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 (Vd) were at the averages of -0.25±0.39 μg m-2 s-1 and 0.29±0.33 cm s-1, respectively. Daytime Vd (0.40±0.38 cm s-1) was obviously higher than in the nighttime (0.17±0.26 cm s-1). Vd 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 Vd were mainly determined by crop growth during wheat growing season, with predominantly contribution of stomatal uptake. Both daytime and nighttime Vd 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 Vd 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.