Evaluation and Application of an ElectricalLow Pressure Impactor in Disjunct Eddy Covariance Aerosol Flux Measurements

A micrometeorological application of an electrical low pressure impactor (ELPI) for the measurement of size-resolved particle fluxes between the surface and the atmosphere is proposed. Particles are introduced into the system for a very short time period through an inlet valve and analyzed in distinct size bins with the ELPI instrument for several seconds. Together with sonic anemometer measurements, size-resolved deposition velocities can be derived employing disjunct eddy covariance (DEC). Laboratory experiments studying ELPI instrument response indicate that a sampling interval of 0.3 s and a measurement interval of at least 5 s yield particle concentration data comparable to continuous sampling, and suitable for DEC. Calculations and Monte-Carlo simulations show that the finite length of the average period as well as Poisson and instrumental noise frequently introduce uncertainties in deposition velocities which may be an order of magnitude larger than the measurement value. In first field measurements with a prototype system, the measured fluxes were often below the detection limit as defined by the measurement uncertainties. At high particle concentrations, the DEC measurements could potentially extend current measurement capabilities to cover submicron particles fluxes in several size classes simultaneously.