Atmos. Chem. Phys., 20, 14407–14417, 2020 https://doi.org/10.5194/acp-20-14407-2020 © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. Size-resolved exposure risk of persistent free radicals (PFRs) in atmospheric aerosols and their potential sources Qingcai Chen1, Haoyao Sun1, Wenhuai Song2, Fang Cao2, Chongguo Tian3, and Yan-Lin Zhang2 1School of Environmental Science and Engineering, Shaanxi University of Science and Technology, Xi’an 710021, China 2Yale – NUIST Center on Atmospheric Environment, International Joint Laboratory on Climate and Environment Change (ILCEC), Nanjing University of Information Science and Technology, Nanjing 210044, China 3Key Laboratory of Coastal Environmental Processes and Ecological Remediation, Yantai Institute of Coastal Zone Research, Chinese Academy of Sciences, Yantai 264003, China Correspondence: Yan-Lin Zhang ([email protected], [email protected]) Received: 16 February 2020 – Discussion started: 1 April 2020 Revised: 14 September 2020 – Accepted: 23 September 2020 – Published: 27 November 2020 Abstract. Environmentally persistent free radicals (EPFRs) 1 Introduction are a new type of substance with potential health risks. EPFRs are widely present in atmospheric particulates, but there is a limited understanding of the size-resolved health Free radicals are atoms or groups containing unpaired elec- risks of these radicals. This study reports the exposure risks trons, such as hydroxyl radicals and superoxide radicals, and and source of EPFRs in atmospheric particulate matter (PM) they usually have strong chemical reactivity and short life- of different particle sizes (< 10 µm) in Linfen, a typical coal- times (Pryor et al., 1986; Finkelstein, 1982). Free radicals burning city in China. The type of EPFRs in fine particles with long lifetimes (months or even years) in the environment (< 2.1 µm) is different from that in coarse particles (2.1– are currently called environmentally persistent free radicals 10 µm) in both winter and summer. However, the EPFR con- (EPFRs), which have received much attention in recent years centration is higher in coarse particles than in fine particles as new environmentally hazardous substances (Vejerano et in summer, and the opposite trend is found in winter. In both al., 2018; Gehling and Dellinger, 2013; Chen et al., 2019c). seasons, combustion sources are the main sources of EPFRs, EPFRs can be used as an active intermediate to catalyze with coal combustion as the major contributor in winter, the production of reactive oxygen species (ROS) by oxy- while other fuels are the major source in summer. Dust con- gen molecules, thus endangering human health (D’Arienzo tributes part of the EPFRs, and it is mainly present in coarse et al., 2017; Thevenot et al., 2013; Harmon et al., 2018; Blak- particles in winter and the opposite in summer. The upper ley et al., 2001; Khachatryan and Dellinger, 2011). Studies respiratory tract was found to be the area with the highest have found that EPFRs are present in different environmen- risk of exposure to EPFRs of the studied aerosols, with an ex- tal media, such as water and soil, and even in the atmosphere posure equivalent to that of approximately 21 cigarettes per (Dellinger et al., 2001; Truong et al., 2010; Vejerano et al., person per day. Alveolar exposure to EPFRs is equivalent to 2012a). 8 cigarettes per person per day, with combustion sources con- A number of studies have investigated the occurrences, tributing the most to EPFRs in the alveoli. This study helps us sources and formation process of EPFRs in atmospheric par- to better understand the potential health risks of atmospheric ticulates in different regions. For example, in the studies PM with different particle sizes. of Rostock in Germany, Taif in Saudi Arabia and Xuan- wei in China, the average concentration of EPFRs in atmo- spheric particulate matter (PM) was reported to be in the range of ∼ 1016–1018 spins g−1 (Wang et al., 2019; Aran- gio et al., 2016; Shaltout et al., 2015). Atmospheric EPFRs are mainly carbon-centered radicals with adjacent oxygen Published by Copernicus Publications on behalf of the European Geosciences Union. 14408 Q. Chen et al.: Size-resolved exposure risk of PFRs in atmospheric aerosols atoms (Gehling and Dellinger, 2013). EPFRs of different 2 Experimental section lifetimes are present in atmospheric PM, with only a few hours for short-lifetime EPFRs and several years for long- 2.1 Sample collection lifetime EPFRs that show no signs of decay (Gehling and Dellinger, 2013; Chen et al., 2019c). Most studies indicate The sampling site for this study is located in Hongdong ◦ 0 ◦ 0 that sources of transportation and combustion may be the pri- (36 23 , 111 40 E) in Shanxi, China. To collect atmospheric mary EPFR sources in atmospheric PM (Wang et al., 2018; particles of different sizes (0–10 µm), this study used a Yang et al., 2017; Chen et al., 2019b). Chen et al. (2018b, Thermo