A grimy film covers the streets, statues, and buildings in our cities. Chemists reported that, when hit with sunlight, this urban grime may release nitrogen oxides (NOx), which produce ozone and smog. Understanding how this chemical film contributes to NOx levels in cities could help environmental scientists produce better models of air quality. Through previous laboratory studies, D. James Donaldson of the University of Toronto had studied how light can trigger unique photochemistry in urban grime to release nitrates, possibly as NOx. To determine if the chemistry happens in cities, he ran a field study in Leipzig, Germany. He and colleagues set up trays of glass beads on a rooftop: Some sat in the shade, and others sat in direct sunlight. Beads that sat in the light had 10% lower nitrate levels than those in the shade. To understand the fate of the lost nitrates, the scientists went back to the lab and put grime-covered glass particles inside a photoreactor. The team observed that NOx was one of the gases released from the illuminated grime.