Assessment of PM2.5-bound nitrogen-containing organic compounds (NOCs) during winter at urban sites in China and Korea☆
Graphical abstract
Introduction
Atmospheric aerosols —defined as suspensions of solid and/or liquid particles in the air— are strongly associated with air quality, human health (Laden et al., 2000; Pope et al., 2002; Watson, 2002; Poschl, 2005; Pope and Dockery, 2006; Shindell et al., 2011; D’Amato et al., 2016; Heo et al., 2016; Requia et al., 2018), and climate change (Weare et al., 1974; Stevens and Boucher, 2012; Rosenfeld et al., 2014; Lee et al., 2017; Zhao et al., 2018; Rosenfeld et al., 2019). Airborne particulate matter (PM) consists of condensed solid-phase aerosol particles and has become a serious environmental issue, particularly in East Asian countries, such as China and Korea, because it severely deteriorates the air quality and may cause damage to the lungs and heart (Ohara et al., 2007; Monks et al., 2009; Liu et al., 2018; Xu et al., 2018). China is often identified as a region with the highest annual concentrations of PM2.5 (i.e., PM with an aerodynamic diameter of less than or equal to 2.5 μm) worldwide, although recent emission control policies have reportedly led to a considerable reduction in the mass of airborne particulates in the Beijing area (Van Donkelaar et al., 2015; Wang et al., 2017; Gui et al., 2019; Fan et al., 2020; Shen et al., 2020; Zhang et al., 2020). Recently, Tran et al. (2018) found that fine airborne particles were the pollutant most strongly related to mortality in Korea. Additionally, Kim et al. (2015) reported that, despite a decrease in the annual average PM2.5 concentration in Seoul, Korea, 1998–2011, the PM2.5-related mortality rate had temporally increased.
During the winter, pollutant emissions from fossil-fuel combustion for heating have a more substantial and widespread impact on air pollution than in the summertime, increasing the average PM2.5 mass concentrations and lowering atmospheric visibility. It has been reported that the PM2.5 concentrations in winter are much higher than those in summer in China, perhaps due to the massive coal consumption during the heating season (Cao et al., 2007; Zhao et al., 2013; Dai et al., 2018; Fan et al., 2020; Shen et al., 2020). Xiao et al. (2015) demonstrated that household heating was a more significant contributor to wintertime air pollution than central heating because the central heating used in industrial settings is generally equipped with PM2.5-regulated control systems. Yang et al. (2019) also reported that residential emissions predominantly contribute to PM2.5 during heavy pollution episodes in China. However, detailed information on hazardous aerosol constituents and their formation mechanisms is lacking due to the complexity and diversity of aerosol components depending on local sources, meteorological conditions, and atmospheric chemistry.
Organic compounds account for up to 50% of the total mass of ambient fine particles, which also contain inorganic compounds (e.g., ammonium, nitrate, and sulfate) (Heintzenberg, 1989; Jimenez et al., 2003; Huang et al., 2014; Seinfeld and Pandis, 2016). Ambient organic aerosols consist of primary organic aerosols (POAs) and secondary organic aerosols (SOAs). POAs are generated through biomass burning, fossil-fuel combustion, traffic, and industrial processes and are directly emitted into the atmosphere, and SOAs are produced by the condensation reaction of gaseous volatile organic compounds (VOCs) or the chemical transformation of primary atmospheric organic compounds (Poschl, 2005; Kroll and Seinfeld, 2008; Hallquist et al., 2009; McNeill, 2015). Of the constituents in organic aerosols, sulfur-containing and nitrogen-containing organics have received the most attention because they can be used to reveal the pollutant sources, aging mechanisms, and transformation processes.
Regarding the various nitrogen-containing organic compounds (NOCs; i.e., R–NO2, R–ONO2, or R–NH2), the NOCs with –NO2 or –ONO2 functional groups (referred to as organonitrates) are generally believed to be formed by atmospheric reactions such as the oxidation of VOCs in the presence of NOx (NOx = NO + NO2) along with various ambient oxidants, including hydroxyl (OH) and nitrate (NO3) radicals and ozone (O3) (Surratt et al., 2007; Ng et al., 2008; Fry et al., 2009; Stone et al., 2009; Zare et al., 2018), whereas nitro-aromatic compounds are believed to be emitted from primary combustion, such as biomass burning (H. Wang et al., 2018). Garnes and Allen (2002) demonstrated that primary emissions and unusual plume chemistry may be important drivers of organonitrate formation. In addition, a variety of amine-containing organic compounds (i.e., NOCs with R–NH2) can be present in the atmosphere, and these are generally emitted from manufacturing processes, biomass burning, or vegetation (Ge et al., 2011; Ferraz et al., 2012). Therefore, details of the chemical composition and characteristics of NOCs can provide insights into the reaction processes that lead to aerosol formation.
