Size-Resolved Chemical Composition of Sub-20 nm Particles from Methanesulfonic Acid Reactions with Methylamine and Ammonia
Size-Resolved Chemical Composition of Sub-20 nm Particles from Methanesulfonic Acid Reactions with Methylamine and Ammonia
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DOI：
10.1021/acsearthspacechem.0c00120
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发表时间：
2020-07-16
影响因子：
3.4
通讯作者：
Smith, James N.
中科院分区：
文献类型：
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作者：
Perraud, Veronique;Li, Xiaoxiao;Smith, James N.
世界各地都观察到了气相前体在大气中形成粒子的现象；然而，我们对主要负责物种和涉及的机制的基本理解仍然不确定。最近的实验室研究表明，甲烷磺酸(CH3SO3H，MSA)与小烷基胺的酸碱反应可能对新颗粒的形成有重要贡献。到目前为止，大多数研究都集中在粒子数密度和尺寸分布的测量以及对最稳定的团簇的量子化学预测上。在这里，我们首次使用热解吸化学电离质谱仪(TDCIMS)测量了在定制流动反应器中MSA与甲胺(MA)在NH3存在或不存在的情况下生成的亚20 nm粒子的尺寸分辨化学成分。对TDCIMS进气口的一种新设计进行了评估，并将颗粒化学成分的测量扩展到直径5 nm，这是迄今报道的最小尺寸。直径小于9 nm的MSA-MA粒子的酸性更强，5 nm粒子的酸碱摩尔比为1.8+/-0.4(1 Sigma)，而较大的MSA-MA粒子为中性粒子。当NH3加入到MSA-MA组合中时，也观察到了类似的酸碱摩尔比变化趋势。在MSA-MA-NH3体系中，尽管气相中NH3的浓度较大(气相MA/NH3的摩尔比接近0.23)，但所有粒径的MA/NH3摩尔比都大于1(最高可达2.6)，表明MA是该体系形成颗粒的关键组分。根据以前对该系统中小团簇的计算，讨论了可能的原因。
Particle formation in the atmosphere from gas-phase precursors has been observed around the world; however, our fundamental understanding of the key species responsible and mechanisms involved remains uncertain. Recent laboratory studies demonstrated that acid-base reactions involving methanesulfonic acid (CH3SO3H, MSA) and small alkylamines may contribute significantly to new particle formation. To date, most of the investigations have been focused on particle number concentration and size distribution measurements in combination with quantum chemistry predictions of the most stable clusters. Here, we present the first measurements of the size-resolved chemical composition of sub-20 nm particles generated in a custom flow reactor from the reaction of MSA with methylamine (MA) in the presence or absence of NH3 using thermal desorption chemical ionization mass spectrometry (TDCIMS). A novel design of the TDCIMS inlet was evaluated, and the measurement of the chemical composition of particles was extended down to 5 nm in diameter, the smallest size yet reported for this method. MSA-MA particles with diameters smaller than 9 nm were found to be more acidic, with an acid/base molar ratio of 1.8 +/- 0.4 (1 sigma) for 5 nm particles compared to the larger particles which were neutral. A similar acid/base molar ratio trend was observed when NH3 was added to the MSA-MA combination. In the MSA-MA-NH3 system, the MA/NH3 molar ratio was higher than 1 (up to 2.6) for all particle sizes despite the much larger concentration of NH3 in the gas phase (the gas-phase MA/NH3 ratio was , similar to 0.23), indicating that MA is a key component in particle formation from this system. The potential reasons for this based on previous calculations of small clusters in this system are discussed.