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Updated: Jun 10, 2025

Computation of Atmospheric Concentrations of Molecular Clusters from ab initio Thermochemistry
Published on: April 8, 2020
Formation of atmospheric molecular clusters containing nitric acid with ammonia, methylamine, and dimethylamine
Dong-Ping Chen1, Wen Ma1, Chun-Hong Yang1
1College of Chemistry and Chemical Engineering, Northwest Normal University, Gansu International Scientific and Technological Cooperation Base of Water-Retention Chemical Functional Materials, Lanzhou, Gansu 730070, China. chendp@nwnu.edu.cn.
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.
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