Boundary-Layer Height Outweighs Wind Speed in Explaining Daily Urban Air Pollution: Unique-Variance Attribution and a City Typology for 10 French Cities

Meteorology drives the day-to-day pollution variation behind short-term health effects, and its misattribution biases exposure estimates and trend normalisation. Wind speed and boundary-layer height (BLH), the main dispersion variables, are strongly collinear (mean r ≈ 0.70, 0.81 on anomalies), with contested relative roles. We quantified their unique variances with semi-partial R2 for NO2, O3, PM10 and PM2.5 in France’s 10 largest cities (2018–2025), across four frames, from pooled levels to deseasonalised, de-trended anomalies, checked with generalised additive models and for wind direction. On pooled levels, BLH carries on average four times the unique variance of wind for NO2 and O3, and eight to ten for particulate matter, whereas their raw correlations with primary pollutants differ only 1.3- to 1.4-fold. This lead is robust for particulate matter, even with direction controlled. For O3, temperature dominates and humidity exceeds BLH on average, plausibly a photochemical signal. For NO2 it is partly seasonal: on anomalies they become comparable, and wind leads significantly only where regional winds are coherent. The NO2 contrasts define three city types, wind-dominated, transitional and BLH-dominated. Both remain significant for NO2 anomalies in every city, so normalisation models should retain both, with city-specific coefficients. BLH should become a first-tier predictor.