Diurnal variation of ozone flux over corn field in Northwestern Shandong Plain of China

High concentration ground-level ozone (O-3) has adverse effects on plant growth and photosynthesis. Compared to the O-3 concentration-based index, the O-3 flux-based (especially stomatal O-3 uptake) index has been considered the better criterion for assessing the impact of ozone on vegetation and ecosystems. This paper reports on a study of O-3 flux using the eddy covariance technique over a corn field in the Northwestern Shandong Plain of China. Diurnal variation of atmospheric O-3 concentration, deposition velocity and flux, and their relationships to environmental factors are analyzed. The results show that: (1) During the observation period (9 August-28 September, 2011), there was a strong diurnal variation of O-3 concentration, with low (16.5 nL L-1) and high (60.1 nL L-1) O-3 mean concentrations observed around 6:30 and 16:00, respectively. Mean O-3 concentrations during daytime (6:00-18:00) and nighttime (18:00-6:00) were 39.8 +/- 23.1 and 20.7 +/- 14.1 nL L-1 (mean +/- std), respectively. The maximum observed concentration was 97.5 nL L-1. The concentration was mainly affected by solar radiation and air temperature. (2) Whether daytime or nighttime, ground-level O-3 flux is always downward. The diurnal course of mean deposition velocity was divided into 4 phases: a low and stable process during nighttime, fast increasing in early morning, relatively large and steady changes around noon, and quickly decreasing in later afternoon. Daytime and nighttime mean deposition velocities were 0.29 and 0.09 cm s(-1), respectively. The maximum deposition velocity was 0.81 cm s(-1). The magnitude of deposition velocity was influenced by the corn growth period, and its diurnal variation was significantly correlated with global radiation and relative humidity. (3) O-3 flux was affected by variations of both O-3 concentration and deposition velocity, with mean O-3 fluxes -317.7 and -70.2 ng m(-2) s(-1) during daytime and nighttime, respectively. There was strong correlation between O-3 flux and CO2 flux or latent heat flux. By comparing the deposition velocities of daytime and nighttime, we infer that stomatal uptake was probably the main sink of ground-level O-3.