One-year measurement of size-resolved particle fluxes in an urban area

Size-resolved particle flux measurements were carried out in an urban area from April 2012 to April 2013. Together with a standard eddy covariance system, two fast optical particle counters have been employed on a 65-meter-high tower in Münster, Germany. Particle number fluxes were directly calculated for particles with diameters from 0.06 to 10 µm within 16 individual size-bins. Whereas particle number concentrations show a distinct yearly pattern with maxima in winter and minima in summer, the flux time series is more multifaceted. Average daily maxima of 3.0e+07 particles m⁻² s⁻¹ occurred during winter while minima of 2.0e+06 particles m⁻² s⁻¹ were observed in fall. The size-resolved measurements revealed that during spring and summer a considerable number of accumulation mode particles deposits while a simultaneous net particle emission occurred, which is mostly driven by particles smaller than 0.12 µm. These bi-directional fluxes lead to a net mass deposition of up to 13.5 µg m⁻² d⁻¹. The tipping-point between the emission and deposition lay between 0.16 and 0.19 µm. In a comprehensive analysis of the flux and concentration time series, the degree of atmospheric stability, the seasons, and the type of source region have been identified as key influences for particle fluxes. Different responses between particle fluxes and concentrations have been found along these drivers.