Saul Thomas D, Tolocka Michael P, Johnston Murray V
Department of Chemistry and Biochemistry, University of Delaware, Newark, 19716, USA.
J Phys Chem A. 2006 Jun 22;110(24):7614-20. doi: 10.1021/jp060639a.
Reactive uptake coefficients for nitric acid onto size-selected (d(ve) = 102 and 233 nm) sodium chloride aerosols are determined for relative humidities (RH) between 85% and 10%. Both pure sodium chloride and sodium chloride mixed with magnesium chloride (X(Mg/Na) = 0.114, typical of sea salt) are studied. The aerosol is equilibrated with a carrier gas stream at the desired RH and then mixed with nitric acid vapor at a concentration of 60 ppb in a laminar flow tube reactor. At the end of the reactor, the particle composition is determined in real time with a laser ablation single particle mass spectrometer. For relative humidities above the efflorescence relative humidity (ERH), the particles exist as liquid droplets and the uptake coefficient ranges from 0.05 at 85% RH to >0.1 near the ERH. The droplet sizes, relative humidity and composition dependencies, are readily predicted by thermodynamics. For relative humidities below the ERH, the particles are nominally "solid" and uptake depends on the amount of surface adsorbed water (SAW). The addition of magnesium chloride to the particle phase (0.114 mole ratio of magnesium to sodium) facilitates uptake by increasing the amount of SAW. In the presence of magnesium chloride, the uptake coefficient remains high (>0.1) down to 10% RH, suggesting that the displacement of chloride by nitrate in fine sea salt particles is efficient over the entire range of conditions in the ambient marine environment. In the marine boundary layer, displacement of chloride by nitrate in fine sea salt particles should be nearly complete within a few hours (faster in polluted areas)-a time scale much shorter than the particle residence time in the atmosphere.
在相对湿度（RH）为85%至10%的条件下，测定了硝酸在尺寸选择的（d(ve)=102和233纳米）氯化钠气溶胶上的反应吸收系数。研究了纯氯化钠以及与氯化镁混合的氯化钠（X(Mg/Na)=0.114，典型的海盐成分）。气溶胶在所需的相对湿度下与载气流达到平衡，然后在层流管反应器中与浓度为60 ppb的硝酸蒸汽混合。在反应器末端，用激光烧蚀单颗粒质谱仪实时测定颗粒成分。对于高于潮解相对湿度（ERH）的相对湿度，颗粒以液滴形式存在，吸收系数范围从85%相对湿度下的0.05到接近潮解相对湿度时大于0.1。液滴尺寸、相对湿度和成分依赖性可以通过热力学轻松预测。对于低于潮解相对湿度的相对湿度，颗粒名义上是“固体”，吸收取决于表面吸附水（SAW）的量。向颗粒相中添加氯化镁（镁与钠的摩尔比为0.114）通过增加表面吸附水的量促进吸收。在存在氯化镁的情况下，吸收系数在低至10%相对湿度时仍保持较高（>0.1），这表明在环境海洋环境的整个条件范围内，细海盐颗粒中硝酸盐对氯化物的置换是有效的。在海洋边界层中，细海盐颗粒中硝酸盐对氯化物的置换在几小时内应该几乎完成（在污染地区更快）——这个时间尺度比颗粒在大气中的停留时间短得多。
