Does Forest City Construction Improve County-Level Air Quality? Evidence from China’s National Forest City Policy

Urban forest policies are increasingly used as nature-based solutions, but their large-scale effects on ambient air quality remain uncertain. This study evaluates China’s National Forest City Construction Policy (NFCC) using a panel of 2076 counties from 2001 to 2021 and a staggered difference-in-differences (DID) design. Annual mean PM2.5 concentration is used as the main outcome. In the preferred specification with county and year fixed effects, NFCC exposure is associated with a reduction of approximately 1.39 μg/m3 in annual mean fine particulate matter (PM2.5). The estimated effect remains negative across most robustness checks, although its magnitude and statistical precision weaken under more restrictive specifications. Both direct county-level designation and prefecture-level exposure are associated with lower PM2.5, with a larger point estimate for directly designated counties. The effect is stronger in counties with higher pre-policy pollution and population density. Spatial analyses further indicate that neighboring NFCC exposure is associated with lower PM2.5, although the estimated separation between own-county and neighboring effects depends on the definition of spatial proximity. Supplementary normalized difference vegetation index (NDVI) results are consistent with increased vegetation greenness following NFCC exposure. These findings suggest that forest-city construction can complement source-oriented pollution control, while the estimated magnitude remains sensitive to regional shocks and assumptions about treatment dynamics.