Influence of Relative Humidity and Irradiation on the Reactive Uptake of Ozone onto Secondary Brown Carbon from Catechol in Thin Films

SITHUMI LIYANAGE, Habeeb Al-Mashala, Katrina Betz, Elijah Schnitzler, Oklahoma State University

     Abstract Number: 331
     Working Group: Aerosol Chemistry

Abstract
Light-absorbing organic aerosol, or brown carbon (BrC), influences climate directly by absorbing and scattering radiation and indirectly by affecting cloud formation and properties. Whether the combination of these effects exerts a warming or cooling effect on Earth’s radiative balance depends on the evolution of the optical properties of BrC induced by multiphase processing by atmospheric oxidants, including ozone, which is governed by the viscosity of the aerosol particles. There is evidence that the viscosity of secondary BrC from phenolic precursors, abundant emissions of biomass burning, is higher than that of primary organic aerosol from biomass burning. Here, we investigate the reactivity of secondary BrC from catechol, in terms of the loss of ozone from the gas-phase over thin films in a coated-wall flow tube. Secondary BrC was generated from the aqueous OH-initiated oxidation of catechol under ultraviolet (UV) irradiation in a photo-reactor. Thin films were prepared from reconstituted, concentrated solutions in ethanol following rotary evaporation of the aqueous reaction mixtures. The effects of relative humidity (RH) and UV irradiation of the thin films were determined. At 0% RH, the uptake coefficient at 4 h of ozone exposure (initially 130 ppm) was less than previously observed for primary BrC from biomass burning, consistent with higher viscosity. The uptake coefficient was even less after UV irradiation. At intermediate RH, the reactive uptake increased significantly, consistent with plasticization of the secondary BrC components, which are still completely water-soluble at the end of the aqueous photo-oxidation experiments. At 75% RH, the thin film deliquesced and coalesced into large droplets, precluding reactive uptake measurements. The implications of these results on the evolution of BrC will be discussed.