Exploring the effects of 3D urban form on urban air quality: Evidence from fifteen megacities in China

•3D urban form was measured based on Conzen's townscape framework and existing metrics.•Our results identified the varied effects of urban form indicators for different pollutants and times.•Most urban form indicators were found to have significant impacts on the concentration of NO2.•Morphological categories of the monitoring site and its surrounding areas matter for air quality.•The sites with ‘valley’ type building height distribution in neighbors have lower PM concentrations.