Comparison of Ozone Fluxes over a Maize Field Measured with Gradient Methods and the Eddy Covariance Technique

Ozone (O3) fluxes were measured over a maize field using the eddy covariance (EC) technique and gradient methods. The main objective was to evaluate the performance of the gradient methods for measuring the O3 flux by comparing them with the EC O3 flux. In this study, turbulent exchange coefficients (K) calculated with three methods were compared. These methods were the aerodynamic gradient (AG) method (in which K is calculated by using wind speed and temperature gradients), the aerodynamic gradient combined with EC (AGEC) method, in which the friction velocity and other variables are based on EC measurements, and the modified Bowen ratio using the EC sensible heat flux and temperature gradient (MBR) method. Meanwhile, the effects of the measurement and calculation methods of the O3 concentration gradient were analyzed. The results showed that: (1) on average, the transfer coefficient computed by the MBR method was 40% lower, and the coefficient determined with the AG method was 25% higher, than that determined with the AGEC method. (2) The gradient method’s O3 fluxes with the MBR, AGEC, and AG methods were 30.4% lower, 11.7% higher, and 45.6% higher than the EC O3 flux, respectively. (3) The effect of asynchronous O3 concentration measurements on the O3 gradient must be eliminated when using one analyzer to cyclically measure two-level O3 concentrations. The accuracy of gradient methods for O3 flux is related to the exchange coefficient calculation method, and its precision mainly depends on the quality of the O3 gradient.