Ozone is a toxic gas in surface air and a major anthropogenic greenhouse gas in the troposphere. Current models systematically overestimate the low surface ozone concentrations measured in the late 19th and early 20th century1,2,3,4. This suggests that current values of the natural ozone background used as baseline for air quality and climate policy are too high. Here we show that inclusion of tropospheric bromine chemistry in two different global models for the preindustrial atmosphere significantly improves agreement with ozone measurements of a century ago, both in terms of magnitude and seasonal variation. A natural surface ozone background of 13 ± 4 ppb is computed for the US, lower than current estimates and implying a greater margin for ozone reduction by emission controls.