The electrification of the atmosphere by particles from bubbles in the sea

Abstract The work in this paper has been directed towards an evaluation of the quantity of electric charge that is separated at the surface of the sea. As the carriers of the charge are particles that arise from the sea by wind and wave action, it was necessary to determine first the flux of particles across the sea and the manner in which the particles originated. Then, laboratory studies were carried out to determine the sign and magnitude of the particle charge. The data from these two separate investigations were combined to obtain an estimate of the charge separation by the world ocean. It was concluded that a majority of the particles that arise from the sea evolve from bubbles that break at the sea surface. These particles come from both the bubble film and the breakdown of a jet which rises at high speeds from the bottom of the collapsing bubble cavity. The film drop flux was computed directly from a knowledge of the world ocean white-cap coverage. The flux of jet drops, on the other hand, was computed indirectly, first by assuming a steady state between the source and the sink of the jet drops and then by computing their fall-out rate by sedimentation, rain washout, and advection over the continents. The charge on the film drops was obtained with a Millikan-continuous diffusion cloud chamber and that on the jet drops by a modified Millikan chamber. The charge Q (esu) on the top jet drop from sea water was found to be positive and a function of both drop radius R (microns) and bubble age T (sec). For bubbles of an age of less than 10 sec this charge can be approximated by the expression Q = 1.76 × 10−8T0.4R1.3. The world ocean charge flow that is carried by film drops was found to be negligible, being far less than 1A. That carried by jet drops, however, was calculated to be about 160 A. This is about 17 per cent of the magnitude of the air-earth current over the world ocean that has been attributed to thunderstorm activity. The jet drop charge flux has been determined for both the winter and summer seasons and as a function of latitude. In both hemispheres and in both seasons the oceanic charge separation attains a maximum at latitudes of 40 to 60° and reaches its highest value of 3.2 × 10−7 esu cm−2 sec−1 at about 50° S. latitude in June–August. Two factors that may modify both the jet drop size and its charge were considered in detail. The first is that of organic surface-active films on the surface of the ocean. Experiments showed that the removal of surface films by jet drops was a function of bubble size and suggested that most of the drops that take part in the charge transfer are sufficiently small that they do not carry away any of the surface-active film. The reason for this appears to be that the small drops from the jets bubbles evolve so rapidly that the surface film does not have time to move into the collapsing bubble cavity and to cover the jet. Calculation showed that the drop ejection velocity was an inverse function of bubble size and attained a value of several thousand centimeters per second for drops from the smaller bubbles. The question of surface films at sea was discussed, and it was shown that one has reason to suspect that film collapse by wind and surface water convergences might prevent them from covering a significant area of the ocean. For these reasons it was concluded that the effects of surface films on the computed charge production were of no significance. The second factor that could modify the jet drop charge is the induction charging that is caused by the earth's electric field, normally 1 V cm−1 at the surface of the sea. This factor, however, was found to be of no great significance, as experiment indicated that the amount of induction charging by such small electric fields was less than 5 per cent of that charge which the drops carried in the absence of a field. Evidence for the 160 A positive current from the world ocean was drawn from two sources. The first, that of space charge measurements in air that had passed over the sea where a great deal of bubbling was occurring, indicated that a net positive space charge was carried by the drops that left the sea in this region. The second source gave evidence that was suggestive though not conclusive. It was shown that the latitudinal distribution of the oceanic electric field had two maxima, one in each hemisphere, in the same latitudes as did the computed oceanic charge separation. But it could not be proved that the oceanic charge separation caused the latitudinal variation in the electric field, for it was shown that such variation could conceivably come about by an appropriate latitudinal variation of the convection current. Finally, it was suggested that the jet drop charge arises from a charge separation of the electric double layer that surrounds the bubble. This separation is thought to be a consequence of laminar shear flow within the rising jet with the result that the water that constitutes the faster moving, positively charged core of the jet ends up as a positively charged jet drop.