The Aerosol Robotic Network (AERONET) is one of the first places that scientists turn when volcanoes, wildfires, pollution plumes, dust storms and many other phenomena-both natural and manmade-make an appearance. The network of ground-based instruments, called sun photometers, measures the many tiny particles blowing about in the atmosphere called aerosols. The particles are often impossible to see with human eyes, but AERONET's sensors can detect their presence by measuring subtle fluctuations in sunlight as the particles reflect and scatter the sun's rays.
GCMD keywords can be found on the Internet with the following citation:
Olsen, L.M., G. Major, K. Shein, J. Scialdone, S. Ritz, T. Stevens, M. Morahan, A. Aleman, R. Vogel, S. Leicester, H. Weir, M. Meaux, S. Grebas, C.Solomon, M. Holland, T. Northcutt, R. A. Restrepo, R. Bilodeau, 2013. NASA/Global Change Master Directory (GCMD) Earth Science Keywords. Version 18.104.22.168.0