This study explores the potential of a novel quantitative CT emphysema air-trapping composite (EAtC) mapping method, implemented in Coreline Soft's AVIEW COPD, for assessing lung function in COPD patients. The method classifies lung parenchyma into three areas - normal, functional air trapping (fAT), and emphysema - based on the analysis of inspiration and expiration CTs in 584 COPD patients. The volume fractions of each area were then compared with clinical parameters and pulmonary function tests (PFTs). Findings revealed that each EAtC class had moderate correlations with forced expiratory volume in 1 second (FEV1) and FEV1/forced vital capacity (FVC). Both fAT and emphysema were significant predictors of FEV1 and FEV1/FVC, with fAT showing better correlations with PFTs than small-airway disease on parametric response mapping. The study concludes that EAtC mapping offers comprehensive lung functional information, potentially serving as an imaging biomarker of lung function in COPD.