Volatility of Ammonium Nitrate in Ultra-viscous Aerosol Droplets by Optical Tweezers

The partitioning of volatile substances in atmospheric particulates is a hot topic in atmospheric science. Ammonium nitrate (NH₄NO₃) is an important inorganic component in atmospheric aerosols, which is a salt with relatively high vapor pressure. Particles containing NH₄NO₃ are equilibrium with gaseous NH₃ and HNO₃ and the partitioning between particle and gas is a strong function of temperature and relative humidity. Atmospheric aerosols have both natural and anthropogenic sources and consist of both organic and inorganic components, and many of them will likely form in semisolid, glassy, and high viscous state in the atmosphere, which show nonequilibrium kinetic characteristics at low relative humidity conditions. In this research, in order to understand the volatility of ammonium nitrate in ultra-viscous aerosol droplets, optical tweezers coupled with cavity-enhanced Raman spectroscopy were employed to observe the volatility of ammonium nitrate in the mixture of NH₄NO₃/MgSO₄ and NH₄NO₃/sucrose droplets. Optical tweezers-stimulated Raman spectroscopy, compared with other suspension techniques, can not only suspend droplets, but also obtain the chemical composition, structure of droplets and other information according to the conventional Raman scattering spectra of droplets. The radius and refractive index of droplets can be calculated according to stimulated Raman-Mie scattering resonance. The advantages of optical tweezers stimulated Raman spectroscopy are that particle radius can be accurately measured, chemical composition, phase and shape can be controlled, and long-term observation can be realized. Here, the radii and refractive indexes of the levitated droplets were determined in real time using the wavelength positions of stimulated Raman spectra, and the effective vapor pressures of ammonium nitrate at different relative humidity (RH) were obtained according to Maxwell equation. For the droplets with ammonium nitrate/sucrose molar ratio of 1:1, the effective vapor pressure of ammonium nitrate decreased with the decrease of RH. When the RH decreased from 70% to 20%, the effective vapor pressure of ammonium nitrate decreased from (3.577±0.82)×10^-5 to (6.55±1.36)×10^-6 Pa, compared to the vapor pressure of pure ammonium nitrate (1.67±0.24)×10^-3 to (6.64±0.3)×10^-3 Pa, the evaporation of ammonium nitrate in the mixed droplet was suppressed by sucrose, especially at low RH. For the mixed droplets with ammonium nitrate/magnesium sulfate molar ratio of 1:1, a similar phenomenon was observed. When the RH decreased from 70% to 40%, the effective vapor pressure of ammonium nitrate decreased from (4.38±0.21)×10^-3 to (8.13±2.34)×10^-5 Pa. The results showed that the effective saturated vapor pressures of ammonium nitrate in ultra-viscous droplets at low humidity were 1~3 orders lower than those of pure ammonium nitrate. Obviously, the volatilization of ammonium nitrate in droplets was inhibited at low relative humidity.