Mixing and transformation of Asian dust with pollution in the two dust storms over the northern China in 2006

Abstract

To study the mixing and transformation of Asian dust with pollution in the two dust storms over the northern China in 2006, both TSP and PM_2.5 samples were collected at three sites of northern China in addition to the dry deposition samples collected in an episode in Beijing. 23 elements, 15 ions, and 16 PAHs in each sample were analyzed. The two dust storms in northern China were observed in April 8–10 (DS1) and April 16–18 (DS2). Compared to DS2, DS1 was weaker and more polluted with stronger mixing between crustal and pollutant aerosols during their long-range transport. The concentrations of pollution species, e.g. pollution elements, ions, and PAHs were higher in DS1 than that in DS2, while the crustal species showed adverse variation. The correlation between chemical species and Al and between PAH(4) and PAH(5,6) further confirmed the stronger chemical transformation and aerosol mixing in DS1 than that in DS2. Back trajectory and chemical analysis revealed that in DS1 the air masses at Beijing were mostly from southern or southwestern direction at lower altitude with much more pollution, while in DS2 the air masses were mostly from the northwestern and northern direction with dust mainly, which explained why there was a stronger mixing of dust with pollution aerosol in DS1 than that in DS2 over Beijing.

Introduction

Dust storm, blowing dust, and floating dust are clarified according to the horizontal visibility degraded by the dust and sand. Dust storm not only deteriorates the air quality and ecological system and accelerates the desertification in source area and pathways, but also affect the local climate by decreasing the visibility and influencing the radiation balance of the atmosphere, which may cause extensive economic loss (Wang et al., 2005a, Wang et al., 2005b). Asian dust, starting from Gobi deserts in Mongolia and northern China and Taklimakan desert in western China (Sun et al., 2005), spreads rapidly over China, Korea (Chung et al., 2003), Japan (Nishikawa et al., 1991), and even to the west coast of North America (Husar et al., 2001, Perry et al., 1999). The crustal dust can mix with pollution aerosol on the pathway and change the chemical composition of the aerosols during its long-range transport. Furthermore, different transport pathway could result in different characteristics of the aerosols (Sun et al., 2005). Northern China located in the eastern Asian monsoon region, where arid and semiarid climate dominate, is one of the four regions of dust storm frequently occurred in the world for the past million years (Yan, 1993). The total area of Gobi desert and desertification land is approximately 1.653 million square kilometers (Wang and Zhu, 2001). Wang et al., 2005a, Wang et al., 2005b concluded that dust storm events gradually increased since 1998 based on the study on the frequencies of dust storm events during 1954–2001. A super dust storm attacked Beijing on March 20, 2002, which was the greatest one ever recorded until then (Sun et al., 2004). Although no such a big dust storm occurred in the next 3 years, another super dust storm attacked Beijing on April 17, 2006, which would verify the conclusion that the dust storm events happened in northern China with a cycle of 3–4 year (Wang et al., 2005a, Wang et al., 2005b). The previous studies in our group (Zhuang et al., 2001, Guo et al., 2004, Wang et al., 2005a, Wang et al., 2005b, Sun et al., 2005) revealed that dust storm not only delivered large amounts of crustal elements but also carried significant quantities of pollutants. In addition, four stages in the dust episode were observed and both crustal and pollutant species increased especially in PM_2.5 during the dust storm, indicating that the dust storm was also a “pollutant” storm (Guo et al., 2004, Sun et al., 2004, Sun et al., 2005). The dust particles could mix well with pollution aerosols, including sulfate, nitrate (Sun et al., 2005, Shen et al., 2007), the pollutant elements, such as Se, Ni, Pb, Br, and Cu (Zhang et al., 2005), sea salt particles (Fan et al., 1996), black carbon (Kim et al., 2004), and volatile organic compounds (Cheng et al., 2006) during the long-range transport. Hou et al. (2006) also reported that dust storm could carry and transport much organic pollutants, such as polycyclic aromatic hydrocarbons (PAH) in dust aerosols. However, Zhang et al. (2005) argued that Asian dust plume and polluted air masses were found not mixed with each other; instead, they were separated in two consecutive air parcels. To date there seems no concurrent measurements of inorganic and organic pollutants during dust storms. In this study both PM_2.5 and TSP samples were collected at three sites of northern China in the spring dust season in 2006 with the measurement of inorganic and organic pollutants. In addition, the dry deposition samples were collected on the day of the super dust fall. This study would provide more evidences of mixing and transformation of Asian dust with pollution aerosols during the long-range transport.