# Photochemical Aging of Indole SOA: Implications for Volatility and Optical Properties

## TL;DR

This study explores how indole-based aerosols change chemically and optically over time due to photochemical aging.

## Contribution

The research reveals how aging affects volatility, chemical composition, and light absorption of indole SOA.

## Key findings

- CHON species dominate 1-day aged SOA but decrease in 5-day aged SOA due to ring-opening reactions.
- INDOH1 absorbs more light than INDOH5, showing significant photobleaching over time.
- Heating increases the refractive index in INDOH1 due to evaporation of nonaromatic compounds.

## Abstract

This study investigates the chemical composition, volatility,
and
optical properties of secondary organic aerosol (SOA) formed from
1 and 5 days of equivalent photochemical oxidation of indole using
a combination of online thermodenuder techniques and offline high-resolution
mass spectrometry (HRMS). Thermodenuder–Aerosol Mass Spectrometer
(TD-AMS) thermograms revealed higher volatility for CH and CHO fragments
(T
50 ∼ 390–395 K) and greater
thermal stability for nitrogen-containing CHON ions (T
50 ∼ 410–415 K). Volatility basis set (VBS)
distributions showed that CHON species dominated the composition of
1-day aged SOA (INDOH1) but were largely depleted in 5-day aged SOA
(INDOH5), indicating extensive oxidative aging associated with ring-opening
reactions and the loss of nitrogen-containing functional groups, as
reflected in the degradation of aromatic species (AImod > 0.67) and reduced π-conjugation. Additionally, INDOH1
exhibited
stronger light absorption than INDOH5, demonstrating significant photobleaching.
The evaporation due to heating affected the complex refractive index
(RI); the imaginary part (k) increased from 0.014
to 0.09 in INDOH1, while it remained below 0.06 in INDOH5. The absorption
enhancement with heating is attributed to the preferential evaporation
of weakly absorbing, nonaromatic compounds, enriching the particle
phase in thermally stable, π-conjugated CHON species. These
results establish a direct link between the volatility, chemical evolution,
and optical properties of indole SOA.