Nice effect of Nature or a serious problem created by pollutants? During the last years, several photos have been posted on web of beautiful red skies over several cities all over the world. An article of The Guardian (https://www.theguardian.com/uk-news/2017/oct/16/apocalypse-wow-saharan-dust-and-iberian-fires-turn-uk-skies-red) shows some photos of this curious event which appears beautiful but could hidden a worrying situation.
The article mentions that the color red was due to Sahara dust also, but it is well common in chemistry that the color red is attributable to Iron-based species, like Iron oxides. Such compounds fluctuate in the atmosphere modifying its color into red one. This effect is present in different environment, for example Mars planet, which is also called “red planet”, is very rich in Iron-based compounds (https://www.thoughtco.com/why-mars-is-red-603792).
This effect is surely present in our planet also, especially near to industrialized cities. Please have a look to the below photo, which I got in Shanghai few months ago. The air seems to have a light red colorful, mainly due to the city pollutants.
Our atmosphere is also rich in optical-active compounds, like CO2, which, depending by the temperature and pressure, change their optical behavior creating a very attractive effect. Other gaseous compounds present at a high concentration in the industrialized environment are the NOx (nitrogen oxides) which are normally red-brown gas. Such gaseous species have the same behavior of CO2 which change the optical reaction according to temperature and pressure.
I got this photo few months back in a famous lake of Switzerland, near the north of Italy, which is a well-known industrialized zone with a concentration of NOx highest in Europe. The nice color red is amazing within a lake context.