A Simulation of the Eddy Accumulation Method for Measuring Pollutant Fluxes

Abstract

“Eddy accumulation” is a variation of standard eddy correlation techniques for determining eddy fluxes by sampling air in two separate systems depending on whether the vertical velocity is positive or negative. In concept, the corresponding eddy flux is determined directly from measurements of the pollutant concentration (or accumulation) difference between the two sampling systems. In practice, the method has not yet been demonstrated for a slowly-depositing pollutant. A numerical simulation of the eddy accumulation technique has been used to test the sensitivity of the method to errors arising from various sources, including sensor orientation, sampling limitations and chemical resolution. These tests were conducted using artificial pollutant concentration signals derived from real meteorological data (obtained above a forest canopy), in order to avoid the possibility of injecting unwanted errors by employing a poor quality pollutant signal. To detect a pollutant deposition velocity of 0.1 cm s⁻¹, it appears necessary to maintain linear sampling characteristics over a dynamic range corresponding to two orders of magnitude of vertical wind speed (the limits are approximately 0.05 σ_w and 5σ_w in any given condition, where σ_w is the standard deviation of the vertical velocity w), to maintain sampling zero offsets to less than 0.02σ_w of equivalent vertical velocity and to resolve chemical concentration differences amounting to about 0.4% in typical conditions.