Ultrafine particles and PM2.5 in the air of cities around the world: Are they representative of each other?

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 numberconcentration (PNC) and PM (mass concentration of particles with aerodynamic diameter 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 . Therefore, PNC and PM measurements are not representative of each other; and regulating PM 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.