Chemical nature of PM_2.5 and PM₁₀ in Xi'an, China: Insights into primary emissions and secondary particle formation

Highlights

• 33% and 42% of ambient SOC and sulfate in winter were from primary emissions.

• Sulfate, OC, and EC have decreased during wintertime since 2003.

• The ratio of NO₃^-/SO₄^2- increased from 0.4 in 2006 to 1.3 in 2014.

Abstract

In Xi'an, a city that frequently experiences serious PM pollution in northern China, 1476 PM₁₀ and 1464 PM_2.5 valid daily filter samples were collected at six sites from December 2014 to November 2015 and analyzed for 29 species. The annual mean PM₁₀ and PM_2.5 concentrations were 149.4 ± 93.1, 108.0 ± 70.9 μg/m³, respectively. Organic carbon (OC) is the predominant PM_2.5 component while crustal material predominated in PM₁₀. Sulfate concentrations, which was the largest component in Xi'an PM in previous studies, were lower than nitrate. Winter sulfate, OC, and elemental carbon (EC) have decreased since 2003, while nitrate remained constant in recent years and the ratio of NO₃⁻/SO₄²⁻ increased from 0.4 in 2006 to 1.3 in 2014. This result suggests that the motor vehicle contribution to PM has increased relative to coal-fired power plant emissions over the past decade. The mass fractions of crustal material, sulfate, and EC in PM_2.5 decreased as the PM_2.5 concentrations increased from “clean” days (<50 μg/m³) to the highest values, while nitrate significantly increased. Despite forming through secondary reactions, the high concentrations of SOC and SO₄²⁻ in winter are attributed to primary emissions and particularly to residential heating and cooking with coal. Primary SOC and SO₄²⁻ accounted for 33% and 42% of their total PM_2.5 concentrations in winter, respectively. Therefore, control measures applied to these primary sources can substantially improve air quality.

Introduction

As one of the areas of the world with the severest air pollution, China's airborne particulate matter pollution has drawn much attention from the government, scientists, and the public (Chan and Yao, 2008; Guo et al., 2014; He et al., 2002; Shao et al., 2006; Song et al., 2017a; Wang et al., 2015b). The high particulate matter concentrations have adverse impacts on visibility, climate change, and human health (Huang et al., 2014; Pope, 2002; Song et al., 2017b). The Chinese State Council released the “Action Plan on Prevention and Control of Atmospheric Pollution” on September 10, 2013 after the unprecedented pollution event in early 2013 (China State Council, 2013). This plan aims to reduce PM concentrations across the country, for example, reducing PM_2.5 in Beijing-Tianjin-Hebei region by up to 25% by 2017 relative to 2012. Although control policies have been developed and implemented, haze pollution is still severe in most cities in China. In 2013, only 4.1% of the Chinese cities attained the annual average PM_2.5 standard (Wang et al., 2017).

Xi'an is a megacity in northwest China with a residential population of 8.7 million and covers an area of about 10,100 square kilometers (Xi'an Statistical Yearbook, 2016), located in the center of the Guanzhong plain, one of the most seriously polluted regions in China (Liu et al., 2016; van Donkelaar et al., 2010; Wang et al., 2015a). The annual average PM_2.5 and PM₁₀ concentrations in Xi'an have never attained the annual grade-Ⅱ limit values set by the national ambient air quality standards (NAAQS) of China since it was promulgated in 2012 (GB3095-2012). Severe pollution from both fine and coarse particles in Xi'an, have received widespread attention from the scientific community (Cao et al., 2012b; Chen et al., 2016; Elser et al., 2016; Huang et al., 2014, 2012; Li et al., 2017, 2016; Liu et al., 2017; Shen et al., 2010; Wang et al., 2014b, Wang et al., 2015a; Xu et al., 2016). Generally, the heating-season runs from November 15 to next March 15, and aggravates the local air pollution in Xi'an (Huang et al., 2012). Huang et al. (2014) reported the source apportionment result of PM_2.5 during a severe haze pollution event in winter in Xi'an and found that a dust-related source contributed up to 46% of PM_2.5 mass. Cao et al. (2005) reported the measurement of atmospheric carbonaceous materials during the fall and winter of 2003 in Xi'an, and found the contribution of coal combustion increased the total winter carbon concentration by up to 44%. The significance of coal combustion primarily used for residential heating and cooking was also reported by Xu et al. (2016), based on the measurement of PM_2.5 chemical composition during the winter of 2006, 2008, and 2010 in Xi'an. Their results showed the contribution of coal combustion decreased from 31% in 2006 to 24% in 2010 but still dominated the PM_2.5 mass. However, all these studies were conducted at a single sampling site in Xi'an for a limited time. These results were augmented by adding information on the spatial and seasonal variability of PM_2.5 with six sampling sites and four seasonal measurement campaigns as reported by Wang et al. (2015a). A recent epidemiologic study conducted in Xi'an suggested that spatial and temporal varying factors might play important roles in modifying the PM_2.5-mortality association (Huang et al., 2012).

Particulate matter concentrations and compositions vary with the nature of the sources and related human activities. The government has strengthened its mitigation strategies to reduce the pollution. Thus, continuing development of effective and efficient mitigation strategies must be based on an updated understanding of the chemical nature of the PM, including data with detailed chemical and spatial scale information. Therefore, the current study utilized four urban sites, one urban background site, and one rural site to determine the ambient PM concentrations across Xi'an and examine the tempo-spatial patterns of particle composition to provide insights into the PM sources.