A quantitative assessment of atmospheric emissions and spatial distribution of trace elements from natural sources in China

Highlights

• Emission of Mn (10, 677t) from natural sources ranks 1st in 15 trace elements in China.

• Emissions of other elements in 2015 range from 7.45 tons (Hg) to 1, 400 tons (Zn).

• Soil & wind-erosion dust and biogenic are identified as the two biggest contributors.

• Soil & wind-erosion dust shared over 95% of total emission of Cr, Co, As, Mn and V.

• Emission intensity per unit land is higher in Southern than that in Northern provinces.

Abstract

Natural sources, such as soil and wind-erosion dust (SWD), biomass open burning (BOB), sea salt spray (SSAS) and biogenic source (BIO), are major contributors to atmospheric emissions of trace elements (TEs) globally. In this study, we used a comprehensive approach to account for area-, production- and biofuel consumption-based emission factor calculation methods, and thus developed an integrated high-resolution emission inventory for 15 types of TEs (As, B, Cd, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Sb, Se, V and Zn) originated from natural sources in China for the year 2015. The results show that national emissions of TEs in 2015 range from 7.45 tons (Hg) to 1, 400 tons (Zn) except for the extremely high emissions of Mn (10, 677 tons). SWD and BIO are identified as the top two source contributors, accounting for approximately 67.7% and 26.1% of the total emissions, respectively. Absolute emissions of TEs from natural sources are high in the Xinjiang, Inner Mongolia and Tibet autonomous regions with large areas of bare soil and desert. However, emission intensity of TEs per unit area in the Southern provinces of China is higher than those in Northern China and Southwestern China, with the Yunnan and Sichuan provinces displaying the highest emission intensity. Our results suggest that controlling SWD can play a significant role in reducing fugitive particulate matter and the associated emissions of TEs from natural sources in China; and desertification control is particularly critical in the Northwest provinces where the majority of deserts are located.

Introduction

East Asia, especially China, is one of the most atmospheric polluted areas in the world (Pacyna et al., 2006b; Tong et al., 2018; Wilson et al., 2006; Zhou et al., 2015). Heavy pollution in China has boosted studies on conventional air pollutants such as sulfur dioxide (SO₂), nitrogen oxides (NO_x), particulate matters (PM) and volatile organic compounds (VOCs), as well as trace elements (TEs) in atmosphere (Li et al., 2017a; Fu and Chen, 2017; Li et al., 2017b). Airborne TEs are principally toxic and harmful heavy metals and semi-metals, which exist in atmosphere with low concentrations. Some TEs act as the interface and catalyst in atmospheric photochemical reaction, making airborne TEs become significant roles in fog-haze formation (Li et al., 2017b; Fu and Chen, 2017; Behera et al., 2015). In recent years, airborne TEs have been linked to various adverse effects on human health, as well as on the ecosystem (Larison et al., 2000; Marsden, 2003; Swaine, 2000). Moreover, when the damages of TEs in the human body can be noticed, they have usually reached an irreversible stage (Senesil et al., 1999; Swaine, 2000). In order to evaluate the adverse effects of atmospheric TEs, quantitative assessments of atmospheric emissions of TEs from anthropogenic sources have been conducted in Europe and America (Pacyna et al., 2007, 2006a; Pacyna and Pacyna, 2001; Streets et al., 2009). For example, mercury which is generally regarded as one of the most important atmospheric trace elements, it is reported that global atmospheric mercury emission has increased sharply since 1950, reached 2300–2600 tons per year in 2010, and will be doubled in 2050 at most (Pirrone et al., 2010; Streets et al., 2018, 2009). In the case of China, Streets et al. (2005) have evaluated the anthropogenic mercury emissions (about 536 tons in 1999) then Wu et al. (2006) updated the time series of mercury inventory from 1995 to 2003. Subsequently, a series of inventories about TEs have been developed and updated for different sectors that include coal combustion (Tian et al., 2014) and municipal solid waste incineration in China (Tian et al., 2012). For example, Tian et al. (2015) have developed an integral emission inventory of 12 TEs from anthropogenic sources in China for the period between 1949 and 2012. Additionally, several specific inventories of TEs focusing on metal smelting, cement plants and some other sectors have also been developed and updated (Hua et al., 2016; Wang et al., 2016a, 2016b; Wang et al., 2017; Xue et al., 2016).

Several comprehensive studies have covered the emissions of various TEs originated from different anthropogenic sources in China (Liu et al., 2018a, 2018b; Tian et al., 2015), of which none of the studies above have included emissions originated from natural sources. Natural sources generally include but are not limited to, soil and wind-erosion dust (SWD), sea salt spray (SSAS), wild fire and biogenic sources (BIO) (EEA, 2016; Nriagu, 1989). Although the emission characteristics of TEs from anthropogenic including both category contribution and spatial distribution have been widely discussed, limited studies have been conducted towards quantitatively assessing emissions of trace elements from natural sources. Additionally, previous studies have usually focused on typical air pollutants from single or limited categories of natural source emissions such as PM or VOCs, leaving comprehensive emission inventories of natural sources as a rare occurrence. Emissions of particulate matters which originated from SWD were used to identify the origin of sandstorms and to evaluate the contribution of dust (Gillette and Passi, 1988; Jiang et al., 2016; Song et al., 2016; Xuan, 1999; Xuan et al., 2000). Biomass open burning (BOB) emits large volume of black carbon (BC), organic carbon (OC), SO₂, NO_x and others in a very short time, some studies have evaluated the contribution of BOB to global biogeochemical processes (Bond et al., 2004; Cao et al., 2005; Friedli et al., 2003; Michelazzo et al., 2010; Tian et al., 2011). Huang et al. (2011) have evaluated mercury emission from biomass burning (including BOB and biofuel combustion) in China, indicated that average mercury emission from biomass burning was 27 tons per year and about 99% of emissions are contributed from mankind activities. BIO are thought as a dominate contributor to VOCs, and detailed national and global inventories based on plant category or crop production have been developed previously (Chi and Xie, 2012; Jiang et al., 2018; Zhang and Xie, 2009). As for emissions of TEs which originating from natural sources in China, a few studies have focused on mercury, showing a release of 574.5 tons per year, contributing to about 10% in global total emission (Pirrone et al., 2010; Wang et al., 2016a, 2016b). However, to the best of our knowledge, research on emission inventories focus on atmospheric TEs from natural sources in China is still very limited.

In this paper, we present a comprehensive primary emission inventory of 15 TEs (As, B, Cd, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Sb, Se, V and Zn) originating from natural sources in China in 2015. Detailed assessment of emissions from four major natural source categories have been estimated with different methods for each provincial administration in Mainland China. We believe this study can improve the understandings of spatial distribution and category contribution of natural source emissions about TEs in China. Furthermore, by comparing with our previous estimation of anthropogenic emissions of TEs, the relative contribution between anthropogenic and natural sources in China has also been assessed.