Environmental Interfaces:

Gas-Aerosol Particle Interface and The Air/Water Interface

Background & Motivation

Sunlight is the primary driving force of gas-phase atmospheric chemistry, but the importance of photochemistry occurring in atmospheric aerosol particles or at the gas-aerosol interface is only beginning to be recognized. The gas–aerosol particle interface is believed to contribute to the growth of secondary organic aerosols in the atmosphere. Despite its importance, the chemical composition of the interface has not been probed directly because of a lack of suitable interface-specific analytical techniques. The growth of aerosol particles is intimately related to chemical reactions in the gas phase and particle phase and at gas–aerosol particle interfaces. While chemical reactions in gas and particle phases are well documented, there is very little information regarding interface-related reactions. The interface of gas–aerosol particles not only facilitates a physical channel for organic species to enter and exit but also provides a necessary lane for culturing chemical reactions. This is mainly due to the fact that there is a lack of suitable in situ interface-sensitive analytical techniques for direct measurements of interfacial properties. The motivation behind this research is to understand how interfaces play a role in the growth of aerosol particles.

Experimental Methods

  • Development of in situ second harmonic scattering (SHS) for direct measurements of molecules of aerosol surfaces and interfaces

  • Development of in situ electronic sum frequency scattering (ESFS) for direct measurements of electronic structures of molecular chromophores at aerosol surfaces and interfaces

  • Development of in situ vibrational sum frequency scattering (VSFS) for direct measurements of vibrational structures of compositions at aerosol surfaces and interfaces

Main Findings

  • We examined interfacial behaviors of aerosol particle constituent molecules under different relative humidity (RH) and salt concentrations. RH not only varies the concentration of solutes inside aerosol particles, but also changes interfacial hydration in local regions. Organic molecules were found to exhibit unique behaviors at the gas-aerosol particle interface relative to the bulk of NaCl particles under different RH conditions. My quantitative analyses showed that the interfacial adsorption free energies remain unchanged and interfacial area increases with relative humidity.

  •  in situ ESFS spectroscopy has enabled us to identify chemical species, molecular configuration, and adsorption behaviors of organic chromophores of aerosol particles in real time. It was found that the aerosol particle surface is more hydrophobic in nature than the underlying particle bulk. Simultaneously, it was shown that the population of organic species at the aerosol surface exhibits different behaviors from the planar air/water interface.

  • We have identified chemical structures of organic species at aerosol surfaces and in bulk phase with VSFS and hyper-Raman scattering (HRS) spectroscopies. It was found that organic molecules are oriented in an ordered fashion at aerosol surfaces. Furthermore, the surface adsorption free energy of organic acids at aerosol surfaces is less negative than that at the air/water interface. This result challenges the long-standing hypothesis that molecular behaviors at the planar air/water interface are the same as those at aerosol particle surfaces.

  •  The air/water interface is a model system. We have examined orientational dynamics of coumarin 153 at the interface. Absolute orientational dynamics in three dimension was mapped with time-resolved sum frequency generation. These results help us understand steric selectivity of chemical reactions at the interface.

  • Interactions of electronic and vibrational degrees of freedom, namely vibronic coupling, is key to understanding photoinduced processes at interfaces. We have investigated vibronic coupling of photoinduced excited states at the air/water interface with our own two-dimensional electronic-vibrational sum frequency generation (2D-EVSFG). It was found that vibronic transitions include Franck-Condon excitation as well as Hertberg-Taylor coupling. Time-dependent 2D-EVSFG experiments have enabled us to reveal intramolecular charge transfer at interfaces. 

Significance and Outlook

Our methods described above open up a new avenue for directly probing surface compositions and chemical aging in the formation of secondary organic aerosols in the atmosphere, as well as providing chemical identifications and analysis of indoor and outdoor viral aerosol particles.


Further research is underway by our group to investigate the droplet size and viscosity dependence of interfacial partitioning, as well as reaction and adsorption rates at droplet surfaces to elucidate the chemical and physical properties of such a unique and important system.

Representative Publications

  • Yuqin Qian, Jesse B. Brown, Tong Zhang, Zhi-Chao Huang-Fu, and Yi Rao*, In Situ Detection of Chemical Compositions at Nanodroplet Surfaces and In-Nanodroplet Phases, The Journal of Physical Chemistry A, 2022 (Just Accepted).

  • Yuqin Qian, Jesse B Brown, Zhi-Chao Huang-Fu, Tong Zhang, Hui Wang, ShanYi Wang, Jerry I Dadap, Yi Rao*, In situ analysis of the bulk and surface chemical compositions of organic aerosol particles, Communications Chemistry, 2022, 5(1), 1-7.

  • Gang-Hua Deng, Yuqin Qian, Tong Zhang, Jian Han, Hanning Chen, Yi Rao*, Two-dimensional electronic–vibrational sum frequency spectroscopy for interactions of electronic and nuclear motions at interfaces, Proceeding of the National Academy of Sciences, 2021, 118 (34), e2100608118.

  • Yuqin Qian, Ganghua Deng, Yi Rao*, In-Situ Spectroscopic Probing of Polarity and Molecular Configuration at Aerosol Particle Surfaces. The Journal of Physical Chemistry Letters, 2020, 11, 6763−6771.


  • Yuqin Qian, Ganghua Deng, Jordan Lapp, Yi Rao*, Interfaces of Gas–Aerosol Particles: Relative Humidity and Salt Concentration Effects, The Journal of Physical Chemistry A, 2019, 123 (29), 6304-6312.


  • Yi Rao*, Yuqin Qian, Ganghua Deng, Ashlie Kinross, Nicholas J. Turro, Kenneth B. Eisenthal*, Molecular rotation in 3 dimensions at an air/water interface using femtosecond time resolved sum frequency generation, The Journal of Chemical Physics, 2019, 150 (9), 094709.


  • Yuqin Qian, Ganghua Deng, and Yi Rao*, In Situ Chemical Analysis of Gas-Aerosol Particle Interface, Analytical Chemistry, 2018, 90 (18), 10967-10973.