Anderson Mark II sampler to collect aerosol sam- 2019b) found that strong atmospheric photochemical effects ples of nine sizes. The samples were collected on a prebaked ◦ in summer and dust particles may also be important sources quartz filter (450 C, 4.5 h), and the sampling dates were as of EPFRs. The process of electron transfer and stabilization follows: in winter, 26 January to 4 February 2017, n D 10; between the surface of metal oxides (such as iron, copper, and in summer, 31 July to 24 August 2017, n D 12. The sam- ◦ zinc and nickel) and substituted aromatic molecules under ples were placed in a −20 C refrigerator prior to analysis. high temperatures is considered to be the main process for the formation of EPFRs in atmospheric particles (Truong, 2010; 2.2 EPFR analysis Vejerano et al., 2012a; Patterson et al., 2013; Vejerano et al., The EPR spectrometer (MS5000, Freiberg, Germany) is used 2011, 2012b). However, the study by Chen et al. (2018a) sug- to detect EPFRs in atmospheric samples. The filters were cut gests that EPFRs in atmospheric particulates are mainly de- into thin strips (5mm × 28mm) and put it into the sample rived from graphite oxide-like substances produced during tank of the quartz tissue cell (the size of the sample tank is combustion. In addition to primary sources such as combus- 10mm×30mm). Then the quartz tissue cell with attached fil- tion, secondary chemical processes in the atmosphere may ter sample was placed in a resonant cavity and analyzed by an also be an important source of EPFRs in atmospheric PM EPR spectrometer. The detection parameters were magnetic (Chen et al., 2019b, d; Tong et al., 2018). field strength, 335–342 mT; detection time, 60 s; modulation Different particle sizes of atmospheric PM pose differ- amplitude, 0.20 mT; number of detections, 1; and microwave ent health risks to humans, depending on the deposition ef- intensity, 8.0 mW. Specific testing protocols have been de- ficiency of the particles and the chemical composition and scribed previously (Chen et al., 2018c). concentrations of hazardous substances they contain (Strak et al., 2012; Valavanidis et al., 2008). Among various hazardous 2.3 Carbon composition analysis substances, EPFRs may also be involved in the toxicity of at- mospheric particulates. Yang et al. (2017) studied the EPFRs The contents of organic carbon (OC) and elemental carbon that are extractable by dichloromethane in different particle (EC) in the filter samples were analyzed using a semicon- sizes in Beijing in winter and found that the concentration of tinuous OC/EC analyzer (Model 4, Sunset Lab. Inc., Ore- EPFRs was the highest in particles with sizes < 1 µm. Aran- gon, USA) with a NIOSH 5040 detection protocol (Lin et gio et al. (2016) found that the concentration of EPFRs in al., 2009). 180 nm particles was the highest in the 56 nm–1.8 µm parti- The water-soluble organic carbon (WSOC) concentration cle size range. Although several studies have examined the was analyzed using an automatic TOC-LCPH analyzer (Shi- particle size distribution of EPFRs, systematic studies have madzu, Japan). The WSOC extraction was performed with not been conducted on the formation process, source and ex- ultrapure water under ultrasonication for 15 min, and all posure assessment of EPFRs in atmospheric particles with WSOC concentrations were blank-corrected. The concentra- different particle sizes. tion of OC in the MSM (methanol-soluble materials) was cal- This study takes Linfen as an example. Linfen is one of the culated as the difference between the OC and WSOC (water- cities in China with the most serious air pollution and is a typ- soluble organic carbon) concentrations. This calculation as- ical coal-burning city. The particle size distribution of EPFRs sumes that all water-insoluble organic carbon (WISOC) in in atmospheric PM in this region was studied using electron the aerosol can be extracted with MeOH, and the rationality paramagnetic resonance (EPR) spectroscopy. The effects of of this assumption has been verified elsewhere (Mihara and particle size and season on the source, formation process and Michihiro, 2013; Liu et al., 2013; Cheng et al., 2016; Chen health risk of EPFRs were revealed. In particular, the com- et al., 2019a). prehensive health risks of EPFRs were evaluated, and it was found that the upper respiratory tract is the area with the 2.4 PAH analysis highest risk of EPFRs’ exposure, which is equivalent to 21 cigarettes per person per day. This study is of great signifi- PAHs were detected using gas chromatography–mass spec- cance for understanding the source and formation process of trometry (GC-MS) on a GC7890B/MS5977A (Agilent Tech- EPFRs in atmospheric particulates as well as for health risk nologies, Clara, CA). Quartz-fiber filter samples (8 mm in assessments. diameter) were cut from each 25 mm quartz-fiber filter sub- strate used on the ELPI impactor stages using a stainless- Atmos.