The flux of ozone to a maize crop and the underlying soil during a growing season

To observe the flux or deposition of ozone above a maize crop, experiments were carried out during the growing season of maize in 1988. The flux of ozone was determined using meteorological techniques. The measurements used in the present study were carried out under atmospheric conditions in which the vertical divergence of the flux of ozone was the dominant term in the mass conservation equation of ozone. That is, under such conditions, the flux measured at a certain height served as a good estimate of the flux at the surface. This was demonstrated in chapter 2 by a scaling exercise of the mass conservation equation and the time dependency of the flux of ozone in order to reveal the importance of the various terms in the equations. The second important term in this scaling is the chemical reactions which produce and destroy ozone (section 2.1.1). However, an accurate estimate of this influence could not be given at first hand. More accurate estimates were made with a model which describes the vertical divergence of the flux of ozone and nitrogen oxides. By scaling the equation of the local time derivative of the flux of ozone (section 2.1.3) it was found that the gradient production and pressure fluctuation term were much larger than the chemical reaction term. From this scaling and the model calculations it was concluded that the chemical reactions did not severely influence the flux of ozone.Three meteorological techniques were used to assess the flux of ozone: the eddy correlation technique, the profile technique and the modified Bowen ratio technique. The theoretical background to these techniques was given in chapter 2. Chapter 3 and 4 presented the experimental outline and the accuracy of the measurements, respectively.It was found that the accuracies of the fluxes were strongly determined by the errors in the differential measurements or the profiles of the variables. The accuracy of the flux of ozone measured with the profile technique was 20-53%. For the modified Bowen ratio technique this was 13-58%. The accuracy of the eddy correlation fluxes was about 20%. 'Ibis was mainly caused by the intermittency of the flux in the 30 min time interval over which they were averaged.A comparison between the three techniques was made for nine days. The profile technique gave systematically lower values for the flux of ozone than the eddy correlation and modified Bowen ratio techniques. A reduction of about 40% of the flux of ozone was found, calculated from the ozone concentration at 6 z om + d and 30 z om + d, during near-neutral and unstable atmospheric conditions. This was caused by a) an inadequate use of the profile technique close to the roughness elements and b) an uncertertainty in the displacement height for ozone. The flux of ozone determined with the modified Bowen ratio technique was moderately consistent with that determined with the eddy correlation technique and no systematic deviations were found. This indicates that: a) the modified Bowen ratio technique is applicable close to the surface, b) sensible and latent heat (water vapour) are transported in. roughly the same way and c) chemical reactions did not cause large systematic deviations, i.e. no large flux divergence between the two techniques existed though the fluxes were measured at different heights.The time integrals over the day of the fluxes of ozone derived with the modified Bowen ratio and the eddy correlation techniques agreed very well. This means that a reliable estimate of the daytime deposition of ozone (accuracy ±10%) was obtained using these techniques. The accuracy of the 30 min values, however, is much smaller (20-50%).In chapter 5 a resistance model was used to deduce the ability of the surface to destroy ozone, expressed in the surface resistance, from the flux measurements. This was done for bare soil as well as the crop - soil system as a whole when the soil was entirely covered by the crop. The resistance of the soil to ozone was dependent on the soil water content, i.e. the soil surface resistance increased with increasing soil water content.In the evaluation of the magnitude of the different parallel sinks of ozone such as the stomata and the soil surface, the conductances of the surface and the crop to ozone were used. An estimate on the crop conductance to ozone i.e. the stomatal uptake of ozone, was made using the analogy to the transpiration of the crop.The surface conductance to ozone was mainly determined by the uptake of ozone by the stomata, the destruction at the soil surface and the transport towards the soil. When the soil surface is wet (i.e. rainfall occurred a few hours prior to the measurements) the surface conductance and the crop conductance to ozone coincided.The conductance of the remaining plant parts (mainly the cuticle) to ozone was small compared to the stomatal or soil conductance to ozone.The exchange of ozone with the soil was mainly determined by the turbulent mixing (expressed by the friction velocity), the stability of the air above the crop and the leaf area density.When the soil surface is not wet (i.e. no rainfall a day before the measurements), the flux of ozone towards the soil can be 25-50% of the total flux of ozone. In such circumstances the flux of ozone should be modelled using a surface resistance in which the soil resistance to ozone, as well as an in- crop aerodynamic resistance are incorporated. This in-crop aerodynamic resistance depends among others on the turbulent mixing above the crop and the leaf area density.A more quantitative analysis of the exchange of ozone with the crop and the underlying soil can be made by using more complex canopy flow models such as those by Meyers and Hicks (1988), Li et al., (1985). In such models the non-local transport of momentum and scalars are described. With these models a more detailed sink distribution of ozone in the crop can also be made using, for instance, measured profiles of ozone in the crop (Raupach, 1989). Another outcome of these models can be a parameterization of the in-crop aerodynamic resistance for use in air pollution dispersion models.In chapter 6 an overview of the deposition of ozone and the governing factors during the growing season of maize were presented. The total deposition of ozone calculated as the time integral of the flux over the entire day, varied from 5-50 mg m -2, with an average of 19.0 mg m -2. The daytime deposition accounted for on average 83% of the total deposition. The deposition during night-time was small compared to the total deposition (17%). The total deposition showed a seasonal pattern. This pattern is largely caused by the seasonal pattern of the concentration of ozone. This is illustrated by the findings that the daytime deposition of ozone can be well estimated by the average concentration of ozone. A better estimate is obtained if the time period is included over which the flux is calculated i.e. the dose of ozone. The main reason for this good estimate is the relatively small fluctuations in the mean daytime surface conductance to ozone. The best estimate of the daytime deposition of ozone is obtained by using the average values of the concentration, the deposition velocity and the time period. This value gives a small underestimate (10%) of the daytime deposition due to some loss in correlation between the deposition velocity and the concentration.The uptake by the crop varied from 2.8 - 25.2 mg m -2, with an average of 12.8 mg m -2. This uptake was 50-100% of the daytime deposition of ozone, with an average of 86%. This uptake can be reasonably estimated with the dose of ozone.To reveal a seasonal trend in the uptake of ozone by the crop, a data series of at least several growing seasons is necessary to obtain full coverage on all wind directions and environmental situations in which the crop was grown. It is especially the coupling of these data to the effects on plants such as a reduction in crop yield that requires very long data series, since the climatic 'noise' on these data is very large. Therefore a more appropriate approach would be to evaluate all available measurements in this field by means of coupled flow - crop growth models. This data set can be used, for example, to verify such models in which the exchange of air pollutants with the crop and the soil is described.