Self-organized Criticality Of PM_2.5 Evolution During Haze Period In Typical City

Atmospheric particulate matter is an important pollutant that causes the haze event to occur,and is also the primary pollutant that currently affects Chinese urban air quality.One of the fine particles matter will not only reduce the atmospheric visibility,but also seriously endanger the physical health of urban residents,which has aroused the extensive attention of the public and governments worldwide.In this background,studying the formation mechanism of PM_2.5 pollution and its evolution and internal dynamics will help us to make new prevention measures to air pollution and improve the forecasting method of ambient air quality,thus reducing its harm to the health of the urban population.In this study,the examined data sets are PM_2.5 hourly concentrations at eight air monitoring stations of Chengdu during haze,from January 20 to January 27,2015.Firstly,the space source of PM_2.5 pollutants during haze was quantitatively analyzed using the HYSPLIT backward trajectory model.Furthermore,the long-range correlation of PM_2.5 space-time evolution and its interaction with external influencing factors were studied by using detrended fluctuation analysis?DFA?and detrended cross-correlation analysis?DCCA?,respectively.And then,on this basis,we explored the dynamic characteristics of PM_2.5 evolution in Chengdu.Meanwhile,based on self-organized criticality?SOC?theory,water avalanche experiment with real physical meaning was designed and implemented.The PM_2.5 concentration evolution was compared with the water avalanche experiment,and the self-organized criticality behavior of PM_2.5 evolution during haze was described.In details,the main jobs and conclusions of this dissertation are as follows.1.The airflow trajectory arriving in Chengdu at different initial heights were tracked by HYSPLIT backward trajectory model,and then clustering analysis.The results show that most of the airflow in Chengdu during haze comes from the short-distance delivery of the local and surrounding urban areas.Compared with the medium and long distance airflow,the proportion of the short-distance airflow is high,and the proportion at each initial height is more than 85%.Moreover,the length of the airflow trajectory is shorter and the movement speed is slower.This indicates that the high concentration PM_2.5 pollutants in Chengdu during haze may mainly come from the evolution of the atmospheric environment system in its local and surrounding areas.2.The persistence of PM_2.5 hourly concentrations at eight air monitoring stations during haze is investigated by DFA method.The results indicated that the evolution of PM_2.5 concentration at each monitoring stations exhibit strong long-range correlation.This means that in the evolution process,the concentration of PM_2.5 at some point in the past will continue to have a certain impact on the future trend of PM_2.5 concentration.The PM_2.5 concentration value in a future time scale will show the same trend as the past value.3.The cross-correlation between PM_2.5 concentration and meteorological factors?precipitation,wind speed?was analyzed by using DCCA method in 2013-2015.The results show that there are some strong interactions between PM_2.5 concentration and precipitation,and wind speed.Both the DCCA curves show two power-law intervals.There is a strong positive correlation between PM_2.5 concentration and precipitation,and wind speed in the previous section,and this correlation exists in a power-law form on a certain time scale,but the overall power-law interval is not long.However,there is a continuous inverse correlation between PM_2.5 concentration and precipitation,and wind speed in the latter part of the long time scale.4.Daily monitoring data of PM_2.5 concentration and air absorbed dose rate from June 1,2013 to May 31,2014 were analyzed by using DCCA method.The results indicated that a notable positive correlation existed between PM_2.5 concentration and air absorbed dose rate,and this relationship display a long-term persistence within a certain time scale.Meanwhile,the contrasting investigation showed that the correlation between PM_2.5 concentration and air absorbed dose rate has significant differences in winter and summer.The DCCA curve of the two exhibits a single power-law form in the winter,and the cross-correlation exponent is 1.015.However,in summer,the DCCA curve of the two is divided into two segments.The corresponding feature time scale of breakpoint is about two months,which it is speculated that the mutation of this correlation structure is mainly associated with the rainfall in summer.Studies have shown that high PM_2.5 concentration may have a significant impact on the radiation environment of the atmosphere during heavy haze.5.The temporal scaling properties of PM_2.5 concentration at eight air monitoring stations during haze were explored by using frequency statistical analysis.The results show that the cumulative frequency statistical distribution follows a good negative power law distribution.This characteristic means that large air pollution events do not play a unique role,but follows the simple rules as the small air pollution events,and the occurrence of air pollution events of all sizes may have the same dynamic mechanism.6.Firstly,based on SOC theory,water avalanche experiment with real physical meaning was designed and implemented.The water avalanche experiment results indicated that the intensity-frequency relationship of the water droplets avalanche scale fluctuation in each group of experiments under different temperatures is subject to the negative power-law distribution.Meanwhile,the study found that the waiting time of water droplet avalanche also well follows stretched exponential distribution.Further,the PM_2.5 concentration evolution was compared with the water avalanche experiment.The results show that the evolution process of PM_2.5 during haze is similar to water droplets avalanche event in water avalanche experiment.The SOC characteristic of high PM_2.5 concentration evolution is the main internal dynamics mechanism in the case of internal forces.It is the stability of this SOC system,so that the PM_2.5 concentration in the local atmospheric environment is maintained at a critical state,which causes the haze pollution to be in a serious situation for a long time.