Direct measurement of turbulent particle exchange with a twin CPC eddy covariance system

Direct measurements of particle number fluxes by eddy covariance (EC) were carried out in the years 2001 and 2002 during the BEWA2000 field experiments. An EC system combining a sonic anemometer and two condensation particle counters was set up and successfully applied above a Norway spruce forest in NE Bavaria, Germany. Particle deposition clearly dominated over emission, with the strongest deposition fluxes occurring during particle formation events identified from submicron particle size distributions. Typical deposition velocities derived from these measurements ranged from −37 to +23 mm s−1. The ultrafine particle fraction (UFP) (3–11 nm diameter) showed different concentration patterns and larger deposition velocities as compared to the particle fraction with larger diameters. Also, particle deposition occurring before noon could be attributed mainly to the UFP, whereas larger particles contributed to turbulent deposition in the afternoon.