In situ measurement of black carbon emission from gasoline powered vehicles

摘要

Motor vehicle emission is the most important source of black carbon (BC) in urban areas. Accurately quantifying BC emission factor (EF) for motor vehicles is a prerequisite for the estimation of BC emission inventory. Traditionally, BC emission factors for vehicles were determined by measuring carbon dioxide (CO_2) and BC at the roadside or chasing a vehicle on-road using the carbon mass balance (CMB) method. However, since BC and CO_2 are dispersed greatly and inhomogeneously by atmospheric turbulence after being exhausted from the tailpipe, quantifying EFs by the traditional method (measuring their increments above background concentrations) would cause large uncertainty. In this study, BC and CO_2 concentrations were measured inside the tailpipe of gasoline vehicles with different engine sizes driving at select speeds and accelerations to determine the respective EFs using the CMB method. BC EFs ranged from 0.10 to 7.63 mg/kg-fuel when the vehicles were moving at speeds of 20-70 km/h. Acceleration is the main cause for increased BC emissions, which resulted in a BC emission value approximately 5 times larger than when moving at constant speeds. The results were also compared with those of recent studies that measured BC emission by chasing vehicles on-road. It shows that the in-situ measurements are about two orders of magnitude lower that of the chasing method, suggesting that measuring location is critical to accurately capture BC EFs for gasoline vehicles. The BC concentration in the exhaust of gasoline vehicles could be only 2-5 times the background concentration, which would cause the concentration increase to not be significantly higher than the background and the fluctuation of background BC concentration would contribute to a large uncertainty of the emission measurement. Condensation in the exhaust has negative impact on the measured EF results. Our dilution tests show that condensation could reduce the measured EF by 30-40%. Considering that the in situ measurements of EFs were two orders of magnitude lower than the chasing methods, it implies that the indirect measurements at the roadside or on the vehicle chasing behind may overestimate BC EF for motor vehicles, especially for gasoline cars that have low BC emissions. Therefore, it is recommended to conduct BC emission measurements at the tailpipe instead of in the atmosphere behind the vehicles. When developing a BC emission inventory for the traffic sector, it is suggested to use the emission factors determined in this study for gasoline vehicles to avoid significant overestimation of BC emission by this type of vehicle.