Size-resolved eddy covariance fluxes of nucleation to accumulation mode aerosol particles over a coniferous forest

Deventer M., Held A., El-Madany T., Klemm O.

Abstract

In late summer 2013, size-resolved particle number concentrations above and below a coniferous forest canopy have been measured at the 'Waldstein' site in NE Bavaria (GER). The application of a fast particle spectrometer (ELPI+) on top of a 30m scaffolding tower allowed the direct calculation of particle number fluxes for 10 separate size stages using the eddy covariance method. To the best of our knowledge this study is the first to report size-resolved and directly measured eddy fluxes and deposition velocities for a wide size range covering nucleation to accumulation mode particles with diameters between 0.006 and 1.4μm. The results suggest that new particle formation (NPF) leads to enhanced particle number fluxes. Overall, a net deposition of -4.1e+07particlesm-2s-1 corresponding to a mean deposition velocity of -0.27cms-1 was observed. Size dependent upward fluxes for ultrafine particles were mainly observed during the morning on NPF days. In total, about 30% of the measured fluxes were upward directed. This study examines correlations between apparent upward fluxes and explanatory variables such as random flux errors and wind direction. Further, a comparison of the measured deposition velocities to latest eddy covariance observations and models was performed. A simple second-order fit to observations for particle sizes between 0.1 and 1μm is presented.