Hygroscopicity of secondary brown carbon aerosol from aqueous photo-oxidation of phenolic precursors

Abstract:

To constrain the climate effects of light-absorbing organic aerosol, or brown carbon (BrC), it is necessary to determine to what extent the indirect effect through aerosol-cloud interactions competes with the direct effect through aerosol-radiation interactions. Towards addressing this need, water uptake onto secondary BrC aerosol was measured in hygroscopic tandem differential mobility analyzer (H-TDMA) experiments. Five phenolic species, selected to form an ordered series varying in the numbers of hydroxy and methoxy substituents (phenol, catechol, pyrogallol, guaiacol, and syringol), were exposed to aqueous hydroxyl radical in a photo-reactor, leading to the formation of secondary BrC. The absorptivity of the BrC was monitored by UV-vis spectroscopy, and the composition was probed by direct injection into a time-of-flight mass spectrometer. Aerosol was generated by atomizing the product mixtures without recirculation. After being passed through a series of diffusion dryers and activated charcoal denuders, particles were selected by size with the first DMA and exposed to a certain relative humidity, stepped between 10 and 90%. Particle growth upon water uptake was measured by the second DMA coupled to a condensation particle counter and used to derive the hygroscopicity parameter, κ. While κ values for the five precursors lie within the range of 0.01-0.07, generally consistent with oxygenated organic aerosol, systematic differences occur, and these are presented in the context of the functionality of the precursors and the composition of the aqueous secondary BrC.