Wavelength-resolved quantum yields for vanillin photochemistry: self-reaction and ionic-strength implications for wildfire brown carbon lifetime†

Abstract

The light absorbing component of organic aerosols, brown carbon (BrC), directly affects climate and can play a role in the oxidative aging of organic aerosols. Recent estimates suggest that globally BrC may have a warming potential that is approximately 20% that of black carbon, and photochemistry from BrC compounds can increase or transform aqueous SOA. Photobleaching of BrC is estimated to occur with a timescale of hours to days, a range that complicates estimates of the effects of BrC on climate and aerosol chemistry. The chemical environment (e.g. pH, ionic strength, and non-BrC organic content) of aqueous aerosols can also affect the reactivity of BrC, potentially altering absorption spectra and reactions of excited states formed upon irradiation. A range of solar illumination sources have been used in studying the photochemistry of BrC compounds, making direct comparisons between results difficult. Higher energy, single wavelength studies (e.g. 308 nm) show much larger quantum yields than broadband studies, indicating wavelength dependent quantum yields for a wide range of atmospherically relevant substituted aromatics. In this work we investigate the wavelength dependence of the quantum yield for loss of a prototypical BrC compound found in wildfire emissions, vanillin, using several narrow band UV-LEDs that span the 295–400 nm range. These wavelength dependent quantum yields will allow a more direct comparison between photochemical experiments with laboratory irradiation sources and actual actinic fluxes. Vanillin photochemical loss rates are concentration-dependent due to direct reaction between triplet excited state and ground state vanillin molecules. The quantum yield for photochemical loss of vanillin can be approximated by a Gaussian decay from 295 nm to ∼365 nm. This function is used to directly calculate the solar zenith angle (SZA) dependence for photochemical loss. Computational results show the presence of two π → π* transitions responsible for the observed UV-vis spectrum and that the rate of intersystem crossing has a wavelength dependence remarkably similar to that of the quantum yield for loss. A strong kinetic salt effect is observed, showing a doubling of the loss rate at high ionic strength.