The
natural formation of silver nanoparticles (AgNPs) via biotic and abiotic pathways in water and soil media contributes
to the biogeochemical cycle of silver metal in the environment. However,
the formation of AgNPs in the atmosphere has not been reported. Here,
we describe a previously unreported source of AgNPs via the reduction of Ag(I) by SO2 in the atmosphere, especially
in moist environments, using multipronged advanced analytical and
surface techniques. The rapid reduction of Ag(I) in the atmospheric
aqueous phase was mainly caused by the sulfite ions formed from the
dissolution of SO2 in water, which contributed to the formation
of AgNPs and was consistent with the Finke–Watzky model with
a major contribution of the reduction–nucleation process. Sunlight
irradiation excited SO2 to form triplet SO2,
which reacted with water to form H2SO3 and greatly
enhanced Ag(I) reduction and AgNP formation. Different pH values affected
the speciation of Ag(I) and S(IV), which were jointly involved in
the reduction of Ag(I). The formation of AgNPs was also observed in
the atmospheric gas phase via direct reduction of
Ag(I) by SO2(gas), which occurred even in 50 ppbv SO2(gas). The natural occurrence of AgNPs in the atmosphere may
also be involved in silver corrosion, AgNP transformation and regeneration,
detoxification of gaseous pollutants, and the sulfur cycle in the
environment.