Turbulent transport of charged aerosol
from The Aerosol Society Newsletter
Looking back at the last article I wrote for this newsletter, I am reminded that the best laid plans always go awry! Or such is the nature of research that they are supposed to.
During the last year, we have perfected a system which makes absolute measurements of the atmospheric electric potential – a wire is fixed parallel to the ground, stretched between poles from which it is insulated. A sensitive electrometer, based on a design by Dr Giles Harrison, is used to measure the potential of the wire as it comes into electrical equilibrium with the surrounding air. This is due to a tiny current of ions and charged aerosol onto the wire. The system is thus sensitive to the local transport of charged aerosol. We are using a vertical array of four wires at 0.5m separation, to observe how the profile changes in different weather and aerosol conditions.
Atmospheric electrical effects can be explained using electrostatics – it is assumed that the charge distribution has a steady state and that electromagnetic effects may be neglected. Poisson’s Law states that the electric field gradient is related to the amount of free charge present which is carried on aerosol. We take advantage of this Law by taking the second derivative of the potential profile which we have measured to give the space charge profile near to the surface. From this the aerosol distribution can be inferred.
I am currently planning an experiment to measure the local wind and temperature conditions alongside the electric potential – this will take place in a rural location near Reading. Ostensibly this is a scientific decision, the site having less obstructions and being more favourable for micrometeorological measurements, but it has been observed that it will be very pleasant to have the picturesque village of Sonning nearby with all its picturesque public houses, together with the liberations of automatic logging…
There will be a difference in aerosol burden between urban Reading and rural Sonning, and I am particularly interested in monitoring Aitken nuclei levels – particles in this size range can carry 80% or more of total space charge. An instrument is being developed which will measure the partitioning of charge between small ions and charged particles. Simultaneous spectral measurements of electric field will go some way to unravelling the interaction between electrical and mechanical forces on charged aerosol, which will have implications for the existing picture of charge transport near the surface.
I am planning to present a paper on this work at the conference in Edinburgh in September – hopefully progress will not be slowed by Sonning’s historical inns.
Janet Barlow, PhD student
Department of Meteorology,
University of Reading.