Multiphase reaction of nitrate radicals with vanillic acid aerosols: kinetics and formation of light-absorbing particles

Given that biomass-burning aerosol emissions have a direct radiative effect on the atmosphere, it is important to understand the chemistry that occurs within wildfire smoke that may change aerosol particle optical properties. To investigate night-time aging chemistry, this laboratory study explores the kinetics of the reaction between gas-phase nitrate radicals (NO₃) and vanillic acid (VA), a functionalized phenol. As breakdown products of lignin, phenolic compounds are the commonly observed components of biomass burning smoke. They are also present in urban air pollution, formed by the oxidation of aromatic precursors. The study was conducted in an aerosol flow tube with a residence time of 15 minutes, where roughly 1.6 pptv of NO₃ was formed by the reaction of NO₂ (21 ppbv) and O₃ (230 ppbv), and VA/ammonium sulfate (AS) solutions were atomized to form particles in the accumulation mode size range. The reaction was monitored by an aerosol mass spectrometer (AMS), which measured nitrated aerosol products, and by a 5-wavelength aethalometer, which observed the optical absorption of aerosol particles. The observed gas-surface kinetics are consistent with a NO₃ reactive uptake coefficient to form a nitrated product of 0.30 ± 0.39 and 0.19 ± 0.12 at respectively RH = 25% ± 5% and 55% ± 5% at 296 K. The aerosol particles became highly absorbing during the reaction in the near ultraviolet (375 nm) and visible (470, 528, and 625 nm) regions. While this change in absorptivity presumably arises via the nitration of the aromatic ring, the reaction drives stronger particle absorption, which extends much more deeply into the visible part of the spectrum than is characteristic of (mono) nitrovanillic acid (NVA), indicative of the formation of complex reaction products. These results demonstrate that night-time atmospheric aging of phenol-containing wildfire smoke and urban particulates will occur rapidly and significantly darken the particles throughout the visible part of the spectrum.