Isomeric Identification of the Nitroindole Chromophore in Indole + NO₃ Organic Aerosol

Oxidation of indole by nitrate radical (NO₃) was previously proposed to form nitroindole, largely responsible for the brown color of indole secondary organic aerosol (SOA). As there are seven known nitroindole isomers, we used chromatographic separation to show that a single nitroindole isomer is produced in the indole + NO₃ reaction and definitively assigned it to 3-nitroindole by comparison with chromatograms of nitroindole standards. Mass spectra of aerosolized 3-nitroindole particles were recorded with an aerosol mass spectrometer and directly compared to mass spectra of SOA from smog chamber oxidation of indole by NO₃ in order to help identify peaks unique to nitroindole (m/z 162, 132, and 116). Quantum chemical calculations were done to determine the energetics of hypothesized indole + NO₃ intermediates and products. The combination of these data suggests a mechanism, wherein a hydrogen atom is first abstracted from the N-H bond in indole, followed by isomerization to a carbon-centered radical in the 3-position and followed by addition of NO₂. Alternative mechanisms involving a direct abstraction of a H atom from a C-H bond or a NO₃ addition to the ring are predicted to be energetically unfavorable from large barriers for the initial reaction steps.