Yifu Ou, Yuan Liang, Donggen Wang
• We identify causal effects of air pollution on within-city mobility in China. • PM2.5 poses an average negative causal impact on within-city mobility in China. • There are substantial spatial variations underlying the PM2.5-mobility nexus. • Positive effects are observed in some cities. • The spatial heterogeneities are mainly driven by pollution and mobility levels. Understanding how air quality influences mobility patterns is crucial for effective transport and environmental management. While existing studies have explored the mobility impacts of air pollution, their findings remain inconsistent, partly due to the dualistic mobility responses (increased population redistribution within the city and reduced mobility for pollution avoidance) during high-pollution episodes. Here, we examine the causal effect of PM 2.5 levels on within-city mobility and explore driving factors of the spatial heterogeneity underlying this effect across 318 Chinese cities from May to November 2023, employing econometric models and explainable machine learning techniques. We find that, on average, a one-standard-deviation (16.99 μg/m 3 ) increase in PM 2.5 level reduces daily trips by approximately 0.28-0.41%, reflecting widespread travel avoidance on heavily polluted days. However, mobility responses exhibit significant spatial heterogeneity. Among sample cities, a negative association between air pollution and mobility has been observed in 68% of them, with the rest suggesting the other way around. The explainable machine learning model results further reveal that this disparity is mainly driven by transboundary air pollution and long-term mobility levels, sociodemographic factors (e.g., age, population, and tertiary industry GDP share), as well as land use features (e.g., population density, geographical area of the city). Our findings offer a scientific basis for continued air pollution mitigation efforts and support the design of targeted land use policies tailored to local conditions.