Freeway bottlenecks often cause traffic capacity drops and speed oscillations that not only 2 compromise traffic performance at the bottlenecks but also likely propagate far backward to 3 break down the upstream traffic. The adverse impacts of bottlenecks include more travel delay, 4 excessive fuel consumption and emissions, and extra safety risks. With the advent of connected 5 and automated vehicles (CAV) technologies, we can control detailed vehicle trajectory shapes 6 within sensing, communication, computation, and physical limits. In this study, a CAV-based 7 trajectory-smoothing concept is proposed to harmonize traffic and improve mobility and 8 environmental impacts. The presented algorithm is applicable to mixed-traffic environments 9 where only a portion of vehicles are CAVs. Simulation analyses are performed to assess the 10 algorithm performance. The results show significant improvement in traffic throughput as well 11 as in fuel consumption and emissions. 12