Continuous monitoring of mixing depth with radon-222 and lidar
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Date
2012-01-31
Journal Title
Journal ISSN
Volume Title
Publisher
Australian Meterological & Oceanographic Society
Abstract
We report on the development of a combined approach using both lidar and radon-222 measurements to
obtain a near-continuous record of mixing depth which can, in turn, be used to assist in the interpretation of
simultaneous trace gas measurements.
Interactions between the land surface and the atmosphere above are moderated by the strength and depth of
mixing in the lower atmosphere which ranges diurnally between several meters at night to over one
kilometer during the day. Elastic backscatter lidar can be used to measure the depth of mixing during the
day, i.e. the height of the planetary boundary layer (PBL), by employing the change in aerosol concentration,
and hence lidar signal, at the boundary between the PBL and the free atmosphere. These measurements are
only possible when the mixing depth is large.
A complimentary approach, based on radon measurements, works well from the time turbulence decays in
the afternoon through till mid morning when mixing depths are too small to be observed using lidar. Radon-
222 is chemically inert and is released from the surface at a relatively constant rate and as such is a natural
passive tracer. Since it is radioactive, with a half-life of 3.8 days, it does not accumulate in the atmosphere.
At horizontally homogeneous inland sites, vertical mixing is the main process affecting near-surface
concentration. An estimate can therefore be obtained of an “equivalent mixing depth” from time-series of
radon concentration measurements, which can themselves be obtained with robust and low-maintenance
instrumentation.
Using two measurement techniques sidesteps the limitations of each to make a combined dataset a useful
component of field studies which seek to understand the exchanges of trace gases between the land surface
and atmosphere.
Description
Keywords
Radon 222, Optical radar, Depth, Gases, Atmospheres, Backscattering, Boundary layers, Datasets
Citation
Griffiths, A., Chambers, S., Parkes, S., & McCabe, M. (2012). Continuous monitoring of mixing depth with radon-222 and lidar. Paper presented at the AMOS 18th National Conference, "connections in climate systems", University of New South Wales, Sydney, 31 Jan - 3 Feb 2012.