The Ice Nucleation Pathway of Different Amorphous Secondary Organic Aerosol – The Role of Oxidation Level and Sulfate Content

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Description

Secondary organic aerosol (SOA) generated from the oxidation of organic gases are ubiquitous in the atmosphere, but their interaction with water vapor and their ice cloud formation potential at low temperatures remains highly uncertain. Laboratory generated SOA from reaction of two volatile organic compound (VOC) precursors, α-pinene and isoprene and one intermediate VOC, longifolene, with hydroxyl radicals are investigated for their onsets of water uptake and ice nucleation, including immersion/condensation freezing and deposition ice nucleation. Water uptake and ice nucleation are determined for temperatures as low as 205 K and for sub and saturated conditions. Furthermore, the the presence of sulfates on water uptake and ice nucleation are assessed. Micro-spectroscopic chemical imaging using scanning electron microscopy with energy dispersive analysis of X‐rays (CCSEM/EDX) and scanning transmission X‐ray microscopy with near edge X‐ray absorption fine structure spectroscopy (STXM/NEXAFS) was used to characterize longifolene SOA particles with and without the presence of sulfuric acid seed particles generated in the Boston College PAM flow reactor with an O/C ratio of about 0.3. Longifolene SOA impacted on solid substrates retained a spherical shape and did not coalesce suggesting that they are highly viscous or glassy at room temperature. When mixed with sulfuric acid having an organic to sulfate ratio of 6:1, the particles deformed and coalesced likely due to the plasticizing effect of the inorganic material. Isoprene and α-pinene SOA with and without the presence of sulfate were found likely to be liquid. The ice nucleation experiments at low temperatures indicate that highly viscous/solid SOA particles require high relative humidity (RH) to take up water and/or nucleate ice. However, the presence of sulfates significantly reduces the RH for which water uptake can be observed. This work emphasizes the complexity of aerosol particle–water vapor interactions as a result of the amorphous organic phase and mixing state.