Reduction of visibility

One of the earliest recognised symptoms of air pollution was the reduction of visibility. The urban smogs (smoke+fog) reported since the 16th century represented an extreme loss of visibility, often to as little as a few metres, where the presence of smoke and sulphur dioxide led to nucleation and growth of fog droplets. Often these smogs were made worse by the presence of ammonia (e.g. from urban livestock, fertiliser factories), which resulted in the formation of ammonium sulphate aerosol, thereby encouraging fog formation. Such pollution episodes would have exacerbated the occurrence of sea fogs in the cities of east Scotland.

With the reduction of urban emissions of smoke, sulphur dioxide and ammonia, such smogs have largely become an issue of the past. By contrast, with the growth of traffic emissions of nitrogen oxides and volatile organic compounds (VOCs), "photochemical smog" has become more frequent. In photochemical smog, nitrogen oxides react with parallel emissions of VOCs in the presence of sunlight to produce a wide ranging chemical cocktail of compounds, such as ozone and PAN (peroxyacetyl nitrate). Ozone concentrations in urban areas are kept low because ozone reacts with nitric oxide (NO) emitted from vehicles and other combustion sources to give nitrogen dioxide (NO2). By contrast, when the plume of air moves out into rural areas, ozone concentrations are raised to much higher than background values, because the rate of ozone production by sunlight is catalysed by the presence of NO2. Eventually, the oxidation of NO2 proceeds to form nitric acid vapour, which rapidly reacts with ammonia in the atmosphere to form small particles. The VOCs emitted by traffic and vegetation are also oxidised in conditions of strong sunlight to form small organic particles in the atmosphere. The loss of visibility occurs as the number of airborne particles increases, and is most obvious during "photochemical episodes", which are promoted in conditions of strong sunshine and low windspeeds.

A further reduction of visibility occurs as these aerosol particles grow. Much of the aerosol that contributes to PM10 and PM2.5 (measured as the respirable portion of particles in the atmosphere) is hygroscopic. This means that that in conditions of high humidity the aerosols grow by absorbing water vapour, although not to the extent that occurs in fog formation. Dry aerosol particles scatter light, and this scattering increases above the deliquescence point, where solid particles absorb water and turn into fine droplets, at around 70-80% relative humidity. Hence visibility may be low on days with high aerosol loading and when humidity is high. Since rain scavenges aerosol particles from the atmosphere, some of the best visibility days occur after strong rain events.

References:

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Brimblecombe P. Long-term trends in London Fog. Science of the Total Environment. 1981 ;22:19-29.

Brimblecombe P, Bowler C. The history of air pollution in York, England. Journal of the Air & Waste Management Association. 1992 ;42:1562-1566.

Brimblecombe P, Rodhe H. Air pollution - Historical trends. Durability of Building Materials. 1988 ;5:291-308.

Eggleston AEJ, Atkins DHF. Results of the Tees-side investigation. Didcot, Oxon, UK: Harwell Laboratory; 1972.

PORG. Ozone in the United Kingdom 1997. London: Department of the Environment; 1997.

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