A large and ubiquitous source of atmospheric formic acid
A large and ubiquitous source of atmospheric formic acid
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DOI：
10.5194/acp-15-6283-2015
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发表时间：
2015-01-01
影响因子：
6.3
通讯作者：
Xu, J.
中科院分区：
文献类型：
--
作者：
Millet, D. B.;Baasandorj, M.;Xu, J.
甲酸(HCOOH)是大气中含量最丰富的酸之一，对降水化学和酸度有重要影响。在这里，我们使用一个化学传输模式(GEOS-Chem CTM)来解释最近在美国东南部上空的空中和地面测量，根据它们对HCOOH源和汇的限制。夏季边界层浓度平均为十亿分之几，比已知的生产和损失途径所能解释的大2-3倍。这表明一个或多个大量缺失的HCOOH来源，并表明当前对碳氢化合物氧化的理解存在关键差距，或者是大量的、未确定的HCOOH直接通量。模型测量比较表明，生物来源(如异戊二烯氧化)是主要的HCOOH来源。解决无法解释的边界层浓度(I)仅基于异戊二烯氧化将需要增加3倍的模型HCOOH产量，或(Ii)仅基于直接的HCOOH排放将需要大约25倍的生物通量增加。然而，这两者都不能解释在人为气团和自由对流层中看到的高HCOOH含量。总的迹象是大量的生物来源与一系列前体中普遍存在的HCOOH的化学生产相结合。需要实验室工作来更好地量化异戊二烯和其他普遍存在的有机物产生羧酸的速率和机制。稳定的Criegee中间体(SCI)提供了大量的HCOOH模型来源，而乙醛互变异构化约占模拟全球负担的15%。由于羧酸也与SCI反应并催化反向互变异构化反应，HCOOH通过这两条途径缓冲自己的产物。根据最新的实验室结果，CH3O2和OH之间的反应可以提供大气中HCOOH的主要来源；但是，包括这种化学作用会降低CH3OOH和NOx：CH3OOH的模型模拟。对SCI和RO2+OH化学制定更好的约束是未来工作的高度优先事项。该模式既没有捕捉到地面空气中HCOOH的大的昼夜振幅，也没有捕捉到它在夜间的反向垂直梯度。这意味着我们目前对边界层动力学调制的沉积的表示有很大的偏差，可能表明HCOOH汇低估了，从而可能是一个更大的缺失源。对于改进HCOOH和相关痕量气体的模拟，以及我们对它们的预算的理解，更有力地处理表面沉积是一个关键需要。
Formic acid (HCOOH) is one of the most abundant acids in the atmosphere, with an important influence on precipitation chemistry and acidity. Here we employ a chemical transport model (GEOS-Chem CTM) to interpret recent airborne and ground-based measurements over the US Southeast in terms of the constraints they provide on HCOOH sources and sinks. Summertime boundary layer concentrations average several parts-per-billion, 2-3 x larger than can be explained based on known production and loss pathways. This indicates one or more large missing HCOOH sources, and suggests either a key gap in current understanding of hydrocarbon oxidation or a large, unidentified, direct flux of HCOOH. Model-measurement comparisons implicate biogenic sources (e. g., isoprene oxidation) as the predominant HCOOH source. Resolving the unexplained boundary layer concentrations based (i) solely on isoprene oxidation would require a 3 x increase in the model HCOOH yield, or (ii) solely on direct HCOOH emissions would require approximately a 25 x increase in its biogenic flux. However, neither of these can explain the high HCOOH amounts seen in anthropogenic air masses and in the free troposphere. The overall indication is of a large biogenic source combined with ubiquitous chemical production of HCOOH across a range of precursors. Laboratory work is needed to better quantify the rates and mechanisms of carboxylic acid production from isoprene and other prevalent organics. Stabilized Criegee intermediates (SCIs) provide a large model source of HCOOH, while acetaldehyde tautomerization accounts for similar to 15% of the simulated global burden. Because carboxylic acids also react with SCIs and catalyze the reverse tautomerization reaction, HCOOH buffers against its own production by both of these pathways. Based on recent laboratory results, reaction between CH3O2 and OH could provide a major source of atmospheric HCOOH; however, including this chemistry degrades the model simulation of CH3OOH and NOx : CH3OOH. Developing better constraints on SCI and RO2 + OH chemistry is a high priority for future work. The model neither captures the large diurnal amplitude in HCOOH seen in surface air, nor its inverted vertical gradient at night. This implies a substantial bias in our current representation of deposition as modulated by boundary layer dynamics, and may indicate an HCOOH sink underestimate and thus an even larger missing source. A more robust treatment of surface deposition is a key need for improving simulations of HCOOH and related trace gases, and our understanding of their budgets.