Abstract
Two episodes with heavy air pollution in Nanjing, China, one in the summer and another one in the winter of 2017, were selected to study aerosol properties using sun photometer and ground-based measurements, together with source region analysis. The aerosol properties, the meteorological conditions, and the source regions during these two episodes were very different. The episodes were selected based on the air quality index (AQI), which reached a maximum value of 193 during the summer episode (26 May–3 June) and 304 during the winter episode (21–31 December). The particulate matter (PM) concentrations during the winter episode reached maximum values for PM2.5/10 of 254 g m−3 and 345 g m−3, much higher than those during the summer (73 and 185 g m−3). In contrast, the value of aerosol optical depth (AOD) at 500 nm was higher during the summer episode (2.52 0.19) than during that in the winter (1.38 0.18). A high AOD value does not necessarily correspond to a high PM concentration but is also affected by factors, such as wind, Planetary Boundary Layer Height (PBLH), and relative humidity. The mean value of the Ångström Exponent (AE) varied from 0.91–1.42, suggesting that the aerosol is a mixture of invaded dust and black carbon. The absorption was stronger during the summer than during the winter, with a minimum value of the single scattering albedo (SSA) at 440 nm of 0.86 on 28 May. Low values of asymmetry factor (ASY) (0.65 at 440 nm and 0.58 at 1020 nm) suggest a large number of anthropogenic aerosols, which are absorbing fine-mode particles. The Imaginary part of the Refractive Index (IRI) was higher during the summer than during the winter, indicating there was absorbing aerosol during the summer. These differences in aerosol properties during the summer and winter episodes are discussed in terms of meteorological conditions and transport. The extreme values of PM and AOD were reached during both episodes in conditions with stable atmospheric stratification and low surface wind speed, which are conducive for the accumulation of pollutants. Potential source contribution function (PSCF) and concentration weighted trajectory (CWT) analysis show that fine mode absorbing aerosols dominate during the summer season, mainly due to emissions of local and near-by sources. In the winter, part of the air masses was arriving from arid/semi-arid regions (Shaanxi, Ningxia, Gansu, and Inner Mongolia provinces) covering long distances and transporting coarse particles to the study area, which increased the scattering characteristics of aerosols.
Highlights
Air quality in China varies throughout the country with local hotspots and high concentrations of aerosols and trace gases in regions with a high degree of industrialization and urbanization and related activities, such as high traffic density, augmented by local geographical and meteorological conditions conducive for the formation of haze [1,2]
The results show that the average concentrations of PM2.5/10 are highest in December (77/131 μg m−3) and lowest in August (16/40 μg m−3)
It is noted that PM2.5/10 concentrations are included in the air quality index (AQI) and, the high AQI in spring and summer is due to the contributions from contaminants other than aerosols
Summary
Air quality in China varies throughout the country with local hotspots and high concentrations of aerosols and trace gases in regions with a high degree of industrialization and urbanization and related activities, such as high traffic density, augmented by local geographical and meteorological conditions conducive for the formation of haze [1,2]. Episodes of haze formation appear to occur more frequently [9] with changing meteorological conditions as one of the major causes [10]. Major factors contributing to aerosol loading during heavy pollution episodes include direct emissions [12], such as from combustion processes, traffic, straw burning, and dust storms, and the formation of aerosol particles from precursor gases. The concentrations may be further augmented due to aerosol dynamical processes and meteorological influences. Chemical, and optical properties of aerosol particles are modified by hygroscopic growth [13]
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