Ozone deposition measurements over wheat fields in the North China Plain

Ozone (O₃) deposition contributes 20% to the annual global tropospheric O₃ loss, affecting surface air quality,, the ecosystem and climate change. Limited by the instrument and method shortage, O₃ deposition in China, experiencing significantly increasing O₃ exposure, was less observed and investigated. Here, we conducted a comprehensive measurement of O₃ deposition over the wheat canopy at a typical polluted agricultural site of North China Plain using a new relaxed eddy accumulation (REA-O₃ flux) system. O₃ deposition flux and velocity (V_d) were at the averages of -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). V_d played a decisive effect on the diel pattern of deposition flux, while O₃ concentration determined the flux variability on the longer timescales. The temporal changes of V_d were mainly determined by crop growth during wheat growing season, with predominantly contribution of stomatal uptake. Both daytime and nighttime V_d exhibited significant increase with decreasing relative humidity, and increasing friction velocity and soil water content, enhanced by higher leaf area index. With rapid increase of soil moisture, simultaneous and following overall increments in V_d were detected, attributed that stomatal conductance increased and opening extended to the night, remarkably strengthening O₃ stomatal uptake, and soil NO emission might be strengthened at moist condition, facilitating non-stomatal O₃ removals at night. The study suggests the leading effects of crop growth on O₃ deposition modulated by environmental condition and the non-negligible influences of nocturnal plant activities, and emphasizes the needs for O₃ deposition observation over different surface and accurate evaluation of O₃ agricultural impacts based on deposition fluxes.