Aerosols at altitudes between 20 and 37 km

The physical appearance, size distributions and concentrations of particles impacted on electron microscope grids at altitudes between 20 and 37 km in the course of 17 flights extending over 14 months are described. It is concluded that many of the particles consist of sulphuric acid with varying degrees of ammoniation, which is least at the lower levels. Many particles at the higher levels had liquid or semi-liquid coatings and contained inclusions which are probably micrometeoritic. It is concluded that the coatings arise from reactions near the mesopause. There was little variation of median diameter with altitude in the range investigated, equivalent sphere diameters being estimated at 0.1 to 0.4 μ. Concentrations, expressed in numbers per cubic centimetre reduced to sea level pressure and temperature, ranged from 2 to 8 (uncorrected for collection efficiency) in the height interval 27 to 37 km, but were significantly less between 24 and 27 km and much more variable below 24 km. There is evidence of the loss of volatile constituents which may have led to an underestimate of particle diameters or concentrations.