Eddy covariance measurements of ozone flux above and below a southern subtropical forest canopy

Abstract

While extensive eddy covariance (EC) measurements of ozone (O3) flux have been reported in American and European forests, such measurements in Asian forests are scarce. Here, we presented the first EC measurements of O3 flux at two levels (above and below the canopy) in a Chinese forest. Above the canopy, O3 always moved downward, with a maximum O3 flux intensity of −15 ~ −10 nmol m−2 s−1 occurring at 9:00–14:00 LT and a maximum O3 deposition velocity of 1.23 cm s−1 occurring at 9:00 LT; both of these values fell to nearly 0 at night. The O3 deposition flux and O3 deposition velocity below the canopy were both lower than those above the canopy. This discrepancy reached the maximum at 9:00–15:00 local time (LT), with the O3 deposition flux and O3 deposition velocity below the canopy being approximately 35 and 42% of those above the canopy, respectively. The O3 flux was well correlated with the CO2 flux and the latent heat flux, suggesting the important role of stomatal uptake in O3 deposition. The O3 deposition velocity increased with the increase in the air temperature, relative humidity, photosynthetically active radiation and friction velocity, but when these meteorological factors exceeded their optimum values, the increase in the O3 deposition velocity tended to be flat. These findings advanced our understanding of the interactions between forests and the atmosphere. This unique dataset is also of great significance for the validation of relevant models concerned with the various impacts of the rapid increase in global O3 concentrations.