Ionic Strength Effect Triggers Brown Carbon Formation Through Heterogeneous Ozone Processing Of Ortho-Vanillin (Vol 55, Pg 4553, 2021)

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-3) 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 (gamma) of O-3 on o-VL ([o-VL] = 1 x 10(-5) mol L-1) increase from. = (1.9 +/- 0.1) x 10(-7) in the absence of Na2SO4 to. = (6.8 +/- 0.3) x 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 O-3. 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-3 reaction with o-VL, in the presence of SO42- 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.