Abstract View
Elucidating Sources of Midday Ultrafine Particles in a Major City
HOSNA MOVAHHEDINIA, Nathan Hilker, Cheol H. Jeong, Greg J. Evans, SOCAAR, University of Toronto
Abstract Number: 612
Working Group: Urban Aerosols
Abstract
Ultrafine particles (UFP; diameter less than 100 nm) can be emitted from anthropogenic sources or produced through atmospheric processes. Vehicular emissions are a major source of UFP in many urban areas. However, new particle formation (NPF) events can also make a significant contribution on some days. These events usually happen around midday and result in the formation of a large number of particles in the atmosphere which then grow and disappear through dispersion, coagulation, and condensation on the pre-existing particles. Many studies have identified these event days by looking at the total number concentration of UFP. However, our study showed that this method can result in incorrect classification of event days. In this study, we analyzed 13 years (2006-2019) of particle size distribution (PSD) data from downtown Toronto, Canada, and identified days that had a large number concentration of UFP around noon but were not NPF events. The average concentration of total UFP around midday on these days, known as midday pollution (MP) days, was about the same level as that for NPF days (MP: 34624 #/cm3, NPF: 33623 #/cm3), but the particle formation and growth were not evident on the PSD images of MP days. In general, MP days were ~4 times more frequent than the NPF days. The frequency of these days has decreased over the last decade (2006: 100 days, 2019: 33 days) while it has been almost constant for NPF days (2006: 10 days, 2019: 8 days). Analysis of other pollutant concentrations provides further evidence that the origin of UFP on these days differs from the NPF days.