Understanding of the chemical and physical properties of particles is an important scientific, engineering, and medical topic that is crucial to air quality, human health, and environmental chemistry. Of special interest are aerosol particles floating in the air for both in-door COVID-19 transmission and atmospheric chemistry.  The growth of bio- and organic-aerosol particles in the air is intimately correlated with chemical structures and their reactions in the gas phase, at aerosol particle surface, and in-particle phase. However, direct measurements of chemical structures at aerosol particle surfaces in the air are lacking. Here we develop a high-performance <i>in situ</i> surface-specific vibrational sum frequency scattering (VSFS) spectrometer to directly identify chemical structures of molecules at aerosol particle surfaces. We achieved few particles detection sensitivity of vibrational structures by with the high-performance technique that directly probing chemical species at surfaces of laboratory-generated aerosol particles in real time. Furthermore, our setup allowed us to simultaneous probe of hyperRaman scattering (HRS) spectra in particle phase. We examined polarized VSFS spectra of propanoic acid at aerosol particle surfaces and in particle bulk. The adsorption free energy of propanoic acid at the aerosol surfaces was found to be much lower than that at the air/water interface. Our approach opens a new avenue in revealing the role of surfaces in the formation of secondary organic aerosols in the atmosphere as well as chemical analysis of in-door COVID-19 aerosol particles.

<i>In situ</i> few particles detection of vibrational structures of organic species both at particle surfaces and in particle phase

ACS Spring 2022

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In situ few particles detection of vibrational structures of organic species both at particle surfaces and in particle phase