Reactivity of Organic Radicals in the Atmospheric Aqueous Phase

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.