Quantifying greenhouse gas (GHG) emissions from large industrial complexes remains challenging due to the coexistence of diverse source types, highly variable operational conditions, fugitive releases, and limited representativeness of bottom-up inventories. These complexities require observational approaches resolving emissions across multiple spatial and temporal scales. To address this gap, this study conducted a coordinated multi-platform observation campaign from 6–14 January 2024 at the Daesan Petrochemical Industrial Complex (DPIC), one of South Korea’s largest petrochemical hubs, which integrated in situ and ground-based column observations, mobile surveys, aircraft measurements, and satellite data across complementary spatial and temporal scales. Ground-based measurements confirmed sustained CO₂ enhancements of 27.72 ppm above background concentrations within the DPIC and observed frequent downwind transport of emissions beyond the complex boundary. Mobile measurements identified combined signatures of combustion-linked emissions and fugitive methane releases at the two largest-emitting facilities, while aircraft emission ratio characteristics corroborated these findings, revealing methane leakage concentrated in the DPIC center alongside increased combustion activity. An annual GHG emission rate of 2,595.45 ± 333.09 GgCO₂eqyr^–1 derived from the two largest-emitting facility hotspots alone accounted for approximately 37.1 % of the nationally reported inventory values, suggesting systematic underestimation of bottom-up accounting frameworks, especially of fugitive methane. Satellite observations further identified methane enhancements in access-restricted areas consistent with ground-based measurements. This study demonstrates that coordinated multi-platform observation strategies provide critical constraints for atmospheric emission verification and a scalable framework for improving the accuracy of the quantification of greenhouse gas emissions in complex industrial environments.