Within-Year PM _2.5 Exposure Structure Provides Mortality-Predictive Information Beyond the Annual Mean

Long-term PM _2.5 exposure is associated with increased mortality and is usually represented by annual mean concentration, which does not capture how concentrations are distributed across days. We tested whether the prespecified PM _2.5 magnitude-rank index (PMRI), a summary of within-year daily concentration structure, improves mortality prediction beyond the annual mean. Daily modeled PM _2.5 estimates were linked to age-adjusted mortality rates for 27,289 county-years from 3,064 U.S. counties during 2003–2011. PMRI is the largest k for which at least k days reached k µg/m³. County-grouped 10-fold cross-validation compared models with annual mean alone versus annual mean plus PMRI, with additional geographic, temporal, cause-specific, and specification analyses. Annual mean PM _2.5 had the highest standalone predictive performance. Adding PMRI increased county-held-out R² from 0.0802 to 0.1391 (ΔR² = 0.0589; 95% CI, 0.0499–0.0677) and reduced RMSE and MAE. Improvement persisted across geographic and temporal validations and all six cause-specific mortality outcomes. Within-year PM _2.5 exposure structure therefore contains reproducible mortality-predictive information not captured by annual mean concentration.