Both Fe(III) and fatty acids are
ubiquitous and important species
in environmental waters. Because they are amphipathic, many fatty
acids are surface active and prone to enrichment at the air–water
interface. Here, we report that by using nonanoic acid (NA) as a model
fatty acid, coexisting Fe(III), even at concentrations as low as 1
μM, markedly enhanced the photochemical release of NA-derived
volatile organic compounds (VOCs) such as octanal and octane into
the air. Further studies indicated that the surface-enriched fatty
acids dramatically increase the local concentration of Fe(III) at
the water surface, which enables Fe(III)-mediated photochemical reactions
to take place at the air–water interface, and the VOCs facilely
produced by fatty acid photooxidation can then be released into the
air. Moreover, the product distribution in the Fe(III)-mediated reactions
was largely different from that in other photochemical systems, and
a mechanism based on photochemical decarboxylation is proposed. Considering
that the coexistence of fatty acids and Fe(III) in the environment
is common, the enhanced photochemical release of VOCs by surface-enriched
fatty acids and Fe(III) may be an important channel for the atmospheric
emission of VOCs, which are known to play an essential role in the
formation of ozone and secondary organic aerosols.