Turbulent transfer of charged aerosol in the atmospheric boundary layer

 

The electric nature of the atmosphere becomes most apparent in the spectacle of a thunderstorm. But exciting electrical events occur even in the blue sky of a fair weather day.

 

A vertical electric field always exists in the atmosphere, such that the electric potential is typically 100 volts at one metre above the surface. The field is strong enough to drive singly charged small ions, of nanometre size, towards or away from the surface according to their sign. This constitutes a vertical electric current called the conduction current. The ions can collide with and become attached to aerosol particles, which are typically three orders of magnitude larger. The charged particles are immobile under electrical forces due to their inertia, but can be transported vertically by the mechanical forces of turbulence. This is called the turbulent current and the aim of my work is to measure it.

 

In previous work there has been some dispute as to its size or importance due to incomplete observation of the factors controlling its magnitude. For example, the strength of the turbulent transport is determined by atmospheric stability, and the current will be smaller in less polluted air, there being fewer charged particles. I plan to monitor micrometeorological parameters and aerosol content alongside electrical measurements to determine:

 

· electric field and conductivity, giving the conduction current

· vertical profile of charge per unit volume a.k.a. space charge, to give the turbulent current

· fast response measurements of electric field and vertical windspeed, from which the turbulent current can be calculated by correlating their fluctuations. This is the first time such micrometeorological electrical measurements have been attempted.

· total vertical current - the sum of the two currents

 

Last summer I made preliminary trials of the instruments - fast response electric fieldmeters were calibrated against a passive wire antenna, which measures electric potential - this was designed and built in the meteorology department; "home-made" space charge meters were compared favourably against values derived from the electric field; work was carried out in developing a conductivity measuring device. A particle sizer and small ion counter have been ordered in readiness for the summer, when I hope to make continuous measurements in a range of atmospheric stabilities and aerosol burdens.

 

As with all fieldwork, the most difficult problems are sometimes the least scientific - considerable thought was put into designing insulators which spiders canít build their webs on, and it was discovered that the average bumble bee carries 45pC of charge - enough to cause great but misplaced excitement at interesting transients in the data!

 

Janet Barlow, PhD student at the Department of Meterology at Reading University. Supervisor - Dr R.G. Harrison.