School of Public and Environmental Affairs, Indiana University , 1315 East 10th Street, Bloomington, Indiana 47405, United States.
Department of Chemistry, Indiana University , 800 East Kirkwood Avenue, Bloomington, Indiana 47405, United States.
Environ Sci Technol. 2017 Sep 5;51(17):9633-9643. doi: 10.1021/acs.est.7b01363. Epub 2017 Aug 10.
Humic acid (HA) is thought to promote NO conversion to nitrous acid (HONO) on soil surfaces during the day. However, it has proven difficult to identify the reactive sites in natural HA substrates. The mechanism of NO reduction on soil surrogates composed of HA and clay minerals was studied by use of a coated-wall flow reactor and cavity-enhanced spectroscopy. Conversion of NO to HONO in the dark was found to be significant and correlated to the abundance of C-O moieties in HA determined from the X-ray photoelectron spectra of the C 1s region. Twice as much HONO was formed when NO reacted with HA that was photoreduced by irradiation with UV-visible light compared to the dark reaction; photochemical reactivity was correlated to the abundance of C═O moieties rather than C-O groups. Bulk electrolysis was used to generate HA in a defined reduction state. Electrochemically reduced HA enhanced NO-to-HONO conversion by a factor of 2 relative to non-reduced HA. Our findings suggest that hydroquinones and benzoquinones, which are interchangeable via redox equilibria, contribute to both thermal and photochemical HONO formation. This conclusion is supported by experiments that studied NO reactivity on mineral surfaces coated with the model quinone, juglone. Results provide further evidence that redox-active sites on soil surfaces drive ground-level NO-to-nitrite conversion in the atmospheric boundary layer throughout the day, while amphoteric mineral surfaces promote the release of nitrite formed as gaseous HONO.
腐殖酸（HA）被认为在白天促进土壤表面上 NO 向亚硝酸（HONO）的转化。然而，已经证明很难识别天然 HA 基质中的反应性位点。通过使用涂层壁流反应器和空腔增强光谱法研究了由 HA 和粘土矿物组成的土壤替代物上 NO 还原的机制。发现 NO 在黑暗中向 HONO 的转化是显著的，并且与从 C 1s 区域的 X 射线光电子能谱确定的 HA 中 C-O 基团的丰度相关。与黑暗反应相比，当 NO 与经紫外线 - 可见光辐照光还原的 HA 反应时，形成了两倍的 HONO；光化学反应性与 C ═ O 基团的丰度相关，而不是 C-O 基团。体积电解用于在定义的还原态下生成 HA。与非还原 HA 相比，电化学还原的 HA 将 NO 转化为 HONO 的转化率提高了 2 倍。我们的研究结果表明，在热和光化学 HONO 形成中，通过氧化还原平衡可互换的对苯二酚和苯醌都有贡献。通过研究模型醌，胡桃醌涂覆的矿物表面上的 NO 反应的实验支持了这一结论。结果进一步证明，土壤表面上的氧化还原活性位点在整个白天驱动大气边界层中地面水平的 NO 向亚硝酸盐的转化，而两性矿物表面促进了形成的亚硝酸盐作为气态 HONO 的释放。
