,the results at 30 days time scale showed winter and spring were the seasons when pollution was much more significantly affected by the weather and season factors,and air quality of Shanghai tended to improve in recent years.Wavelet analysis results in this work could also be verified by the real observation and statistics of Shanghai air pollution at the same time.Secondly,MannKendall method was employed to detect trends of the sequence change of PM10,SO2 and NO2 in recent 12 years.The three sequences all showed a extremely significant decline tendency,and the average decline rates of PM10,SO2 and NO2 sequences were -0289 5/100 d,-0293 5/100 d and -0164 1/100 d,respectively.Thirdly,MannKendall method was used to identify the mutations of the sequences.Mutation positions occurred at the start as the trend of each sequence changed.The sequence mutation of PM10 pollution began from July 3,2006,SO2 from March 21,2010,and NO2 from April 18,2006.Thereafter,the three sequences showed significant downward trend.Although there were local fluctuations,the air quality tended to get better.PM10 and NO2 pollution reduced since the summer of 2006,and SO2 pollution reduced after the spring of 2010.Though SO2 pollution was the most serious in 2007,but it later reduced year by year.Before 2010,SO2 sequence showed a significant upward trend over a long period of time,indicating that the shortterm decline might be just normal fluctuation in the overall upward trend for a long time,and rise and fall of the sequence repeatedly formed the characteristics of multiple time scales.Both the wavelet and MannKendall methods had their advantages and drawbacks,and the analysis results of the two methods could verify and complement each other to get more accurate conclusion,providing reference for the urban atmospheric environment monitoring and protection.Highspeed economic developments of Shanghai brought tremendous environmental pressure.With the rapid growth of automotive vehicles,the increased road dust density and exhaust poisnous emissions became the source of air pollutants of the urban areas in Shanghai.Despite many measures taken and efforts made on air pollution control by the Shanghai administration,air pollution in Shanghai remains a problem to address.Finally,proposals were put forward to improve the urban air quality in Shanghai in the future