Mixing States of Aerosol Particles in Urban Haze

Secondary organic aerosols (SOA) are a large fraction of PM_2.5 mass and contribute to extreme haze events, reducing visibility and impairing human health, especially in the Northern China Plain. It has been observed that laboratory generated and field collected SOA material can undergo liquid-liquid phase separation (LLPS), however this has never been directly observed in single ambient aerosol particles. Oligomers are a significant component of atmospheric SOA typically having a molecular mass of >200 g mol^-1. These large molecules can be produced via multiphase chemical processes and, when soluble in the aerosol phase, may lead to interesting phase separation behavior.

We conducted a campaign in Beijing during which PM_2.5 was analyzed using matrix-assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF-MS) to observe oligomers at significant quantities. Aerosol particle samples collected before, during and at the peak of a pollution event were targeted. We have evidence that oligomers were the result of multiphase chemistry at high relative humidity. Single particles were probed for chemical morphology and mixing states using X-ray spectro-microscopy to characterize the numbers of particles mixed with inorganic matter, organic matter or soot. Using an environmental microchamber, we subjected single ambient particles to humidity cycles and observed any LLPS to occur. We also quantify the humidity required for LLPS to occur. Our data links oligomeric material having different solubility than e.g. inorganic hygroscopic components with LLPS, giving rise to a clear constraint for urban haze. The results will give statistically significant information about particle mixing state for aerosol population having different oligomer content, humidity history, LLPS behavior and pollution levels.