Conservation properties of oil fog used as an atmospheric tracer

Abstract Oil fog used as a tracer of atmospheric motions is usually considered to be conserved in the sense of negligible evaporation loss. However, small drops can evaporate at a rapid pace. Lidar measurements of optical backscatter from a fog plume containing drops of a medium oil, which is typical of those suggested for battlefield obscuration, suffered significant losses in daytime. Backscatter from drops of a heavy oil with low volatility were nearly conserved. The physics of the evaporation process for oil drops in a diffusing plume are summarized. The rate and extent of evaporation depend on the molecular weight of the oil, on the temperature, and on the amount of plume dispersion. If a partially evaporating oil fog is to be detected optically, the signal is better conserved if drop sizes are large enough to be in the Mie rather than the Rayleigh scatter regime. Researchers should confirm the conservative property of an oil fog tracer if the analysis requires accurate measurements of concentration.