Pollutant-specific effects of urban air policy: Causal evidence from South Korea's Clean Air Act

Source abstract

Urban air pollution remains a pressing challenge—especially in rapidly growing Asian cities—where population and economic expansion outpace environmental governance. We provide policy-attributable, pollutant-specific causal evidence from the Seoul Metropolitan Area (SMA), to our knowledge, the first causal quantification of Phase 2 of Korea's Clean Air Act (CAA) by pollutant. Using a Difference-in-Differences (DID) design paired with the Synthetic Control Method (SCM) over a longer pre-period to triangulate identification and address each method's vulnerabilities, we find that the CAA reduced NO₂ in the SMA by approximately 1.4 ppb relative to other regions, while PM₁₀ remained unchanged. This divergence reflects pollutant behavior: NO₂—primarily from local mobile sources—responds to localized interventions, whereas PM₁₀, shaped by secondary formation and long-range transport, does not. Further analysis indicates that vehicle-related interventions were effective, whereas industry-focused measures were constrained by design and enforcement gaps. Results highlight the need for pollutant-specific strategies in urban air governance: localized measures can effectively target short-lived pollutants such as NO₂, whereas pollutants like PM₁₀ require coordination across jurisdictions. Our estimates provide decision-relevant benchmarks for SMA-like megacities, demonstrating what comprehensive urban policy packages can, and cannot, deliver across pollutants.