Thinking about the question and answers for Does the Moon have any oxygen in its atmosphere? begs the question, despite being considerably smaller than the Moon and further away from the sun, does Ceres have a similar atmosphere as the Moon?
In general, asteroids experience the same fundamental processes as the moon, the solar, micrometeorite and photons from the sun are striking them and sputtering material off to form an exosphere and except the moon occasionally passes through the Earth's magnetotail. So there must be some sort of exosphere, but its density and composition will vary with distance from the sun and composition of the asteroid.
A 1992 ultraviolet study of Ceres found OH emission, consistent with escaping water, and suggesting a ice cap.