ABSTRACT
References .......................................................................................................................... 173
Ice particles in the upper troposphere can be in the form of cirrus clouds, subvisible cirrus, or
aircraft condensation trails (contrails). The cirrus and subvisible cirrus are highly variable
with particle sizes ranging from 2 to 3000mm diameter [1] and surface area densities ranging from 20 to 20,000mm2/cm3 [2]. Further, studies indicate that 30% of the Earth is covered in cirrus at any time and that subvisible cirrus may be present in the tropics 75% of the time [2].
The aircraft contrail particles are typically smaller in size and have higher number densities
than natural cirrus [3]. One satellite study over Europe showed that contrails may be present
in the sky 60% of the time, with coverages ranging from <0.1% to 8% [4]. Although the exact
conditions necessary for cirrus and contrail nucleation are not yet known, these particles will
provide ample sites for heterogeneous chemical reactions in the upper troposphere.
