Multiphase Formation of Secondary Aerosol Mediated by Particulate Nitrate Photolysis

粒相硝酸鹽光解反應介導的二次氣溶膠形成過程

Student thesis: Doctoral Thesis

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Award date11 Nov 2022

Abstract

Due to the rapid economic development and accelerated urbanization, enhanced emissions from anthropogenic activities have led to poor air quality in China in the past few decades. Some Chinese megacities have experienced severe and persistent haze events with high concentrations of PM2.5. Nitrate, sulfate, and secondary organic aerosol (SOA) are important constituents of PM2.5. In particles, nitrate photolysis can generate OH radicals and NO2 at λ<290 nm, but also N(III) (NO2-/HONO) and O(3P) at λ>290 nm. These oxidants can potentially oxidize precursors to form secondary aerosol through multiphase reactions. This thesis examines the formation of sulfate and SOA mediated by particulate nitrate photolysis.

Our earlier work studied sulfate formation from SO2 oxidation by OH radicals and NO2 produced from nitrate photolysis at 254 nm (Environ. Sci. Technol. Lett. 2019, 6, 86–91). In chapter 2, we describe photolysis experiments at an atmospherically relevant wavelength of 300 nm to further evaluate the role of particulate nitrate photolysis in sulfate formation. We found that NO2 and N(III) pathways were the major contributors to sulfate at 300 nm, but the contribution of the OH pathway is minor. Built on this work, we further investigate the sulfate production rate during the nitrate photolysis in the presence of halides in chapter 3. The sulfate production rate was enhanced as the initial molar ratio of [Cl-]0/[NO3-]0 increased due to the halide-induced enhancement of nitrate photolysis. In addition, higher enhancement factors of sulfate production rate were observed in the presence of bromide and iodide ions due to their higher surface propensity. Our findings open new perspectives on the increased atmospheric oxidative capacity, resulting in the formation of secondary aerosols during nitrate photolysis.

Building on the works on sulfate formation promoted by nitrate photolysis, we also explored the effect of nitrate photolysis in forming SOA. Glyoxal has been reported to be an important precursor in forming SOA in literature. Herein, we investigate the impact of nitrate photolysis on the SOA formation by using glyoxal as a precursor. Firstly, we performed the experiments with premixed particulate glyoxal + NaNO3, and found that formate is the major product from the oxidation of glyoxal by OH radicals, as will be discussed in chapter 4. Such a chemical system is relevant to aged sea-salt aerosols. This result is inconsistent with previous studies in the literature, where low-volatile organic acids (e.g., oxalic acid and glyoxylic acid) are the main products in aqueous-phase oxidation. We attributed the differences to the different oxidative environments in aerosols and bulk liquid phase. The present study was conducted in a system with glyoxal concentration at the M level. Besides, NO2 radicals can also be produced from nitrate photolysis in addition to the OH radicals, whereas previous studies mainly focused on the OH radicals as the sole oxidant from H2O2 photolysis. The exact mechanisms leading to such differences warrants further investigation. We also examined premixed glyoxal + NH4NO3 to explore the effect of nitrate photolysis in forming SOA in the presence of “Brown Carbon” (BrC), as will be discussed in chapter 5. In this case, glyoxal reactions include photooxidation and its reaction with dissolved ammonia (from ammonium ions) to produce BrC. We found that glyoxal photooxidation does not retard its reaction with ammonia in forming BrC. In addition, the photosensitization induced by the formed BrC (e.g., imidazole-2-carboxaldehyde and 2,2’-biimidazole) and nitrate photolysis both contribute to the decay of glyoxal. A significantly enhanced glyoxal decay rate by a factor of ~12 was observed in the presence of both nitrate photolysis and photosensitization compared to that under dark. Phase separation was observed during glyoxal oxidation, which is likely attributed to the oxidation of imidazole, but detailed chemical characterizations of the organic phase are needed to identify its composition in the future.

In the final chapter, we conclude this thesis and propose the potential future directions that can be further explored.

    Research areas

  • Nitrate Photolysis, Sulfate, Glyoxal, Secondary Organic Aerosol, Brown Carbon, Photosensitization