Optical properties of vehicular brown carbon emissions: Road tunnel and chassis dynamometer tests.

Brown carbon (BrC), as an important light-absorbing aerosol, significantly impacts regional and global climate. Vehicle emission is a nonnegligible source of BrC, but the optical properties of BrC emitted from vehicles remain poorly understood. This study evaluates the absorption Ångström exponent (AAE) of traffic-related light-absorbing aerosols (i.e., AAE) and the absorption emission factor (EF) of vehicular BrC via chassis dynamometer tests and a road tunnel measurement in Tianjin, China. AAE are estimated as 0.98-1.33 and 1.11 ± 0.001 for tested vehicles and on-road vehicle fleet, respectively. The AAE of vehicular BrC (AAE) is 3.83 ± 0.092 for on-road vehicle fleet. The vehicle technology updates effectively reduce the EF of vehicular BrC. Among the four tested China 5 and China 6 gasoline vehicles in the chassis dynamometer tests, BrC EF of China 5 gasoline direct injection vehicle is the highest, while China 6 mixing fuel injection vehicle exhibits the lowest EF. The BrC EF of on-road vehicle fleet at 370 nm wavelength are 0.081 ± 0.0058 m kg for mixed fleet, 0.074 ± 0.018 m kg for gasoline vehicles (GVs), and 1.66 ± 0.71 m kg for diesel vehicles (DVs) in the tunnel measurement. EF of GV fleet in the road tunnel is higher than China 5 and China 6 vehicles, as China 1-4 vehicles accounted for 26.8% of the total vehicle fleet in the tunnel. EF of vehicular BrC are lower than those from biomass burning and coal combustion emissions. The light absorption of BrC from GVs and DVs accounts for 7.2 ± 2.1% and 1.5 ± 0.77% of total traffic-related absorption at 370 nm, respectively. Our study provides optical features of BrC from vehicle source and could contribute to estimating the impacts of vehicular aerosol emissions on global and regional climate change.