The interpretation of measurements of surface exchange of nitrogen oxides: correction for chemical reactions

The interpretation of measurements of the land-atmosphere exchange of NO and NO 2 is complicated by the interference of chemical reactions in air. In the absence of measurements to test correction procedures a one-dimensional surface layer model was constructed to generate concentrations and fluxes of relevant trace gases in the surface layer to simulate measurement data. The model contains a detailed description of surface exchange processes and a comprehensive submodel describing relevant reactions in air including hydrocarbons. To provide realistic conditions in the simulation runs important parameters and boundary conditions to drive the model were obtained from a large field experiment. The results of calculations with the model were used to test and compare several existing and new procedures to correct flux measurements. In testing the model important parameters such as air concentrations and surface fluxes were varied over an order of magnitude in order to compare the procedures in a broad range of conditions. The calculations showed that the true surface flux of NO and NO 2 may differ from the uncorrected flux by, on average, 20% during the day to 40% at night. Correction procedures from the literature yielded incorrect results with differences as large as 100% between the modelled and true surface fluxes because basic assumptions were not valid. A new, simple, correction procedure is described which derives fluxes that differ by less than 5% from the true flux during the day and less than 20% at night. All studied procedures were then applied to field data to derive the true surface flux of NO and NO 2 above grassland.

Response of a grassland ecosystem to air pollutants: III--The chemical climate: Vertical flux densities of gaseous species in the atmosphere near the ground

The establishment of element balances for ecosystems presupposes a knowledge of the amounts of the respective element exchanged between the ecosystem and the atmosphere near the ground by determining their vertical flux densities. Any adequate approach to calculate flux densities of gaseous species in the atmosphere has to use micrometeorological techniques. The authors applied the ratiometric method, which is described in detail. Results of flux density calculations for sulfur dioxide, nitrogen dioxide, nitric oxide and ammonia obtained during two years of measurement are discussed with regard to their role in element balances. Flux densities and deposition velocities of ozone are interpreted as functions of meteorological parameters as well as of the biological activity of the canopy. The latter is characterized by the flux density and flux-to-concentration ratio of carbon dioxide.