Coexistence of three liquid phases in atmospheric aerosol particles

Abstract:

Submicron aerosol particles are ubiquitous in the atmosphere and play an important role for air quality and climate. Primary organic aerosol (POA), secondary organic aerosol (SOA), and secondary inorganic aerosol (SIA) constitute a significant mass fraction of these particles. POA, SOA, and SIA can become internally mixed within the same particle though different processes including coagulation, gas-to-particle partitioning, cloud processing, and multiphase chemical reactions. To predict the role of these internally mixed particles in climate, air quality, and atmospheric chemistry, information on their phase behaviour is needed. For instance, a particle with a single homogeneous liquid phase can have different scattering properties, reaction rates, and uptake kinetics compared to a particle with multiple liquid or solid phases.

In the current study Nile red, a solvatochromic dye, and fluorescence microscopy is used to determine the phase behavior of POA+SOA+SIA particles. Squalane was used as a proxy of POA, and ammonium sulfate was used as SIA. As proxies of SOA, we used 1 of 23 different oxidized organic molecules. We show that three phases frequently coexist within these particles, and that the phase behaviour strongly depends on the oxygen-to-carbon ratio of the SOA proxies. Experiments with SOA generated by dark ozonolysis of α-pinene in an environmental chamber are consistent with these observations. Thermodynamic and kinetic modelling was also used to investigate the atmospheric impacts of our experimental results.