Potential and Limitations of an Electrical Low Pressure Impactor in Disjunct Eddy Covariance Aerosol Flux Measurements

A micrometeorological application of an electrical low pressure impactor (ELPI) is proposed for the measurement of size-resolved particle fluxes between the surface and the atmosphere. This extends flux measurement capabilities to cover submicron particles in several size classes simultaneously. The disjunct eddy covariance system combines a sonic anemometer, an ELPI and a valve-controlled particle sampling unit. Depending on the valve setting, ambient air or filtered particle-free air is sampled and introduced into the impactor. For disjunct eddy covariance measurements, ambient air is sampled only during a very short sampling interval (~ 0.1 s) in measurement intervals of several seconds. The integrated ELPI signal is representative for the particle size distribution during the short sampling interval. This information may be correlated with fast measurements of the vertical wind speed to obtain turbulent fluxes of submicron particles simultaneously in several size classes. A prototype system has been tested in lab and field experiments in order to evaluate the technical limitations of this approach. Tests show that different valve switch cycles do not affect the concentration measurements. However, longitudinal diffusion processes within the sampling lines lead to signal dilatation. In addition, reliable measurements require a minimum number of charged particles impacting on each stage. This leads to the exclusion of some size bins in typical field applications. Nevertheless, the system showed its potential for size-resolved flux measurements under favorable conditions. Travel support by the ACCENT Access to Infrastructures program is gratefully acknowledged.