The widespread occurrence of nocturnal NOx accumulation under weak‑wind conditions revealed by a dense air quality monitoring network in Japan

Understanding the influence of meteorological conditions on surface nitrogen oxide (NO x ) concentrations is important. However, interpreting increases in NO x concentrations at night remains challenging under weak-wind conditions. These increases can be caused by changes in emissions, horizontal transport, or accumulation within a stable nocturnal boundary - layer. To elucidate the processes controlling nighttime increases in NO x concentrations under weak‑wind conditions, we analyze a region‑wide nighttime NO x enhancement over Ibaraki Prefecture (Japan) using a dense regional air quality monitoring network. Hourly NO x concentrations and wind speed from 31 general ambient monitoring stations were analyzed. NO x concentrations peaked within a 1–2 h window across the monitoring network during the nighttime and early ‑ morning, primarily under weak wind speeds (< 1 m s − 1 ). The wind directions differed among stations, while NO x concentration peaks were observed at similar times across the region. These spatial and temporal characteristics are inconsistent with organized horizontal transport as the primary driver and support nighttime accumulation under weak atmospheric ventilation. The results show that dense surface monitoring networks enable diagnosis of nighttime NO x increases that cannot be inferred reliably from individual monitoring sites.