The impact of ammonium on the distillation of organic carbon in PM2.5
Graphical abstract
Introduction
Carbonaceous aerosols are an important component (20–50%) of ambient particulate matter (PM) (Cao et al., 2007; Han et al., 2016; Kanakidou et al., 2005; Putaud et al., 2010) and have attracted considerable attention among atmospheric environment researchers due to their crucial effects on air quality, climate change, and human health (Bikkina et al., 2016; Bond et al., 2013; Cao et al., 2004; Watson, 2002; Zhang et al., 2008). Organic carbon (OC), which is formed by both direct emissions (namely, primary organic carbon, POC), including biomass burning, fossil fuel combustion, vehicle exhaust, plant material, soil and dust suspension, and secondary formation from the oxidation of volatile organic compounds (VOCs) or POC in the atmosphere (Han et al., 2008; Pio et al., 2011; Turpin and Huntzicker, 1995), is the important contributor to carbonaceous aerosols. To date, thousands of organic compounds, including alkanes, polycyclic aromatic hydrocarbons (PAHs), hopanes, steranes, and esters, have been identified in OC (Mauderly and Chow, 2008; Ruiz-Jimenez et al., 2011). However, the identified molecules contribute less 30% of the total OC mass in PM2.5 (Aiken et al., 2008; Alier et al., 2013; Goldstein et al., 2008; Hallquist et al., 2009; Yang et al., 2020), even though many sophisticated techniques, such as thermal desorption aerosol gas chromatography mass spectrometry (TAG-GC–MS) (Goldstein et al., 2008), thermodenuder Aerosol Mass Spectrometer (TD-AMS) (Huffman et al., 2009; Lee et al., 2010), two-dimensional gas chromatography (GC × GC) (Liu and Phillips, 1991) and liquid chromatography electrospray ionization orbitrap (LC-ESI-Orbitrap) (Wang et al., 2017), have been developed. Since acquiring complete compositional information regarding OC remains a long-standing challenge in OC measurement, analytical technologies based on the bulk properties of OC, such as thermal desorption-based methods, are still useful and widely used for OC measurements.
In thermal desorption-based technologies, such as TAG-GC–MS (Williams et al., 2006), Filter Inlet for Gases and AEROsols (FIGAERO) (Lopez-Hilfiker et al., 2014) and OC/EC analysis, PM is first collected on a substrate, and then the desorbed components or oxidation products are detected at a fixed or programmed temperature. OC and EC can be further thermally classified into eight fractions based on desorption temperature during OC/EC analysis, i.e., four OC fractions (OC1, OC2, OC3, and OC4 in a He atmosphere), a pyrolyzed carbon fraction (PyOC, determined when the laser light attained its original intensity after O2 was added), and three EC fractions (EC1, EC2, and EC3 in a 2% O2/98% He atmosphere) (Chen et al., 2017; Zhang et al., 2018). The source of carbonaceous aerosols can be further analyzed based on the distribution of these distillates because of the specific OC/EC ratio and/or the characteristic distribution of the distillates from a given source (Chow et al., 2004; Hwang and Hopke, 2007; Watson et al., 2005). For example, the OC/EC ratios in samples obtained from coal combustion, automobile exhaust, and biomass combustion were 12.0, 4.1, and 60.3, respectively (Cao et al., 2005; Wang et al., 2018). Sources of carbonaceous aerosols including gasoline/diesel vehicle exhaust, coal combustion, biomass burning, and specific diesel vehicle exhaust were apportioned based on Positive Matrix Factorization (PMF) analysis of OC components in Haikou, China (Liu et al., 2018). In addition, the single carbonaceous thermal distribution (CTD) from the converted CO2 can also be used to understand the source and chemical composition of carbonaceous aerosols (Bae et al., 2019; Bae et al., 2014).
