We use a series of state-of-the-art experimental techniques to study the ultrafast (on the scale of femtoseconds, millionths of a billionth of a second) dynamics that underpin photochemical reactions, with a particular focus on reactions of atmospheric relevance. Some of this work takes place at international free-electron laser facilities, using intense short pulses of X-ray laser light. Complementary work will take place in our lab using ultrafast optical lasers.