Formic acid (HCOOH) is an important component of atmospheric acidity but its budget is poorly understood, with prior observations implying substantial missing sources. Here, we combine pole-to-pole airborne observations from the Atmospheric Tomography Mission (ATom) with a chemical transport model (GEOS-Chem CTM) and back-trajectory analyses to provide the first global in situ characterization of HCOOH in the remote atmosphere. ATom reveals sub-100 ppt HCOOH concentrations over most of the remote oceans, punctuated by large enhancements associated with continental outflow. Enhancements correlate with known combustion tracers and trajectory-based fire influences. The GEOS-Chem model underpredicts these in-plume HCOOH enhancements, but elsewhere, we find no broad indication of a missing HCOOH source in the background free troposphere. We conclude that missing nonfire HCOOH precursors inferred previously are predominantly short-lived. We find indications of a wet scavenging underestimate in the model consistent with a positive HCOOH bias in the tropical upper troposphere. Observations reveal episodic evidence of ocean HCOOH uptake, which is well-captured by GEOS-Chem; however, despite its strong seawater undersaturation, HCOOH is not consistently depleted in the remote marine boundary layer. Over 50 fire and mixed plumes were intercepted during ATom with widely varying transit times and source regions. HCOOH:CO-normalized excess mixing ratios in these plumes range from 3.4 to >50 ppt/ppb CO and are often over an order of magnitude higher than expected primary emission ratios. HCOOH is thus a major reactive organic carbon reservoir in the aged plumes sampled during ATom, implying important missing pathways for in-plume HCOOH production.

中文翻译：

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远程大气中的 HCOOH：来自大气断层扫描 (ATom) 机载观测的约束

甲酸 (HCOOH) 是大气酸度的重要组成部分，但对其预算知之甚少，先前的观察表明大量缺失来源。在这里，我们将来自大气断层扫描任务 (ATom) 的极对极空中观测与化学传输模型 (GEOS-Chem CTM) 和反向轨迹分析相结合，以提供 HCOOH 在偏远大气中的第一个全球原位表征。ATom 揭示了大部分偏远海洋的 HCOOH 浓度低于 100 ppt，不时出现与大陆流出相关的大幅增强。增强与已知的燃烧示踪剂和基于轨迹的火灾影响相关。GEOS-Chem 模型低估了这些羽状 HCOOH 增强，但在其他地方，我们没有发现背景自由对流层中缺少 HCOOH 源的广泛迹象。我们得出结论，先前推断的缺失的非火灾 HCOOH 前体主要是短暂的。我们发现模型中的湿清除被低估的迹象与热带对流层上层的正 HCOOH 偏差一致。观测揭示了海洋 HCOOH 吸收的偶发证据，这已被 GEOS-Chem 很好地捕捉到；然而，尽管海水严重欠饱和，但 HCOOH 在遥远的海洋边界层并没有持续耗尽。在 ATom 期间拦截了 50 多个火和混合羽流，传输时间和源区域差异很大。这些羽流中 HCOOH:CO 归一化的过量混合比范围为 3.4 至 >50 ppt/ppb CO，并且通常比预期的初级排放比高一个数量级。因此，HCOOH 是 ATom 期间采样的老化羽流中的主要反应性有机碳库，