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

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 (T₅₀ ∼ 390-395 K) and greater thermal stability for nitrogen-containing CHON ions (T₅₀ ∼ 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 (AI_mod > 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.