Impact of filtering methods on ultrafine particles turbulent fluxes by eddy covariance

In the present work, a comparison between Linear Detrending (LDT) and a Recursive Digital Filter (RDF) inremoving the low-frequency contribution to the vertical turbulent fluxes was performed. The two methods wereapplied in order to obtain a correct evaluation of ultrafine particles, sensible heat and momentum turbulentfluxes. Exchange velocity was also evaluated separating the positive cases (the so called deposition velocity Vd)from negative cases (named emission velocity Ve). The low-frequency time scales (?c) required by the RDF wereobtained by means of an ogive analysis of turbulent fluxes for different atmospheric stability conditions (i.e.unstable, stable and neutral). RDF was applied also with a constant low-frequency time scale (RDF300, ?c = 300s).The stationarity test proposed by Mahrt (1998 - MST98) has been applied to momentum, kinematic temperatureand particle number fluxes before and after applying LDT and RDF methods, in order to investigate the impact offiltering criteria on stationarity of time series. Results emphasised that there were no significant differences instationary cases for different filtering procedures. The comparison analysis on the main turbulent variableshighlighted that wider discrepancies occurred between LDT and RDF300, showing on average an increase inturbulent number particles flux obtained through RDF methods, especially in unstable atmospheric conditions.On the other hand, a mean decrease for momentum and sensible heat fluxes was observed. Filtering proceduresled to a slight increase of exchange velocity, although an underestimation occurred for emission and depositionvelocities when considered separately