Study on the Water Transfer of Magnesium Acetate Aerosols Led by the Rapid and Slow Change of Relative Humidity

WANG Na, PANG Shu-feng, ZHANG Yun-hong

Institute of Chemical Physics, School of Chemistry, Beijing Institute of Technology, Beijing 100081, China

Study on the Water Transfer of Magnesium Acetate Aerosols Led by the Rapid and Slow Change of Relative Humidity

WANG Na, PANG Shu-feng*, ZHANG Yun-hong

Institute of Chemical Physics, School of Chemistry, Beijing Institute of Technology, Beijing 100081, China

A combination of vacuum FTIR spectrometer (Vertex 80v, Bruker, German) and novel relative humidity (RH) adjusting equipment,which provides the pressure by pure water vapor, is used to study the hygroscopicity of magnesium acetate (Mg(CH3COO)2) aerosols.The RH can change not only rapidly but also slowly by the RH adjusting equipment.Because the RH is decided by the pure vapor, the real-time RH can be gained by calculating the integrated intensity of a feature band of vapor in an IR spectrum.Such the synchronism between FTIR spectrum and RH canbe ensured.The high-quality spectra of aerosols are obtained and the water peak and feature peaks of Mg(CH3COO)2are analyzed during the slow and rapid RH changing process.The result shows that the areas of acetate ions and water decreases continuously at constant high RHs.After a slow cycle of RH (1.05×104minutes), the water area decreases from 1.5 to 1.1, which means that the water content decreases after a cycle of RH.This phenomenon is reported at first up to date.The detailed analysis suggests that the hydrolysis of Mg(CH3COO)2at high RH produces acetic acid, which was put out from the aerosols owing to the decrease of the pressure around the aerosols droplets.Furthermore, the dynamic hygroscopicity of Mg(CH3COO)2aerosols is studied by changing RH as a pulse mode.It reveals that there is only water transfer hysteresis and no water loss after a pulse (10 seconds) when the RH is above 70%.Compared to slow process, it can be concluded that the hydrolysis reaction rate is slower than that of a pulse RH.The water transfer limited on rapid process should rise from some species on the surface of aerosols.

Magnesium acetate aerosol; Water transfer process; Evaporation

Hygroscopicity of the aerosol, which is defined as the water uptake or loss with the change of relative humidity (RH), is a key factor to affect the size of the aerosols and the Earth’s radiative forcing[1].The most reports about hygroscopicity are the physical and chemical properties of aerosols under equilibrium state, which is the thermodynamic[2-6].While the other important problem is the velocity of mass transfer and the immediate species accompanying mass transfer, which is the kinetic[7-9].Usually, some observations in dynamic process cannot be found in the thermal stable state, which is important in predicting the physical and chemical properties of aerosols in real environment[10-13].

In this communication, the pressure of the sample cell is provided by a pure vapor, which was put to or from the surface of aerosols leading to the rise or decrease of the RH.The RH can change not only rapidly but also slowly.As the pressure of vapor changes, the FTIR spectra are collected synchronously by the rapid scan measurement mode.The RH values can be acquired by calculating the integrated area of a feature band of vapor in an IR spectrum.Figure 1 describes the RH adjusting system and the representative rates of RH change.By this pressure equipment combined to rapid scan FTIR technique, the rapid-change of RH can reach 19.24% per 0.36 second by a pulse mode [show in Figure 1(c)], while for slow change of RH, the velocity of RH change is only 0.7% per 274.5 second [shown in Figure 1(b)].By the rapid scan mode, a spectrum can be acquired every 0.1 second.

Fig.1 (a) Scheme of the RH adjusting system and the changing trend of RH as a function of experimental time in a slow process(b) and rapid process (c)

At first, the hygroscopicity of magnesium acetate aerosols is studied by a slow change of RH.The RH and water area as a function of time have been shown in Figure 2(a).It can be found that the water area is 1.5 at the first 80% RH, and decreases to 1.1 at the end 80% RH after a cycle of RH.It means that some water drop after 1.05×104seconds.Specially, the water area decreases at the terrace of RH above 60%.So the quantitative water and acetate ions are calculated by the integrated areas of their feature bands as the RH keeps constant.Figure 2(b) gives the calculated results at 90%, 80% and 70% RH, respectively.It shows that acetate ions and water content decreases continuously at constant RH.It is particularly obvious at 90% and 80%RH.This phenomenon is observed at first up to date.In order to confirm the result, we performed the other experiment, in which the RH was adjusted by the nitrogen/vapor mixed gas.However, the water content keeps constant at the same RH.The previous work[14-15]has reported that magnesium acetate aerosol entered the gel state below 60% RH.Above 70% RH, the aerosols are liquid state.In usual, water transfer keeps synchronous with the RH in the liquid state.The present continuous water-loss at high RH suggested the hydrolysis of Mg(CH3COO)2droplets.Mg(CH3COO)2is a salt composed of a weak acid and a weak base.So it is in equilibrium with hydrated cations and dissociated acetate/acetic acid in aqueous solution.Because the decrease in RH was achieved by vapor off the droplets in present experiment, the acetic acid can be blown away by the vacuum pump.The hydrolysis reaction can cost water, which leads to decrease of water continuously.