It should be noted that PM2.5 is a mixture of organics and inorganics in which carbonaceous carbon coexists with the matrix of other components. It has been recognized that some inorganic salts can promote the formation of secondary organic aerosol (SOA) and influence the volatility of OA through heterogeneous reactions (Ervens et al., 2011; Faust et al., 2017; Ortiz-Montalvo et al., 2014). During EC/OC analysis, Yu et al. (2002) observed enhancement of charring of cellulose and starch when they were mixed with ammonium bisulfate, which was explained by the promotion of dehydration reactions through acid catalysis. It has also been demonstrated that inorganic compounds found indigenously within biomass can promote the formation of char (Sekiguchi and Shafizadeh, 1984). On the other hand, several studies have found that the presence of NaCl (Kirchstetter and Novakov, 2007) and metal salts (Wang et al., 2010) can reduce the combustion temperature of EC. Despite these insights, it is still unclear how inorganic salts affect the distribution of OC1–OC4 in ambient particles during OC/EC analysis. In particular, the concentration of PM2.5, as well as that of sulfate, declined substantially in most regions of China (Li et al., 2020; Wang et al., 2020) after the central government executed the Air Pollution Prevention and Control Action Plan in September 2013, while the ammonium and nitrate fractions in PM2.5 increased (Xu et al., 2019). Therefore, it is vital to understand the interaction between inorganic components and OC in PM2.5. In this study, observations of OC in downtown Beijing were performed by an OC/EC analyzer (Model-4, Sunset Lab. Inc.) based on the thermal-optical method. The influences of chloride, nitrate, sulfate, and ammonium (measured by a time-of-flight aerosol chemical speciation monitor, ToF-ACSM, Aerodyne Co. Ltd., USA) on the distribution of OC distillates are discussed. This work will help us understand both the evolution of the PM2.5 composition at different pollution levels and the effect of the inorganic matrix on thermal-desorption-based OC measurements.
Section snippets
Field measurements
Ground-based observations were performed at the Aerosol and Haze Laboratory/Beijing University of Chemical Technology Station (AHL/BUCT Station) (Lat. 39°56′31″ and Lon. 116°17′52″) from September 1, 2020 to November 31, 2020 in downtown Beijing, China. The details of the station were described in our previous work (Chu et al., 2021; Liu et al., 2020; Zhou et al., 2019). The instruments used in this work are shown in Table S1.
The OC and EC concentrations were determined semi-continuously using
Dependence of OC distillates on the pollution level
OC1–OC4 represent carbonaceous components distilled at different temperatures (Bae et al., 2004). Five overlapping peaks (from Peak 1 to Peak 5 in Figs. 1 and S4, due to the shorter analysis time of the NIOSH protocol (Wu et al., 2016)) were observed in the typical desorption curve (analysis lapse time of 1–400 s). Peak fitting was conducted to identify peak positions (Fig. S4) and quantify these components. Peak 5 was mainly located in the EC region and remained very stable in terms of both
Conclusions
Our results demonstrate that OC fractions showed slightly transition from low volatile to high volatile during pollution cycles. Moreover, a matrix effect that can lead to redistribution of different OC fractions during thermal desorption process was observed. These factors led to the increased of low volatile OC fractions (OC4). Laboratory studies further supported the assumption that ammonium plays a key role in the redistribution of OC. Therefore, in addition to providing insights into the
CRediT authorship contribution statement
Zemin Feng: Writing - original draft, Conducting experiments, Data curation. Feixue Zheng: Investigation, Data curation. Chao Yan: Investigation, Data curation. Peng Fu: Investigation, Data curation. Yusheng Zhang: Investigation, Data curation. Zhuohui Lin: Investigation, Data curation. Jing Cai: Investigation, Data curation. Wei Du: Investigation, Data curation. Yonghong Wang: Investigation, Data curation. Juha Kangasluoma: Investigation, Data curation. Federico Bianchi: Investigation, Data
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.
Acknowledgments
This research was financially supported by the National Natural Science Foundation of China (41877306, 92044301), the Ministry of Science and Technology of the People's Republic of China (2019YFC0214701), the Academy of Finland via the Center of Excellence in Atmospheric Sciences (272041, 316114, and 315203) and the European Research Council via ATM-GTP 266 (742206) and ERA-PLANET (www.era-planet.eu) transnational project SMURBS (www.smurbs.eu) (grant agreement no. 689443), funded under the EU
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2022, Science of the Total EnvironmentCitation Excerpt :Jiang et al. (2020) also put forward that the main emission source of PM2.5 and PM10 is dust, and it involves industrial processes for VOCs and CO. OC is an important contributor to carbonaceous aerosols (Feng et al., 2022). Carbonaceous aerosols are an important component (20–50 %) of ambient particulate matter (PM) (Han et al., 2016).