ZiCOS-L is a network of low-cost sensors in Zurich (Switzerland) to monitor carbon dioxide (CO₂) concentrations. After correcting for drift and checking the sensor performance, we found that local factors like traffic, public events and vegetation affect CO₂ levels. Even though the sensors have higher uncertainties than other sensors, the lower cost allows for a denser network with detailed insights into CO₂ levels across the city, helping cities track emissions and support climate action plans.