Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory , Richland, Washington 99354, United States.
Environ Sci Technol. 2016 Aug 16;50(16):8649-60. doi: 10.1021/acs.est.6b01915. Epub 2016 Jul 28.
Nitrous acid (HONO) accumulates in the nocturnal boundary layer where it is an important source of daytime hydroxyl radicals. Although there is clear evidence for the involvement of heterogeneous reactions of NO2 on surfaces as a source of HONO, mechanisms remain poorly understood. We used coated-wall flow tube measurements of NO2 reactivity on environmentally relevant surfaces (Fe (hydr)oxides, clay minerals, and soil from Arizona and the Saharan Desert) and detailed mineralogical characterization of substrates to show that reduction of NO2 by Fe-bearing minerals in soil can be a more important source of HONO than the putative NO2 hydrolysis mechanism. The magnitude of NO2-to-HONO conversion depends on the amount of Fe(2+) present in substrates and soil surface acidity. Studies examining the dependence of HONO flux on substrate pH revealed that HONO is formed at soil pH < 5 from the reaction between NO2 and Fe(2+)(aq) present in thin films of water coating the surface, whereas in the range of pH 5-8 HONO stems from reaction of NO2 with structural iron or surface complexed Fe(2+) followed by protonation of nitrite via surface Fe-OH2(+) groups. Reduction of NO2 on ubiquitous Fe-bearing minerals in soil may explain HONO accumulation in the nocturnal boundary layer and the enhanced [HONO]/[NO2] ratios observed during dust storms in urban areas.
亚硝酸（HONO）在夜间边界层中积累，在那里它是白天羟基自由基的重要来源。尽管有明确的证据表明，NO2 在表面上的多相反应是 HONO 的来源，但机制仍知之甚少。我们使用涂壁流动管测量了环境相关表面（铁（氢）氧化物、粘土矿物和来自亚利桑那州和撒哈拉沙漠的土壤）上的 NO2 反应性，以及对底物的详细矿物学特征分析，表明土壤中含铁矿物对 NO2 的还原作用可能是 HONO 的一个更重要的来源，而不是假定的 NO2 水解机制。NO2 向 HONO 的转化程度取决于底物中 Fe(2+)的含量和土壤表面酸度。研究考察了 HONO 通量对底物 pH 的依赖性，结果表明，在土壤 pH < 5 的范围内，NO2 与表面水膜中存在的 Fe(2+)(aq)之间的反应形成 HONO，而在 pH 5-8 的范围内，HONO 来自于 NO2 与结构铁或表面络合的 Fe(2+)的反应，随后通过表面 Fe-OH2(+)基团将亚硝酸盐质子化。土壤中普遍存在的含铁矿物对 NO2 的还原作用可能解释了夜间边界层中 HONO 的积累以及在城市地区尘暴期间观察到的[HONO]/[NO2]比值升高的现象。
