In situ analysis of the bulk and surface chemical compositions of organic aerosol particles

Abstract

Understanding the chemical and physical properties of particles is an important scientific, engineering, and medical issue that is crucial to air quality, human health, and environmental chemistry. Of special interest are aerosol particles floating in the air for both indoor virus transmission and outdoor 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 surfaces and in-particle phases. However, direct measurements of chemical structures at aerosol particle surfaces in the air are lacking. Here we demonstrate in situ surface-specific vibrational sum frequency scattering (VSFS) to directly identify chemical structures of molecules at aerosol particle surfaces. Furthermore, our setup allows us to simultaneously probe hyper-Raman scattering (HRS) spectra in the particle phase. We examined polarized VSFS spectra of propionic acid at aerosol particle surfaces and in particle bulk. More importantly, the surface adsorption free energy of propionic acid onto aerosol particles was found to be less negative than that at the air/water interface. These results challenge the long-standing hypothesis that molecular behaviors at the air/water interface are the same as those at aerosol particle surfaces. Our approach opens a new avenue in revealing surface compositions and chemical aging in the formation of secondary organic aerosols in the atmosphere as well as chemical analysis of indoor and outdoor viral aerosol particles.

Estimates of Future New Particle Formation under Different Emission Scenarios in Beijing

“…The structure, mass transport kinetics, and heterogeneous chemical reactions at the surface of particles will be distinct from those in the bulk. 39 , 40 The highly oxygenated organic content of new particles in Beijing results in a high hygroscopicity, 41 and thus, they will have a liquid water interface at the surface. Soluble organic material will dissolve in this surface more readily than in other particles, 42 , 43 with subsequent reactions producing highly oxygenated material in the particle phase, 43 which will itself be hygroscopic.…”

Emerging investigator series: surfactants, films, and coatings on atmospheric aerosol particles: a review

“…Notably, most of the work described above focuses on the air-water interface. Rao and co-workers [105][106][107][108][109] introduced nonlinear optical spectroscopy specically for the air-particle interface. Overcoming the challenges of low signal and small particle size, they pioneered second harmonic scattering (SHS) to determine the surface density and adsorption free energy of small organic molecules, including crystal violet and trans-4-[4-(dibutylamino)styryl]-1-methylpyridinium iodide, at the surface of $100 nm NaCl particles.…”

“…108 The authors attributed the difference to surface curvature. In a follow-up study with aerosol particles containing the more environmentally-relevant species propionic acid, Qian et al 109 combined VSFS to probe the particle surface with hyper-Raman scattering to probe the particle bulk. The surface adsorption free energy of propionic acid was signicantly more negative at the air-water interface (À15.38 AE 0.11 kJ mol À1 ) than at the particle surface (À12.69 AE 0.28 kJ mol À1 ).…”

A Molecular Dynamic Study of the Effects of Surface Partitioning on the OH Radical Interactions with Solutes in Multicomponent Aqueous Aerosols

“…Surfactants at the air–water interface orient themselves with the hydrophobic chain toward the gas phase, thus forming an outer molecular layer at high concentrations . Preferred molecular orientation near the air–water interface is also observed using surface-specific electronic sum frequency scattering (SFS) for smaller organic solutes such as alcohols, hexafluoro-2-propanol, and more recently for malachite green and propionic acid. , Molecular orientation is known to have an effect on photochemical activity , and is also likely to affect reactivity by making certain organic functional groups more or less available to surface reactive species. Although the orientation of molecules at the air–water surface may appear to be intuitive, the extent of the molecular alignment and the size of the interfacial area have been investigated only for a limited number of solutes, and their effects on reactivity remain mostly unquantified.…”

“…Aqueous aerosols are known to control cloud nucleation − and to affect the climate, air quality, and human health. − A complete understanding of their role in atmospheric phenomena remains challenging as the properties of nanometric aqueous particles differ greatly from those of a bulk solution. − The chemical evolution of atmospheric aerosols as well as their ability to initiate cloud formation is mostly controlled by the properties of the gas–water interface. , Phenomena such as microconfinement, high surface electric field, preferred molecular orientation, ,,, and lower water densities at the interface , affect reaction rates , and photochemical processes, thus enhancing certain reaction channels while suppressing others. − Because atmospheric aerosols contain a wide range of solutes, it has become important to improve our understanding of how chemical composition changes surface reactivity …”