Reactivity of Organic Radicals in the Atmospheric Aqueous Phase

在化学系环境化学科学项目的支持下，杰西·克罗尔教授和威廉·格林教授将对大气中小液滴中活性化学物质的命运进行实验和建模研究。有机（含碳和含氢）化合物从各种各样的来源排放到大气中，包括车辆、野火和植物。这些化合物会对空气质量和气候产生重大影响，因为它们在大气中发生化学反应，形成臭氧和细颗粒物等产物。许多这些反应发生在气相，但也有一些发生在悬浮在空气中的液态水滴（云滴、雾滴和细颗粒）中。然而，发生在水滴中的有机化学通常知之甚少。因此，本项目致力于研究液态水中有机自由基（有机氧化反应中的关键反应物质）的化学性质。实验室研究将涉及自由基的生成及其反应产物的测量，而计算研究将侧重于预测化学反应的速率和产物。研究生和本科生将参与这项研究，该团队将参加麻省理工学院范围内的研究机会项目，该项目针对传统上代表性不足的群体的学生。所产生的软件将广泛提供。有机过氧（RO2）和氧（RO）自由基将在水相中通过自由基前体的光解产生，从而可以控制它们的化学结构和浓度。产品将使用气溶胶质谱法和化学电离质谱法进行实时测量。这些自由基的产物已经得到了大量的研究，尽管主要是在气相和散装有机相。气溶胶中这些自由基的化学性质得到的研究少得多，主要局限于使用气相色谱法测量反应产物的脉冲辐射分解实验。最初的工作将集中在优化生成这些自由基的技术上。后续的化学研究将首先在散装水相中进行，然后在悬浮的亚微米液滴中进行。同时，计算工作（速率系数计算和化学动力学建模）有望提供对潜在机制的深入了解，并有助于将结果推广到更广泛的反应和条件中。水相自由基化学可能与气相有很大的不同：液态水的存在会极大地影响基本反应的动力学和热力学，并引入在气相中无法发生的新反应途径。溶解颗粒的独特化学环境引入了额外的化学效应，与气-颗粒分配和高离子强度有关。这项工作将通过对水相自由基化学的实验室和计算研究来研究所有这些影响，目的是提高我们对这些重要自由基中间体的基本反应性、大气命运和最终影响的理解。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

With support from the Environmental Chemical Sciences Program in the Division of Chemistry, Professors Jesse Kroll and William Green will pursue experimental and modeling studies of fate of reactive chemical species in small droplets in the atmosphere. Organic (carbon- and hydrogen-containing) compounds are emitted into the atmosphere from a wide range of sources, including vehicles, wildfires, and plants. These compounds can have major impacts on air quality and climate, because in the atmosphere they undergo chemical reactions, forming products that such as ozone and fine particle matter. Many of these reactions occur in the gas phase, but some occur within liquid water droplets suspended in the air (cloud droplets, fog droplets, and fine particles). However, the organic chemistry occurring in water droplets is generally poorly understood. Therefore, this project addresses the chemistry of organic radicals (key reactive species in organic oxidation reactions) within liquid water. Laboratory studies will involve the generation of radicals and measurements of their reaction products, and computational studies will focus on predicting rates and products of the chemical reactions. Graduate and undergraduate students will be involved in this research, and the team will participate in MIT-wide research opportunity programs aimed at students from traditionally underrepresented groups. The software generated will be made widely available. Organic peroxy (RO2) and oxy (RO) radicals will be generated in the aqueous phase by the photolysis of radical precursors, allowing for control of their chemical structure and concentration. Products will be measured in real time using aerosol mass spectrometry and chemical ionization mass spectrometry. The products of such radical species have received considerable study, though primarily in the gas phase and bulk organic phase. The chemistry of such radicals in aerosols has received substantially less study, mostly limited to pulse radiolysis experiments that used gas chromatography to measure reaction products. Initial work will focus on optimizing techniques to generate these radicals. Subsequent studies of the chemistry will be carried out first in the bulk aqueous phase and then within suspended submicron droplets. In parallel, computational work (rate coefficient calculations and chemical kinetic modeling) is expected to provide insight into underlying mechanisms and will help generalize results to a wider range of reactions and conditions. Aqueous-phase radical chemistry may be substantially different from that in the gas phase: the presence of liquid water can dramatically affect the kinetics and thermodynamics of elementary reactions, as well as introduce new reaction pathways that cannot occur in the gas phase. The unique chemical environment of deliquesced particles introduces additional chemical effects, associated with gas-particle partitioning and high ionic strengths. This work will investigate all these effects, via laboratory and computational studies of aqueous-phase radical chemistry, with the aim of improving our understanding of the fundamental reactivity, atmospheric fate, and ultimate impacts of these important radical intermediates.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.