Atmospheric aerosol particles are a major air pollutant in urban cities. To reduce and prevent the generation of aerosols, the first step would be understanding their origin and generation mechanism, then finding a feasible method to interrupt it in a cost-effective way. In many cases, aerosols reveal themselves as a collection of particles. The collective properties of aerosols can be considered as the sum of the individual properties of comprising particles. Some bulk-only characterization techniques, such as optical methods and chromatography, provide information of sizes, shapes and chemical compositions of aerosol particles. However, when it comes to individual particles, other complimentary techniques have to be applied. For example, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and time-of-flight mass spectrometer (ToFMS) have been utilized to analyze topography and chemical compositions of aerosol particles, but all of them require a high vacuum testing environment. Based on the comparisons above, we can see that current technical approaches lack the ability to non-invasively characterize individual aerosol particles with high spatial resolution under ambient environment. Herein we report a new approach for aerosol particles characterization via a type of scanning probe microscopy, the peak force infrared (PFIR) microscopy that provides simultaneous infrared and mechanical responses at sub 10 nm spatial resolution. PFIR microscopy allows infrared nano-imaging at a spatial resolution of 10 nm, mechanical information mapping through peak force tapping (an operation mode of atomic force microscope from Bruker), and collection of infrared spectra at individual sample locations. Modulus mapping shows how rigid the sample is, and the value of adhesion shows how sticky the sample is towards AFM probe, thus revealing the heterogeneity in a cluster of aerosol particles. By collecting infrared spectra at multiple locations of one particle, we can identify different components in one intra-particle. For example, if different types of C=O are observed in a globular aerosol particle, we can deduce that oxidized hydrocarbons likely play a role in the nucleation of organic aerosols. Besides organic compositions, urban aerosols also contain inorganic species, such as (NH4)2SO4 from the oxidation of SO2 by NO2. In this presentation we will include the application of PFIR on characterizing inorganic compositions as well.

Nanoscale spectroscopic and mechanical characterization of individual aerosol particles with peak force infrared microscopy

256th National Meeting (2018)

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