In the 46,208 square kilometre area that forms the Delhi-NCR, there are only 51 air quality monitoring stations.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/delhi-55-more-stations-monitor-pollution
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/indian-express-new-delhi
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/delhi
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/noida
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/air-quality-monitoring
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/central-pollution-control-board-cpcb
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/environment-pollution-control-authority-epca
[8] http://admin.indiaenvironmentportal.org.in/category/thesaurus/delhi-pollution-control-committee-dpcc