We recommend the spatial distributions of particle concentrations and trends with time be monitored on a global basis. A network of about 100 stations is required to give representative data for the whole atmosphere. For all optical measurements an accuracy of 5 percent is required to obtain meaningful data.

Because of the wide range of particle sizes, different methods have to be used simultaneously, in particular:

  1. … [transmissivity]

  2. … [horizontal extinction]

  3. … [trends for particles < 0.1 µm]

  4. … [oceans: continue electrical conductivity measurements]

  5. Trends in the concentration of ice and cloud nuclei should be continuously monitored. Very careful selection and perhaps improvement of existing methods are necessary.

We recommend increased research on the refractive index (including absorption) of atmospheric particles in relation to their composition, origin, size, and shape and its change with increasing pollution for short-and long-wave radiation. Also, more data on particle growth with humidity are needed to understand the influence on the refractive index with relative humidity.

We recommend comprehensive comparative studies of the radiation fields for clean and polluted atmospheres in order better to identify the effects of short-and long-wave radiation on the atmosphere and its modification with increasing pollution.

We recommend measurements of albedo and other radiative properties of clouds and fogs in unpolluted and polluted areas, in conjunction with sufficiently complete measurements of cloud microstructure. These very important measurements require operation of instrumented aircraft.

We recommend studies of the effects of pollution on the refractive index of cloud droplets and ice crystals.

We recommend theoretical study of the integrated effects of pollution on radiative properties of clouds and fogs.

We recommend that comprehensive field studies directed toward the resolution of the question of the effect of particle concentration on frequency, type, and intensity of clouds and precipitation be designed and implemented.

We recommend that at suitable time intervals (about 2 years) the refractive index and the detailed size distribution of particles be determined at selected places in both clean and polluted air.

We recommend that objective methods for monitoring cloud cover by satellites or other means and for monitoring changes of cloud cover over large areas of the atmosphere be developed and implemented.

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