Reference

Geiger, R., Aron, R. H. and Todhunter, P. (2003) The Climate Near the Ground, 6th edn. Rowman & Littlefield, Lanham, MD, 584pp.

Figure 12.20 Sunshine in and around London. (A) Mean monthly bright sunshine recorded in the city and suburbs for the years 1921 to 1950, expressed as a percentage of that in adjacent rural areas. This shows clearly the effects of winter atmospheric pollution in the city. (B) Mean monthly bright sunshine recorded in the city, suburbs and surrounding rural areas during the period 1958 to 1967, expressed as a percentage of the averages for the period 1931 to I960. This shows the effect of the 1956 Clean Air Act in increasing the receipt of winter sunshine, in particular in central London.

Figure 12.20 Sunshine in and around London. (A) Mean monthly bright sunshine recorded in the city and suburbs for the years 1921 to 1950, expressed as a percentage of that in adjacent rural areas. This shows clearly the effects of winter atmospheric pollution in the city. (B) Mean monthly bright sunshine recorded in the city, suburbs and surrounding rural areas during the period 1958 to 1967, expressed as a percentage of the averages for the period 1931 to I960. This shows the effect of the 1956 Clean Air Act in increasing the receipt of winter sunshine, in particular in central London.

Sources: (A) After Chandler (1965); (B) After Jenkins (1969), reprinted from Weather, by permission of the Royal Meteorological Society. Crown copyright ©.

African tank battles of the Second World War disturbed the desert surface to such an extent that the material subsequently deflated was visible in clouds over the Caribbean. Soot aerosols generated by the Indonesian forest fires of 1999 September 1997 and March 2000 were transported across the region.

The background concentration of fine particles (PM10, radius <10 |m) currently averages 20 to 30 |g m-3 in the British countryside but daily average values regularly exceed 50 |g m-3, and occasionally exceed 100 |g m-3 in industrial cities near ground level. The greatest concentrations of smoke generally occur with low wind speed, low vertical turbulence, temperature inversions, high relative humidity and air moving from the pollution sources of factory districts or areas of high-density housing. The temporal character of domestic heating and power demands causes city smoke pollution to take on striking seasonal and diurnal cycles, with the greatest concentrations occurring at about 08:00 hours in early winter (Figure 12.19). The sudden morning increase is also partly a result of natural processes. Pollution trapped during the night beneath a stable layer a few hundred metres above the surface may be brought back to ground level (a process termed fumigation) when thermal convection sets off vertical mixing.

The most direct effect of particulate pollution is to reduce visibility, incoming radiation and sunshine. In Los Angeles, aerosol carbon accounts for 40 per cent of the total fine particle mass and is the major cause of severe visibility decreases, yet it is not routinely monitored. Half of this total is from vehicle exhausts and the remainder from industrial and other stationary fuel burning. Pollution, and the associated fogs (termed smog), used to cause some British cities to lose 25 to 55 per cent of incoming solar radiation during the period November to March. In 1945, it was estimated that the city of Leicester, England, lost 30 per cent of incoming radiation in winter, as against 6 per cent in summer. These losses are naturally greatest when the sun's rays strike the smog layer at a low angle. Compared with the radiation received in the surrounding countryside, Vienna lost 15 to 21 per cent of radiation when the sun's altitude is 30°, but the loss rises to 29 to 36 per cent with an altitude of 10°. The effect of smoke pollution is dramatically illustrated in Figure 12.20, which compares conditions in London before and after enforcement of the UK Clean Air Act of 1956. Before 1950, there was a striking difference of sunshine between the surrounding rural areas and the city centre (see Figure 12.20A), which could mean a loss of mean daily sunshine of sixteen minutes in the outer suburbs, twenty-five minutes in the inner suburbs and forty-four minutes in the city centre. It must be remembered, however, that smog layers also impeded the re-radiation of surface heat at night and that this blanketing effect contributed to higher night-time city temperatures. Occasionally, very stable atmospheric conditions combine with excessive pollution production to give dense smog of a lethal character. During the period 5 to 9 December 1952, a temperature inversion over London caused a dense fog with visibility of less than 10 m for forty-eight consecutive hours. There were 12,000 more deaths (mainly from chest complaints) during December 1952 to February 1953 compared with the same period the previous year. The close association of the incidence of fog with increasing industrialization and urbanization was evident in Prague, where the mean annual number of days with fog rose from seventy-nine during 1860 to 1880 to 217 during 1900 to 1920.

The use of smokeless fuels and other controls cut London's total smoke emission from 1.4 X 108 kg (141,000 tons) in 1952 to 0.9 X 108kg (89,000 tons) in 1960. Figure 12.20B shows the increase in average monthly sunshine figures for 1958 to 1967 compared with those of 1931 to 1960. Since the early 1960s annual average concentrations of smoke and sulphur dioxide in the UK have fallen from 160 ppm and 60 ppm, respectively, to below 20 ppm and 10 ppm in the 1990s.

Visibility in the UK improved at many measuring sites during the late twentieth century. In the 1950s and 1960s, days with visibility at midday in the lowest 10th percentile were in the 4 to 5 km range, whereas in the 1990s this had improved to 6 to 9 km. Annual average 12 UTC visibility at Manchester airport was 10 km in 1950, but near 30 km in 1997. The improvements are attributed to improved vehicle fuel efficiency and catalytic converter installation in the 1970s.

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