Chemical characterization of organic aerosol from Chinese cities using high resolution mass spectrometry
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Abstract
Atmospheric aerosols are strongly related to air quality, global climate and human health. They influence the Earth’s radiative balance directly by scatting and absorbing solar radiation and indirectly by nucleating cloud droplets. Aerosol particles can also have adverse effects to the human respiratory and cardiovascular systems. Within the last decades, China has experienced severe and persistent haze pollution with the fine particulate matter (PM) reaching unprecedentedly high levels across many cities, which is influencing the regional air quality and million people’s health. Hence, understanding the chemical composition and sources of the PM during the haze events is essential for assessing the effects of polluted aerosols.
The aim of this work is to characterize the chemical composition of organic aerosol (OA) samples collected from different Chinese cities during the haze events in January 2014 based on ultrahigh resolution mass spectrometry (UHRMS) and to present a comprehensive overview of OA in Chinese urban regions. The first part of this study was the development of an analytical method to analyze the aerosol samples with diameters of PM ≤ 2.5 μm (PM2.5) collected in a Chinese city (Beijing) and a central European city (Mainz, Germany). In combination with ultrahigh performance liquid chromatography (UHPLC), the UHRMS-Orbitrap technique coupled with electrospray ionization (ESI) was able to identify thousands of organic compounds at the molecular level. The comparison of the chemical composition of OA in Beijing and Mainz showed that OA in Beijing was more unsaturated and contained more aromatics than OA in Mainz, indicating that combustion emission (e.g., automotive exhaust emission) played a more important role in Beijing OA. The second part of this study focused on the identification and comparison of chemical components in OA collected from three representative Chinese cities using again the UHPLC-Orbitrap MS method. The three Chinese cities Changchun (CC), Shanghai (SH) and Guangzhou (GZ) locate, respectively, in the northeast, central east and southeast regions of China. The results showed that the major organic compounds with high concentrations in all cities had similar chemical composition and were dominant by aromatics, while OA in SH and GZ showed a higher similarity. Compared to the OA in SH and GZ, OA in CC obviously experienced less oxidation processes and contained more polycyclic aromatics, which were probably generated from the coal burning for the residential heating in winter in northeast China. Finally, an important organic species named organosulfates (OSs) in OA samples from Beijing and Mainz were characterized using the UHPLC-Orbitrap MS method. By summarizing the elemental composition information of OSs generated in previous OSs chamber simulation experiments, a diagram for the prediction of OSs precursors was created. Based on this OSs precursor diagram, OSs in Mainz OA is suggested to be mainly derived from isoprene or other small molecules with carbon atom number less than five, while OSs in Beijing OA was generated from both isoprene and anthropogenic precursors (e.g., long chain alkanes and aromatics).
In conclusion, the UHPLC-Orbitrap MS method developed in this work has been proved to be effective to identify the complex chemical composition in OA. This thesis offers a comprehensive overview about the molecular composition of OA in different Chinese cities, which helps to better understand their chemical characteristics and sources as well as provide the chemical database for haze control strategies in China.