The characteristics and mixing states of PM2.5 during a winter dust storm in Ningbo of the Yangtze River Delta, China.

Dust particulates play an essential role for the nucleation, hygroscopicity and also contribute to aerosol mass. We investigated the chemical composition, size distribution and mixing states of PM2.5 using a single-particle aerosol mass spectrometer (SPAMS), Monitor for AeRosols and Gases (MARGA), and off-line membrane sampling from 2018.1.24 to 2018.2.20 at a coastal supersite in Ningbo, a port city in Yangtze River Delta, China. During the study campaign, the eastern part of China had experienced a wide range of cooling, sandstorm, and snowfall processes. The entire sampling campaign was categorized into five sub-periods based on the levels of PM2.5 and the ratios of PM2.5/PM10, namely clean (T1), heavy pollution (T2), light pollution (T3), dust (sandstorm) (T4) and cleaning pollution (T5) period. After comparing the average mass spectrum for each period, it shows that the primary ions, such as Ca2+and SiO3-, rarely coexist with each other within a single particle, but secondary ions generally coexist with these primary ions. Furthermore, the coexistence of each two different ions within a particle does not show distinct variation for the whole study periods. All these suggest that the absorption and partitioning of gaseous contaminants into the surface of primary aerosol through heterogeneous reactions are the major pathways of aging and growth of aerosol; and the merging of particles through collisions usually is insignificant. Although the absolute concentrations of nitrate and sulfate all increased with the PM2.5 concentrations, the relative equivalent concentrations of NO3- and SO42- displayed opposite trends; the relative contribution of sulfate decreased and that of nitrate increased as the increase of pollution. During the dust period, the relative equivalent concentrations of calcium and/or potassium ions in PM2.5 are significantly higher. This study provided deep insights about the mixing states and characteristics of particulate after long-range transport and a visualization tool for aerosol study.

[Pollution Characteristics and Potential Source Contributions of Gaseous Elemental Mercury (GEM) During Summer and Autumn in Ningbo City].

Gaseous elemental mercury (GEM) is commonly known as a hazardous heavy metal in the atmosphere and is harmful to living organisms. GEM is chemically stable and has a long residence time in the atmosphere; hence, it can be transported over long distances with air masses and is regarded as a global pollutant. We study the transportation and transformation mechanisms of GEM and its potential anthropogenic and natural contribution sources. GEM, conventional atmospheric pollutants, and meteorological parameters were monitored at a coastal site in Ningbo during the summer and autumn of 2017. The results were as follows. ① The concentrations of GEM ranged from 0.97-10.95 ng·m-3 and the mean and standard deviation (SD) were (2.32±0.90) ng·m-3, whereby the mean summer concentration was lower than the mean autumn concentration. ② The diurnal variations of GEM, ozone (O3), and gaseous oxidized mercury (GOM) during summer/autumn and sunny/rainy days suggest that higher levels of O3 and that strong solar radiation accounted for the rapid photochemical oxidation of GEM. The intensity of oxidation on sunny days was higher than that on rainy days. ③ Correlation analysis showed that GEM was significantly positively correlated with PM2.5 (R=0.65, P<0.01), PM10 (R=0.47, P<0.01), NO2 (R=0.46, P<0.01), and CO (R=0.57, P<0.01). Local and regional sources of GEM were mainly related to fossil fuel combustion. ④ The photochemical oxidation rate of GEM was influenced by the concentrations of oxidants (e.g., O3), gas-particle partitioning between GEM and particles, and light extinction effects of PM2.5, water vapor, and NO2. ⑤ Potential source contribution analysis (PSCF) indicated that the northwestern Zhejiang Province (including Ningbo City), the southern Anhui Province, and most of Jiangxi Province constitute a triangular area that is a potential source contribution to NBUEORS atmospheric GEM pollution during the summer. Local, regional, and long-range sources all had strong impacts on GEM pollution. During the autumn, the potential sources were mainly in the northern Zhejiang Province, and the source was smaller than that during the summer. GEM pollution during the autumn was mainly influenced by local and regional sources. Therefore, the control of atmospheric GEM pollution in the Yangtze River delta should apply inter-regional prevention and comprehensive control strategies in order to reduce atmospheric mercury pollution.