Researchers who studied the heavily polluted air of Shanghai, China, between 2014 and 2016 report the chemical conditions that led to the surprising formation of new particles in the city's atmosphere, particles that likely worsened regional air quality. The study reveals new insights into the puzzling process of new particle formation (NPF) in urban environments, where such particulates are not expected to form as readily as in cleaner air. NPF is a global phenomenon that greatly contributes to the concentration of atmospheric aerosol particles, which in turn affects local air quality as well as regional and perhaps even global climate. 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. To better understand this discrepancy, Lai Yao et al. performed long-term observations of NPF events in Shanghai, China, between March 2014 and February 2016. Using a variety of instruments, which measured atmospheric chemistry and the molecular priorities of newly formed aerosols, NPF events were recorded in two surveys during this time period. The two datasets revealed the chemical and physical mechanisms behind the observed events and suggest that the formation of secondary aerosols likely occurs through the sulfuric acid-dimethtlamine-water (H2SO4-DMA-H2O) nucleation - a conclusion largely supported by experimental laboratory studies. 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.