Molecular Analysis of Secondary Organic Aerosol and Brown Carbon from the Oxidation of Indole

Abstract. Indole (ind) is a nitrogen-containing heterocyclic volatile organic compound commonly emitted from animal husbandry and from different plants like maize with global emissions of 0.1 Tg y^-1. The chemical composition and optical properties of indole secondary organic aerosol (SOA) and brown carbon (BrC) are still not well understood. To address this, environmental chamber experiments were conducted to investigate the oxidation of indole at atmospherically relevant concentrations of selected oxidants (OH radicals and O₃) with/without NO₂. In the presence of NO₂, the SOA yields decreased by more than a factor of two but the mass absorption coefficient at 365 nm (MAC₃₆₅) of ind-SOA was 4.3 ± 0.4 m² g^-1, which was 5 times higher than that in experiments without NO₂. In the presence of NO₂, C₈H₆N₂O₂ (identified as 3-nitroindole) contributed 76 % to the all organic compounds detected by a chemical ionization mass spectrometer, contributing ~50 % of the light absorption at 365 nm (Abs₃₆₅). In the absence of NO₂, the dominating chromophore was C₈H₇O₃N contributing to 20–30 % of Abs₃₆₅. Indole contributes substantially to the formation of secondary BrC and its potential impact on the atmospheric radiative transfer is further enhanced in the presence of NO₂, as it significantly increases the specific light absorption of ind-SOA by facilitating the formation of 3-nitroindole. This work provides new insights into an important process of brown carbon formation by interaction of two pollutants, NO₂ and indole, mainly emitted by anthropogenic activities.