Spatiotemporal characteristics of ozone and the formation sensitivity over the Fenwei Plain.

High surface ozone (O) levels affect human and environmental health. The Fenwei Plain (FWP), one of the critical regions for China's "Blue Sky Protection Campaign", has reported severe O pollution. This study investigates the spatiotemporal properties and the causes of O pollution over the FWP using high-resolution data from the TROPOspheric Monitoring Instrument (TROPOMI) from 2019 to 2021. This study characterizes spatial and temporal variations in O concentration by linking O columns and surface monitoring using a trained deep forest machine learning model. O concentrations in summer were 2-3 times higher than those found in winter due to higher temperatures and greater solar irradiation. The spatial distributions of O correlate with the solar radiation showing decreased trends from the northeastern to the southwestern FWP, with the highest O values in Shanxi Province and the lowest in Shaanxi Province. For urban areas, croplands and grasslands, the O photochemistry in summer is NO-limited or in the transitional regime, while it is VOC-limited in winter and other seasons. Reducing NO emissions would be effective for decreasing O levels in summer, while VOC reductions are necessary for winter. The annual cycle in vegetated areas included both NO-limited and transitional regimes, indicating the importance of NO controls to protect ecosystems. The O response to limiting precursors shown here is of importance for optimizing control strategies and is illustrated by emission changes during the 2020 COVID-19 outbreak.