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 <10%) conditions. Aerosol bulk composition was determined using extractive electro-spray ionization mass spectrometry, allowing for high chemical and temporal identification of oxidation products, i.e., monomers and dimers, present within the particles. Under dry conditions, particles (both with and without iron) were found to contain a higher fraction of monomers, compared to dimers. Whereas under wet conditions the monomer/dimer ratio was smaller when iron was present. This suggests that iron-catalyzed functionalization reactions are favoured under wet conditions. Furthermore, when iron was present in the seed particles the lifetimes of monomers and dimers varied greatly, where the signal for some organic species (e.g., C19s and C20s) was observed to decrease rapidly (t_1/2 ~ 25 min.) following SOA formation under wet conditions, while only slow decay was observed under dry conditions (t_1/2 ~ 110 min.). This suggests that iron-catalyzed reactions are limited by diffusion of organic molecules under dry conditions. Overall, our results elucidate the key role of transition metals, such as iron, in altering the chemical composition of SOA particles during atmospheric transport. Such effects need to be considered to correctly reflect atmospheric aging of ambient SOA particles that are internally mixed with e.g., mineral dust, when predicting their role for air pollution and climate in atmospheric models.