Correlation between PM2.5 and Particle Number Concentrations in Four Major Cities: Toronto, Los Angeles, Helsinki and London

Gaining more knowledge on how different particulate metrics are related would help in successfully controlling particulate matter (PM) concentrations in the ambient air. This study focused on the mass and number concentrations, particularly PM2.5 or mass concentration of particles with diameter of <2.5 µm in order to illustrate that mitigating PM2.5 would not necessarily reduce the ultrafine particles (UFP) concentration. Particles with diameter <0.1 µm are best quantified by the particle number concentration (PNC). The two parameters, PM2.5 and PNC are affected by different drivers, therefore may vary spatially and temporally between cities. PM2.5 is relatively more homogenous within an air shed while PNC is more variable depending mainly on the distribution of the combustion emission sources. To better understand these two important metrics and demonstrate their similarities and differences, this study aims to provide quantitative information on the relationship between ambient PM2.5 and PNC in four cities: Toronto, Canada; Los Angeles, USA; Helsinki, Finland; and London, UK. All these cities are located in the temperate region though Helsinki and Toronto are classified under Moist Continental Mid-Latitude Climate while London and Los Angeles are classified under Moist Subtropical Mid-Latitude climate based on the Köppen-Geiger system. Urban areas are particularly interesting because high population density implies that considerable amount of anthropogenic pollutants are produced in any city.