Molecular Characterization of Brown Carbon Chromophores in Atmospherically Relevant Samples and Their Gas‐Particle Distribution and Diurnal Variation in the Atmosphere

By developing a nontarget tandem mass spectrometry experiment workflow, we identified the most probable structures of 100 brown carbon (BrC) chromophores in five types of atmospherically relevant samples including ambient aerosols, biomass burning aerosols, road tunnel aerosols, toluene secondary organic aerosol and rainwater. These chromophores were classified into nine groups, that is, 31 nitrophenols, 12 benzoic acids, 11 oxygenated polycyclic aromatic hydrocarbons, 10 phenols, 10 aryl amides/amines, 9 phenylpropene derivatives, 9 coumarins and flavonoids, 5 pyridines, and 3 nitrobenzoic acids. Among them, aryl amides/amine chromophores, which were mostly found in rainwater, were reported for the first time. The identified chromophores accounted for 20.9%–83.5% of total light absorption over 300–400 nm wavelength or total ion intensity in the 5 types of samples. We identified the characteristic chromophore groups in each sample type and identical chromophores among different sample types, especially between the source samples and ambient aerosols. Gas-particle distribution and diurnal variation of 13 BrC compounds in the atmosphere were measured by an online high-resolution chemical ionization mass spectrometer equipped with a Filter Inlet for Gases and Aerosols. It was found that gas-particle distribution of BrC compounds was dictated by their functional group, oxygen and carbon atom numbers. The BrC compounds containing nitro-group had higher nighttime concentrations, while those without nitro-group had higher daytime concentrations.