Ronald and Maxine Linde Center for Global Environmental Science, California Institute of Technology, Pasadena, California 91125, USA.
The Hakubi Center for Advanced Research, Kyoto University, Kyoto 606-8302, Japan.
Faraday Discuss. 2013;165:407-20. doi: 10.1039/c3fd00040k.
In tropospheric chemistry, secondary organic aerosol (SOA) is deemed an end product. Here, on the basis of new evidence, we make the case that SOA is a key reactive intermediate. We present laboratory results on the catalysis by carboxylate anions of the disproportionation of NO2 'on water': 2NO2 + H2O = HONO + NO(9-) + H+ (R1), and supporting quantum chemical calculations, which we apply to reinterpret recent reports on (i) HONO daytime source strengths vis-à-vis SOA anion loadings and (ii) the weak seasonal and latitudinal dependences of NO(x) decay kinetics over several megacities. HONO daytime generation via R1 should track sunlight because it is generally catalyzed by the anions produced during the photochemical oxidation of pervasive gaseous pollutants. Furthermore, by proceeding on the everpresent substrate of aquated airborne particulates, R1 can eventually overtake the photolysis of NO2: NO2 + hv = NO + O(3P) (R2), at large zenith angles. Thus, since R1 leads directly to *OH-radical generation via HONO photolysis: HONO + hv = NO + OH, whereas the path initiated by R2 is more circuitous and actually controlled by the slower photolysis of O3: O3 + hv (+H2O) = O2 + 2OH, the competition between R1 and R2 provides a mechanistic switch that buffers *OH concentrations and NO2 decay (via R1 and/or NO2 + *OH = HNO3) from actinic flux variations.
在对流层化学中，二次有机气溶胶（SOA）被认为是最终产物。在这里，基于新的证据，我们提出 SOA 是一种关键的反应中间体的观点。我们介绍了关于羧酸根阴离子催化二氧化氮在水中歧化反应的实验室结果：2NO2 + H2O = HONO + NO(-) + H+ (R1)，并提供了支持的量子化学计算，我们将其应用于重新解释最近关于（i）HONO 白天源强与 SOA 阴离子负荷之间的关系，以及（ii）在几个大城市中，NO(x)衰减动力学的弱季节性和纬度依赖性的报告。由于 R1 通常由光化学氧化过程中产生的阴离子催化，因此通过 R1 产生的 HONO 白天生成应与阳光跟踪，因为它通常由光化学氧化过程中产生的阴离子催化。此外，由于 R1 可以在一直存在的空气传播颗粒物的水合基质上进行，因此它最终可以取代 NO2 的光解：NO2 + hv = NO + O(3P) (R2)，在大天顶角处。因此，由于 R1 通过 HONO 光解直接导致 *OH 自由基的生成：HONO + hv = NO + OH，而 R2 引发的途径更加迂回，实际上受较慢的 O3 光解控制：O3 + hv (+H2O) = O2 + 2OH，因此 R1 和 R2 之间的竞争提供了一种机制开关，缓冲了 *OH 浓度和 NO2 衰减（通过 R1 和/或 NO2 + *OH = HNO3）免受光化通量变化的影响。
