Measurement of concentrations of natural ice nuclei

If the currently forecast global warming occurs, the distribution of clouds and their properties are likely to change also and interact with the warming. The distribution of precipitation will also be affected. One known influence on cloud properties and the formation of precipitation is the concentration of airborne particles on which ice forms in supercooled clouds, known as ice nuclei. The possibility that their concentrations may also change with time must be examined if the predictions of warming are to be accurate. If they are of biogenic origin, as now seems possible, there is the possibility of feedback between climate and ice nucleus production. It is argued here that there is at present only one feasible method of measuring ice nucleus concentrations on a sufficiently wide scale, with sufficient frequency and at various altitudes. In this method, the particles are caught on membrane filters and the ice nuclei detected in a thermal diffusion chamber. The defects of this technique are reviewed and it is concluded that with suitable precautions it is adequate for the task.