Aqueous OH-initiated photooxidation of smoke extracts from maize straw and coal combustion: optical character and molecular composition

Abstract. Aqueous-phase •OH photodegradation of coal- and maize-derived smoke extracts was investigated to elucidate their optical and molecular transformations. Parallel factor analysis of excitation-emission matrix fluorescence spectra identified one humic-like and two protein-like substances. FT-ICR MS revealed that CHO (74.5 % for maize, 58.9 % for coal) and CHON (24.1 % for maize, 11.8 % for coal) compounds dominated both smoke extracts, whereas sulfur-containing species were more abundant in coal smoke (29.4 %) than in maize (1.4 %). The aqueous •OH photooxidation enhanced molecular saturation and reduced aromaticity, reflected by lower double bond equivalent and aromaticity index values. Lignin-like compounds decreased, whereas lipid- and aliphatic-like fractions increased, indicating transformation of aromatic species into more saturated products. Distinct photodegradation pathways were observed for coal and maize extracts based on changes in resistant, degraded, and newly formed molecules. Reactive species contributed to WSOC degradation in the order •OH > ³C > ¹O₂, with contributions of 86.4 %, 12.8 %, and 0.8 % for coal extracts, and 80.9 %, 16.0 %, and 3.1 % for maize extracts, respectively. Increased oxalic acids, CHO₂⁺ fragments, and declining pH values during the first 5 h indicated substantial formation of carboxylic acids. Measurements from aerosol mass spectrometry showed increasing oxidation indicators during this early stage, confirming enhanced oxidation of aqueous secondary organic aerosol. Oxidative potential, assessed by dithiothreitol consumption, initially increased and then declined, while its normalization by water-soluble organic carbon increased, likely due to the formation of nitrogen-containing compounds in coal smoke and reactive quinones in maize smoke, respectively. Continued photodegradation led to decreases in light absorption and total fluorescence intensity. Overall, this study improves understanding of aqueous-phase photochemical processing of smoke-derived water-soluble organic matter and supports more accurate representation of these processes in atmospheric models, contributing to better assessments of smoke aging impacts on air quality and climate.