K.A. Nicoll and R.G. Harrison

Electric charge in the atmosphere results from the generation of molecular cluster ions by cosmic rays, and natural radioactivity from the Earth’s surface.  Ions are the smallest components of the aerosol size spectrum, with diameters typically around 1-10nm.  Their removal arises from self (ion-ion) recombination, and attachment to aerosol particles. As a result of the large potential difference between the oppositely charged ionosphere and the Earth’s surface, ions permit a small current, known as the air-earth conduction current density Jc, to flow vertically, and through cloud layers.  The effect of a cloud is to create conductivity boundaries. With Jc, these boundaries generate layers of charge above and below the cloud (Tinsley, 2000), leading to electrification of cloud droplets and aerosol particles, which are present in abundance in these areas.  Some microphysical processes are electrically influenced, particularly scavenging of aerosol particles by water droplets (Tripathi and Harrison, 2002).  It is possible that modulation of the aerosol charge around clouds may lead to observable changes in cloud properties.  This may have consequences for Earth’s climate, as clouds play a vital role in the regulation of the climate system, through both warming and cooling of the planet.  Recent modeling work (Zhou and Tinsley, 2007) simulated charged layers around stratiform cloud, but in-situ measurements are needed.  An instrument capable of this is currently under development.  In this paper we present some preliminary results concerning the measurement of layers of charged particles in the troposphere.

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