A study of the contribution of pollution to visibility in a radiation fog: Fid-Bau Portal

A study of the contribution of pollution to visibility in a radiation fog

Garland, J.A. / Branson, J.R. / Cox, L.C.

Abstract Measurements of the visibility, droplet size distribution, liquid water content, chemical composition of the air and cloud nucleus concentration were made at ground level during a radiation fog. The results were compared to demonstrate consistency between the various methods employed. In particular agreement between integrating nephelometer and transmissometer measurements of visibility confirmed the applicability of calculated corrections for forward scattered light losses in the former instrument. A method of calculating the contribution of hygroscopic materials to the extinction coefficient in fog was developed. Calculated values based on the chemical composition of the aerosol agreed with the contribution due to the small droplet fraction obtained from the size distributions. About a half of the extinction coefficient was accounted for by hygroscopic components in the aerosol, although pollution levels were moderate. In fogs with higher aerosol concentrations this contribution to the extinction could be even more significant. In contrast with previous experience the maximum radius of droplets present was only about 20 μm. This could be explained in terms of the short period available for droplet growth before sampling. Unfortunately the chemical samplers, 800 m from the site of visibility measurement, were not in fog during most of the period and the results cannot be used to look for chemical changes occurring in the fog.