Integrating portable air cleaners to reduce mechanical ventilation energy under CO₂-based control: Twin-chamber experiments and multi-city analysis of energy–IAQ trade-offs

• Twin-chamber tests minimised bias and offered a quantitative basis for comparison. • MV and PAC removed particles more effectively than ozone. • MV performance was stable, while PACs were sensitive to ACHs and indoor sources. • No coupling effects between MV and PAC were found. • Enthalpy differences and seasonal shifts in dominant pollutants affecting energy use. Mechanical ventilation (MV) in public buildings is typically designed for minimum ventilation rates, which challenges the global carbon-reduction goals. While portable air cleaners (PACs) are widely used as supplementary devices, their potential as partial substitutes for ventilation—maintaining acceptable indoor air quality (IAQ) with reduced energy demand—remains underexplored. This study integrated controlled twin-chamber experiments with multi-city modelling to quantify energy-IAQ trade-offs of MV and PAC systems at different air change rates (ACHs) and explore integrated MV–PAC strategies. The experiments aimed to: (1) quantify the cleaning performance and energy burden of MV-only and PAC-only systems across representative pollutants (ultrafine particles, PM 0.3–10 , ozone, TVOC, and benzene series) using the Levenberg–Marquardt optimization; (2) assess the effects of air distribution, ACH, and emission sources; and (3) validate a theoretical MV–PAC coupling model. Five scenarios were developed based on CO₂-derived clean air demand, each representing a different PAC-to-MV substitution ratio. Two key decision factors—the dominant pollutant type and the indoor–outdoor enthalpy difference—were applied to analyze system performance across 15 cities spanning various climate zones and seasons. Results indicated that both MV and PAC systems removed particles more effectively than ozone, with PAC performance sensitive to ACH and more effective when indoor emissions dominated. Integrated MV-PAC system achieved up to 50% energy savings in summer while maintaining pollutant removal above 80%, with optimal configurations varying by climate and pollutant profiles. These findings support informed decision-making for integrating PACs with MV systems, promoting more flexible and energy-efficient ventilation strategies.