Molecular revealing of mixing states of aerosol particles using ToF-SIMS spectra and imaging analysis

Fine particulate matter (PM), especially PM_2.5 (PM with the aerodynamic diameter less than 2.5 μm), has been raising concern due to its potential health risks. Many analysis tools have been used to study chemical components of PM2.5 particles, elucidating that PM2.5 particles are made up of complex chemical components. However, determination of the fine structure of the PM2.5 particles, such as mixing states, has been challenging. For example, popularly-used Particle into Liquid Sampler (PILS) and Aerosol Mass Spectrometer (AMS) are bulk analysis tools, which can't provide enough information for the fine structure. Sophisticated imaging tools, including Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM) can provide high lateral resolution (a few nm to sub-nm level) topographic and elemental images. However, only limited chemical information, such as bulk elemental information, is available. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is a powerful surface analysis tool. It can simultaneously provide elemental, isotopic and molecular information with part per million (ppm) sensitivity. More importantly, it is very surface sensitive, and its information depth is less than a few nanometers. Also, ToF-SIMS can provide molecular imaging with submicron lateral resolution. Such capabilities are unique in aerosol research. In this work, a combination of ToF-SIMS surface analysis and traditional bulk analysis provided critical information to elucidate the mixing states of aerosol particles. PM_2.5 aerosol samples in a typical Beijing winter pollution case were used as a model system. Our data show that chemical components of the PM_2.5 hardly change from low to medium pollution situations, and only single-component particles form (external mixing). Under severe pollution situation, the major chemical component change is increasing of (NH₄)₂SO₄, which can form relatively large particles (5+ microns); while external mixing is still the dominant mixing state for all chemical components.