Added Value and Geographic Transferability of Same-Day ERA5 Reanalysis for PM2.5 Onset Prediction Across Chinese Cities

Whether meteorology adds information for emerging fine particulate matter (PM2.5) pollution may depend on the baseline and evaluation population. We assessed next-day crossings above 75 µg m−3 among currently non-exceeding days in 12 development cities and six additional Chinese cities. Additive logistic models were trained on 2016–2021 data, selected using 2022 data and frozen for retrospective evaluation through 16 September 2026. In 2023–2024, same-day ERA5 surface meteorology increased average precision (AP) from 0.307 to 0.358 across 7651 city-days and 268 onsets. The unrounded paired increment was 0.050409 (95% calendar-block interval 0.020–0.088). Exploratory co-pollutant enrichment reduced the increment to 0.029; city-specific seasonal adjustment left it uncertain. Additional daily ERA5 boundary-layer height did not establish a further ranking benefit. Consistent transfer to the six additional cities was not established. In the six added cities, pooled and descriptive event-weighted within-city increments had opposite signs in partial 2026. That panel contained only 21 onsets in partial 2026, with uncertainty sensitive to calendar-block choices. These retrospective reanalysis results support informative baselines and local validation with predictors available at issuance.