For the soup made from peas, see Pea soup. Pea souper in Western Sydney, Australia. Pea soup, or a pea souper, also known as a black fog, killer fog or smog is a very thick and often yellowish, greenish, or blackish fog caused by air pollution that contains soot particulates and the poisonous gas sulfur dioxide.he fog plays a role in Michael Crichton's 1975 The Great Train Robbery On the evening of January 9th, a characteristic London “pea-soup” fog, heavily mixed with soot, blanketed the town. Clean Willy Williams, easing down Tooley Street, one eye to the facade façade Of London Bridge, station was not sure he liked the. fog