The Characteristics and Contributing Factors of Air Pollution in Nanjing: A Case Study Based on an Unmanned Aerial Vehicle Experiment and Multiple Datasets

Shudao Zhou,Min Wang,Zhanhua Liu,Ao Shen,Shuling Peng

doi:10.3390/atmos9090343

Abstract

Unmanned aerial vehicle (UAV) experiments, multiple datasets from ground-based stations and satellite remote sensing platforms, and backward trajectory models were combined to investigate the characteristics and influential mechanisms of the air pollution episode that occurred in Nanjing during 3–4 December 2017. Before the experiments, the position of the detector mounted on a UAV that was minimally disturbed by the rotation of the rotors was analyzed based on computational fluid dynamics (CFD) simulations. The combined analysis indicated that the surface meteorological conditions—high relative humidity, low wind speed, and low temperature—were conducive to the accumulation of PM2.5. Strongly intense temperature inversion layers and the low thickness of the atmospheric mixed layer could have resulted in elevated PM2.5 mass concentrations. In the early stage, air pollution was affected by the synoptic circulation of the homogenous pressure field and low wind speeds, and the pollutants mainly originated from emissions from surrounding areas. The aggravated pollution was mainly attributed to the cold front and strong northwesterly winds above 850 hPa, and the pollutants mostly originated from the long-distance transport of emissions with northwesterly winds, mainly from the Beijing‒Tianjin‒Hebei (BTH) region and its surrounding areas. This long-distance transport predominated during this event. The air pollution level and aerosol optical depth (AOD) were positively correlated with respect to their spatial distributions; they could reflect shifts in areas of serious pollution. Pollution was concentrated in Anhui Province when it was alleviated in Nanjing. Polluted dust, polluted continental and smoke aerosols were primarily observed during this process. In particular, polluted dust aerosols accounted for a major part of the transport stage, and existed between the surface and 4 km. Moreover, the average extinction coefficient at lower altitudes (<1 km) was higher for aerosol deposition.

Highlights

With the recent social modernization and industrialization coincident with the rapid increase in energy consumption and the intense emissions of air pollutants, major cities throughout China have frequently suffered from serious regional air pollution
Air pollution events are essentially subject to the impacts of emission sources and regional transport characteristics in addition to the atmospheric diffusion capacity [10], which is primarily related to meteorological conditions and synoptic situations
The characteristics and causes of air pollution episodes, such as haze pollution, have been widely analyzed in many studies based on observations, measurements, and numerical calculations, the results of which could provide a theoretical basis for the prevention and effective emergency warning of pollution events and the reduction in emissions

Summary

Introduction

With the recent social modernization and industrialization coincident with the rapid increase in energy consumption and the intense emissions of air pollutants, major cities throughout China have frequently suffered from serious regional air pollution. Air pollution events are essentially subject to the impacts of emission sources and regional transport characteristics in addition to the atmospheric diffusion capacity [10], which is primarily related to meteorological conditions and synoptic situations. The characteristics and causes of air pollution episodes, such as haze pollution, have been widely analyzed in many studies based on observations, measurements, and numerical calculations, the results of which could provide a theoretical basis for the prevention and effective emergency warning of pollution events and the reduction in emissions. Research suggests that the accumulation, transport, and dissipation of pollutants are affected by meteorological elements, including wind speed, temperature, relative humidity, and precipitation [11,12]. Long-term strong temperature inversion layers and lower atmospheric mixed layer height could contribute to the inhibition of diffusion conditions, resulting in continuous accumulation of pollutants [17]

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