Light absorption by brown carbon (BrC) is dynamic due to atmospheric aging processes, leading to complex and poorly constrained effects on photochemistry and climate. In this study, a smog chamber was used to simulate the heterogeneous ozone (O-3) aging of soot particles. Twelve aging times and seven O-3 concentrations were set to investigate the effects of aging degree on BrC light absorption. The results showed that light absorption by BrC was enhanced after O-3 aging, but followed a non-monotonic trend with an initial increase and subsequent decrease. An aging time of 60 min and O-3 concentration of 1.2 ppm were optimal for enhancing BrC absorption, where the contribution of BrC to total absorption and the contribution of BrC relative to black carbon absorption at 370 nm of ozonized soot were 23.0 +/- 1.8% and 30.0 +/- 3.0%, respectively, much greater than those of fresh soot (8.1 +/- 1.1% and 8.8 +/- 1.3%, respectively). The absorption Angstrom exponent (AAE) and delta C (Delta C) of ozonized soot at 60 min ranged from 1.18 +/- 0.01 to 1.31 +/- 0.03 and from 13.5 +/- 7.0 to 24.3 +/- 13.5 mu g m(-3), respectively, and were greater than those of fresh soot (1.12 +/- 0.02 and 8.0 +/- 0.8 mu g m(-3)), but also showed non-monotonic trends, suggesting the formation of BrC during O-3 aging. Comparative results indicated that AAE might be a better BrC indicator for soot than Delta C. The non-monotonic trend was tentatively explained by changes in organic carbon, oxygenated functional groups and conjugated structures, as well as polycyclic aromatic hydrocarbon (PAH) degradation and oxygenated PAH formation. The relative intensities of oxidative formation and degradation of chromophores may determine BrC evolution during O-3 aging. This study will be useful for clarifying BrC evolution in the atmosphere and estimating its radiative forcing. (C) 2020 Elsevier Ltd. All rights reserved.

Soot particles positively influence radiative forcing due to their strong absorption. Because of their chain-like structure, aggregated soot particles become more compact with the aging process, and the monomers or particles are always covered by water coatings. The optical parameters of two typical soot-water mixtures (i.e., an aggregate with core-shell monomers and a soot aggregate inside a water droplet) at 550 nm were investigated using the superposition T-matrix method, with a focus on the impact of the morphology and water coating of soot aggregates. For the soot aggregate with core-shell monomers, a relationship among the fractal dimension, relative humidity (RH) and monomer number was established and used to calculate optical parameters. The intensity of forward scattering declined with the increasing RH. The Cext, Csca, Cabs and SSA are much more insensitive to RH under higher RH conditions (RH > 90%) than at a lower RH level. In addition, hygroscopic shrinkage and the thickness of water coating have stronger effects on the optical properties of larger aggregated soot at higher RH than lower RH. For another mixing state, the soot aggregate inside a water droplet, the morphology of the soot core plays an important role in the optical properties when the thickness of the water shell is small. When the diameter ratio of the water droplet to the aggregated soot (D_ratio) changes from 1.2 to 2.8, Cext difference increases from 0.23 mu m(2) to 2.36 mu m(2) for particles with N = 100 and 500, whereas the SSA difference decreases from 0.12 to 0.01. If the agglomerated structure of the soot core is not considered, the Cext, Csca and SSA will be underestimated for a relatively small D_ratio of 1.2. Ignoring the soot core in the water droplet could introduce large errors into the calculation of the optical parameters, and ignoring the structure of the aggregated soot core could enlarge the errors.