光镊技术研究硝酸铵在超粘气溶胶中的挥发性

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

大气颗粒物中挥发性物质的气粒分配问题是大气科学研究的热点.选择典型的高粘度态模型体系、硝酸铵/蔗糖体系以及硝酸铵/硫酸镁体系,利用光镊-受激拉曼光谱技术原位获得液滴的自发拉曼和受激拉曼信号,同时观察回音壁(WGM)模式,利用米氏散射理论对一系列的WGMs峰位在给定范围内的粒子半径和折射率进行模拟计算,通过Maxwell方程精确计算了两个体系中硝酸铵在不同相对湿度(RH)下的有效饱和蒸汽压值,结果表明,在低湿度下的超粘态液滴中硝酸铵的有效饱和蒸汽压比纯硝酸铵的饱和蒸汽压低1~3个数量级.显然,低相对湿度下,液滴中硝酸铵的挥发受到了抑制.

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.