Particles from air bursts are smaller than 10–25 micrometers, usually in the submicrometer range. They are composed mostly of iron oxides, with smaller proportion of aluminium oxide, and uranium and plutonium oxides. Particles larger than 1–2 micrometers are very spherical, corresponding to vaporized material condensing into droplets and then solidifying. The radioactivity is evenly distributed throughout the particle volume, making total activity of the particles linearly dependent on particle volume. About 80% of activity is present in more volatile elements, which condense only after the fireball cools to considerable degree. For example, strontium-90 will have less time to condense and coalesce into larger particles, resulting in greater degree of mixing in the volume of air and smaller particles. The particles produced immediately after the burst are small, with 90% of the radioactivity present in particles smaller than 300 nanometers. These coagulate with stratospheric aerosols. Coagulation is more extensive in the troposphere, and, at ground level, most activity is present in particles between 300 nm and 1 µm. The coagulation offsets the fractionation processes at particle formation, evening out isotopic distribution.