Size-resolved microelectrical effects of fine aerosol on nocturnal ozone: Evidence from field observations and...

Abnormal nocturnal ozone (O₃) enhancement has been frequently observed worldwide, but its driving mechanisms under stable nocturnal atmospheric conditions remain insufficiently understood. While meteorological transport is recognized as a dominant statistical predictor, the role of particulate electrical properties in nocturnal O₃ chemistry has rarely been considered. Based on summertime ground-based observations in Xi'an during 2023-2025, this study investigated the association of particle charge-to-mass ratio (CMR) on nocturnal O₃ variations using generalized additive model (GAM), partial least squares structural equation modeling (PLS-SEM), and a simplified interfacial contribution model. Results showed that nocturnal O₃ anomalies were consistently accompanied by elevated particulate CMRs, with values approximately 2-3 times higher than those on normal nights. A pronounced size-dependent association was identified: significant fitted responses were mainly confined to fine particles of 0.30-0.94 μm, with the strongest response occurring in the 0.30-0.58 μm fraction, whereas coarser particles (>0.94 μm) showed no statistically significant contribution. Furthermore, this promoting effect was strongly modulated by relative humidity, operating predominantly under low-RH conditions (RH < 50%) and substantially attenuated at higher RH. Observation-constrained estimates indicated that the interfacial formation contribution ratio was higher in the high-CMR group than in the low-CMR baseline, particularly under low-RH conditions. These findings suggest that fine-particle electrical properties are conditionally associated with nocturnal O₃ persistence under low-RH conditions, possibly through gas-particle interfacial processes. This highlights particulate electrical properties as a potential supplementary factor in urban nocturnal O₃ chemistry.