Dark aging of iron containing alpha-pinene secondary organic aerosol

Secondary organic aerosol (SOA) can undergo atmospheric aging processes that alter their impact on climate, air quality and human health. Transition metals, such as iron, can age SOA particles through catalytic chemical reactions within the condensed phase. Iron-containing particles originating from e.g., mineral dust, often become internally mixed with SOA, forming iron-containing SOA particles through various atmospheric processes, such as coagulation, condensation or cloud processing. When acidic organic vapors condense on iron-containing mineral dust particles, they can cause dissolution of minerals followed by iron-organic complex formation. Iron-organic complexes are common in atmospheric particles and can generate reactive oxygen species within a particle through dark peroxide and photochemical reactions (i.e., Fenton chemistry), leading to further aging of the particles by functionalization or fragmentation of organic species. Such particle-phase aging processes can considerably change the particle chemical composition. However, detailed understanding of these compositional changes is lacking to date, and hence considerable uncertainties still exist regarding the impact aged particles have on air quality and climate.Here, we present detailed information on the chemical composition of iron-containing SOA particles and how it evolves over time. Particles were produced by forming SOA via α-pinene ozonolysis on both ammonium sulfate or iron-containing seed particles in an atmospheric simulation chamber under dark conditions. This allowed us to probe the impacts of iron on dark e.g., peroxide reactions and aerosol aging in the absence of photochemical driven Fenton chemistry, i.e., simulating nocturnal aging processes. Experiments were also conducted under both wet (relative humidity (RH) >80%) and dry (RH