10.1021/acs.est.8b00910.s001 Provat K. Saha Provat K. Saha Ellis S. Robinson Ellis S. Robinson Rishabh U. Shah Rishabh U. Shah Naomi Zimmerman Naomi Zimmerman Joshua S. Apte Joshua S. Apte Allen L. Robinson Allen L. Robinson Albert A. Presto Albert A. Presto Reduced Ultrafine Particle Concentration in Urban Air: Changes in Nucleation and Anthropogenic Emissions American Chemical Society 2018 nucleation events Reduced Ultrafine Particle Concentration intensity frequency ambient ultrafine particles Pittsburgh source particle size distribution measurements Anthropogenic Emissions Nucleation UFP concentrations background particle growth rates 15 years weekday 2018-05-18 00:00:00 Journal contribution https://acs.figshare.com/articles/journal_contribution/Reduced_Ultrafine_Particle_Concentration_in_Urban_Air_Changes_in_Nucleation_and_Anthropogenic_Emissions/6392456 Nucleation is an important source of ambient ultrafine particles (UFP). We present observational evidence of the changes in the frequency and intensity of nucleation events in urban air by analyzing long-term particle size distribution measurements at an urban background site in Pittsburgh, Pennsylvania during 2001–2002 and 2016–2017. We find that both frequency and intensity of nucleation events have been reduced by 40–50% over the past 15 years, resulting in a 70% reduction in UFP concentrations from nucleation. On average, the particle growth rates are 30% slower than 15 years ago. We attribute these changes to dramatic reductions in SO<sub>2</sub> (more than 90%) and other pollutant concentrations. Overall, UFP concentrations in Pittsburgh have been reduced by ∼48% in the past 15 years, with a ∼70% reduction in nucleation, ∼27% in weekday local sources (e.g., weekday traffic), and 49% in the regional background. Our results highlight that a reduction in anthropogenic emissions can considerably reduce nucleation events and UFP concentrations in a polluted urban environment.