Abstract The nonlinear chemical processes involved in ozone production (P(O3)) have necessitated using proxy indicators to convey information about the primary dependence of P(O3) on volatile organic compounds (VOCs) or nitrogen oxides (NOx). In particular, the ratio of remotely sensed columns of formaldehyde (HCHO) to nitrogen dioxide (NO2) has been widely used for studying O3 sensitivity. Previous studies found that the errors in retrievals and the incoherent relationship between the column and the near-surface concentrations are a barrier in applying the ratio in a robust way. In addition to these obstacles, we provide calculational-observational evidence, using an ensemble of 0-D photochemical box models constrained by DC-8 aircraft measurements on an ozone event during the Korea-United States Air Quality (KORUS-AQ) campaign over Seoul, to demonstrate the chemical feedback of NO2 on the formation of HCHO is a controlling factor for the transition line between NOx-sensitive and NOx-saturated regimes. A fixed value (~2.7) of the ratio of the chemical loss of NOx (LNOx) to the chemical loss of HO2+RO2 (LROx) perceptibly differentiates the regimes. Following this value, data points with a ratio of HCHO/NO2 less than 1 can be safely classified as NOx-saturated regime, whereas points with ratios between 1 and 4 fall into one or the other regime. We attribute this mainly to the HCHO-NO2 chemical relationship causing the transition line to occur at larger (smaller) HCHO/NO2 ratios in VOC-rich (VOC-poor) environments. We then redefine the transition line to LNOx/LROx~2.7 that accounts for the HCHO-NO2 chemical relationship leading to HCHO = 3.7 × (NO2 – 1.14 × 1016 molec.cm-2). Although the revised formula is locally calibrated (i.e., requires for readjustment for other regions), its mathematical format removes the need for having a wide range of thresholds used in HCHO/NO2 ratios that is a result of the chemical feedback. Therefore, to be able to properly take the chemical feedback into consideration, the use of HCHO = a × (NO2 – b) formula should be preferred to the ratio in future works. We then use the Geostationary Trace gas and Aerosol Sensor Optimization (GeoTASO) airborne instrument to study O3 sensitivity in Seoul. The unprecedented spatial (250 × 250 m2) and temporal (~every 2 h) resolutions of HCHO and NO2 observations form the sensor enhance our understanding of P(O3) in Seoul; rather than providing a crude label for the entire city, more in-depth variabilities in chemical regimes are observed that should be able to inform mitigation strategies correspondingly.

中文翻译：

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重新审视 HCHO/NO2 比率在 KORUS-AQ 活动期间高臭氧事件中使用高分辨率机载遥感观测推断臭氧对其前体的敏感性的有效性

摘要 臭氧产生 (P(O3)) 所涉及的非线性化学过程需要使用代理指标来传达有关 P(O3) 对挥发性有机化合物 (VOC) 或氮氧化物 (NOx) 的主要依赖性的信息。特别是，甲醛 (HCHO) 与二氧化氮 (NO2) 的遥感柱的比率已广泛用于研究 O3 敏感性。先前的研究发现，反演中的错误以及色谱柱与近地表浓度之间的不连贯关系是以稳健的方式应用该比率的障碍。除了这些障碍之外，我们还提供了计算观测证据，使用受 DC-8 飞机在首尔上空进行的韩美空气质量 (KORUS-AQ) 运动期间臭氧事件测量约束的一组 0-D 光化学箱模型, 证明 NO2 对 HCHO 形成的化学反馈是 NOx 敏感和 NOx 饱和状态之间过渡线的控制因素。NOx (LNOx) 的化学损失与 HO2+RO2 (LROx) 的化学损失之比的固定值 (~2.7) 可明显区分这些制度。根据该值，HCHO/NO2 比率小于 1 的数据点可以安全地归类为 NOx 饱和状态，而比率在 1 和 4 之间的点属于一种或另一种状态。我们主要将这归因于 HCHO-NO2 化学关系，导致在富含 VOC（VOC 贫乏）的环境中以较大（较小）HCHO/NO2 比率出现过渡线。然后我们将过渡线重新定义为 LNOx/LROx~2.7，这说明 HCHO-NO2 化学关系导致 HCHO = 3.7 × (NO2 – 1.14 × 1016 molec.cm-2)。尽管修订后的公式是局部校准的（即，需要对其他区域进行重新调整），但其数学格式不需要在 HCHO/NO2 比率中使用广泛的阈值，这是化学反馈的结果。因此，为了能够正确地考虑化学反馈，在未来的工作中应该优先使用 HCHO = a × (NO2 – b) 公式。然后，我们使用地球静止微量气体和气溶胶传感器优化 (GeoTASO) 机载仪器来研究首尔的 O3 敏感性。传感器前所未有的空间（250 × 250 m2）和时间（~每 2 小时）分辨率的 HCHO 和 NO2 观测值增强了我们对首尔 P(O3) 的理解；而不是为整个城市提供一个粗糙的标签，