Combined stationarity index for the estimation of turbulent fluxes of scalars and particles in the atmospheric surface layer

In this work the performances of three different tests widely used in scientific community for assessing turbulent flow stationarity have been tested and compared. The stationarity tests have been applied to turbulent fluxes of different scalars (temperature, carbon dioxide and water vapour concentration, ultrafine particle number concentration), collected over a wide range of surface roughness conditions, from almost smooth surfaces to surfaces with different degrees of complexity: suburban, urban and forested canopies. The application of the selected tests to the scalar and particle turbulent fluxes measured above the different experimental sites has frequently shown an ambiguous identification of non-stationarity in turbulent flux records, regardless of the surface conditions and of the transported quantity. The potential impact of surface characteristics, transported scalar, atmospheric stratification and micrometeorological conditions on test performances has been investigated. Finally a new operative stationarity index obtained by the combination of two tests has been proposed to assess the second-order stationarity of turbulence time series.