Triplet State Formation of Chromophoric Dissolved Organic Matter in Atmospheric Aerosols: Characteristics and Implications

Abstract. There is chromophore dissolved organic matter (CDOM) in the atmosphere, which may form triplet-state chromophoric dissolved organic matter (³CDOM*) to further driving the formation of reactive oxygen species (ROS) under solar illumination. ³CDOM* contributes significantly to aerosol photochemistry and plays an important role in aerosol aging. We quantify the ability to form ³CDOM* and drive the formation of ROS by primary, secondary and ambient aerosols. Biomass combustion has the strongest ³CDOM* generation capacity and the weakest vehicle emission capacity. Ambient aerosol has a stronger ability to generate ³CDOM* in winter than in summer. Most of the triplet states generation conform to first-order reaction, but some of them do not due to the different quenching mechanism. The structural-activity relationship between the CDOM type and the ³CDOM* formation capacity shows that the two types of CDOM identified, which similar to the nitrogen-containing chromophores contributed 88 % to the formation of ³CDOM*. The estimated formation rate of ³CDOM* can reach ~ 100 μmol m⁻³ h⁻¹ in the atmosphere in Xi'an, China, which is approximately one hundred thousand-times the hydroxyl radical (•OH) production. This study verified that ³CDOM* drives at least 30 % of the singlet oxygen (¹O₂) and 31 % of the •OH formed by aerosols using the spin trapping and electron paramagnetic resonance technique.