State Key Laboratory of Organic Geochemistry and Guangdong Key Laboratory of Environmental Protection and Resources Utilization, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, China.
Wolfson Atmospheric Chemistry Laboratories, Department of Chemistry, University of York, York YO10 5DD, U.K.
Environ Sci Technol. 2020 Jul 7;54(13):7798-7806. doi: 10.1021/acs.est.0c00526. Epub 2020 Jun 12.
Formic acid (HCOOH), one of the most important and ubiquitous organic acids in the Earth's atmosphere, contributes substantially to atmospheric acidity and affects pH-dependent reactions in the aqueous phase. However, based on the current mechanistic understanding, even the most advanced chemical models significantly underestimate the HCOOH concentrations when compared to ambient observations at both ground-level and high altitude, thus underrating its atmospheric impact. Here we reveal new chemical pathways to HCOOH formation from reactions of both O and OH with ketene-enols, which are important and to date undiscovered intermediates produced in the photo-oxidation of aromatics and furans. We highlight that the estimated yields of HCOOH from ketene-enol oxidation are up to 60% in polluted urban areas and greater than 30% even in the continental background. Our theoretical calculations are further supported by a chamber experiment evaluation. Considering that aromatic compounds are highly reactive and contribute ca. 10% to global nonmethane hydrocarbon emissions and 20% in urban areas, the new oxidation pathways presented here should help to narrow the budget gap of HCOOH and other small organic acids and can be relevant in any environment with high aromatic emissions, including urban areas and biomass burning plumes.
甲酸（HCOOH）是地球大气中最重要、最普遍的有机酸之一，它对大气酸度有很大的贡献，并影响水相中的依赖 pH 值的反应。然而，根据目前的机制理解，即使是最先进的化学模型，与地面和高空的环境观测相比，也大大低估了 HCOOH 的浓度，因此低估了其对大气的影响。在这里，我们揭示了 O 和 OH 与烯酮-烯醇反应形成 HCOOH 的新化学途径，这些烯酮-烯醇是芳香族化合物和呋喃类化合物光氧化过程中产生的重要且迄今为止尚未被发现的中间体。我们强调，从烯酮-烯醇氧化产生的 HCOOH 的估计产率在污染的城市地区高达 60%，甚至在大陆背景下也大于 30%。我们的理论计算得到了一个腔室实验评估的支持。考虑到芳香族化合物具有很高的反应性，约占全球非甲烷碳氢化合物排放量的 10%，在城市地区占 20%，因此这里提出的新氧化途径应该有助于缩小 HCOOH 和其他小分子有机酸的预算缺口，并在包括城市地区和生物质燃烧羽流在内的任何具有高芳香族化合物排放的环境中都具有相关性。
