Rainald Borck, Philipp Schrauth

• We use panel data from Germany to analyze the effect of population density on urban air pollution (nitrogen oxides, particulate matter, ozone, and an aggregate index for bad air quality [AQI]). To address unobserved heterogeneity and omitted variables, we present long difference/fixed effects estimates and instrumental variables estimates, using historical population and soil quality as instruments. Using our preferred estimates, we find that the concentration increases with density for NO2 with an elasticity of 0.25 and particulate matter with elasticity of 0.08. The O-3 concentration decreases with density with an elasticity of -0.14. The AQI increases with density, with an elasticity of 0.11-0.13. We also present a variety of robustness tests. Overall, the paper shows that higher population density worsens local air quality.