Similarities and Differences in the Temporal Variability of PM2.5 and AOD Between Urban and Rural Stations in Beijing

Disong Fu,Yunfei Wu,Minzheng Duan,Huaigang Zhou,Huizheng Che,Xiangao Xia,Zhiqiang Ma,Xiaoling Zhang,Weijun Quan,Zijue Song,Weiwei Pu

doi:10.3390/rs12071193

Abstract

Surface particulate matter with an aerodynamic diameter of <2.5 μm (PM2.5) and column-integrated aerosol optical depth (AOD) exhibits substantial diurnal, daily, and yearly variabilities that are regionally dependent. The diversity of these temporal variabilities in urban and rural areas may imply the inherent mechanisms. A novel time-series analysis tool developed by Facebook, Prophet, is used to investigate the holiday, seasonal, and inter-annual patterns of PM2.5 and AOD at a rural station (RU) and an urban station (UR) in Beijing. PM2.5 shows a coherent decreasing tendency at both stations during 2014–2018, consistent with the implementation of the air pollution action plan at the end of 2013. RU is characterized by similar seasonal variations of AOD and PM2.5, with the lowest values in winter and the highest in summer, which is opposite that at UR with maximum AOD, but minimum PM2.5 in summer and minimum AOD, but maximum PM2.5 in winter. During the National Day holiday (1–7 October), both AOD and PM2.5 holiday components regularly shift from negative to positive departures, and the turning point generally occurs on October 4. AODs at both stations steadily increase throughout the daytime, which is most striking in winter. A morning rush hour peak of PM2.5 (7:00–9:00 local standard time (LST)) and a second peak at night (23:00 LST) are observed at UR. PM2.5 at RU often reaches minima (maxima) at around 12:00 LST (19:00 LST), about four hours later (earlier) than UR. The ratio of PM2.5 to AOD (η) shows a decreasing tendency at both stations in the last four years, indicating a profound impact of the air quality control program. η at RU always begins to increase about 1–2 h earlier than that at UR during the daytime. Large spatial and temporal variations of η suggest that caution should be observed in the estimation of PM2.5 from AOD.

Highlights

Beijing, the capital of China, has frequently suffered from worsening air pollution episodes, especially in recent years [1,2,3,4]
A novel time-series analysis tool developed by Facebook, Prophet, is used to investigate the holiday, seasonal, and inter-annual patterns of PM2.5 and aerosol optical depth (AOD) at a rural station (RU) and an urban station (UR) in Beijing
Using collocated hourly PM2.5 and AOD datasets spanning more than 10 years at Beijing urban (UR) and rural (RU) stations, the objective of this study is to present a closer look at the temporal variabilities of PM2.5 and AOD, as well as their relationship at diurnal to inter-annual scales

Summary

Introduction

The capital of China, has frequently suffered from worsening air pollution episodes, especially in recent years [1,2,3,4]. The diurnal and seasonal variability of PM2.5 in the Beijing urban area has been studied based on observations of 1–3 years, which showed diurnal maxima during the latter half of the night to early morning and a seasonal peak in winter [11,12]. The AOD uncertainty of the Aerosol Robotic Network (AERONET [16], https://aeronet.gsfc.nasa.gov/) is 0.01~0.02 [17]. Diurnal variation of AOD based on 15 years worthy of AERONET observation data in the North China Plain (NCP) showed that AOD increased gradually from early morning to later afternoon [18]. Based on long-term observations of 50 China Aerosol Remote Sensing Network (CARSNET) sites, Che et al [19] found that annual mean AOD at 440 nm (AOD 440nm) increased from remote/rural sites (0.12) to urban sites (0.79)

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Conclusion