Urea-enhanced nucleation in the atmosphere: Mechanistic insights from MSA, SA, and ternary MSA-MA pathways

Abstract

New particle formation (NPF) is a fundamental process governing atmospheric aerosol production and, consequently, climate and air quality. Despite the extensive study of acid-base interactions in nucleation, the role of organic nitrogen compounds, particularly urea, remains inadequately explored. Here, we employ quantum chemical calculations and kinetic simulations via the Atmospheric Cluster Dynamics Code (ACDC) to elucidate the mechanisms by which urea modulates nucleation processes. Our investigation reveals that urea, through its dual hydrogen-bonding capability and proton-accepting -C=O group, stabilizes clusters formed with methanesulfonic acid (MSA), sulfuric acid (SA), and nitric acid (NA). In binary systems, MSA-Urea clusters exhibit intermediate stability, higher than those of MSA-NH₃ but lower than MSA-MA. Notably, in ternary MSA-MA-Urea systems, urea markedly enhances nucleation rates, particularly under low-temperature conditions. These results indicate that urea can compensate for the low ambient concentrations of more potent bases such as methylamine (MA), thus playing a pivotal role in atmospheric nucleation. Our findings underscore the need to incorporate organic nitrogen chemistry into atmospheric models to improve predictions of aerosol formation and its broader climatic impacts.