Because you don't mention which specific text you are reading to learn the myth of Persephone and Demeter I am not sure what specific imagery that author is using to tell the story. That said, the story of Persephone and Demeter is the story of why the seasons change, so there may be language that appeals to any or all of the senses that relates to that. Also, Persephone travels to Hades and while there eats the pomegranate seeds, so there may be imagery related to that place and that piece of fruit.
The myth says that Persephone was kidnapped by Hades, the god of the underworld). She would have been allowed to return to earth if she did not consume anything when she was there, but she was presented with a beautiful pomegranate and couldn't resist its juicy seeds, so she ate them and was therefore condemned to stay in Hades forever. Persephone's mother, Demeter was the goddess of the harvest. Because she was so saddened over the loss of her daughter she turned the weather weather cold and no crops grew. Realizing that this was going to destroy mankind, the gods intervened and made a compromise: Persephone would return to the earth for half of the year and would spend that other half in the underworld. Demeter agreed. Hence, Demeter brings warmth and growth to the earth in the spring and summer when Persephone is with her, and ends the growing season with colder weather in the fall and winter. In a written version of the story you should look for images of the various seasons or change in seasons, for images of the scary, dreary atmosphere on earth in the winter and of the underworld at all times, and of the temptation of the pomegranate fruit. I would assume that if you are being asked about atmosphere that the imagery does, in fact, create the positive or negative atmosphere of those places and those times of year.