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

Abstract

Secondary organic aerosol (SOA) from indole was produced in a smog chamber under different relative humidity (RH = <2%, 25% and 50%) and with different oxidants (O₃, OH, OH + NO_x, and NO₃). 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 NO₃ 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/cm³ for indole + O₃ SOA to 1.33 g/cm³ 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 + NO₃ 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

EDITOR:

• Hans Moosmüller

Disclosure statement

No potential conflict of interest was reported by the authors.

Author contributions

YR and SAN designed the experiments; JM, NNA, LTF, and VJB did the smog chamber experiments; QF did the OFR experiments; PL, JL, AL, and VJB recorded off-line mass-spectrometric data; JM, QF and VJB analyzed the data and wrote the manuscript; all authors edited the manuscript.

Additional information

Funding

The UCI group acknowledges financial support from National Science Foundation (NSF) grant AGS-1853639. JM and VJB acknowledge support from the NSF graduate research fellowship program. WIS and PU groups acknowledge partial support from the US National Science Foundation/US-Israel Binational Science Foundation (BSF Grant No. 2020656, NSF Grant No. AGS-2039985) at the time of this paper preparation. YR acknowledges partial support by the Israel Science Foundation (grant #928/21).