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Poor linear correlation between PNC and PM2.5 was observed in an international study

Poor linear correlation between PNC and PM2.5 was observed in an international study

Particle number concentration and PM2.5 mass concentration measurements are not representative of each other and regulating PM2.5 does little to reduce particle number concentration.

Therefore if the number concentration or health effects of ultrafine particles want to be diminished, separate limits for PM2.5 and particle number concentration of ultrafine particles are needed.

A broad international consortium study led by Queensland University of Technology studied if mitigating only particle mass, as the existing air quality measures do, ultimately leads to reduction in ultrafine particles (UFP, particle diameter Dp <0.1µm). 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 PM2.5 (mass concentration of particles with aerodynamic diameter < 2.5 µm) and factors that influence their concentrations.

A generally poor linear correlation between PNC and PM2.5 was observed. Therefore, this study concluded that PNC and PM2.5 measurements are not representative of each other; and regulating PM2.5 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.

Hourly average PNC and PM2.5 were acquired from 10 cities located in North America, Europe, Asia, and Australia over a 12-month period. Large variation in the annual mean concentrations particle number concentrations (8.0 x 103 - 19.5 x 103 particles/cm-3) and particle mass (7.0 to 65.8 µg/m-3) were observed in the cities. PNC showed a more distinct diurnal trend compared with PM2.5, attributed to the high contributions of UFP from vehicular emissions to PNC (Particle Number Concentration). The variation in both PNC and PM2.5 due to seasonality was linked also to the cities' geographical location and features

For more information:

Senior Scientist Hilkka Timonen, Finnish Meteorological Institute, tel. +358 50 380 2864,

Reference: Alma Lorelei de Jesus, Md Mahmudur Rahman, Helen Thompson, Luke Knibbs, Cheol-Heon Jeong, G. Evans, Wei Nie, Aijun Ding, Liping Qiao, Li Li, Harri Portin, Jarkko V. Niemi, Hilkka Timonen, Krista Luoma, Tuukka Petaja, Markku Kulmala, Michal Kowalski, Annette Peters, Josef Cyrys, Luca Ferrero, Maurizio Manigrasso, Pasquale Avino, Giorgio Buonanno, Cristina Reche, Xavier Querol, David Beddows, Roy Harrison, Mohammad H. Sowlat, Constantinos Sioutas, Lidia Morawska (2019), Ultrafine particles and PM2.5 in the air of cities around the world: are they representative of each other?, Environment International, accepted.

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