Independent and interaction effects of manufacturing agglomeration on air pollution: evidence from subsectoral and multi-pollutant analysis

Air pollution (AP) poses a major public health threat in rapidly industrializing regions like China, where manufacturing agglomeration (MA) drives both economic growth and environmental impacts. While extensive studies have examined the MA–AP relationship, consensus on its environmental externalities remains elusive. Current research lacks the granularity to guide low-pollution development, due to broad sector classifications, limited pollutant coverage, neglect of subsector interactions and inter-pollutant dependencies, and coarse spatial scales. These methodological limitations undermine both the accuracy and policy relevance of existing findings. Addressing these gaps, this study introduces a multi-pollutant and subsectoral perspective to analyze the spatial relationship between MA and AP in the Beijing–Tianjin–Hebei (BTH) region, China. Using the location quotient (LQ), correlation-based screening, and geographically weighted regression (GWR), this research assessed 18 manufacturing subsectors and multiple pollutants from both independent-effect and interaction-effect dimensions. Results revealed a clear north–south pollution divide, with heavier pollution in the south, while MA exhibited strong southeast–northwest spatial gradients and marked subsectoral variation. Independent-effect estimates revealed significant spatial heterogeneity in the effects of both overall and subsector-level agglomeration on the Air Quality Index (AQI) and six key pollutants. The integrated analysis demonstrated that synergistic pollution from subsector interactions was confined to specific sector–pollutant linkages, whereas inter-pollutant dependencies played a more prominent role. These findings underscore the limitations of uniform industrial policies. A differentiated strategy framework was developed, spanning two levels—synergistic interaction regulation and independent-effect strategies—and two dimensions—manufacturing type and developmental stage, together with a multi-pollutant coordinated control strategy centered on shared emission source identification. This research provides a theoretical and practical foundation for region-specific and sector-specific environmental governance, with broader relevance for comparable industrial zones.