The characteristics and size of lung-depositing particles vary significantly between high and low pollution traffic environments

Currently, only the mass of sub 2.5 μm and sub 10 μm particles (PM_2.5, PM₁₀) in ambient air is regulated and monitored closely, but the same increase in PM_2.5 can cause different degrees of health effects in different cities (sometimes more harmful effects per unit mass in less polluted cities) (Li et al., 2019). In addition to mass concentration, other measurement metrics are needed to connect particle pollution data and health effects. In our measurements made in traffic-influenced environments in Helsinki, Finland (a relatively clean city), and Delhi-National Capital Region (Delhi-NCR), India (a polluted area), we noted a large difference in the median particle size for lung-deposited surface area (LDSA). In Helsinki, the median size was 80 nm, corresponding to soot particles emitted from diesel engines. However, the median size increased to 190 nm during a long-range transport event of air mass. In Delhi-NCR, surprisingly, the median size was even larger, 410 nm. These larger particles were likely to originate from regional sources rather than local traffic. The LDSA to PM_2.5 ratio for particles in Helsinki was 2–4 times the amount in Delhi-NCR, potentially linked with the higher toxicity of a unit of particulate mass in Helsinki.