Characteristics Of Urban-Rural Differences In Air Pollution And Its Driving Factors In China

Air pollution is one of the world’s greatest health and environmental problems,which is particularly serious in developing countries.The rapid expansion of urban areas,and the increase in population and energy consumption,caused significant changes in the urban atmospheric environment and gained widespread attention.In the current stage,all people are exposed to an atmospheric environment that exceeds safety standards in China.Influenced by urban development,air pollution management and natural environment,urban-rural differences in air pollution show remarkable spatial and temporal heterogeneity.Currently,China has fully established an air environment monitoring network,but there is still an urban-rural imbalance in the number of monitoring stations and related researches.In 2021,World Health Organization（WHO）further emphasized the health hazards of PM2.5 and NO2 and tightened their guidelines,meanwhile,PM2.5 and NO2 are the primary air pollutants in China.Therefore,this study took PM2.5 and NO2 as the primary research objects,from the perspective of sustainable development of urban and rural areas,the spatial and temporal characteristics of the urban-rural difference in PM2.5 and NO2 were studied.Supported by ground monitoring stations and satellite remote sensing air pollutants data,we conducted research on the impact of the natural environment,urban form and emission reduction on urban and rural air pollution based on quantitative evaluation of the spatial and temporal variability of urban-rural differences of primary air pollutants in China.Furthermore,based on the characteristics of PM2.5 and population distribution in urban and rural areas at the prefecture-level city scale,the demands for additional air pollution monitoring stations in old urban areas,new urban areas and rural areas in China were further evaluated from the perspective of balanced development of urban and rural areas.The main conclusions of this paper are as follows:（1）The average concentrations of PM2.5 and NO2 in China in 2019 were 30.1 and19.3μg/m³,respectively,and there were no city’s PM2.5 and NO2 concentrations that reached the global air quality guideline values（AQG 2021）issued by WHO in 2021.In general,air pollutants’concentrations were higher in urban than rural areas,and the concentrations of PM2.5 and NO2 in urban areas were higher than that in rural areas by14.5%and 14.9%,respectively.Due to the influence of air pollution transport and resuspended dust,PM2.5 concentrations were higher in rural areas of most cities in Xinjiang.It is worth noting that the increase of emission intensity in urban areas,the difference in human activity intensity between urban and rural areas,the increase in vegetation cover,and the complexity of topographic terrain all contribute significantly to amplifying the urban-rural difference in air pollutants.In addition,this study found that the urbanization rate of land had a significant effect on reducing the urban-rural difference in NO2,while a significant relationship was not found in PM2.5.（2）Urban form had a significant effect on PM2.5 and NO2 concentration in urban and rural areas in China.Urban PM2.5 concentration was mainly influenced by the sprawl and number of urban patches,and NO2 was mainly influenced by the sprawl of urban patches.Compared with urban areas,rural PM2.5 concentration was more likely to be influenced by urban patches’morphological characteristics,and fragmentation of urban patches could facilitate the transport of air pollutants between urban and rural areas.The simulation results based on the locally weighted regression model showed that the high level of urbanization had an ameliorating effect on urban and rural PM2.5,while a weak ameliorating effect on NO2 was found.It should be noted that the development and expansion of small cities were more likely to worsen PM2.5 and NO2pollution.In addition,forest land could play an important role in mitigating air pollution as well as reducing the transport of air pollution from urban to rural areas.In the urban expansion planning of small cities,regulating the number of urban patches and avoiding excessive fragmentation of urban patches will be conducive to mitigating urban and rural air pollution.（3）The change trends of main air pollutant concentrations were similar in urban and rural areas at the national scale from 2016 to 2020,but the decreases of PM2.5 and NO2 in rural areas were lower than that in urban areas,and the urban-rural differences in PM2.5 and NO2 were gradually reduced.In addition,the provincial decrease of PM2.5showed a significant linear relationship with the air pollution emission reduction plan（the reduction ratio of emission planned for 2020）（urban:R²=0.62,Rural:R²=0.76）,the stricter the pollution emission control,the greater the PM2.5 concentration decreased.It is worth noting that there was a significant difference in the response of urban and rural PM2.5 to the air pollution emission reduction plan,with the plan improving PM2.5more in urban areas.In contrast,the NOx emission reduction control plan failed to show a remarkable effect.When the NOx emission reduction plan was>10%,the NO2decrease only started to increase with a more stringent emission control.The harmonic model simulation results showed that the improvement of PM2.5 during the 13^th Five-Year Plan period was mainly due to pollution reduction control,and the decrease of NO2 was mainly due to traffic control during the COVID-19 epidemic（the contribution of the COVID-19 epidemic was 66.7%）.The effectiveness of NOx reduction during the13^th Five-Year Plan would be overestimated if only the NO2 pollution concentration in2020 was used as the evaluation indicator.（4）At present,China’s national monitoring stations（NMS）for air quality were excessively concentrated in old urban areas—90%of the NMS were distributed in the buffer zone within 2500 m of old urban areas,and the air quality monitoring stations in new urban and rural areas were relatively insufficient.The comparative analysis of PM2.5 concentrations from NMS and remote sensing data revealed that the monitoring data based on NMS concentrated in old urban areas would overestimate the urban or national scale PM2.5 pollution levels.Based on temporal and spatial variability of PM2.5,population distribution and current stations number in each region,this study assessed the demand index for additional NMS for the atmospheric environment in different urbanization level areas in 336 prefecture-level cities in China,and the results showed that the region with high demand for additional NMS including new urban areas in northern China（North and Northwest China）,as well as the rural areas in Hebei,Henan and Shandong provinces,with each region requiring 2 to 5 additional monitoring stations.