Isokinetically sampled laminar flow for eddy-correlation sampling of particulate matter

Rapid measurements of particulate matter (PM) concentrations have been used to determine atmosphere-land fluxes, including those of particulate carbon, using the eddy correlation method. For practical reasons, most instruments used to measure PM cannot be mounted on a tower near a sonic anemometer. Therefore atmospheric samples are commonly drawn through a sampling line to a concentration sensor located at the base of the tower. These sampling lines are usually designed for turbulent flow in order to reduce frequency attenuation and transmit concentration fluctuations at frequencies of 1 Hz or higher. Here we consider frequency attenuation of aerosol particles isokinetically subsampled near the center of parabolic laminar flow where differences in velocity in the radial direction are small. Experiments were conducted in which a step change was made in the concentration of 350 nm particles in steady laminar flow. Aerosol concentrations were measured at the end of an 8.3 m straight sampling line at 10 Hz using an Aerodyne Quadrupole Aerosol Mass Spectrometer (Q-AMS). Measured attenuation for these experiments was less than 1% at 5 Hz, suitable for eddy correlation measurements. We also present theoretical calculations of aerosol particle attenuation with large Schmidt number for laminar flow, turbulent flow, and isokinetically sampled laminar flow. Results from the calculations are in agreement with the experiments. Using the same geometry, maximum frequency response in the isokinetically sampled laminar flow (Re = 1200) was calculated to be approximately 4 orders of magnitude higher than for the main laminar flow and 1.5 orders of magnitude higher than for turbulent flow (Re = 10000).