Fig.2 (a) The RH and water area in the Mg(CH3COO)2aerosols with the time (b) The water and acetate content change at the RH of 90%, 80% and 70% terraces

In order to understand the water-loss in detail at high RHs, another experiment has been performed, where the RH changes very quickly by three pulses, shown in Figure 3(a).Because the magnesium acetate aerosols are liquid above 70% RH, water transfer should be synchronous with RH.However, it can be found the hysteresis in humidification than dehumidification process.While water content keeps constant at the highest RH after three pulsed RH cycles.The results show that there are some species on the surface of magnesium acetate aerosols produced in a pulse RH cycle, which limit the water transfer in a short time[16].When the RH is raised, the species can dissolve, so the water content can reach the original value after several pulsed RHs.In addition, it also can be demonstrated that the time scale of hydrolysis reaction is longer than that spent by a pulse RH cycle (10 seconds).

As a conclusion, the hygroscopicity of magnesium acetate aerosols has been studied with the various RH.The results show that the hydrolysis reaction of magnesium acetate takes place and acetic acid evaporates owing to the subpressure of sample cell in a slow RH change process.Because the time of hydrolysis reaction is longer than a pulse (10 seconds), water-loss cannot be found after a pulse-RH change process.And the short-timed species are formed on the surface of magnesium acetate droplets which led to the hysteresis of water-uptake than water-loss in the pulsed rise of RH.

Fig.3 (a) The changing trends of relative humidity and relevant water content along with experimental time and (b) the water content of pulse 1(black), pulse 2(red), and pulse 3(blue) as a function of RH of magnesium acetate

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*通讯联系人

O657.3

A

利用不同的湿度调节方法研究醋酸镁气溶胶中水的传质过程

王 娜，庞树峰*，张韫宏

北京理工大学化学学院，化学物理研究所，北京 100081

利用一种自制的湿度调节设备与真空红外光谱仪相结合, 提出了一种研究醋酸镁气溶胶的吸湿性和传质动力学的新方法。湿度调节装置通过改变纯水汽压力来调节样品室的相对湿度，这样可以实现湿度以不同的速度发生变化。与红外光谱手段相结合，湿度缓慢变化，让气溶胶时刻处于准稳态过程，可以研究气溶胶在热力学稳态的吸湿性质。湿度脉冲式改变，可以研究气溶胶的动态吸湿性质以及传质动力学过程。红外光谱的扫描方式随着湿度的改变速度进行调节。由于实验中的相对湿度由纯水气提供，因此通过对红外方光谱中纯水气的特征吸收峰面积的定量计算获得与光谱同步的相对湿度值。研究发现，当湿度稳定在一个高湿度时，醋酸根和液态水的峰面积在持续下降。并且首次发现经过一个准稳态的湿度循环后(1.05×104s)，在80%RH条件下，水峰面积由1.5降低至1.1。通过改变实验方案并对其结果进行对比可知，在高湿度条件下，醋酸镁发生水解，生成的醋酸由于样品室内的负压而挥发，因此引起气溶胶含水量下降。采用脉冲发式快速改变样品室的湿度，一个湿度脉冲循环需要10 s。计算脉冲循环过程中气溶胶的含水量发现，在湿度高于70%RH时，气溶胶含水量没有减少。但是加湿过程的水传输比去湿过程的水传输快。这些现象说明醋酸镁的水解速率小于湿度的脉冲改变速率。在湿度快速变化的过程中，气溶胶液滴表面难溶物的形成引起水传输发生受阻现象。

醋酸镁气溶胶； 水传质动力学； 挥发

2015-02-04，

2015-06-11)

Foundation item：Supported by the NSFC (41175119, 21373026, 21473009)

10.3964/j.issn.1000-0593(2016)05-1581-04

Received：2015-02-04; accepted：2015-06-11

Biography：WANG Na, (1989—), female, graduate student of Beijing Institute of Technology e-mail: wangna.jz@163.com *Corresponding author e-mail: sfpang@bit.edu.cn