Sulfate Formation via Photosensitized Reactions of Biomass Burning Compounds

Description

Sulfate (SO42-), which is primarily produced from the oxidation of sulfur dioxide (SO2), is a key component of atmospheric particulate matter (PM), for example, during severe haze events in China. However, traditional air quality models fail to reproduce the sulfate production during heavy haze episodes. This discrepancy has been partially addressed by the introduction of other sulfate formation pathways, mainly via multiphase and heterogeneous processes. Still, further studies on missing processes that can give new insights on sulfate formation are imperative. Photosensitizers, which are ubiquitous in biomass burning (BB) particles, have been gaining growing interests in initiating novel reactions in atmospheric chemistry. The absorption of solar radiation by a photosensitizer generates triplet excited states (3C*) that can initiate various chemical reactions that would otherwise not occur at the ground state. Recently, Wang et al. (2020) proposed a photosensitized pathway for sulfate formation involving a direct reaction between S(IV) species and3C* of humic-like substances (HULIS) and BB-derived photosensitizers, although the exact mechanism is uncertain. This is the only study on photosensitized sulfate formation thus far and so further investigations are warranted particularly on the effects of different atmospheric conditions (e.g., ionic strength, relative humidity, RH).In this proposed project, we will evaluate sulfate formation via SO2oxidation by BBderived photosensitizers in the aqueous and particle phases. Representatives of different classes (e.g., aromatic carbonyls) of BB-derived organics and real BB samples will be used as3C* precursors. Aqueous-phase experiments will be performed at3C* precursors concentrations expected for cloud or fog drops in areas impacted by BB (i.e., µM levels). UV/Vis spectrophotometry and ultra-high-performance liquid chromatography with photodiode array detector (UHPLC-PDA) will be used to obtain the absorption spectra and decay rates of3C*, respectively. will be used to obtain the absorption spectra and decay rates of3C*, respectively. We will also systematically study the photosensitization reactions in the particle phase (3C* precursors at >mM levels and high ionic strength >10 M) under controlled RH conditions using flow cell/in situ Raman spectroscopy. For both aqueous and particle phases, UHPLC coupled with quadrupole time-of-flight mass spectrometry (UHPLC-qToF-MS) and ion chromatography will be used to identify the composition (e.g., chromophores, carbonylcontaining compounds) of real BB samples and reaction products and estimate the sulfate concentration, respectively.The proposed study will provide information on the atmospheric kinetics of S(IV) oxidation to SO42-in the presence of BB-derived3C* precursors in both cloud droplets and aerosols at different RH conditions. The kinetic parameters will be inputted into a kinetic model for comparison with other known mechanisms in terms of importance to sulfate production.