光镊技术观测悬浮丙二酸/硝酸钠液滴中丙二酸的分配过程

Observation of the Repartitioning of Malonic Acid in Levitated Malonic Acid/NaNO3/H2O Droplets by Optical Tweezers

半挥发性有机物(SVOCs)在颗粒间的重新分配过程,对于理解大气颗粒物的生长和沉降具有重要意义。光镊-受激拉曼光谱技术,相比于其他悬浮技术,不但可以悬浮液滴,而且能获得液滴的常规拉曼散射光谱,得到液滴的化学组成和结构等信息,根据受激拉曼米氏散射共振可以计算出液滴的半径和折射率随环境的变化。光镊受激拉曼光谱技术的优势,体现在颗粒半径可以精确测量,化学组成、相态和形态可控,并可实现长时间观测。采用光镊-受激拉曼光谱技术,观测了不同摩尔比(OIR)的丙二酸/硝酸钠的悬浮液滴,与样品池内壁沉积的颗粒中丙二酸的重新分配过程。发现当OIR为1∶1时,光镊悬浮的液滴和样品池沉积的液滴在整个观测相对湿度(RH)内均未有硝酸钠晶体析出,在恒定RH下,悬浮液滴的半径随着丙二酸的蒸发而缓慢减小。当OIR为1∶2及1∶3,即硝酸钠的含量较多时,在RH分别低于52.5%,58%的条件下,悬浮液滴的半径在恒定RH下并没有减小反而逐渐增加,这表明丙二酸在悬浮液滴与周围样品池沉积的液滴之间发生了重新分配,归因于低RH下,样品池表面沉积的液滴中,硝酸钠经历了异相成核而出现了结晶,而悬浮液滴始终保持溶液状态。因此样品池表面沉积颗粒物的丙二酸蒸汽压,远大于没有结晶的悬浮液滴中丙二酸的蒸汽压,从而使得丙二酸由样品池内壁上的颗粒物挥发转移至悬浮液滴中,使得悬浮液滴的半径增大。这对于解释SVOC在外混的不同相态颗粒间的重新分配过程是一个很好的模型。

Exploring the repartitioning process of semi-volatile organic compounds(SVOCs)between particles is important for understanding the growth and shrinkage of atmospheric particulates.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.In this study,optical tweezers coupled with cavity-enhanced Raman spectroscopy was used to observe the repartitioning of malonic acid between a levitated malonic acid/NaNO 3/H 2O droplet and surrounding malonic acid/NaNO 3/H 2O droplets deposited on the inner wall of a chamber with different organic to inorganic molar ratios(OIRs).It was found that when the OIR was 1∶1,no NaNO 3 crystallization occurred both for levitated and deposited dropletsin the whole relative humidity(RH)range,the radius of the levitated droplet decreased slowly due to the evaporation of malonic acid from the levitated droplet at constant RHs.When the OIRs were 1∶2 and 1∶3,the radius of the suspension droplets did not decrease but increased gradually when the RH was lower than 52.5%and 58%respectively.This indicated that the repartitioning of malonic acid occurred.The reason was that NaNO 3 in the deposited droplets was crystallized when RH was lower than 52.5%for 1∶2 or 58%for 1∶3.So the vaporpressure of malonic acid at the surface of deposited droplets was higher than that of the levitated droplet which always remained as liquid droplet without NaNO 3 crystallization,resulting in the transfer of malonic acid from the deposited ones to the levitated one.This is a good model for explaining the repartitioning process of SVOC among different phase particles states.