A closure experiment in atmospheric science is a combination of different measurement techniques to describe the current state of the investigated system as fully as possible, and to find inaccuracies in one or some of the methods involved.
The comparison of different types of measurement often involves model calculations, which may also be tested in this process.
An aerosol closure experiment might look like this: Additionally, measurements can be performed within the plume or outside, by means of remote sensing, from the surface, from aircraft, and from satellites in space.
Algorithms exist, for example, to predict the optical properties from a given particle shape, size distribution, and index of refraction.
Closure is achieved if the predictions of such an algorithm agree (within the limits of measurement uncertainty) with the optical properties that have been derived from the independent radiation measurements; if the lidar information about the vertical distribution of the aerosol sums up to the total optical depth determined by the sun photometer; etc.