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

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.