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Atmospheric pollution

The veil of two cities

In 1752, Benjamin Franklin sent a metal key on its famous kite flight to demonstrate the electrical nature of lightning. The study of atmospheric electricity subsequently burgeoned, and writing in Atmospheric Environment (37, 5319–5324; 2003) R. G. Harrison and K. L. Aplin describe how they have mined records of one aspect of later historical research. They show that electrical measurements made at the Eiffel Tower in the 1890s can be used to estimate the levels of Parisian smoke pollution at that time.

The atmosphere's electrical system results from a combination of thunderstorm activity and the slight ionization of air. The upshot is an electrical potential difference between the ionosphere (at an altitude of some 60 km and more) and the Earth's surface, which causes a small current to flow. The potential gradient close to the ground is an indicator of the electrical state of the atmosphere, and in the nineteenth century it was commonly measured in several European cities.

In clean air, the potential gradient has a distinctive diurnal cycle known as the Carnegie curve. Local aerosol pollution alters this gradient. So if that effect can be separated from the influence of the global electrical circuit at atmospherically clean sites, it becomes possible to infer pollution levels from potential-gradient measurements. But that is easier said than done.

To analyse electrical measurements made at the top of the Eiffel Tower, Harrison and Aplin applied a relationship between smoke pollution and potential gradient that had emerged from their earlier historical study for Kew, near London, using data from 1863. But they also needed an absolute calibration that was specific for Paris. For this, the authors conducted a modelling study that enabled them to identify the time of day when the top of the Eiffel Tower was in clean air and when it was in the urban boundary layer — the atmospheric layer affected by urban pollution. At times when the tower was in clean air, the observed electrical variations can be attributed to the Carnegie curve alone, allowing an absolute calibration of the electrical data based on the tabulated Carnegie values. At times when it was in the urban boundary layer, the higher potential gradient signals the presence of smoke pollution.

The approach allowed Harrison and Aplin to estimate smoke pollution at both the top and the bottom of the Eiffel Tower in the 1890s. They calculate that pollution at ground level had an autumnal daily peak of 60±30 μg m−3. Their earlier study gives a value of 170±50 μg m−3 for London. In the 1860s the UK Clean Air Act lay almost 100 years in the future. So in the late nineteenth century it is likely that, as a place to live, Paris had the edge over its English counterpart in more than just cancan and cabaret.

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