Aqueous Phase Formation of Photooxidants during Nitrate Photolysis and Brown Carbon Excitation

Abstract

Secondary organic aerosol (SOA) contribute to the haze events and reduced visibil- ity in the atmosphere. Atmospheric aqueous phases, i.e., fog/cloud droplets and aerosol liquid water (ALW), is an important media for the chemical processing of water-soluble and polar compounds, leading to the formation of aqueous SOA. Formation and evo- lution of SOA are largely driven by the photooxidants. In this thesis, we aim to reveal the radicals released from nitrate photolysis and two overlooked photooxidants, singlet oxygen (¹O₂^* ) and organic triplets (³C^* ), from photo-excitation of ambient brown car- bon (BrC) in the aqueous phases.

The hydroxyl radical (•OH) plays key role in the photooxidation of substantial amounts of species. Nitrate is an ubiquitous and abundant ion in atmosphere. Upon solar irradiation, nitrate undergoes photolysis and releases •OH and complex reactive species including nitric oxide (NO•) and nitrogen dioxide (NO₂•). In Chapter 2, we quantify the pH-dependent formation of •OH during NH₄NO₃ photolysis and evaluate the nitrate-mediated photodegradation of three carboxylic acids. The pH-dependent photodegradation of carboxylic acids were due to combined effects of pH-dependent •OH formation from nitrate photolysis and the differences in •OH reactivities of their dissociated vs. undissociated states. In Chapter 3, kinetic and SOA mass yields were measured for the nitrate-mediated photooxidation of four green leaf volatiles (GLVs). The comparison between fog/cloud conditions and ALW-like conditions highlights the distinct differences in SOA formation from biogenic volatile organic compounds.

Photoexciation of BrC leads to the formation of ³C^* and ¹O₂^* , which are efficient aqueous oxidants towards electron-rich or unsaturated species. The inclusion of ³C^* and ¹O₂^* in atmospheric aqueous photoreactions has not been extensively considered in both laboratory experiments and model calculations. In Chapter 4, we measured the steady-state concentrations and quantum yields of ³C^* and ¹O₂^* in the illuminated aqueous extracts of ambient PM_2.5 collected year-round in Hong Kong. Distinct sea- sonal variations in their production were attributed to the differences in quantities and optical properties of BrC. However, it is difficult to apply our reported results to other regions and for models given the wide ranges in their values. This motivates our last work in Chapter 5 using linear regression models to predict the quantities of ¹O₂^* and ³C^* with straightforward characterization of BrC.

The results from this work help to close the gaps between ambient measurement and model estimation of the formation and evolution of SOA in the condensed aqueous- phases.

Date of Award	2 Sept 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Ern Mei Theodora NAH (Supervisor)