Climate mitigation policies for cities must consider air quality impacts

Air pollutants are coemitted with carbon dioxide (CO2) during the combustion of carbon-based fuels used to generate energy. By reducing air pollution in cities, we can improve the quality of life for millions of people, reduce the number of premature deaths, and mitigate the climate impacts of CO2 emissions. Here, we discuss the links between CO2 and coemitted air pollutants released from fossil and biofuels in mobile and stationary combustion and the impact of these emissions on air quality in cities. We highlight important examples of how policies to reduce CO2 emissions can either degrade or improve urban air quality, depending on how they are implemented. Key recommendations are to (1) prioritize the removal of carbon-fuel based combustion, especially coal and wood burning; (2) electrify transportation, especially for colder climates and heavy-duty vehicles; (3) remove sulfur from all fuels; and (4) leverage existing air quality monitoring networks to increase density of CO2 concentration measurements in cities worldwide.