# Photochemical Degradation of Iron Citrate in Anoxic Viscous Films Enhanced by Redox Cascades

## TL;DR

This study explores how iron citrate in organic aerosol particles degrades under UV light, with reactions influenced by humidity and oxygen availability.

## Contribution

The study reveals how redox cascades enhance photochemical degradation of iron citrate in viscous films under anoxic conditions.

## Key findings

- CO2 production decreases with lower relative humidity due to anoxia in viscous films.
- Reactive oxygen species drive FeII reoxidation, enabling multiple redox cycles.
- Photooxidation is limited by O2 diffusion under dry conditions and by chemical reactions under high humidity.

## Abstract

Iron contained in
atmospheric aerosol particles can form complexes
with organic ligands and initiate photochemical reactions that alter
the composition and physicochemical properties of the particles. Depending
on the temperature and humidity, organic particles exist in different
phase states, which affects reactant diffusivity and chemical reaction
rates. We performed coated-wall flow-tube experiments using citric
acid films doped with iron as proxies for secondary organic aerosols.
We quantified the CO2 production under UV irradiation as
a function of time and relative humidity (RH) and observed a pronounced
decrease of CO2 production with decreasing RH. The kinetic
multilayer model of aerosol surface and bulk chemistry (KM-SUB) and
a Monte Carlo-based global optimization method were applied to all
measured data to determine the underlying effects of mass transport
and chemical reactions. The model analysis revealed that after an
initial rapid reaction, photooxidation becomes limited by the reoxidation
of FeII. Under dry conditions (RH < 65%), the reoxidation
of FeII is kinetically limited by the supply of O2, as slow diffusion in the viscous organic matrix leads to anoxia
in the interior of the film. At high humidity (RH > 85%), mass
transport
limitations cease, resulting in full O2 saturation, and
photooxidation becomes limited by the chemical reaction of FeII with oxidants. Reactive oxygen species play a key role in
FeII reoxidation and thus in perpetuating photooxidation
chemistry. A single O2 molecule triggers a redox cascade
from O2 to HO2, H2O2,
and OH, leading to ≈3 cycles of the FeII/FeIII redox pair. Our model and kinetic parameters provide new
insights and constraints in the interplay of microphysical properties
and photochemical aging of mixed organic–inorganic aerosol
particles, which may influence their effects on air quality, climate,
and public health.