Many mass spectrometry methods using various ionization sources provide bulk composition of airborne particles, but little is known about the surface species that play a major role in determining their physicochemical properties that impact air quality, climate, and health. The present work shows that the composition of surface layers of atmospherically relevant submicron organic particles can be probed without the use of an external ionization source. Solid dicarboxylic acid particles are used as models, with glutaric acid being the most efficient at generating ions. Coating with small diacids or products from α-pinene ozonolysis demonstrates that ions are ejected from the surface, providing surface molecular characterization of organic particles on the fly. This unique approach provides a path forward for elucidating the role of the surface in determining chemical and physical properties of particles, including heterogeneous reactions, particle growth, water uptake, and interactions with biological systems.

Inlet ionization mass spectra were collected with a triple quadrupole mass spectrometer (Waters, Xevo TQ-S) with the ion source removed. Direct analysis in real time mass spectra were collected on the same mass spectrometer using a DART ion source (IonSense) equipped with the Vapur interface. Particle diameter measurements were made with a scanning mobility particle sizer (TSI, Inc.) using an electrostatic classifier model 3080, long differential mobility analyzer model 3081, and a condensation particle counter model 3776). Data were processed with MassLynx and Igor Pro (Wavemetrics).

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