Green infrastructure and air pollution: evidence from highways connecting two megacities in China

Following market liberalization, the vehicle population in China has increased dramatically over the past few decades. This paper examines the causal impact of the opening of a heavily used high speed rail line connecting two megacities in China in 2015, Chengdu and Chongqing, on air pollution. We use high-frequency and high spatial resolution data to track pollution along major highways linking the two cities. Our approach involves the use of a novel augmented regression discontinuity in time approach that incorporates machine learning to inform our specification choice in the first stage. Our estimates show that CO is reduced by 6.4% and PM_2.5 by 7.1% along the main affected highway. These findings are supported using a difference-in-differences approach.