Impact of the Nitrogen Emission Control Area policy on reducing NO₂ concentrations in coastal cities: evidence from a spatial difference-in-differences model

Nitrogen Dioxide (NO2) is an important air pollutant a major air pollutant emitted by ships. This study investigates the effect of the Nitrogen Emission Control Area (NECA) policy on NO₂ concentrations in coastal cities of the United States, employing a spatial difference-in-differences model and NO₂ Vertical Column Densities (VCDs). Results show that the NECA policy reduces NO₂ VCD by 4.9% in coastal cities and indirectly decreases NO₂ levels in adjacent inland areas. A lag effect is evident, with the full impact materialising in the second year post-implementation. The policy’s emission reduction benefits are particularly significant for East Coast coastal cities. The insights derived from this study can offer strategic recommendations for the future management of ship emissions.