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• A framework to assess the effects of time-dependent regulations was established. • Special period regulation (SPR) effectively reduced the air pollution level. • There are heterogeneous impacts of the SPR across cities. • The effects of SPR are negative across 24 h and weak from 12 pm to 2–3 pm. • SPR achieves a "win-win" status about environmental protection and economic growth. This study examines the impacts of time-dependent environmental regulations on air pollution during the autumn and winter. This was done by exploiting a quasi-experimental setting where a special period regulation was implemented in the Changsha-Zhuzhou-Xiangtan region (CZT, located within Hunan Province in southern China), but not in cities surrounding CZT over the same period. Using hourly level data from October 1, 2014, to September 30, 2017, and by employing a difference-in-differences model, we found that the special period regulation significantly reduced PM 2.5 concentrations, the Air Quality Index, and the probability of the occurrence of severely polluted days in CZT. Furthermore, we found that while this regulation had significant negative impacts on air pollution in Changsha, the impacts were not robust in Zhuzhou and Xiangtan. Our results further revealed that the regulatory effects were significantly negative over a 24-hour time period, but the effects temporarily weakened from 12 pm to 2–3 pm. We also confirmed that such regulations can be used to achieve a "win-win" situation in terms of environmental protection and economic growth. Thus, the results suggest that time-dependent environmental regulations will be valuable in combination with existing policies to improve air quality during specific periods. [ABSTRACT FROM AUTHOR]