Understanding the chemical and physical properties of aerosol particles is crucial for air quality, human health, and environmental chemistry, particularly concerning indoor viral aerosol transmission. The growth of bio- and organic aerosols is intricately linked to chemical structures and reactions in gas, particle, and interfacial phases. However, direct measurements of surface chemical structures in airborne aerosol particles have been limited. Atmospheric particles engage in various heterogeneous and multiphase processes, including surface organic processing by gas-phase oxidants, hygroscopic water uptake, and cloud droplet activation. Aerosol surfaces mediate these interactions, and their properties (structure, mass transport kinetics, molecular dynamics, and heterogeneous chemical reactions) differ significantly from the bulk. For instance, surface pH influences heterogeneous chemistry, optical properties, and ice/cloud nucleation. While studies suggest organic materials tend to accumulate at aerosol surfaces, altering composition, surface chemistry, and physical properties, direct investigation of surface chemistry in submicron aerosols has been lacking, relying instead on indirect observations and modeling. Previous studies primarily involved ex situ measurements of collected particles. Recent in situ attempts using second harmonic scattering (SHS) and electronic sum frequency scattering (ESFS) provided evidence of organic molecules at aerosol surfaces, but lacked chemical specificity. This research aims to directly identify organic species and their surface properties in real-time using vibrational sum frequency scattering (VSFS) spectroscopy, comparing adsorption of organic species to both planar and spherical surfaces.

The literature extensively covers the importance of understanding aerosol particle properties for various scientific and health-related reasons. Several studies highlight the role of heterogeneous chemistry at the air-aqueous interface in atmospheric processes, focusing on the impact of organic films. The influence of these films on the reactions at the surface has been recognized. Other research explores the evolution of organic aerosols in the atmosphere. Significant advancements have been made in heterogeneous atmospheric aerosol chemistry through laboratory studies of water droplets. Previous studies of aerosol surfaces largely relied on ex situ measurements of field-collected samples. The development of in situ techniques like SHS and ESFS has provided insights into molecular behaviors at aerosol surfaces, offering data on organic molecule presence, surface populations, polarity, and configurations. However, these techniques lack the chemical specificity necessary to fully identify the surface species. This gap motivated the current study’s development of a VSFS method.

The study employed a novel experimental setup combining vibrational sum frequency scattering (VSFS) and hyper-Raman scattering (HRS) spectroscopy for in situ analysis of aerosol particles. A femtosecond amplifier laser system, pumping an optical parametric amplifier, generated a tunable mid-IR light beam (2500-4500 nm). A spectrally narrow picosecond pulse (1025 nm) was obtained using an etalon to filter the broadband femtosecond 1025 nm laser. The picosecond (1025 nm) and broadband IR light beams were non-collinearly incident on the aerosol samples. VSFS signals were collected at a wide angle (2θ ~60 degrees) and focused onto a spectrometer with a CCD detector. HRS signals were simultaneously collected at 0° using a second spectrometer and CCD detector. Laboratory-generated aerosol particles were produced using a constant output atomizer from a seed solution (0.5 M NaCl) with varying concentrations of propionic acid. Particle density was controlled by diluting the aerosol stream with humidified N2 gas. Polarized VSFS spectra (HHH, VVH, VHV, HVV) were measured to determine molecular orientations. Concentration-dependent experiments were conducted to study adsorption isotherms for propionic acid at both aerosol surfaces and the air/water interface, comparing VSFS and VSFG intensities (for air/water interface). The Langmuir model was applied to quantitatively assess surface adsorption ability. Similar experiments were performed using butanoic acid to investigate the effect of chain length on adsorption free energy. For the air-water interface measurements, a separate femtosecond laser amplifier system with an OPA was used to generate 3250 nm IR and 800 nm pulses using a reflection geometry.

The VSFS technique successfully identified chemical structures of propionic acid at the surfaces of aerosol particles. Three main peaks in the HHH-polarized VSFS spectrum at 2887.3, 2950.9, and 2991.8 cm⁻¹ were assigned to the symmetric stretching mode of -CH3 (CH3-ss), Fermi resonance, and the asymmetric stretching mode of -CH3 (CH3-as), respectively. Simultaneous HRS measurements from the same particles showed corresponding peaks but with different relative intensities, indicating different orientations of molecules in the bulk phase. Concentration-dependent experiments showed that VSFS intensity is non-linearly related to bulk propionic acid concentration, further confirming surface specificity. However, VSFS intensity was linearly proportional to particle density, confirming the surface origin of the signal. Polarized VSFS spectra revealed preferential orientations of propionic acid molecules at the aerosol surface; only HHH and VVH polarizations showed signals, indicating molecular ordering. Quantitative analysis using the Langmuir model compared surface adsorption free energy of propionic acid at the aerosol surface (-12.69 ± 0.28 kJ mol⁻¹) and the air/water interface (-15.38 ± 0.11 kJ mol⁻¹), showing lower adsorption ability at the curved particle surface. Similar experiments with butanoic acid yielded a more negative adsorption free energy (-15.96 ± 0.24 kJ mol⁻¹), demonstrating an increase in adsorption strength with increasing carbon chain length.

The results directly demonstrate the utility of in situ VSFS for characterizing the chemical composition and molecular orientations of organic molecules at aerosol surfaces. The observation that the adsorption free energy of propionic acid is less negative at the aerosol surface compared to the air/water interface directly challenges the assumption that these two interfaces are equivalent. This is a significant finding, as many atmospheric models have used data from the air/water interface to represent aerosol surface behavior. The differences in adsorption behavior might arise from the curvature of the aerosol particles, altering molecular interactions. The non-linear relationship between VSFS intensity and propionic acid concentration further emphasizes the importance of surface-specific analysis in understanding aerosol chemistry. The observed molecular orientation provides crucial information for predicting the rates and yields of chemical reactions at aerosol particle surfaces. This work highlights the need to revisit previous atmospheric chemistry modeling that relied solely on data from planar surfaces.

This study successfully demonstrated the in situ analysis of chemical structures at aerosol surfaces using VSFS spectroscopy. Simultaneous HRS analysis provided complementary bulk information. Key findings include the observation of preferential molecular orientation at the surface, and the surprisingly lower adsorption free energy of propionic acid on aerosol particles compared to the air/water interface. This work challenges established assumptions in atmospheric chemistry modeling and highlights the importance of surface-specific analysis. Future research should focus on expanding the technique to study a broader range of organic species and atmospheric conditions, investigating the influence of other factors like relative humidity and particle size, and developing quantitative models to explain the observed orientational behavior. The insights gained from this research could greatly improve our understanding of secondary organic aerosol formation and impact atmospheric chemistry models.

While this study provides valuable insights into the chemical composition and molecular orientations of organic molecules on aerosol surfaces, there are several limitations. The quantitative analysis of molecular orientation using VSFS was challenging. The theoretical framework used did not fully account for refractive index mismatching and scattering effects from small particles, which may have affected the accuracy of orientation estimations. The study focused primarily on propionic and butanoic acids; further studies are needed to expand the range of molecules studied and to investigate the effects of different functional groups. The use of laboratory-generated aerosols, while providing controlled conditions, may not perfectly represent the complexity of real-world atmospheric aerosols.