Experiment-theory hybrid method for studying the formation mechanism of atmospheric new particle formation

Atmospheric aerosols have a significant influence on climate change through their effect on the cloud lifetime and the radiative balance of the Earth's atmosphere. Despite its importance, the mechanism of aerosol nucleation is still poorly understood. Based on the low-energy structure of cluster molecules, quantum chemical (QC) computations can help us to directly calculate the formation mechanism of atmospheric clusters and formation rates at the molecular level. However, deciphering the formation mechanism of pre-nucleated clusters, especially those close to the critical size (∼1.7 nm), remains extremely challenging because many millions of configuration spaces might need to be explored to find the low-energy structure. We present a new idea that establishes a comprehensive experimental and computational hybrid calculation protocol to integrate experimental data, isomer distributions, hydrogen bond interactions, and interaction sites for exploring the configuration spaces and clarifying the nucleation mechanism of acid-base clusters, whose maximum size can exceed 1.7 nm. This protocol can effectively and accurately explore the configuration space of complex large nucleation clusters on the potential energy surface (PES). The consistency of the cluster concentration and the formation rate between the experiment and the in situ measurement is much better than that of the previous studies and proves its accuracy. In addition, we found that malic acid (MA) can enhance sulfuric acid-dimethylamine (SA-DMA)-based particle formation rates in the atmospheric boundary layer, for example, in Shanghai and Beijing in the summer, with an enhancement of up to 700- and 135-fold, respectively. The enhancement in atmospheric particle formation by MA is critical for new particle formation in the boundary layer with relatively low SA and DMA concentrations and at high temperature. This model greatly improves our understanding of the complex aerosol nucleation mechanism of large-scale multicomponent cluster molecules.