Atomic Force Microscopy-Infrared Spectroscopy of Individual Atmospheric Aerosol Particles: Subdiffraction Limit Vibrational Spectroscopy and Morphological Analysis

Abstract

Chemical analysis of atmospheric aerosols is an analytical challenge, as aerosol particles are complex chemical mixtures that can contain hundreds to thousands of species in attoliter volumes at the most abundant sizes in the atmosphere (∼100 nm). These particles have global impacts on climate and health, but there are few methods available that combine imaging and the detailed molecular information from vibrational spectroscopy for individual particles <500 nm. Herein, we show the first application of atomic force microscopy with infrared spectroscopy (AFM-IR) to detect trace organic and inorganic species and probe intraparticle chemical variation in individual particles down to 150 nm. By detecting photothermal expansion at frequencies where particle species absorb IR photons from a tunable laser, AFM-IR can study particles smaller than the optical diffraction limit. Combining strengths of AFM (ambient pressure, height, morphology, and phase measurements) with photothermal IR spectroscopy, the potential of AFM-IR is shown for a diverse set of single-component particles, liquid-liquid phase separated particles (core-shell morphology), and ambient atmospheric particles. The spectra from atmospheric model systems (ammonium sulfate, sodium nitrate, succinic acid, and sucrose) had clearly identifiable features that correlate with absorption frequencies for infrared-active modes. Additionally, molecular information was obtained with <100 nm spatial resolution for phase separated particles with a ∼150 nm shell and 300 nm core. The subdiffraction limit capability of AFM-IR has the potential to advance understanding of particle impacts on climate and health by improving analytical capabilities to study water uptake, heterogeneous reactivity, and viscosity.

Microscopic Observations of Core‐Shell Particle Structure and Implications for Atmospheric Aerosol Remote Sensing

“…The following computational scenarios are presented: Scenario 1, Δ shell /r total is a continuous function of the total radius (Δ shell is size resolved), as defined by equation (2); Scenario 2, Δ shell /r total is a fixed mean value for all sizes of particles (i.e., 0.44 for urban case and 0.32 for desert case); and Scenario 3, Δ shell / r total is discretized in two mean values, one for fine and one for coarse mode particles, as presented in section 3.4. It should be mentioned that some studies (Bondy, Kirpes, et al, 2017;Grassian & Tivanski, 2018) showed that particles can be also coated by much thin thickness of shell than the ones assumed 8. Histograms of axis ratio of particles with halo (core part only, denoted as particle cores) and particles without halo (denoted as not coated particles) for (a) urban and (b) desert cases.…”

Aerosol Mixing State: Measurements, Modeling, and Impacts

“…More recently, AFM studies have incorporated more detailed analysis of specific properties, such as surface tension (Lee et al, , ) and hygroscopicity (Ghorai et al, ; Morris et al, ). An exciting advancement for AFM has been coupling with a scanning IR laser to conduct photothermal IR spectroscopy (Bondy et al, ; Kwon et al, ; Or et al, ). The operating principle of AFM‐IR is that when the tunable IR laser interacts with the sample at a frequency where it absorbs, the sample expands slightly causing the oscillation of the cantilever to change, and that signal can be converted back to an IR spectrum.…”

Physicochemical Characteristics of Individual Aerosol Particles during the 2015 China Victory Day Parade in Beijing

“…For example, various "bulk" sample analyses conducted in the laboratory have confirmed the liquid-liquid phase separation do exist for the secondary particles [41,46,47]. The presence of the organic core and inorganic shell in individual particles have been confirmed in the laboratory by using optical microscopy and Raman spectroscopy [38,41,42], atomic force microscopy with infrared spectroscopy [48], and environmental scanning electron microscopy and scanning transmission X-ray microscopy [49].…”