Direct measurements of chemical structures at aerosol particle surfaces in the air are lacking. This paper demonstrates in situ surface-specific vibrational sum frequency scattering (VSFS) to directly identify chemical structures of molecules at aerosol particle surfaces and simultaneously probe hyper-Raman scattering (HRS) spectra in the particle phase. Polarized VSFS spectra of propionic acid at aerosol particle surfaces and in the particle bulk were examined. The surface adsorption free energy of propionic acid onto aerosol particles was found to be less negative than that at the air/water interface. This challenges the hypothesis that molecular behaviors at the air/water interface are the same as those at aerosol particle surfaces. The 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.

Yuqin Qian, Jesse B. Brown, Zhi-Chao Huang-Fu, Tong Zhang, Hui Wang, ShanYi Wang, Jerry I. Dadap, Yi Rao