Abstract. In the aqueous phase, fine particulate matter can form reactive species (RS)
that influence the aging, properties, and health effects of atmospheric
aerosols. In this study, we explore the RS yields of aerosol samples from
a remote forest (Hyytiälä, Finland) and polluted urban locations
(Mainz, Germany; Beijing, China), and we relate the RS yields to different
chemical constituents and reaction mechanisms. Ultra-high-resolution mass
spectrometry was used to characterize organic aerosol composition, electron
paramagnetic resonance (EPR) spectroscopy with a spin-trapping technique was
applied to determine the concentrations of ⚫OH,
O2⚫-, and carbon- or oxygen-centered organic radicals, and
a fluorometric assay was used to quantify H2O2. The aqueous
H2O2-forming potential per mass unit of ambient PM2.5
(particle diameter < 2.5 µm) was roughly the same for all
investigated samples, whereas the mass-specific yields of radicals were
lower for sampling sites with higher concentrations of PM2.5. The
abundances of water-soluble transition metals and aromatics in ambient
PM2.5 were positively correlated with the relative fraction of
⚫OH and negatively correlated with the relative fraction of
carbon-centered radicals. In contrast, highly oxygenated organic molecules
(HOM) were positively correlated with the relative fraction of
carbon-centered radicals and negatively correlated with the relative
fraction of ⚫OH. Moreover, we found that the relative fractions
of different types of radicals formed by ambient PM2.5 were comparable
to surrogate mixtures comprising transition metal ions, organic
hydroperoxide, H2O2, and humic or fulvic acids. The interplay of
transition metal ions (e.g., iron and copper ions), highly oxidized organic
molecules (e.g., hydroperoxides), and complexing or scavenging agents (e.g.,
humic or fulvic acids) leads to nonlinear concentration dependencies in
aqueous-phase RS production. A strong dependence on chemical composition
was also observed for the aqueous-phase radical yields of
laboratory-generated secondary organic aerosols (SOA) from precursor
mixtures of naphthalene and β-pinene. Our findings show how the
composition of PM2.5 can influence the amount and nature of
aqueous-phase RS, which may explain differences in the chemical reactivity
and health effects of particulate matter in clean and polluted air.
Formation and fragmentation of gas-phase ion-molecule complexes of transition-metal ions with organic molecules containing two functional groups
