Quantification of Exposure to Black Carbon and NOx inside Vehicles when Driving through a Road Tunnel

This study aimed to quantify the concentrations of NOx and black carbon (BC) inside vehicle cabin in relation to outside concentrations while driving through a 3.8 km long road tunnel, Sodra Lanken, in Stockholm. 10 different vehicles of different types, brands and sizes were chosen as the test fleet. Every vehicle was run back and forth 8 times through the road tunnel, four times in each direction. Temperature inside vehicles was set to 20°C and fan speed was set to auto. The instruments for measuring NOx were two model 405 nm NO2/NO/NOx Monitor™ from 2B technologies, and BC was measured using two Aethalometer, microAeth® model AE51 from AethLabs. In addition, CO2 was also measured using Senseair K33 LP T/RH CO2-sensor. All instruments were battery powered. From the speed of which the pollutants decreased after exiting the tunnel, turnover time for the air in the tested vehicles was calculated to be between 1 and 2 minutes. One passage through the tunnel took about 4 minutes. The total exposure in the passenger compartment varied depending on the car model. The measurements of BC showed that a driver is exposed to between 20% and 80% of the exposure compared driving in a roofdown convertible through the tunnel. For NOx, the exposure in the car compartment was significantly higher, and almost no difference compared to driving with the roof down. Approximately 20%–40% of the BC exposure occurred after the vehicles had exited the tunnel. For NOx, about 30%-45% of the inside car exposure occurred outside the tunnel. The BC to NOx ratio inside the tunnel was calculated to 12.2 ± 1.1 mg BC g-1 NOx-1. Compared to the corresponding ratio in the emission model HBEFA 4.0, which is is 4.3 mg BC g-1 NOx-1, the measured ratio was three times higher.