Photoaging of phenolic secondary organic aerosol in the aqueous phase: evolution of chemical and optical properties and effects of oxidants
Authors: Jiang Wenqing , Niedek Christopher , Anastasio Cort , Zhang Qi 
Published: June 2023

Abstract. While gas-phase reactions are well established to have
significant impacts on the mass concentration, chemical composition, and
optical properties of secondary organic aerosol (SOA), the aqueous-phase
aging of SOA remains poorly understood. In this study, we performed a series
of long-duration photochemical aging experiments to investigate the
evolution of the composition and light absorption of the aqueous SOA (aqSOA)
from guaiacyl acetone (GA), a semivolatile phenolic carbonyl that is common
in biomass burning smoke. The aqSOA was produced from reactions of GA with
hydroxyl radical (•OH-aqSOA) or a triplet excited state of
organic carbon (3C∗-aqSOA) and was then photoaged in water under
conditions that simulate sunlight exposure in northern California for up to
48 h. The effects of increasing aqueous-phase •OH or 3C∗
concentration on the photoaging of the aqSOA were also studied. High-resolution aerosol mass spectrometry (HR-AMS) and UV–Vis spectroscopy were
utilized to characterize the composition and the light absorptivity of the
aqSOA and to track their changes during aging. Compared to •OH-aqSOA, the 3C∗-aqSOA is produced more
rapidly and shows less oxidation, a greater abundance of oligomers, and
higher light absorption. Prolonged photoaging promotes fragmentation and the
formation of more volatile and less light-absorbing products. More than half
of the initial aqSOA mass is lost, and substantial photobleaching occurs
after 10.5 h of prolonged aging under simulated sunlight illumination
for 3C∗-aqSOA and 48 h for •OH-aqSOA. By performing
positive matrix factorization (PMF) analysis of the combined HR-AMS and
UV–Vis spectral data, we resolved three generations of aqSOA with distinctly
different chemical and optical properties. The first-generation aqSOA shows
significant oligomer formation and enhanced light absorption at 340–400 nm.
The second-generation aqSOA is enriched in functionalized GA species and has
the highest mass absorption coefficients in 300–500 nm, while the
third-generation aqSOA contains more fragmented products and is the least
light absorbing. These results suggest that intermediately aged phenolic
aqSOA is more light absorbing than other generations, and that the light
absorptivity of phenolic aqSOA results from a competition between brown
carbon (BrC) formation and photobleaching, which is dependent on aging time.
Although photoaging generally increases the oxidation of aqSOA, a slightly
decreased O/C of the •OH-aqSOA is observed after 48 h of
prolonged photoaging with additional •OH exposure. This is likely
due to greater fragmentation and evaporation of highly oxidized compounds.
Increased oxidant concentration accelerates the transformation of aqSOA and
promotes the decay of BrC chromophores, leading to faster mass reduction and
photobleaching. In addition, compared with •OH, photoaging by
3C∗ produces more low-volatility functionalized products, which
counterbalances part of the aqSOA mass loss due to fragmentation and
evaporation.