Surface Enhanced Optical- and Atomic Force Microscope- Photothermal Infrared and Raman Microspectroscopy Enables Observation of Individual Ultrafine Aerosol Particles

YAO XIAO, Ziying Lei, Andrew Ault, University of Michigan

     Abstract Number: 590
     Working Group: Instrumentation and Methods

Abstract
Atmospheric aerosol particles have a large impact on climate by scattering solar radiation and nucleating cloud droplets and ice crystals. These climate impacts are driven by single particle physicochemical properties and non-destructively characterizing individual particles spectroscopically <1 μm in diameter remains challenging. In previous studies, our group has successfully characterized submicron particles using Optical Photothermal Infrared Spectroscopy (O-PTIR), Raman microspectroscopy, and Atomic Force Microscope - Photothermal Infrared Spectroscopy (AFM-PTIR) to characterize aerosol particle under ambient temperature and pressure. However, prior PTIR measurements of individual particles still struggled with detecting key species within submicron (< 1 µm) for O-PTIR/Raman and ultrafine (< 0.1 µm) for AFM-PTIR. Herein, we apply surface enhanced IR spectroscopy (SEIRS) to detect trace organic and/or inorganic species in particles below the diameters previously measured by these instruments, respectively. Gold substrates were used to enhance the signal due to enhanced reflection and plasmon resonances. Organic and inorganic functional groups were characterized in laboratory-generated and ambient aerosol particles with aerodynamic diameters < 500 nm and 20 nm for O-PTIR/Raman and AFM-PTIR, respectively. Spectral mapping at a specific wavenumber was also used to characterize SEIRS spatial resolution. The results showing enhanced detection limits and spatial scales for detecting key species in aerosol particles with SEIRS demonstrate improved chemical characterization of aerosol particles below the traditional size range of infrared microscopy.