Winter emission factors of ultrafine total and solid particle numbers and PM2.5 near a major arterial road in Tokyo

Road-traffic particulate emissions are regulated by limits expressed as emission factors per driving distance. However, these limits are difficult to compare directly to atmospheric concentrations. This study measured particulate and gaseous pollutant concentrations simultaneously in winter of 2023 at two locations near a major ring road in Tokyo, one roadside and the other 100 m back. Traffic-related emissions of PM 2.5 (fine particulate matter, particle size ≤ 2.5 μm), total particle number (TPN 10–100 nm ), and solid particle number (SPN 10–100 nm ) were not correlated with traffic volume. Marked differences at the two sites confirmed that traffic-related emissions affected the roadside atmospheric environment, although this impact was evident only in hourly data. Emission factors for TPN 10–100 nm and SPN 10–100 nm , estimated using concentration differences between sites and CO 2 /CO and NO x /CO ratios, yielded factors of 1.61 × 10 13 ± 4.5 × 10 12 and 4.78 × 10 12 ± 2.2 × 10 12 #/km/veh., respectively. Barium in PM 2.5 was used to estimate brake particle emission factors (2.05 × 10 9 ± 6.6 × 10 8 #/km/veh., 0.01% of TPN 10–100 nm ; 1.64 × 10 9 ± 5.2 × 10 8 #/km/veh., 0.03% of SPN 10–100 nm ), revealing only a small contribution to total traffic-derived particle emissions. The traffic-derived PM 2.5 emission factor was 20.1 ± 6.5 mg/km/veh., with brake particle–derived PM 2.5 contributing only 0.94 mg/km/veh. (4.7%). These findings support previous studies showing no correlation between roadside PM 2.5 and traffic volume, and highlight the importance of high-resolution, simultaneous roadside and background measurements for evaluating traffic-derived particulate emissions. • Particulate pollutants were measured both at roadside and background in Tokyo • Emission factors for TPN 10–100 nm and SPN 10–100 nm were around 10 13 and 10 12 #/km/veh, respectively • Brake particle number emission factors were around 10 9 #/km/vehicle • Brake particles contributed 4.7% (0.94 mg/km/vehicle) to roadside PM 2.5