Characteristics of Aerosol Chemical Compositions and Size Distributions during a Long-Range Dust Transport Episode in an Urban City in the Yangtze River Delta

• Main Authors: Zhengxu Gao, Xiaoling Wang, Lijuan Shen, Hua Xiang, Honglei Wang
• Format: Article
• Language: English
• Published: MDPI AG 2019-02-01
• Series: Atmosphere
• Subjects: dust transport episode; the Yangtze River Delta; water soluble ions; size distribution; PM
• Online Access: https://www.mdpi.com/2073-4433/10/2/68

A long- and large-range heavy dust episode occurred from 3 to 8 May 2017 in China. To explore the impacts of this long-range dust transport episode on the chemical compositions and size distributions of urban aerosols, such instruments as an online analyzer for monitoring aerosols and gases (MARGA) and a wide-range particle spectrometer (WPS) were mainly used to monitor chemical components, such as PM_2.5 and aerosol size distributions in the range of 10 nm to 10 μm, in Nanjing in this study. During the dust episode, the average concentrations of PM_2.5 and PM₁₀ and ions of Ca²⁺, Mg²⁺, Cl^−, SO₄^2−, NO₃^−, and NH₄⁺ were 66.2, 233.9, and 1.1, 1.5, 1.1, 11.4, 7.8 and 4.4 μg·m^−3, which were 4.4, 5.8, 3.7, 15, 1.38, 1.84, 1.66 and 1.83 times higher than the values observed before the episode and 2.2, 3.3, 5.5, 5.0, 1.57, 1.97, 1.39 and 1.69 times the levels after the episode. The dusts were demonstrated to have differential impacts on the water-soluble gases in the air. During the dust episode, the concentrations of HCl, SO₂ and NH₃ were comparably low, while the HNO₂ and HNO₃ concentrations were high. The diurnal variations in pollutants, including SO₂, HNO₃, Cl^−, Ca²⁺, Mg²⁺, PM_2.5 and PM₁₀, were strongly impacted by the dust episode. However, those variations in NH_3, NO₃^−, SO₄^2− and NH₄⁺ were only slightly influenced. Pollutants were distinctively featured in the various dust stages. The concentration of HNO₂ was relatively high in the earliest stage but was substituted by those of SO₂, PM₁₀, PM_2.5, Ca²⁺, Mg²⁺ HNO₃ and Cl^− in the explosion stage. The aerosol number concentrations exhibited unimodal distributions in the earliest and explosion stages but showed bimodal distributions in the duration and dissipation stages. Additionally, the aerosol size distributions were observed to shift to larger particle segments in different dust stages. The surface area concentrations exhibited four peaks in different dust stages and exhibited trimodal distributions in the non-dust episode. The surface area concentration of fine particles first increased during the earliest stage, while that of coarse particles first decreased during the dissipation stage.