posted on 2021-08-02, 15:06authored byRujing Yin, Chao Yan, Runlong Cai, Xiaoxiao Li, Jiewen Shen, Yiqun Lu, Siegfried Schobesberger, Yueyun Fu, Chenjuan Deng, Lin Wang, Yongchun Liu, Jun Zheng, Hongbin Xie, Federico Bianchi, Douglas R. Worsnop, Markku Kulmala, Jingkun Jiang
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 the neutral clusters and the naturally charged
ion clusters during NPF periods in urban Beijing. In ion clusters,
we observed pure sulfuric acid (SA) clusters, SA–amine clusters,
SA–ammonia (NH3) clusters, and SA–amine–NH3 clusters. However, only SA clusters and SA–amine clusters
were observed in the neutral form. Meanwhile, oxygenated organic molecule
(OOM) clusters charged by a nitrate ion and a bisulfate ion were observed
in ion clusters. Acid–base clusters correlate well with the
occurrence of sub-3 nm particles, whereas OOM clusters do not. Moreover,
with the increasing cluster size, amine fractions in ion acid–base
clusters decrease, while NH3 fractions increase. This variation
results from the reduced stability differences between SA–amine
clusters and SA–NH3 clusters, which is supported
by both quantum chemistry calculations and chamber experiments. The
lower average number of dimethylamine (DMA) molecules in atmospheric
ion clusters than the saturated value from controlled SA–DMA
nucleation experiments suggests that there is insufficient DMA in
urban Beijing to fully stabilize large SA clusters, and therefore,
other basic molecules such as NH3 play an important role.