Light-absorbing carbonaceous aerosols (LACs), including black carbon (BC) and brown carbon (BrC), significantly influence Earth's radiative balance and global climate. However, their atmospheric aging processes and associated optical evolution remain insufficiently understood. In this study, in situ photochemical aging of ambient LACs under varying relative humidity (RH) conditions was simulated using an oxidation flow reactor (OFR). The distinct absorption evolution of BC and BrC was investigated, and the underlying mechanisms were explored. BC absorption primarily decreased under low-RH aging but significantly increased under high-RH aging. This contrasting behavior can be attributed to RH-dependent changes in BC coating processes: the dominant loss of preexisting coatings at low RH versus enhanced formation of secondary species that preferentially coat BC under high RH. Notably, BC absorption enhancement is more sensitive to nitrate, ammonium, and secondary organic aerosol (SOA) formation than to sulfate. BrC exhibited optical bleaching under both RH conditions; however, the bleaching rate was substantially accelerated under high RH at comparable photochemical ages within the range of below 5 equiv atmospheric aging days. This is primarily due to a 2-fold increase in the aqueous-phase photo-oxidative degradation of BrC chromophores derived from biomass-burning sources, whereas nonbiomass BrC showed RH-independent bleaching. These findings show that RH strongly modulates the chemical and optical aging of LACs, with important implications for their direct radiative forcing and better representation in climate models.

Soot particles released in the atmosphere have long been investigated for their ability to affect the radiative forcing. Although freshly emitted soot particles are generally considered to yield only positive contributions to the radiative forcing, atmospheric aging can activate them into efficient cloud condensation or ice nuclei, which can trigger the formation of persistent clouds and ultimately provide a negative contribution to the radiative forcing. Depending on their residence time in the atmosphere, soot particles can undergo several physical and chemical aging processes that affect their chemical composition, particle size distribution and morphology, and ultimately their optical and hygroscopic properties. The impact of the physical-chemical aging on the properties of soot particles is still difficult to quantify, as well as their effect on the radiative forcing of the atmosphere.This work investigates the hygroscopic properties of chemically aged soot particles obtained from the combustion of aviation fuel, and in particular the interplay between aging mechanisms initiated by two widespread atmospheric oxidizers (O-3 and SO2). Activation is measured in water supersaturation conditions using a cloud condensation nuclei counter. Once particle morphology and size distribution are taken into account, the hygroscopicity parameter kappa is derived using kappa-K & ouml;hler theory and correlated to the change of the chemical composition of the particles aged in a simulation chamber. While fresh soot particles are poor cloud condensation nuclei (kappa < 10(-4)) and are not significantly affected by either O-3 or SO2 at the timescale of the experiments, rapid activation is observed when they are simultaneously exposed to both oxidizers. Activated particles become efficient cloud condensation nuclei, comparable to the highly hygroscopic particulate matter typically found in the atmosphere (kappa = 0.2-0.6 at RH = 20 %). Statistical analysis reveals a correlation between the activation and sulfur-containing ions detected on the chemically aged particles that are absent from the fresh particles.

Effective density (peff) is an important property describing particle transportation in the atmosphere and in the human respiratory tract. In this study, the particle size dependency of peff was determined for fresh and photochemically aged particles from residential combustion of wood logs and brown coal, as well as from an aerosol standard (CAST) burner. peff increased considerably due to photochemical aging, especially for soot agglomerates larger than 100 nm in mobility diameter. The increase depends on the presence of condensable vapors and agglomerate size and can be explained by collapsing of chain-like agglomerates and filling of their voids and formation of secondary coating. The measured and modeled particle optical properties suggest that while light absorption, scattering, and the single-scattering albedo of soot particle increase during photochemical processing, their radiative forcing remains positive until the amount of nonabsorbing coating exceeds approximately 90% of the particle mass.