Formation of atmospheric molecular clusters containing nitric acid with ammonia, methylamine, and dimethylamine†

Abstract

This study investigates the formation of atmospheric molecular clusters containing ammonia (NH₃, A), methylamine (CH₃NH₂, MA), or dimethylamine (CH₃NHCH₃, DMA) with nitric acid (HNO₃, NA) using quantum mechanics. The Atmospheric Cluster Dynamic Code (ACDC) was employed to simulate the total evaporation rate, formation rate, and growth pathways of three types of clusters under dry and hydrated conditions. This study evaluates the enhancing potential of A/MA/DMA for NA-based new particle formation (NPF) at parts per trillion (ppt) levels. The results indicate that A/MA/DMA can enhance NA-based NPF at high nitric acid concentrations and low temperatures in the atmosphere. The enhancing potential of MA is weaker than that of DMA but stronger than that of A. Cluster growth predominantly follows the lowest free energy pathways on the acid–base grid, with the formation of initial acid–base dimers (NA)(A), (NA)(MA), and (NA)(DMA) being crucial. Hydration influences the evaporation rate and formation rate of clusters, especially for initial clusters. When the humidity is at 100%, the formation rate for NA–A, NA–MA, and NA–DMA clusters can increase by approximately 10⁹, 10⁷, and 10⁴-fold compared to the corresponding unhydrated clusters, respectively. These results highlight the significance of nitric acid nucleation in NPF events in low-temperature, high-humidity atmospheres, particularly in regions like China with significant automobile exhaust pollution.