In the present study, ambient fine particles were collected at two cities in China and Korea (Beijing and Gwangju, respectively) simultaneously during the winter. The underivatized PM2.5 was extracted using dichloromethane (DCM) and investigated using a 15-T Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer, which has emerged as a powerful tool for identifying the molecular composition of organic material in complex mixtures, such as environmental matrices (e.g., water, soil, or snow samples) (Antony et al., 2014; Lobodin et al., 2014; Guigue et al., 2016; Ksionzek et al., 2016; Bianco et al., 2018), to elucidate the chemical characteristics of the PM2.5-derived organic compounds present at both sites. Recently, FT-ICR mass spectrometers equipped with an electrospray ionization (ESI) or an atmospheric pressure photoionization (APPI) interface have been widely used to determine the molecular composition of organic aerosols to obtain more complete information on the organic molecule components with complicated polarity and functionality (Jiang et al., 2014; Kourtchev et al., 2014; Noziere et al., 2015; Choi et al., 2017; Kuang et al., 2018; Choi et al., 2019; Giorio et al., 2019). The resulting data were analyzed to characterize the chemical composition of the organic matter (OM) present in the fine aerosol particles. In particular, the proportions and potential sources of the NOCs in aerosol-derived OM and their relationship with the ambient inorganic components were determined. This in-depth chemical characterization of the OM found in urban atmospheric aerosols provides broad insight into the nature of wintertime airborne pollutants.
Section snippets
Aerosol sampling
Aerosol samples were taken at 24 h intervals simultaneously in Beijing and Gwangju simultaneously during winter (3 January–1 February 2018). The locations of the sampling sites are shown in Fig. 1. The Beijing site was located on the roof of a building on the Changping campus of Peking University (PKU CP; 40.14°N, 116.11°E), 38 km northeast of urban Beijing. The Changping area is known to be highly influenced by coal combustion, dust, and vehicle emissions from neighboring settlements and the
Wintertime PM2.5 concentrations at the Beijing and Gwangju sites
The molecular composition of PM2.5-derived OM depends on the pollutant origin, such as different types of fuel (i.e., coal, gasoline, and diesel), and meteorological conditions (e.g., wind velocity and direction, air temperature and humidity, and atmospheric pressure and stability), which can affect the ambient phase transformation of primary organic molecules. Therefore, the PM2.5 was collected in Beijing and Gwangju simultaneously in January 2018 to comprehensively investigate its organic
Conclusions
In this paper, we present a highly resolved characterization of nonpolar and semipolar OM components in organic aerosols collected during winter at two urban sites: Beijing, China and Gwangju, Korea. Detailed analysis of the molecular changes in the urban organic aerosols using an ultrahigh-resolution mass spectrometer, with additional information on the chemical composition of PM2.5, allows the regional characteristics of ambient organic aerosols and their formation mechanisms to be more fully
Funding
This study was supported by the National Strategic Project-Fine Particle of the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (MSIT), the Ministry of Environment (ME), and the Ministry of Health and Welfare (MOHW) (NRF-2017M3D8A1092223) and KBSI (C030222) grants.
CRediT authorship contribution statement
Kyoung-Soon Jang: Conceptualization, Validation, Writing - original draft, Writing - review & editing. Mira Choi: Formal analysis, Data curation. Minhan Park: Formal analysis, Data curation. Moon Hee Park: Formal analysis. Young Hwan Kim: Validation, Data curation. Jungju Seo: Validation, Data curation. Yujue Wang: Data curation, Writing - original draft. Min Hu: Data curation, Writing - original draft. Min-Suk Bae: Data curation, Writing - original draft. Kihong Park: Writing - review &
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We thank Min Sung Kim for providing support for NMDS analysis of the airborne OM samples, and Gun Wook Park for the help with preparing the bar graphs showing the compound class-dependent chemical properties in the Supporting Information. Data used in our analysis are available for download under the following link: https://doi.org/10.5281/zenodo.3841762.
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This paper has been recommended for acceptance by Admir Créso Targino.