Ozone deposition onto bare soil: A new parameterisation

The variables controlling ozone deposition onto bare soil are still unknown and it is necessary to understand this pathway well, as it represents a significant sink for ozone. Eddy-covariance measurements of ozone (O3) fluxes were performed over bare soils in agricultural land. Three datasets with contrasted meteorological conditions and soil nitric oxide (NO) emissions were used to study the factors controlling soil deposition. It is considered that ozone deposition can be represented with an aerodynamic resistance (Ra), a quasi-laminar boundary layer resistance (Rb O3Rb O3), and an additional resistance, named soil resistance (Rsoil). Although it is assumed in previous studies that soil resistance is a function of soil water content (SWC) and could be considered constant as variation of SWC at monthly scale are generally weak, the results of this study indicate that SWC is not the main factor controlling Rsoil which shows daily and hourly variations. The main factor controlling soil resistance is the surface relative humidity which is positively correlated with Rsoil, contrary to non stomatal resistance onto canopies which show a negative correlation with relative humidity. The relationship between Rsoil and the surface relative humidity is probably due to a decrease in the surface available for ozone deposition, due to an increasing adsorption of water molecules onto the ground with relative humidity. A new parameterisation of Rsoil was established, where Rsoil is a function of the surface relative humidity only (Rsoil = Rsoil min × e(k×RHsurf), and Rsoil min = 21 ± 1.01 s m−1 and k = 0.024 ± 0.001, mean ± SD). The measured and parameterised ozone deposition velocities agree well when soil NO emissions are negligible. However, when there are large soil NO emissions, the parameterised ozone deposition strongly underestimates the measured deposition velocity even if the chemical destruction of ozone by reaction with NO in the air column was evaluated to be negligible. This suggests that soil NO emissions enhance soil ozone deposition by chemical reaction at or near the soil surface. The new parameterisation allows a better estimation of soil deposition, especially during daytime when Rsoil is overestimated using previously published parameterisations. It is an important step towards a better parameterisation of the non-stomatal uptake of ozone.

Research highlights
► The study reports ozone fluxes measured over one agricultural field during periods with bare soil.
► Soil resistance was found to depend on surface relative humidity.
► Soil resistance was found to increase with relative humidity.
► A new parameterisation of soil resistance is proposed.