Photolytic Transformation of Soluble and Colloidal Components in Atmospheric Brown Carbon

Atmospheric Brown Carbon (BrC), composed of light-absorbing soluble and colloidal organic constituents, is emitted predominantly during biomass-burning events. Quantification of its impacts on Earth’s radiative balance remains challenging due to dynamic chemical transformations of BrC occurring after ejection into the atmosphere. Exposure to UV radiation facilitates the fragmentation of BrC compounds, typically resulting in a progressive loss of light absorption properties, a process commonly referred to as photobleaching, which varies significantly among individual BrC components. To investigate these transformations, a laboratory-generated BrC mixture was photolyzed using a custom-built photoreactor, and UV–vis absorbance spectra were recorded throughout the experiment. Aliquots of the photolyzed mixture were subsequently separated into filtrate and residue subfractions via vacuum filtration. The molecular composition of the soluble filtrates and insoluble residues was analyzed using temperature-programmed desorption (TPD) combined with direct analysis in real time (DART) ambient ionization coupled to a high-resolution mass spectrometer (HRMS). Detected species were evaluated based on their modified aromaticity index and saturation mass concentration (volatility) values. These molecular descriptors indicated that more polar, solvent-soluble BrC constituents produce less aromatic and more volatile photoproducts upon irradiation, underscoring their higher photolability. Conversely, the insoluble constituents exhibited minimal changes in both aromaticity and volatility, contributing to the retention of particle-phase mass and slowing the overall volatilization of the BrC mixture. Volatility assessments and photochemical calculations suggest that the photolabile BrC constituents persist in the particle-phase for approximately 11–40 atmospheric irradiation hours before undergoing conversion into smaller, more volatile gas-phase photoproducts. The opposing effects of soluble and insoluble colloidal fractions yield a net-zero change in overall atmospherically relevant BrC absorption quantified as the solar-flux-weighted mass absorption coefficients. The photodegradation rates derived from this study can inform atmospheric models to improve predictions of BrC contributions to aerosol–radiation interactions relevant to the radiative forcing of Earth’s climate.