Direct determination of highly size-resolved turbulent particle fluxes with the disjunct eddy covariance method and a 12 – stage electrical low pressure impactor

[摘要] During summer 2007, turbulent vertical particle mass and number fluxes weremeasured for a period of 98 days near the city centre of Münster innorth-west Germany. For this purpose, a valve controlled disjunct eddycovariance system was mounted at 65 m a.g.l. on a military radiotower. The concentration values for 11 size bins with aerodynamic diameters(D50) from 0.03 to 10 μm were measured with an electrical low pressureimpactor. After comparison with other fluxes obtained from 10 Hzmeasurements with the classical eddy covariance method, the loss ofinformation concerning high frequent parts of the flux could be stated asnegligible. The results offer an extended insight in the turbulentatmospheric exchange of aerosol particles by highly size-resolved particlefluxes covering 11 size bins and show that the city of Münster acts as arelevant source for aerosol particles.

Significant differences occur between the fluxes of the various particlesize classes. While the total particle number flux shows a pattern which isstrictly correlated to the diurnal course of the turbulence regime and thetraffic intensity, the total mass flux exhibits a single minimum in theevening hours when coarse particles start to deposit.

As a result, a mean mass deposition of about 10 mg m⁻² per day wasfound above the urban test site, covering the aerosol size range from 40 nmto 2.0 μm. By contrast, the half-hourly total number fluxes accumulatedover the lower ELPI stages range from −4.29×10⁷ to +1.44×10⁸ particles m⁻² s⁻¹ and are clearly dominated bythe sub-micron particle fraction of the impactor stages with diametersbetween 40 nm and 320 nm. The averaged number fluxes of particles withdiameters between 2.0 and 6.4 μm show lower turbulent dynamics duringdaytime and partially remarkably high negative fluxes with mean depositionvelocities of 2×10⁻³ m s⁻¹ that appear temporary duringnoontime and in the evening hours.

[学科分类] 大气科学