Attenuation of Fine Particle Concentration Fluctuations in Turbulent and Isokinetically Sampled Laminar Flows
Abstract
Rapid measurement of gas and particulate concentrations have been used with synchronous wind velocity measurements to determine atmosphere-land fluxes using the eddy correlation method. For valid flux calculations, the concentration measurements must be made with a frequency of at least 1 Hz, and atmospheric samples are commonly drawn through a sampling line to a concentration sensor located at the base of a tower. Isokinetic subsampling from the center of a laminar flow is a widely-used technique to minimize loss of particles to sampling line walls. Here we evaluate frequency attenuation of fine particle concentration fluctuations in this design. Step changes in concentrations were introduced into stable laminar flow and particle concentrations were measured at 10 Hz using an Aerodyne Quadrupole Aerosol Mass Spectrometer (Q-AMS). The experimental response times and transfer functions for particle concentration fluctuations, averaged over the isokinetic region, were calculated and the results are compared with the transfer functions for the cup-mixing average concentration in laminar and turbulent flows. For a 10 m long sampling line with an inner diameter of 0.5 in, the frequency response was greater than three orders of magnitude higher in the isokinetic region of a laminar flow (isokinetic radius ratio = 0.06, Re = 1300) compared to the analogous sampling line operated under turbulent flow (Re = 10000). A theoretical transfer function for fine particle concentration fluctuations, integrated over the isokinetic region, was developed to evaluate the experimental results.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- Bibcode:
- 2007AGUFM.A33B1193G
- Keywords:
-
- 0305 Aerosols and particles (0345;
- 4801;
- 4906);
- 0315 Biosphere/atmosphere interactions (0426;
- 1610);
- 0394 Instruments and techniques;
- 0470 Nutrients and nutrient cycling (4845;
- 4850)