Chappuis absorption

Compared to the absorption of ultraviolet light by the ozone layer, known as the Hartley and Huggins absorptions, Chappuis absorption is distinctly weaker. Along with Rayleigh scattering, it contributes to the blue color of the sky, and is noticeable when the light has to travel a long path through the Earth's atmosphere. For this reason, Chappuis absorption only has a significant effect on the color of the sky at dawn and dusk, during the so-called blue hour. It is named after the French chemist James Chappuis (1854–1934), who discovered this effect.

History

James Chappuis was the first researcher (in 1880) to notice that light passing through ozone gas has a blue tint. He attributed this effect to absorption in the yellow, orange, and red parts of the light spectrum. The French chemist Auguste Houzeau had already shown in 1858 that the atmosphere contains traces of ozone, so Chappuis presumed that ozone could explain the blue color of the sky. He was certainly aware that this was not the only possible explanation, since the blue light that can be seen from Earth's surface is polarised. Polarization cannot be explained by light absorption by ozone, but can be explained by Rayleigh scattering, which was already known by Chappuis's time. Contemporary scientists thought that Rayleigh scattering was sufficient to explain the blue sky, and so the idea that ozone could play a role was eventually forgotten.

In the early 1950s, Edward Hulburt was conducting research on the sky at dusk, to verify theoretical predictions on the temperature and density of the upper atmosphere on the basis of scattered light measured at the Earth's surface. The basic idea was that after the Sun passes under the horizon, it continues to illuminate the upper layers of the atmosphere. Hulburt wished to relate the intensity of light reaching the Earth's surface through Rayleigh scattering to the abundance of particles at each altitude, as the sunlight passes through the atmosphere at different heights over the course of sunset. In his measurements, performed in 1952 at Sacramento Peak in New Mexico, he found that the intensity of measured light was lower by a factor of 2 to 4 than the predicted value. His predictions were based on his theory, and on measurements that were made in the upper atmosphere only a few years before by rocket flights launched not far from Sacramento Peak. The magnitude of the deviation between prediction and photometric measurements made on Sacramento Peak precluded mere measurement error. Until then, theory had predicted that the sky at the zenith during sundown should appear blue-green to grey, and the color should shift to yellow during dusk. This was obviously in conflict with daily observation that the blue color of the sky in the zenith at dusk changes only imperceptibly. As Hulburt knew about the absorption by ozone, and as the spectral range of Chappuis absorption had been more precisely measured only a few years before by the French couple Arlette and Étienne Vassy, he made an attempt to account for this effect in his calculations. This brought the measurements completely into agreement with the theoretical predictions. The results of Hulburt were repeatedly confirmed in the following years. Indeed, not all color effects at dusk in clear sky can be explained by the deeper layers. To this end it is probably necessary to account for spectral extinction by aerosols in theoretical simulations.

Independently of Hulburt, the French meteorologist Jean Dubois had proposed a few years before that Chappuis absorption had an effect on another color phenomenon of the sky at dusk. Dubois worked on the so-called "Earth's shadow" in his doctoral thesis in the 1940s, and he hypothesized that this effect could also be attributed to Chappuis absorption. However, this conjecture is not supported by more recent measurements.