Spatiotemporal Patterns Of The Major Air Pollutant And Causes Of The Heavy Pollution Period Based On Multi-Source Data In The Yangtze River Delta

With the rapid development of China’s economy and the accelerating process of urbanization,high-intensity haze pollution episodes frequently occurred in China.It is well known that the air pollution could bring huge economic losses and threats on human health.It will also affect the transportation and exchange of water and energy,and thus have a certain impact on climate change.Air pollution has attracted widespread public concerns.The Yangtze River Delta（YRD）is the largest metropolitan region in China.Its massive urbanization has detrimentally affected the urban environment and regional air quality,which have led YRD become one of the four major haze areas in China.Therefore,understanding the spatiotemporal characteristics of major air pollutants and investigating its influencing factors have been very important tasks.Besides,identifying the effects of urbanization on regional air quality and understanding to what extent urban design can improve or degrade urban air quality are also vital for sustainable development.In this paper,we mainly focus on:（1）exploring the spatiotemporal characteristics of air quality and major air pollutants in the YRD based on ground-based observation and satellite data;（2）investigating how the regional air quality varied with the urbanization,including urban form and comprehensive urbanization indexes;（3）examining the spatial and temporal variation of the severe air pollution events over the YRD,and analyzing the effects of meteorological factors and regional transport on the heavy pollution processes;（4）investigating the feasibility of using the AOD data retrieved by the Geostationary Ocean Color Imager（GOCI）to quantitatively estimate hourly PM_2.5 concentrations during winter haze episodes in the YRD.The main findings of this study are as follows:（1）The annual average AQI was 79,with the highest AQI appeared in winter and the lowest AQI appeared in summer.PM_2.5,PM₁₀ and O₃ were three primary pollutants in the YRD.Moreover,the air quality of the southern areas in the YRD was generally better than that of the northern parts.The highest AQI appeared in Xuzhou city of Jiangsu province（AQI:101）,and the lowest AQI appeared in Zhoushan city of Zhejiang province（AQI:46）.Besides,based on the satellite data of PM_2.5 and the Mann-Kendall（M-K）trend analysis results,it indicated that about 75%of the YRD showed a significant increasing trend of PM_2.5 during 1998²015,especially for the northern parts of Jiangsu province.While the PM_2.5 concentrations in the southern mountainous areas of Zhejiang province were relatively stable for many years.（2）Urban form had a significant effect on urban air quality in the YRD through the size and shape of urban patches.PARA_MN（Mean Perimeter-area Ratio）,ENN_MN（Mean Euclidean Nearest Neighbor Distance）,CA（Total Urban Area）and NP（Number of Urban Patches）had the most significant impacts on the air quality.In addition,the land use configuration was an important indicator to describe the urban air quality.When the buffer distance was 25 km,the air quality showed the highest correlation with the forest coverage.It suggested that a high forest coverage rate could contribute to better air qualities.Furthermore,we developed a spatial and temporal statistical model to explore the impacts of comprehensive urbanizations on PM_2.5patterns in the YRD during 1998-2015.The results indicated that the CV R² of the Linear Mixed Effect（LME）model was 0.87 with a regression slope of 0.88.The urban population,GDP proportion of the second industry,built-up areas,total road areas,number of students in colleges and universities,and total retail sales of consumer goods were positively associated with PM_2.5 levels.On the other hand,the proportion of the third industry employment,GDP ratio of the primary industry,forest areas,and number of hospital beds were negatively associated with PM_2.5.（3）By using the meteorological information and backward trajectory model,we investigated the effects of meteorological factors and regional transport on typical severe air pollution episodes.The results showed that the heavy pollution episodes（AQI>200）occurred in the YRD with a frequency of 2.1%,mainly concentrated in January and December,with PM_2.5.5 as the primary contributor.Spatially,compared to the southern region,northern YRD suffered more heavy pollution days,especially for Xuzhou city and Changzhou city in Jiangsu Province.We then analyzed three representative heavy air pollution episodes,during these episodes,the predominant wind was of northwesterly direction,and the atmosphere was more stable as indicated by lower wind speeds and high relative humidity.Analysis of the backward trajectories and frequency distributions showed that the airflow from the northwest had an important influence on the transport of air pollutants in the YRD.（4）We developed a three-stage spatial and temporal statistical model,using GOCI AOD and fine mode fraction,as well as corresponding monitoring PM_2.5 concentrations,meteorological and land use data on a 6-km modeling grid with complete coverage in time and space.The results showed that the 10-fold cross-validation R² was 0.72 with a regression slope of 1.01 between observed and predicted hourly PM_2.5 concentrations.We further analyzed two representative large regional episodes,i.e.,a“multi-process diffusion episode”related to unfavorable meteorological conditions during December21²6,2015 and a“Chinese New Year episode”related to human activities during February 7⁸,2016.We concluded that AOD retrieved by geostationary satellites could serve as a new valuable data source for analyzing the heavy air pollution episodes.