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Environ Sci Technol. 2018 Jun 19;52(12):6798-6806. doi: 10.1021/acs.est.8b00910. Epub 2018 May 30.

Reduced Ultrafine Particle Concentration in Urban Air: Changes in Nucleation and Anthropogenic Emissions.

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Center for Atmospheric Particle Studies , Carnegie Mellon University , 5000 Forbes Avenue, Doherty Hall , Pittsburgh , Pennsylvania 15213 , United States.
Department of Civil, Architectural and Environmental Engineering , University of Texas at Austin , Austin , Texas 78712 , United States.


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 SO2 (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.

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