Large-scale citizen science protocol provides high-resolution nitrogen dioxide values while enhancing community knowledge and collective action.

We present an already tested protocol from a large-scale air quality citizen science campaign (xAire, 725 measurements, see Ref. [1]). A broad partnership with 1,650 people from communities including 18 primary schools in Barcelona (Spain) provided the capacity to obtain unprecedented high-resolution NO2 levels. Communities followed the protocol to select measurement points and obtain NO2 levels from outdoor locations n=671, playgrounds n=31, and inside school buildings (primarily classrooms) n=23. Data was calibrated and annualized with concentration levels from the city´s automatic air quality monitoring reference stations [2].