Babbage | Air pollution

Sunsets and scientists

By M.S.L.J.

POLLUTION plagued London last week and citizens were encouraged to stay inside. The rich colours of sunset paintings by Turner (as seen above), Reynolds and Gainsborough in the city’s Tate Gallery could be enjoyed nevertheless. A new study shows that these works also depict environmental information helpful to scientists: they can be used to estimate pollution levels in the Earth’s atmosphere over past centuries.

The study, led by Christos Zerefos from the Academy of Athens,was published late last month in Atmospheric Chemistry and Physics. It analysed 124 sunset pictures from the gallery and revealed that pollution, particularly when caused by the absorption and dispersal of light by ash and gas particles released during historical volcanic eruptions, made depictions of sunsets more red.

The study corroborated findings of research from 2007 which suggested that the ratios of red to green in the paintings correlated well with the aerosol optical depth—a measure of the extinction of sunlight by dust—at the time in which they were made.

The colour ratio consequently told the research team about the transparency of the sky at a given time and how far light could travel. The findings adopt an alternative and indirect use of the Ångström law of atmospheric physics that describes the varied effects of aerosols on the different wavelengths of solar light (seen as different colours).

To take an example, after Indonesia’s Tambora volcano erupted in 1815, painters in Europe depicted the colours of the sky changing through their more fiery sunsets. Turner’s works were found to have been influenced as particles altered light for three years after the eruption.

Dr Zerefos’s team first compared the ratios of red to green in high resolution and low resolution images of a selection of works painted between 1500-2000. This allowed them to check whether their reliance on web-based images of sunset paintings would affect results. As both sets showed similar ratios of red to green they proceeded to examine the 124 Tate paintings.

Earlier estimates of aerosol optical depths at 550 nm, the wavelength of green light in the visible spectrum, were used alongside research from 2005 and 2007 to compile a time series measuring them for the time period under scrutiny. The study found significant links between the levels of particles in the air, as shown in the colour ratios of the Tate works, and the time series.

This was further backed up when the team compared 50-year averages of aerosol optical depths with the total sulphate in ice core over time (deposits of the salt increase after volcanic activity) and the total stratospheric level of air particles. They excluded the year of a known large eruption from their calculations and the three following it, however.

Concerns over the style, mood and age of the Tate paintings, and whether their colour could have faded, were also considered. The authors reckon that as their results were not based on true colours, but ratios, and because they used a large number of different paintings by various artists, their findings are still helpful in detecting levels of pollution across time. Their focus on European mid-latitude painters also ensured that the angle of depictions could not affect results significantly either.

To test their conclusions another way, the researchers hired Panayiotis Tetsis, an artist, to paint pictures of successive sunsets in June 2010 on the island of Hydra in Greece—nice work if you can get it thinks Babbage. The painter was unaware that the site and timing had been chosen to coincide with the passage of a Saharan dust outbreak.

The red to green ratios of his pictures before and after the dust event (analysed as the Tate paintings were) matched well with measurements of aerosol optical depth collected on site, and with photographs taken there too. As expected, his later paintings used a more ruddy palette.

Overall the study suggests that air pollution increased greatly after rich countries began to industrialise. While this finding is not surprising, the artistic route taken to it by scientists is. But as Albert Einstein wisely pointed out, however, “all religions, art and sciences are branches of the same tree”.

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