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 (O) with a liquid film of -vanillin (-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 O on -VL ([-VL] = 1 × 10 mol L) increase from γ = (1.9 ± 0.1) × 10 in the absence of NaSO to γ = (6.8 ± 0.3) × 10 at = 0.2 mol L, and then, it decreases again. The addition of NO ions only slightly decreases the uptakes of O. 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 O reaction with -VL, in the presence of SO and NO ions. The addition of NO ions favors the formation of nitrooxy (-ONO) or oxygenated nitrooxy group of organonitrates, which are components of brown carbon that can affect both climate and air quality.