The formation of new atmospheric particulates—i.e., de novo rather than as direct emissions—via nucleation and growth is a major source of aerosol pollution in terms of number concentration. In spite of a great deal of research, it is still a puzzle why and how such new particule formation (NPF) occurs in a highly polluted urban atmosphere.
Previous research has suggested that NPF is extremely sensitive, occurring only under specific atmospheric conditions and requiring clean air, free from large numbers of pre-existing aerosols, which tend to suppress the new particle formation process.
However, recent observations reveal substantial rates of NPF occurring in some heavily-polluted megacities, despite the heavy loads of ambient particles there, contradicting these understandings.
Now, a team of researches has investigated new particle formation in Shanghai and, in a paper published in Science, describes the conditions that make this process possible.
Using a variety of instruments which measured atmospheric chemistry and the molecular priorities of newly formed aerosols, NPF events were recorded in two surveys between March 2014 and February 2016.
The two datasets revealed the chemical and physical mechanisms behind the observed events and suggest that the formation of secondary aerosols likely occurs through sulfuric acid-dimethtlamine-water nucleation—a conclusion largely supported by experimental laboratory studies.
According to both observation and theoretical arguments, NPF usually requires a relatively high sulfuric acid (H2SO4) concentration to promote the formation of new particles and a low preexisting aerosol loading to minimize the sink of new particles. We investigated NPF in Shanghai and were able to observe both precursor vapors (H2SO4) and initial clusters at a molecular level in a megacity.
High NPF rates were observed to coincide with several familiar markers suggestive of H2SO4–dimethylamine (DMA)–water (H2O) nucleation, including sulfuric acid dimers and H2SO4-DMA clusters. In a cluster kinetics simulation, the observed concentration of sulfuric acid was high enough to explain the particle growth to ~3 nanometers under the very high condensation sink, whereas the subsequent higher growth rate beyond this size is believed to result from the added contribution of condensing organic species.—Yao et al.
The authors suggest that the large atmospheric NPF events observed in China are the result of the large emissions of precursor gases, like sulfur dioxide, ammonia, and other volatile organic compounds, and that reductions in the emission of these compounds are crucial to reducing the formation of secondary aerosols.
Lei Yao, Olga Garmash, Federico Bianchi, Jun Zheng, Chao Yan, Jenni Kontkanen, Heikki Junninen, Stephany Buenrostro Mazon, Mikael Ehn, Pauli Paasonen, Mikko Sipilä, Mingyi Wang, Xinke Wang, Shan Xiao, Hangfei Chen, Yiqun Lu, Bowen Zhang, Dongfang Wang, Qingyan Fu, Fuhai Geng, Li Li, Hongli Wang, Liping Qiao, Xin Yang, Jianmin Chen, Veli-Matti Kerminen, Tuukka Petäjä, Douglas R. Worsnop, Markku Kulmala, Lin Wang (2018) “Atmospheric new particle formation from sulfuric acid and amines in a Chinese megacity” Science Vol. 361, Issue 6399, pp. 278-281 doi: 10.1126/science.aao4839