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

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.