Marine new particle formation (NPF) affects climate and radiative forcing globally by seeding clouds. Nucleation is critical for NPF, yet revealing its mechanisms remains technically challenging. Recently, iodic acid (IA), the key nucleating precursor over oceans, has been identified, but the observed nucleation rate is hard to explain by itself. Despite much attention on iodine species, little is known about the impacts of others. Given atmospheric complexity, the well-known and widespread acid stabilizer, dimethylamine (DMA) may combine chemically with IA via acid-base reactions, but how they act is unknown. Hence, we conceived this work to provide the molecular-level insights to IA-DMA nucleation process.
In this study, we adopted quantum chemical (QC) calculations and multi-step structural sampling to locate stable cluster conformers. Next, cluster properties were calculated as input to the Atmospheric Cluster Dynamics Code (ACDC) to explore clustering kinetics by solving the birth-death equation without fitted parameters. Finally, we can obtain the cluster formation rate, steady-state concentration and formation path.
Our QC results suggest that DMA can structurally stabilize IA via hydrogen bonding, halogen bonding, and even proton transfer, resulting in a barrierless clustering process. In ACDC simulations, the IA cluster formation is highly sensitive to DMA of only a few ppt. Such an efficient amine-stabilization mechanism may provide theoretical evidence for intensive marine NPF events. Further analysis of clustering mechanism, we find that more stable IA-DMA clusters dominate nucleation process rather than pure IA clusters. Thus, the enhancement of DMA on IA cluster formation cannot be ignored, especially in regions near the source of IA and DMA emissions. Interestingly, the facilitation effect of DMA has a seasonal character, especially in summer, with a positive correlation with temperature.
Overall, our study highlights the important role of DMA in stabilizing IA, thus facilitating particle formation. The proposed IA-DMA nucleation mechanism may help to explain the missing source of iodine particles, consequently the rapid marine NPF events. We are looking forward to experimental work to verify the efficient stabilization effect of DMA on IA. In future study, the other iodine-bearing components should also be considered together.
Sincere thanks to my co-authors: An Ning, Ling Liu, Shaobing Zhang, Fangqun Yu, Lin Du, and Maofa Ge, for their valuable guidance. This work is funded by the National Science Fund for Distinguished Young Scholars [Grant no. 22225607] and the National Natural Science Foundation of China [Grant no. 21976015].