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 NO2 is complicated by the interference of chemical reactions in air. In the absence of measurements to test correction procedures a o ne-dimensional surface layer model was constructed to generate concent rations and fluxes of relevant trace gases in the surface layer to sim ulate measurement data. The model contains a detailed description of s urface exchange processes and a comprehensive submodel describing rele vant reactions in air including hydrocarbons. To provide realistic con ditions in the simulation runs important parameters and boundary condi tions to drive the model were obtained from a large field experiment. The results of calculations with the model were used to test and compa re several existing and new procedures to correct flux measurements. I n testing the model important parameters such as air concentrations an d surface fluxes were varied over an order of magnitude in order to co mpare the procedures in a broad range of conditions. The calculations showed that the true surface flux of NO and NO2 may differ from the un corrected flux by, on average, 20% during the day to 40% at night. Cor rection procedures from the literature yielded incorrect results with differences as large as 100% between the modelled and true surface flu xes because basic assumptions were not valid. A new, simple, correctio n 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 NO2 above grassland.