Evidence for phase separation in atmospheric particle samples

In the atmosphere, a large fraction of submicron atmospheric particles can contains both organic and sulfate material. As the relative humidity cycles in the atmosphere from high to low values, these particles possibly could go through a range of phase transitions including phase separation. However these phase transitions are considered uncertain in part because only approximately 10% of the organic content in these particles has been identified at a molecular level. Using optical and fluorescence microscopy, we directly observe the coexistence of two non-crystalline phases (an organic-rich phase and a sulfate-rich phase) in particles generated from atmospheric samples and samples from environmental chambers. These observations suggest that atmospheric particles can undergo phase separation forming an organic-rich and sulfate-rich phase without the need for a crystallization event.