Ozone deposition measurements over wheat fields in the North China Plain: variability and related factors of deposition flux and velocity

Ozone (O ₃) deposition is closely related to air quality, ecosystem and climate changes. Due to the instrument and method shortage, O ₃ deposition was less observed and investigated in China, experiencing significantly increasing O ₃ exposure. Here, we conducted a comprehensive measurement of O ₃ deposition over the wheat canopy at a typical polluted agricultural site in the North China Plain using a newly developed relaxed eddy accumulation system. For the main wheat growing season in 2023, O ₃ deposition flux and velocity (V _d) averaged -0.25 ± 0.39 μg m ^-2 s ^-1 and 0.29 ± 0.33 cm s ^-1, respectively. Daytime V _d (0.40 ± 0.38 cm s ^-1) was obviously higher than in the nighttime (0.17 ± 0.26 cm s ^-1). The temporal changes of V _d were mainly determined by crop growth, with predominant contribution of stomatal uptake. Both daytime and nighttime V _d exhibited significant increases with decreasing relative humidity, and increasing friction velocity and soil water content, enhanced by higher leaf area index. With rapid increases of soil moisture, simultaneous and following overall increments in V _d were detected, attributed to remarkably strengthening O ₃ stomatal uptake under increased stomatal conductance and extended opening to the night, and more non-stomatal O ₃ removal at night resulted from strengthened soil NO emission at moist conditions. This study confirms the leading effects of crop growth on O ₃ deposition modulated by environmental conditions and the non-negligible influences of nocturnal plant activities, and emphasizes the needs for O ₃ deposition observation over different surfaces and accurate evaluation of O ₃ agricultural impacts based on deposition fluxes.