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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Kinetic study of heterogeneous reaction of deliquesced NaCl particles with gaseous HNO3 using particle-on-substrate stagnation flow reactor approach
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Kinetic study of heterogeneous reaction of deliquesced NaCl particles with gaseous HNO3 using particle-on-substrate stagnation flow reactor approach

机译：衬底上停滞流反应器法研究潮解性NaCl颗粒与HNO3气态反应的动力学

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摘要

Heterogeneous reaction kinetics of gaseous nitric acid with deliquesced sodium chloride particles NaCl(aq) + HNO3(g) -> NaNO3(aq) + HCl(g) were investigated with a novel particle-on-substrate stagnation flow reactor (PS-SFR) approach under conditions, including particle size, relative humidity, and reaction time, directly relevant to the atmospheric chemistry of sea salt particles. Particles deposited onto an electron microscopy grid substrate were exposed to the reacting gas at atmospheric pressure and room temperature by impingement via a stagnation flow inside the reactor. The reactor design and choice of flow parameters were guided by computational fluid dynamics to ensure uniformity of the diffusion flux to all particles undergoing reaction. The reaction kinetics was followed by observing chloride depletion in the particles by computer-controlled scanning electron microscopy with energy-dispersive X-ray analysis (CCSEM/EDX). The validity of the current approach was examined first by conducting experiments with median dry particle diameter (D) over bar (P) = 0.82 mu m, 80% relative humidity, particle loading densities 4 x 10(4) <= N-s <= 7 x 10(6) cm(-2) and free stream HNO3 concentrations 2, 7, and 22 ppb. Upon deliquescence the droplet diameter (D) over bar (d) approximately doubles. The apparent, pseudo-first-order rate constant determined in these experiments varied with particle loading and HNO3 concentration in a manner consistent with a diffusion-kinetic analysis reported earlier (Laskin, A.; Wang. H.; Robertson, W. H.; Cowin, J. P.; Ezell, M. J.; Finlayson-Pitts, B. J. J. Phys. Chem. A 2006, 110 10619). The intrinsic, second-order rate constant was obtained as k(II) = 5.7 x 10(-15) cm(3) molecule(-1) s(-1) in the limit of zero particle loading and by assuming that the substrate is inert to HNO3. Under this loading condition the experimental, net reaction uptake coefficient was found to be gamma(net),,, = 0. 11 with an uncertainty factor of 3. Additional experiments examined the variations of HNO3 uptake on pure NaCl, a sea salt-like mixture of NaCl and MgCl2, (Mg-to-Cl molar ratio of 0. 114) and real sea salt particles as a function of relative humidity. Results show behavior of the uptake coefficient to be similar for all three types of salt particles with (D) over bar (P) similar to 0.9 pm over the relative humidity range 20-80%. Gaseous HNO3 uptake coefficient peaks around a relative humidity of 55%, with gamma(net) well over 0.2 for sea salt. Below the efflorescence relative humidity the uptake coefficient declines with decreasing RH for all three sea salt types, and it does so without exhibiting a sudden shutoff of reactivity. The uptake of HNO3 on sea salt particles was more rapid than that on the mixture of NaCl and MgCl2 and uptake on both sea salt and sea salt-like mixture was faster than on pure NaCl. The uptake of HNO3 on deliquesced, pure NaCl particles was also examined over the particle size range of 0.57 <= (D) over bar (p) <= 1.7 mu m (1.1 <= (D) over bar (d) <= 3.4 mu m) under a constant relative humidity of 80%. The uptake coefficient decreases monotonically with an increase in particle size. Application of a resistance model of reaction kinetics and reactant diffusion over a single particle suggests that, over the range of paticle size studied, the uptake is largely controlled by gaseous reactant diffusion from the free stream to the particle surface. In addition, a combined consideration of uptake coefficients obtained in the present study and those previously reported for substantially smaller droplets ((D) over bar (d) similar to 0.1 mu m) (Saul, T. D.; Tolocka, M. P.; Johnston, M. V. J. Phys. Chem. A 2006, 110, 7614) suggests that the peak reactivity occurs at a droplet diameter of similar to 0.7 mu m, which is immediately below the size at which sea salt aerosols begin to notably contribute to light scattering.
机译：用新型的基体上滞流器研究了气态硝酸与潮解氯化钠颗粒NaCl（aq）+ HNO3（g）-> NaNO3（aq）+ HCl（g）的非均相反应动力学在与粒径，相对湿度和反应时间有关的条件下，该方法与海盐颗粒的大气化学直接相关。通过在反应器内的停滞流撞击，使沉积在电子显微镜栅格基板上的颗粒在大气压和室温下暴露于反应气体。反应堆的设计和流动参数的选择受计算流体动力学的指导，以确保扩散通量对所有反应颗粒均一。反应动力学之后，通过具有能量色散X射线分析（CCSEM / EDX）的计算机控制的扫描电子显微镜观察颗粒中氯化物的消耗。首先通过进行实验来检验当前方法的有效性，该实验的中值是干粉直径（D）超过bar（P）= 0.82微米，相对湿度为80％，颗粒装载密度4 x 10（4）<= Ns <= 7 x 10（6）cm（-2）和自由流HNO3浓度2、7和22 ppb。潮解后，条（d）上的液滴直径（D）大约翻倍。在这些实验中确定的表观伪一级速率常数随颗粒载荷和HNO3浓度的变化而变化，其方式与先前报道的扩散动力学分析一致（Laskin，A .; Wang。H .; Robertson，WH; Cowin， JP; Ezell，MJ; Finlayson-Pitts，BJJ Phys.Chem.A 2006，110 10619）。在零颗粒负载的限制下并假设基底为k（II）= 5.7 x 10（-15）cm（3）分子（-1）s（-1），从而获得了固有的二级速率常数。对HNO3呈惰性。在这种加载条件下，实验的净反应吸收系数为gamma（net），= 0. 11，不确定因素为3。其他实验检查了纯NaCl（一种海盐样）对HNO3吸收的变化。 NaCl和MgCl2的混合物（Mg与Cl的摩尔比为0. 114）和真实的海盐颗粒随相对湿度的变化而变化。结果表明，在相对湿度20-80％范围内，所有三种类型的盐颗粒的吸收系数的行为都相似，其中（D）超过bar（P）接近0.9 pm。气态HNO3的吸收系数在相对湿度为55％时达到峰值，而海盐的γ（净值）远大于0.2。在风化相对湿度以下，所有三种海盐类型的吸收系数均随RH的降低而降低，并且这样做不会表现出突然的反应性关闭。海盐颗粒上HNO3的吸收要比NaCl和MgCl2混合物上的要快，海盐和类海盐混合物上的HNO3的吸收要比纯NaCl快。还检查了在0.57 <=（D）超过条形（p）<= 1.7微米（1.1 <=（D）超过条形（d）<= 3.4）的粒径范围内，潮解的纯NaCl颗粒上HNO3的吸收μm）在80％的恒定相对湿度下。吸收系数随着粒径的增加而单调降低。反应动力学和反应物在单个颗粒上扩散的阻力模型的应用表明，在所研究的颗粒尺寸范围内，吸收率很大程度上受气态反应物从自由流向颗粒表面的扩散控制。此外，综合考虑了本研究中获得的吸收系数和先前报道的小得多的液滴（（D）超过条形图（d），类似于0.1微米）的吸收系数（Saul，TD; Tolocka，MP; Johnston，MVJ Phys） Chem。A 2006，110，7614）表明峰反应性发生在类似于0.7μm的液滴直径处，该液滴直径直接低于海盐气溶胶开始显着促进光散射的大小。

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