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.
A common example for closure experiments are
aerosol
An aerosol is a suspension of fine solid particles or liquid droplets in air or another gas. Aerosols can be natural or anthropogenic. Examples of natural aerosols are fog or mist, dust, forest exudates, and geyser steam. Examples of ant ...
studies. Aerosols can be studied from space, from aircraft, and from the surface. The different properties of aerosols (chemical composition, particle size, particle number concentration, optical absorptivity, thermal emissivity, index of refraction, pattern of light scattering, etc.) require very different kinds of measurement techniques. Different instruments have been developed to perform each technique, and researchers usually specialize in one or a few of them. Due to the logistic efforts required to bring different groups of researchers with different measurement techniques together, such experiments stand out among routine experiments that are more focused on one or a few techniques and have fewer participants.
An aerosol closure experiment might look like this:
* Individual aerosol particles are sampled, and their size and shape are determined under an electron microscope;
* A
lidar
Lidar (, also LIDAR, or LiDAR; sometimes LADAR) is a method for determining ranges (variable distance) by targeting an object or a surface with a laser and measuring the time for the reflected light to return to the receiver. It can also be ...
emits a laser beam into the aerosol plume and detects the backscatter signal;
* An aerosol flow is led through optical detectors that record the size distribution of the particles;
* The sunlight attenuated by the aerosol is measured by a
Sun photometer
A sun photometer is a type of photometer conceived in such a way that it points at the sun.
Recent sun photometers are automated instruments incorporating a sun-tracking unit, an appropriate optical
system, a spectrally filtering device, a photod ...
, yielding the
optical depth
In physics, optical depth or optical thickness is the natural logarithm of the ratio of incident to ''transmitted'' radiant power through a material.
Thus, the larger the optical depth, the smaller the amount of transmitted radiant power throug ...
of the aerosol layer;
* the spectral
absorption
Absorption may refer to:
Chemistry and biology
*Absorption (biology), digestion
**Absorption (small intestine)
*Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials
* Absorption (skin), a route by which ...
of sunlight, as well as the
scattering
Scattering is a term used in physics to describe a wide range of physical processes where moving particles or radiation of some form, such as light or sound, are forced to deviate from a straight trajectory by localized non-uniformities (including ...
properties of the aerosol are measured by spectrometers;
* The extent and total reflectivity of the aerosol plume are observed by a satellite radiometer.
Additionally, measurements can be performed within the plume or outside, by means of
remote sensing
Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object, in contrast to in situ or on-site observation. The term is applied especially to acquiring information about Ear ...
, from the surface, from aircraft, and from satellites in space. Different viewing geometries again introduce complexity, which can be assessed in closure experiments.
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.
References
Quinn, P. K., and D. J. Coffman (1998), Local closure during the First Aerosol Characterization Experiment (ACE 1): Aerosol mass concentration and scattering and backscattering coefficients, J. Geophys. Res., 103(D13), 16,575–16,596, {{doi, 10.1029/97JD03757.
http://hdl.handle.net/10068/250054
Atmospheric sciences