Can Urea Be a Seed for Aerosol Particle Formation in Air?

Urea is ubiquitous in rainwaters and aqueous aerosols in different environments. However, its atmospheric fate and the exact mechanism on how it influences new particle formation remain completely unexplored. Herein, we have used quantum chemical calculations and Born-Oppenheimer molecular dynamics simulations to explore the potential role of urea in the particle formation events. The results suggest that urea binds more strongly to common acidic precursors than ammonia or monoamines and is capable of binding at most two molecules of an acidic precursor or hydroperoxyl radical via hydrogen bonding interactions. The molecular dynamics simulations suggest that the complex of urea with an acidic precursor or hydroperoxyl radical on the water droplet is stabilized by intermolecular and interfacial hydrogen bonding interactions over the simulation time scale of 10-15 ps. An important implication of these results is that urea may contribute toward the particle formation in marine environments as well as in Asia where the usage of urea for the agricultural activities has increased dramatically over last few decades. Though there is at the moment no evidence of urea being present in the atmospheric gas-phase, we hope our work would inspire field measurements for detecting urea in the gas-phase.