Ionic Strength Effect Triggers Brown Carbon Formation through Heterogeneous Ozone Processing of Ortho-Vanillin

• Published in: Environmental science & technology Vol. 55; no. 8; pp. 4553 - 4564
• Main Authors: Wang, Yiqun, Mekic, Majda, Li, Pan, Deng, Huifan, Liu, Shiyang, Jiang, Bin, Jin, Biao, Vione, Davide, Gligorovski, Sasho
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 20-04-2021
• Subjects: Aerosols; Air quality; Anthropogenic Impacts on the Atmosphere; Aromatic compounds; Benzaldehydes; Biomass burning; Carbon; Cyclotron resonance; Fourier transforms; Ionic strength; Ionization; Ions; Mass spectrometry; Mass spectroscopy; Osmolar Concentration; Ozone; Ozone - analysis; Sodium sulfate; Vanillin; Wall flow
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/acs.est.1c00874

Methoxyphenols are an important class of compounds emerging from biomass combustion, and their reactions with ozone can generate secondary organic aerosols in the atmosphere. Here, we use a vertical wetted wall flow tube reactor to evaluate the effect of ionic strength on the heterogeneous reaction of gas-phase ozone (O3) with a liquid film of o-vanillin (o-VL) (2-hydroxy-3-methoxybenzaldehyde), as a proxy for methoxyphenols. Typical for moderately acidic aerosols, at fixed pH = 5.6, the uptake coefficients (γ) of O3 on o-VL ([o-VL] = 1 × 10–5 mol L–1) increase from γ = (1.9 ± 0.1) × 10–7 in the absence of Na2SO4 to γ = (6.8 ± 0.3) × 10–7 at I = 0.2 mol L–1, and then, it decreases again. The addition of NO3 – ions only slightly decreases the uptakes of O3. Ultrahigh-resolution electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) reveals that the formation of multicore aromatic compounds is favored upon heterogeneous O3 reaction with o-VL, in the presence of SO4 2– and NO3 – ions. The addition of NO3 – ions favors the formation of nitrooxy (−ONO2) or oxygenated nitrooxy group of organonitrates, which are components of brown carbon that can affect both climate and air quality.