Atmospheric aerosols are closely related to various environmental effects, including climate change, visibility degradation, and public health threat. This thesis research consists of two parts. The first part focuses on online characterization of nitrogenous aerosols, and the second part involves a comparative study of aerosol aging potentials at suburban sites in Northern and Southern China.

Nitrogenous aerosols, including inorganic nitrogen (IN) and organic nitrogen (ON), are relevant to ecosystem evolution, secondary aerosol formation, and biotoxicity. Pollution episodes driven by the major IN species, nitrate (NO₃^-), has been increasingly reported in Chinese cities, particularly in densely populated urban areas. But analysis on enhanced urban NO₃^- formation and its controlling fact...[ Read more ]

Atmospheric aerosols are closely related to various environmental effects, including climate change, visibility degradation, and public health threat. This thesis research consists of two parts. The first part focuses on online characterization of nitrogenous aerosols, and the second part involves a comparative study of aerosol aging potentials at suburban sites in Northern and Southern China.

Nitrogenous aerosols, including inorganic nitrogen (IN) and organic nitrogen (ON), are relevant to ecosystem evolution, secondary aerosol formation, and biotoxicity. Pollution episodes driven by the major IN species, nitrate (NO₃^-), has been increasingly reported in Chinese cities, particularly in densely populated urban areas. But analysis on enhanced urban NO₃^- formation and its controlling factors at a high time resolution (e.g., hourly) is currently rather sparse. In addition, while analytical method for aerosol IN species is achievable, direct quantification method of ON is so far unavailable, hindering the quantitative assessment of aerosol nitrogenous aerosols.

First, urban nitrate formation is probed via examining the concentration gradients in NO₃^- and its precursors through concurrent real-time measurements at a pair of urban and suburban sites in Hong Kong over a two-month period. The comparative analysis indicates that the high NO_x levels largely explained the excessive NO₃^- concentrations in the urban site, with the gas phase HNO₃ formation reaction contributing significantly during the daytime and the N₂O₅ hydrolysis pathway playing a prominent role during nighttime. This work presents a first quantitative analysis that unambiguously shows local formation of NO₃^- in urban environments as a driver for urban episodic PM_2.5 pollution, suggesting effective benefits of lowering urban NO_x.

Second, we have made a method breakthrough in measuring bulk aerosol IN and ON. We develop the first online simultaneous quantification method of aerosol IN and ON based on programmed thermal evolution and chemiluminescent detection of nitrogenous aerosols coupled with multivariate curve resolution analysis. Atmospheric abundances of IN and ON at a suburban site in Hong Kong were quantified using the online technique, which was validated through comparing IN measured by ion chromatography analysis of offline filters. Further source apportionment analysis using positive matrix factorization (PMF) revealed both primary emission and secondary formation sources of ON. This method breakthrough improves the quantitative assessment of aerosol nitrogen at the diurnal scale and provides scientific insights to assist formulation of source-specific control policies on nitrogen-containing aerosols.

Another piece of my thesis work concerns characterization of aerosol aging potentials and key factors and mechanisms influencing secondary aerosol formation. Aerosol mass spectrometer was used to characterize submicron aerosols before and after aging in a Gothenburg Potential Aerosol Mass (Go:PAM) reactor at two suburban sites in northern and southern China. Under constant OH exposure, organic aerosol (OA) enhancement dominated the total aerosol mass increment at both sites. The suburban Beijing site exhibited higher OA oxidation potential with more enhancement of the less oxygenated secondary OA, implying that aerosol aging was more sensitive to the abundant locally emitted primary OA and volatile organic compound precursors. On the contrary, air masses in suburban Hong Kong were already in a higher oxidation state, and aerosol aging mainly escalated the degree of oxygenation of OA. Among the inorganic species, NO₃^- was enhanced the most during atmospheric oxidation at both locations, highlighting the importance of controlling NO_x precursors in mitigating secondary inorganic pollution.