To#nbsp;answer this and similar questions about how urban air "works", we#nbsp;conduct experiments from time to#nbsp;time. In#nbsp;one of#nbsp;the first such experiments, we#nbsp;installed one air quality monitor on#nbsp;each floor of#nbsp;a#nbsp;12-storey building in#nbsp;the center of#nbsp;a#nbsp;metropolis to#nbsp;determine the dependence of#nbsp;pollutant concentrations on#nbsp;height.