Accelerated chemical reactions in microdroplets are of great interest, however, the mechanism and the principle of the phenomena remain elusive. Malonic acid (MA) is a dicarboxylic acid widely found in atmospheric organic aerosols (OAs) over oceans and urban areas. Recent research revealed MA exhibits keto-enol tautomerization in deliquesced droplets and the equilibrium ratio of keto and enol forms vary over the relative humidity (RH) conditions. In this study, an environment controlled electrodynamic balance (EDB) coupled with Mie scattering imaging and Raman spectroscopy was used to levitate single charged MA droplets and investigate the reaction kinetics of keto-enol tautomerization of MA and the effect of RH and droplet size to the tautomerization rates. Hydrogen-deuterium isotopic exchange rates of alpha carbon of MA were measured to estimate keto-enol tautomerization rates. The result showed tautomerization rates are faster for higher RH and smaller droplets. Step-by-step isotopic exchange model was applied to determine enolization rate of MA at 90% RH, yielding a value 10-fold higher than known value from bulk solution study. Findings of this study suggests that accelerated keto-enol tautomerization of MA can play an important role for aging of OAs in nature, as the enol-MA can undergo series of chemical reactions due to the reactive C=C bond.

Investigating unusual chemical reactions in single levitated aerosol using Raman spectroscopy

ACS Spring 2022

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