The changes in mobility across Europe due to the COVID-19 lockdowns had consequences for air quality. We compare what was experienced to estimates of "what would have been" without the lockdowns. Nitrogen dioxide (NO₂), an important vehicle-sourced pollutant, decreased by a third. However, ozone (O₃) increased in response to lower NO₂. Because NO₂ is decreasing over time, increases in O₃ can be expected in European urban areas and will require management to avoid future negative outcomes.