Effect of pH on the Optical Properties of Brown Carbon (BrC) Aerosols and Climate Change During Aging Process by Hydroxyl Radicals

YANRU HUO, Thomas Preston, McGill University

     Abstract Number: 116
     Working Group: Burning Questions of Aerosol Emissions, Chemistry, and Impacts from Wildland-Urban Interface (WUI) Fires

Abstract
Brown carbon (BrC) emitted from wildfires represents a significant fraction of atmospheric light-absorbing aerosols to impact its radiative forcing and climate. However, the optical properties of BrC during the aging process by ·OH and the effects of pH are still not very clear. To address this gap, six nitrophenols are chosen as model compounds to investigate their ·OH aging mechanism, kinetics, ultraviolet-visible spectrum, and complex refractive index (RI) in the aqueous at different pH values using computational chemistry. Reactivity exhibits a positive correlation with the electron-withdrawing ability of side-chain substituent groups of the aromatic rings. At 298 K, the reaction rate constant for neutral species + ·OH reactions is in the range of 6.17×10⁴–8.04×10⁹ M⁻¹ s⁻¹, while that for deprotonated species + ·OH reactions is 1.21×10¹⁰–5.91×10¹⁰ M⁻¹ s⁻¹. Deprotonated species are more reactive than neutral species. Neutral and weakly alkaline conditions favor the aqueous phase aging of BrC. Aging predominantly yields more light-absorbing polyhydroxy compounds and quinones, whereas the formation of less light-absorbing ring-opening products is limited under the ·OH-initiated aging pathway alone. Aging products of nitrophenol BrC formed during the daytime exhibit resistance to photobleaching. Owing to their high complex RI and strong light absorption, these products significantly absorb solar energy, leading to localized atmospheric heating and potentially enhancing the greenhouse effect.