Secondary organic aerosol (SOA) is key to our climate, affecting the Earth’s radiative balance both indirectly and directly. Understanding the chemical composition and properties of SOA is crucial to accurately predict their concentrations and ultimately their impact on climate model predictions. Multiphase atmospheric reactions have been found to form many low-volatility, high-molecular-weight oligomeric species. Although oligomers constitute a large portion of SOA, they are not well understood. It is difficult for analytical methods to detect these species due to their high molecular weight. Herein, we present an improved method using matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) to determine the oligomeric content of SOA particles. As a solid-state method, MALDI-MS opens the door for analyzing directly impacted size-separated SOA, which allows tracking particulate oligomer formation or degradation as a function of particle size. We explore coupling this technique with single-particle Raman spectroscopy to obtain functional group information and inform SOA mixing state. We have applied the method to analyze SOA proxy standards and particles formed from reactive uptake of IEPOX onto acidic ammonium sulfate seed particles during atmospheric chamber experiments. We compare the oligomeric content of the particles over the course of aging by OH radical. Our work will provide insight about the formation of oligomers in the atmosphere, which will allow better modeling of their climatic impact.

Matrix-assisted laser desorption mass spectrometry (MALDI-MS) as a method for tracking oligomeric content in secondary organic aerosol

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