Hygroscopicity of Secondary Brown Carbon Aerosol from Aqueous Photo-Oxidation of Phenolic Precursors

To understand the impact of light-absorbing organic aerosol, also called brown carbon (BrC), it is necessary to determine the extent to which the direct effect through aerosol- radiation interactions and the indirect effect through aerosol-cloud interactions change during its atmospheric residence time. Toward addressing this need, the light absorption and water uptake of secondary BrC aerosol produced from phenolic compounds, abundant biomass burning emissions, were measured. Phenol, catechol, and pyrogallol were selected to form a homologous series, varying in the number of hydroxyl substituents, and they were exposed to aqueous hydroxyl radical in a photoreactor, leading to the formation of secondary BrC. The absorptivity of the BrC was monitored by UV-vis spectroscopy; the hygroscopicity was determined using a hygroscopic tandem differential mobility analyzer. The absorptivity of the secondary BrC increased within 8 h of photo-oxidation and then began decreasing. After 24 h of photo oxidation, at an atmospherically relevant OH exposure of 2.2 x 10-10 mol s L-1, the hygroscopicity parameters for BrC from phenol, catechol, and pyrogallol were similar, i.e., 0.13 +/- 0.02, 0.10 +/- 0.02, and 0.13 +/- 0.02, respectively, so BrC from phenolic compounds exhibits similar water uptake regardless of the functionalization of the precursor. After 36 and 48 h of continued photo-oxidation, during which the product mixture exhibited further whitening, the hygroscopicity parameter of secondary BrC from catechol did not change. These observations suggest that the changes in absorptivity (related to the direct effect) of secondary BrC produced from phenolic precursors are greater than the changes in hygroscopicity (related to the indirect effect) upon atmospheric aging.