Atmospheric secondary organic aerosol (SOA) can undergo physical and chemical aging processes that alter their impact on climate, air quality and health. Transition metals, such as iron, can age SOA through catalytic chemical reactions within the particle phase, leading to direct and indirect reaction with condensed-phase organics. Iron containing particles (originating from mineral dust, and particles emitted from combustion and industrial processes), can become internally mixed with SOA, forming iron containing SOA particles through various atmospheric processes, such as coagulation, condensation or cloud processing. When organic vapors condense on iron containing mineral dust particles, they can become acidic causing dissolution of minerals followed by iron-organic complex formation; subsequently generating reactive oxygen species (ROS) within a particle, e.g., through Fenton chemistry, and further functionalization or fragmentation of organic compounds. Here, we present data from atmospheric simulation chamber experiments, where SOA was generated from condensation of alpha-pinene ozonolysis products on iron containing seed particles under dark conditions. Aerosol bulk composition was determined using electro-spray ionization mass spectrometry (EESI-MS), allowing for high resolution chemical and temporal identification of oxidation products, i.e., monomers and dimers, present within the particles. In addition, experiments were conducted under both wet (relative humidity (RH) &gt;80%) and dry (RH &lt;10%) conditions to probe the impact aerosol mixing state (i.e., core-shell versus well mixed) has on the condensed phase aging of particles, e.g., diffusion limitations. Under dry conditions, particles contained a higher fraction of monomers, compared to dimers, whereas under wet conditions this ratio was smaller. Furthermore, the lifetimes of monomers and dimers varied greatly, where the signal for some species (e.g., C19s and C20s) decreased rapidly (t1/2 ~ 25 min) following SOA formation under wet conditions, while slower decay was observed under dry conditions (t1/2 ~ 110 min). The implications of this on aerosol aging will be discussed.

Dark aging of iron containing alpha-pinene secondary organic aerosol

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