Application of Micrometeorological Methods for Determining Fluxes of Nitrogen Compounds

For many applications of determining the deposition or emission of a trace gases or aerosols the landscapes, micrometeorological methods are often employed to determine the flux. These methods are often used because the fluxes can resolved at time scales that are on the order of hours. This is particularly important because there is often a diurnal pattern of trace gas fluxes and measurements are needed in order to resolve the diurnal nature of the flux. The particular method used depends on the accuracy of the concentration measurement and the frequency at which the measurement can be made. For those gases in which concentrations can be measured at a frequency of at least 1 Hz, the eddy covariance method is the preferred technique and fluxes are usually computed at 30 minute intervals. Although newer cutting edges methodologies, are being developed for fast measurements, there are relatively few chemical species for which fast response sensors are available. Those include O3, SO₂, CO2 and NO. There are more sensors available that can measure trace gas concentrations, but that are not fast response. Flux/gradient methods are then often employed with such sensors to determine fluxes. These include the Bowen ratio, modified Bowen ratio, and aerodynamic method. Essentially, the fluxes and corresponding gradients of well know surrogates such as heat and water vapor, are used to compute the fluxes of other constituents based on their gradients over some specified interval. These methods require accurate measurements such that small differences in concentrations can be resolved. Typically, differences less that 5% of the mean concentration are required.

More recently, the conditional sampling (relaxed eddy accumulation) method has been used to measure trace gas and aerosol fluxes. With this method, sample collectors or “accumulators” are used to selectively sample large “updrafts” and “downdrafts” of which the difference in proportional to the flux. Filterpacks and annual denuders are often used in this configuration. Similar to gradient methods, accurate differences are required in order to obtain a statistically significant flux. Special care must be made to monitor flow rates and extraction volumes. Examples of each will be presented with a focus on the application towards nitrogen compounds, especially ammonia.