A multiyear concentration and flux-based O₃ risk assessment of winter wheat in China: Trends and drivers

Surface ozone (O₃) is a major factor limiting wheat production in China, especially in the North China Plain (NCP), which produces nearly 60% of national winter wheat output. In this study, concentration-based (AOT40) and flux-based (POD₁₂) metrics were used to assess O₃ risks from 2016 to 2023 by combining O₃ monitoring data, observational phenology and ERA5 meteorological data. Mean AOT40 was 9400 ppb h in the NCP, while mean POD₁₂ averaged 1.74 mmol m^-2, confirming NCP as China's main O₃ risk hotspot. POD₁₂-based national annual wheat yield and economic losses were 17.76 million metric tons (Mt) and USD 6.58 billion, lower than the 21.60 Mt and USD 8.03 billion estimated using AOT40. Principal component analysis identified vapor pressure deficit (VPD) and light as the leading drivers of spatial limitation on stomatal conductance. VPD limited O₃ uptake especially in the NCP, a trend that furthermore has increased in recent years. Although these physiological constraints have reduced flux-based O₃ risk in key provinces, overall O₃ exposure (i.e., AOT40) continues to rise. This multiyear, process-based assessment provides the most recent national evaluation of wheat O₃ vulnerability, highlighting the importance of using flux metrics for realistic risk mapping and supporting adaptive management strategies under the changing climatic conditions.