Duan, J., Huang, R. J*, Lin, C., Qu, J., Liu, J., Huang, W., Zhan, Y., Yuan, W., Wang, T., Zhou, L., Xu, W., Liu, Q., Liu, Z.; Lou, S., Yang, H., Huang, D. D., Huang, C., Wang, H., Relative Humidity Modulates Photochemical Aging of Light-Absorbing Carbonaceous Aerosols: Insights from Ambient Oxidation Flow Reactor. Environ. Sci. Technol., 59(45), 24481–24491, 2025
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.
A research group led by Prof. HUANG Rujin from the Institute of Earth Environment (IEE) of the Chinese Academy of Sciences simulated in-situ photochemical aging of ambient LACs under varying relative humidity (RH) conditions 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 pre-existing 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 equivalent atmospheric aging days. This is primarily due to a twofold increase in the aqueous-phase photo-oxidative degradation of BrC chromophores derived from biomass burning source, whereas non-biomass 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 a better representation in climate models.
This study, published in Environmental Science & Technology, highlighted the important roles of relative humidity in the photochemical aging of LACs under real atmosphere.
Figure 1. Absolute and relative changes in total light absorption coefficients (abs(λ), Mm−1) as a function of atmospheric-equivalent photochemical age (days) during OFR aging under low-RH (a, c) and high-RH (b, d) conditions.
Figure 2. Source- and RH-dependent BrC aging under atmospherically relevant conditions simulated by OFR.
This work was supported by the National Natural Science Foundation of China (NSFC) (grant no. 42525301, 42430708, and 42407155), the Key Research Program of Frontier Sciences from the Chinese Academy of Sciences (ZDBS-LY-DQC001), the Innovation Capability Support Program of Shaanxi (2024RS-CXTD-40), the New Cornerstone Science Foundation through the XPLORER PRIZE, and the Natural Science Basic Research Program of Shaanxi Province (2023-JC-QN-0319).