Nonlinear Effects of Relative Humidity on the Light Absorption of Water-Soluble Brown Carbon: Molecular-Level Constraints under Different Pollution Levels

Abstract. Brown carbon (BrC) plays an important role in aerosol light absorption, yet its relative humidity (RH) dependent optical evolution remains poorly constrained. Based on field observations and ultra-high-resolution mass spectrometry, this study reveals that the light absorption of water-soluble BrC is nonlinearly regulated by the interplay between pollution level and RH. The mass absorption efficiency at 365 nm shows a biphasic response to RH. It increases with RH, reaching a maximum in the range of approximately 50%–60% RH, and then decreases when RH exceeds this threshold. Molecular-level analysis reveals that RH may alter the chemical fate of water-soluble BrC chromophores, steering their formation and attenuation pathways while concurrently reshaping their optical absorption profiles. In the low-to-moderate RH regime, liquid-phase chemical processing drives the accumulation of chromophores. Beyond the inflection point, increasing aerosol liquid water activates several synergistic suppression pathways, including oxidative bleaching, hydrolytic cleavage of N/S-containing chromophores, and physical dilution. High pollution levels may partially offset the RH-driven suppression effects. These findings challenge static parameterizations of BrC in models and highlight the necessity of incorporating RH-driven molecular restructuring into radiative forcing assessments.