Regime-dependent sensitivity of the atmospheric potential gradient to anthropogenic air pollution in São Paulo, Brazil

Ground-based measurements of the atmospheric potential gradient (PG) are sensitive to near-surface electrical conductivity and can reflect anthropogenic perturbations under carefully defined fair-weather (FW) conditions. This study investigates the coupling between PG and major air pollutants in São Paulo, Brazil, using a long-term dataset (2018–2024) combining rooftop electric field measurements with hourly concentrations of CO, SO₂, NO, NO₂, NOₓ, and PM₁₀. FW periods were identified using surface meteorological screening and satellite-based cloud masking. The analysis reveals a reproducible hierarchy in PG–pollutant coupling controlled by the dynamical regime of the atmospheric boundary layer. During nocturnal stable conditions, primary combustion-related gases (NOₓ, NO, CO) exhibit the strongest associations with PG, with median Pearson correlation coefficients (r) on the order of r ≈ 0.6, representing the central tendency of a regime-dependent distribution. In contrast, PM₁₀ shows weaker and more variable coupling. Across all seasons, PG–pollutant correlations strengthen at night and weaken markedly during the afternoon mixing maximum, with enhanced wind ventilation further suppressing coupling strength. Large-scale emission perturbations support the anthropogenic origin of these relationships. During the 2020 COVID-19 lockdown, FW PG exhibits a persistent shift toward lower values relative to the multi-year baseline while preserving the diurnal phase structure. In contrast, a short (11-day) truck drivers' strike in 2018 produces no statistically distinguishable PG response. These results indicate that PG does not behave as a universal proxy for urban air pollutant concentrations in this megacity environment, but rather as a regime-dependent electrical indicator whose sensitivity to anthropogenic emissions emerges preferentially under stable boundary-layer conditions.