Acid–Base Clusters during Atmospheric New Particle Formation in Urban Beijing

Molecular clustering is the initial step of atmospheric new particle formation (NPF) that generates numerous secondary particles. Using two online mass spectrometers with and without a chemical ionization inlet, we characterized neutral clusters naturally charged ion during NPF periods in urban Beijing. In clusters, observed pure sulfuric acid (SA) SA-amine SA-ammonia (NH3) SA-amine-NH3 clusters. However, only SA were form. Meanwhile, oxygenated organic molecule (OOM) by nitrate bisulfate Acid-base correlate well occurrence sub-3 nm particles, whereas OOM do not. Moreover, increasing cluster size, amine fractions acid-base decrease, while NH3 increase. This variation results from reduced stability differences between SA-NH3 which supported both quantum chemistry calculations chamber experiments. The lower average number dimethylamine (DMA) molecules than saturated value controlled SA-DMA nucleation experiments suggests there insufficient DMA Beijing to fully stabilize large therefore, other basic such as play an important role.