Relaxed eddy accumulation simulations of aerosol number fluxes with various proxy scalars

The vertical transport of aerosol particles in the atmospheric boundary layer has become an important research area in many disciplines, e.g. in aerosol-cloud-climate-interactions, or investigations of the nutrient and pollutant input into ecosystems. In the absence of sensors sufficiently fast for direct eddy covariance measurements, relaxed eddy accumulation based on conditional sampling of atmospheric particles into separate reservoirs has been used for size-resolved aerosol flux and aerosol constituent flux measurements.

In this study, eddy covariance measurements of aerosol number fluxes from the Canopy Horizontal Array Turbulence Study (CHATS) in Dixon, CA are presented and compared to relaxed eddy accumulation simulations. The validity of the similarity assumption of different scalars such as aerosol number concentration, temperature and water vapor density will be evaluated and discussed.

From the high-resolution time series of these scalars, b factors are calculated using various definitions of updraft and downdraft conditions, i.e. different wind deadbands. Depending on the selected deadband, the sampling process is concentrated towards strong updrafts and downdrafts, which increases the scalar concentration difference between the updraft and downdraft reservoirs. This is compensated by a decrease in the corresponding b factor. While functional relationships of this dependence have been suggested for various scalars including temperature, CO₂, and H₂O, we are not aware of previous investigations of this relationship with regard to ultrafine aerosol number concentrations.

This study contributes to a better understanding of the turbulent transport of aerosol particles in the boundary layer, and selecting appropriate proxy scalars in future relaxed eddy accumulation applications for aerosol flux measurements.