Ultrafine particles and PM 2.5 in the air of cities around the world: are they representative of each other?

Alma Lorelei de Jesus, Md Mahmudur Rahman, Mandana Mazaheri, Helen Thompson, Luke D. Knibbs, Cheol Jeong, Greg Evans, Wei Nei, Aijun Ding, Liping Qiao, Li Li, Harri Portin, Jarkko V. Niemi, Hilkka Timonen, Krista Luoma, Tuukka Petäjä, Markku Kulmala, Michal Kowalski, Annette Peters, Josef CyrysLuca Ferrero, Maurizio Manigrasso, Pasquale Avino, Giorgio Buonano, Cristina Reche, Xavier Querol, David Beddows, Roy M. Harrison, Mohammad H. Sowlat, Constantinos Sioutas, Lidia Morawska*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

40 Citations (Scopus)
196 Downloads (Pure)


Can mitigating only particle mass, as the existing air quality measures do, ultimately lead to reduction in ultrafine particles (UFP)? The aim of this study was to provide a broader urban perspective on the relationship between UFP, measured in terms of particle number concentration (PNC) and PM 2.5 (mass concentration of particles with aerodynamic diameter < 2.5 μm) and factors that influence their concentrations. Hourly average PNC and PM 2.5 were acquired from 10 cities located in North America, Europe, Asia, and Australia over a 12-month period. A pairwise comparison of the mean difference and the Kolmogorov-Smirnov test with the application of bootstrapping were performed for each city. Diurnal and seasonal trends were obtained using a generalized additive model (GAM). The particle number to mass concentration ratios and the Pearson's correlation coefficient were calculated to elucidate the nature of the relationship between these two metrics. Results show that the annual mean concentrations ranged from 8.0 × 10 3 to 19.5 × 10 3 particles·cm −3 and from 7.0 to 65.8 μg·m −3 for PNC and PM 2.5 , respectively, with the data distributions generally skewed to the right, and with a wider spread for PNC. PNC showed a more distinct diurnal trend compared with PM 2.5 , attributed to the high contributions of UFP from vehicular emissions to PNC. The variation in both PNC and PM 2.5 due to seasonality is linked to the cities' geographical location and features. Clustering the cities based on annual median concentrations of both PNC and PM 2.5 demonstrated that a high PNC level does not lead to a high PM 2.5 , and vice versa. The particle number-to-mass ratio (in units of 10 9 particles·μg −1 ) ranged from 0.14 to 2.2, >1 for roadside sites and <1 for urban background sites with lower values for more polluted cities. The Pearson's r ranged from 0.09 to 0.64 for the log-transformed data, indicating generally poor linear correlation between PNC and PM 2.5 . Therefore, PNC and PM 2.5 measurements are not representative of each other; and regulating PM 2.5 does little to reduce PNC. This highlights the need to establish regulatory approaches and control measures to address the impacts of elevated UFP concentrations, especially in urban areas, considering their potential health risks.

Original languageEnglish
Pages (from-to)118-135
Number of pages18
JournalEnvironment International
Early online date21 May 2019
Publication statusPublished - 1 Aug 2019


  • Particle number concentration
  • PM2.5
  • Urban aerosol

ASJC Scopus subject areas

  • Environmental Science(all)


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