P-phenylenediamines (PPDs) and 6PPD-quinone in tunnel PM: From the perspective of characterization, emission factors, and health risks.

P-phenylenediamines (PPDs) and a quinone derivative (6PPD-Q), as antioxidants added to tires, can inevitably enter into the environment during tire wear emission, posing potential health and ecological risks. However, investigation on their pollution characteristics in PM is still lacking, especially for high-pollution scenarios, such as tunnels. Herein, we investigated the pollution characteristics and emission factors, as well as the correlation analysis and daily intakes of PM-bound PPDs and 6PPD-Q in tunnel. The results indicated heavy PPDs and 6PPD-Q pollution were observed in tunnel PM, with the concentration at the two tunnel sites being 3.83 and 8.73 times higher than those at the urban site, respectively. PPDs were negatively correlated to relative humidity and positively to temperature. Emission factors of 6PPD and 6PPD-Q were 3013.54 and 1466.67 ng·veh·km for large vehicles. PPDs and 6PPD-Q were most harmful to children, and annual exposure dosages at the tunnel sites were 4.64 times higher than those at the urban site. This study conducted a comparison of PPDs and 6PPD-Q in urban and tunnel environments for the first time. Our findings clarified the key factors to reduce the pollution of PPDs in tunnel and supported policy-making for emission reduction of PPDs and 6PPD-Q.