Direct determination of highly size-resolved turbulent particle fluxes with the disjunct eddy covariance method and a 12 – stage electrical low pressure impactor A. Schmidt and O. Klemm Institute of Landscape Ecology – Climatology, University of Münster, Robert-Koch-Str. 26, 48149 Münster, Germany
Abstract. During summer 2007, turbulent vertical particle mass and number fluxes were
measured for a period of 98 days near the city centre of Münster in
north-west Germany. For this purpose, a valve controlled disjunct eddy
covariance system was mounted at 65 m a.g.l. on a military radio
tower. The concentration values for 11 size bins with aerodynamic diameters
(D50) from 0.03 to 10 μm were measured with an electrical low pressure
impactor. After comparison with other fluxes obtained from 10 Hz
measurements with the classical eddy covariance method, the loss of
information concerning high frequent parts of the flux could be stated as
negligible. The results offer an extended insight in the turbulent
atmospheric exchange of aerosol particles by highly size-resolved particle
fluxes covering 11 size bins and show that the city of Münster acts as a
relevant source for aerosol particles.
Significant differences occur between the fluxes of the various particle
size classes. While the total particle number flux shows a pattern which is
strictly correlated to the diurnal course of the turbulence regime and the
traffic intensity, the total mass flux exhibits a single minimum in the
evening hours when coarse particles start to deposit.
As a result, a mean mass deposition of about 10 mg m−2 per day was
found above the urban test site, covering the aerosol size range from 40 nm
to 2.0 μm. By contrast, the half-hourly total number fluxes accumulated
over the lower ELPI stages range from −4.29×107 to +1.44×108 particles m−2 s−1 and are clearly dominated by
the sub-micron particle fraction of the impactor stages with diameters
between 40 nm and 320 nm. The averaged number fluxes of particles with
diameters between 2.0 and 6.4 μm show lower turbulent dynamics during
daytime and partially remarkably high negative fluxes with mean deposition
velocities of 2×10−3 m s−1 that appear temporary during
noontime and in the evening hours.
Citation: Schmidt, A. and Klemm, O.: Direct determination of highly size-resolved turbulent particle fluxes with the disjunct eddy covariance method and a 12 – stage electrical low pressure impactor, Atmos. Chem. Phys., 8, 7405-7417, doi:10.5194/acp-8-7405-2008, 2008.