Mineralogical similarities and differences of dust storm particles at Beijing from deserts in the north and northwest

Highlights

• Clay and quartz were the main minerals in the dust storm particles.

• Northerly dust storm particles contain more feldspathic and less carbonate minerals than northwesterly ones

• Sulfate rarely observed on the dust particles will be gradually produced after the dust storms.

Abstract

Dust storm particles have been one of the important contributors to global aerosol loading, affecting human health and climate system. Beijing, a megapolitan city, experienced two severe dust storms in spring of 2015, with maximum hourly-mean PM₁₀ mass concentrations exceeding 1000 μg/m³. The first dust storm (Dust 1) was from east area of Gobi Desert about 850 km in the north of Beijing and the second (Dust 2) was from west area of Gobi Desert about 1500 km in the northwest of Beijing. Morphologies and elemental compositions of dust particles were identified using high-resolution electron microscopy. The statistical analysis showed that the number fractions of mineral dust particles during the two dust storm episodes were 85.3% and 95.4%, respectively. Clay minerals were the most abundant among mineral particles, with a number fraction larger than 50%, followed by quartz particles (17.3% and 14.8%) and feldspar. Feldspar and carbonate particles accounted for 14.8% and 3.4% of mineral particles in Dust 1, and 9.9% and 13.6% in Dust 2, with the difference due to the different source areas. When the dust storms directly migrated to Beijing, the occurrence of S-containing mineral particles and the relative weight ratio of S in individual mineral particles were extremely low, indicating limited production of sulfate on the dust-storm particles in the atmosphere, regardless of the differences of source areas, migration paths, and mineralogical components. After the peaks of dust storms passed, the occurrence of S on the mineral particles were much higher, although the relative weight ratios of S in the mineral particles was still very small. This result suggests that most of the mineral particles underwent heterogeneous reactions, but the reaction rates were low.

Introduction

Airborne mineral dust, which mainly originates from arid and semi-arid regions, can be transported in the atmosphere across continents (Shao et al., 2008; Yu et al., 2020). Mineral dust is one of the important contributors to global aerosol loading (Bibi et al., 2020). It was estimated that the annual global emissions of airborne mineral dust with diameter less than 10 μm accounted for 1490 ± 160 Tg of the total particulate mass loading in the atmosphere (Zender et al., 2003). Dust particles are involved in many important processes in the global climate system (Buseck and Posfai, 1999). They can absorb and scatter solar radiation (Unga et al., 2018), act as cloud condensation nuclei once they are coated with soluble species (Buseck and Posfai, 1999; Kelly et al., 2007), and act as ice nuclei (Lambert et al., 2008). Dust particles transported over long distances can be deposited into oceans and forests acting as nutrients and affecting biogeochemical cycles (Li et al., 2017; Wang et al., 2020a; Yu et al., 2015).

Dust storms in northern China frequently occur in spring (Xie et al., 2019), and spring dust storms account for more than half of the total dust storms (Yang et al., 2019). The Gobi Desert and Taklamakan Desert in the East Asia emit about 140–220 Tg mineral dust annually (Querol et al., 2019). Dust particles can cross the continental China and be transported to Korea and Japan (Chun et al., 2008), and even to the North Pacific in springtime (Arimoto et al., 2006). Previous research has confirmed that Asian dust aerosols can be transported from northwest China to the Arctic within 5 days, crossing eastern China, Japan and Siberia before reaching the Arctic (Huang et al., 2015). Beijing, as a metropolitan city, might be on the migration path of the mineral dust to the Arctic. Therefore, detailed knowledge of mineral dust on Beijing is important for understanding the air pollution in Beijing and its downstream areas.

It has been confirmed that the mineral particle surface can act as an important sink for atmospheric trace gases, enhancing the formation of sulfate and nitrate during polluted haze periods (Wan et al., 2016; Wang et al., 2016, Wang et al., 2017a). However, field studies have shown different results about sulfate and nitrate formation on dust particles during severe dust storm periods. Several previous studies have found substantial sulfate and nitrate formation on the surface of mineral dust after long-distance transport (Huang et al., 2010; Tang et al., 2004; Wang et al., 2017a, Wang et al., 2018; Yu et al., 2020; Zhao et al., 2007). On the contrary, some measurements on the composition of the Asian mineral dust after long-range transport showed that most of the mineral dust particles were not altered chemically (Hu et al., 2016; Wang et al., 2020b; Wu et al., 2017; Zhang et al., 2005). Therefore, there are still some uncertainties about the heterogeneous reaction rates on the surface of mineral dust. The influence factors of heterogeneous reaction rates on the surface of mineral dust should be discussed in more detail.

The Gobi Desert has an area of more than 1.3 million square kilometers and most of the researchers classified the dust storms sourced from Gobi Desert as Asian dust. To our knowledge, there are limited information on the comparison of individual mineralogical components in Beijing from different source areas of Gobi Desert and on how the mineralogical components affect the heterogeneous reactions of mineral dust during their transport. High resolution transmission electron microscopy (TEM) coupled with energy dispersive X-ray spectroscopy (EDS) can identify the morphology, elemental composition and mixing state of individual aerosol particles (Li et al., 2016). Beijing experienced two severe dust storms in spring of 2015, with their maximum hourly-mean concentrations of PM₁₀ exceeding 1000 μg/m³. The sky became yellow and the visibility became extremely low. People had to wear masks when going out under such weather conditions. These dust particles had further deteriorated air quality in Beijing and its downstream areas. The severe dust storms observed in Beijing had different dust sources and different migration paths. In this paper, we have compared the similarities and differences in mineralogy of dust particles of these two dust storm episodes, and based on the elemental compositions, sulfation characterization of mineral dust was further discussed.