GEOS-CF Evidence for Regional Near-Neutral and Weak Day–Night Reversal Signals in Surface Ozone

Traditionally, near-surface ozone (O3) concentrations are higher during the day than at night. However, analysis of global hourly O3 reanalysis data from 2018–2025 reveals that a day–night reversal (higher nighttime than daytime concentrations) is emerging in many regions. Globally, the day–night O3 difference is diminishing, with a notable reversal already evident in parts of India, China, and selected cities in Western Europe. This shift challenges long-held assumptions about O3 diurnal patterns and has important implications for understanding regional chemistry, air quality assessment, and policy development.