[Secondary Organic Aerosol Mass Yield and Characteristics from 4-ethylguaiacol Aqueous·OH Oxidation: Effects of...

Aqueous-phase chemical processing, as an essential formation pathway of secondary organic aerosol (SOA), has attracted widespread attention from within atmospheric chemistry fields. Due to the complicated reaction nature, reaction mechanisms, and product characteristics of aqueous-phase chemical processing, its contribution to the SOA budget is still not fully understood. In this work, we investigate how the initial concentration (0.03-3 mmol·L^-1) of 4-ethylguaiacol affects SOA formation of aqueous·OH photochemical oxidation. We use soot-particle aerosol mass spectrometry (SP-AMS) to monitor SOA mass yield and oxidation character, and gas chromatography-mass spectrometry (GC-MS) and ion chromatography (IC) to measure products and organic acids. Additionally, we use ultraviolet visible spectroscopy (UV-vis) and high-performance liquid spectrometry (HPLS) to track the formation of light-absorbing products such as humic-like substances (HULIS). Our research indicated that the range of the O/C ratio of EG-aqSOA measured by the SP-AMS exhibited increasing trends with increased reaction time 0.42-0.61 (0.03 mmol·L^-1), 0.49-0.84 (0.3 mmol·L^-1), and 0.49-0.63 (3 mmol·L^-1). Dimers (C₁₆H₁₈ O₂⁺, m/z 302) via SP-AMS were obviously higher under a higher initial concentration, thereby demonstrating that the oligomerization reaction proceeded more easily. The absorption at 250 nm recorded by UV-vis was distinctly enhanced, which might be attributed to new light-absorbing products with absorbance at 250 nm. Furthermore, the HULIS concentration increased with reaction time, in accordance with enhancement of absorbance in the 300-400 nm region, thus suggesting that aqueous-phase processing formed brown carbon. Small organic acids, including formic acid, malic acid, and oxalic acid, were detected by IC in all reaction solutions, with the highest concentration being for formic acid. GC/MS detected ketone, an OH monomer, and dimers in the aqSOA, which further indicates that functionalization and oligomerization took place.