A most important aspect of effects of air pollutants on air quality is their impact upon visibility. As far as can be determined this is the first visibility study performed in the UK in which the influence of individual pollutants, the local microclimate and the long range transport of air pollutants have been simultaneously examined over the whole visibility range. Techniques were developed to analyse the influence of the relative humidity on the light scattering efficiency of hygroscopic particles, and to examine their nature and size distributions. A regression model that specifically included the influence of hygroscopic particles and the relative humidity accounted for over 80% of the total variance in visibility, and showed that over 50% of the visibility degradation was due solely to the presence of sulphate particles. The constant of the regression implied a background visibility of over 60km for Teesside, whereas a mean of 12km was recorded. Original results were obtained by examining the relationship of atmospheric stability and thermal structure to visibility and pollutant behaviour using an acoustic sounder. This was used to investigate the conditions created by the local sea breeze which was associated with a substantial proportion of the summertime hazes. Wind rose analysis of the role played by emissions from Teesside industries together with comparisons of visibility levels at nearby locations showed that 20-40% of the light extinction was due to local factors. An airstream categorisation scheme was developed to study the role of long range transport of air pollutants and other influences on visibility. The study was extended across the British Isles to produce convincing evidence of large scale visibility degradation, due principally to sulphates derived from sulphur dioxide emissions, from sources as distant as 1500km. At Teesside the elimination of local air pollutant emissions would result in a significant but limited visibility improvement. A most effective means of controlling visibility degradation would be the reduction of sulphur dioxide emission to the atmosphere on an international scale. The resulting improvement in the visibility levels experienced at Teesside and in the UK in general would be in the order of 100% or more.

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