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Secondary organic aerosol (SOA) from indole was produced in a smog chamber under different relative humidity (RH = &lt;2%, 25% and 50%) and with different oxidants (O 3 , OH, OH + NO x , and NO 3 ). The mass absorption coefficients were measured by extracting SOA in methanol and taking absorption spectra in the 200-700 nm range. Chromophoric compounds were tentatively identified by liquid chromatography – spectrophotometry – mass spectrometry. One of the strongest chromophores was nitroindole, produced in high abundance by both NO 3  and OH + NO x  oxidation. Indole SOA was also prepared in an oxidation flow reactor (OFR) using the same set of oxidants under variable RH conditions. Densities of the OFR-generated SOA particles ranged from 1.05 g/cm 3  for indole + O 3  SOA to 1.33 g/cm 3  for indole + OH SOA. The real ( n ) and imaginary ( k ) refractive indices were quantified using a broadband cavity-enhanced spectrometer (BBCES) over the 360–425 nm spectral range. The values of  k  ranged from 10 −2  to 10 −1  at 405 nm, which is comparable to those for humic-like substances and at least an order of magnitude larger than  k  values of SOA produced from other biogenic and anthropogenic volatile organic compounds. The  k  values for the indole + NO 3  SOA had a strong and non-monotonic dependence on RH, suggesting two competing reaction mechanisms for the chromophores, highlighting the complex effect of water on the absorption coefficient of SOA from heterocyclic aromatic precursors. Strong absorption of indole SOA may contribute to the degradation of visibility near its emission sources, which include animal husbandry facilities and stressed plants. Copyright © 2023 American Association for Aerosol Research

Light absorption and scattering properties of indole secondary organic aerosol prepared under various oxidant and relative humidity conditions